Underground to inseam (UIS) borehole gas drainage is of significance for improving mining safety and turning hazardous methane into a clean gas resource. However, in the horizontal UIS borehole sealing by adopting cement-grout-based sealing method, an upper gap frequently occurs in the sealing section after cement-grout solidification, which could potentially be an air-leakage path and consequently decrease drainage efficiency. Despite extensive previous investigations on borehole sealing, very few studies have been reported in addressing the upper gap issue to enhance drainage performance. In this study, the air-leakage flow models in concentric ring and off-centre ring seal gap of horizontal borehole were established. Then by adopting new expansive composite sealing (ECS) material, a double-phase-grouting (DPG) sealing method was developed to seal the upper gap. Based on theoretical fundamentals of seal-gap leakage, procedures and explanations of DPG sealing method were discussed. Finally, coalmine field trials were conducted to examine effects of DPG sealing method. Results show that: (1) there are positive correlations between air leakage quantity into borehole and the gap width, borehole diameter, pressure differences and eccentricity ratio. While the air leakage amount is negatively related to fluid viscosity and sealing-section length; and (2) because of adopting ECS material and the second-phase grouting specially designed for upper gap sealing, DPG sealing method is capable of sealing upper gap well to improve sealing quality. It mainly reflects in increasing drained gas concentration and prolonging the period of high-methane-concentration drainage. In comparison with traditional sealing method, the mean gas flow with DPG sealing method averagely rises by 30%. Outcomes of this study are expected to provide an effective way to solve upper gap issue and enhance drainage performance of horizontal UIS boreholes.

NGL fractionation is one of the most energy-intensive facilities in hydrocarbon processing plants playing an important role in the petrochemical supply chain. Innovative schemes for multicomponent distillation processes made the opportunity to attain more justifiable fractionation processes. The current study presents a systematic procedure to design and optimize both simple and complex distillation schemes based on the separation matrix approach and applies it to a NGL feedstock. A modular design approach is presented for the ultimate rigorous evaluation. The genetic algorithm with a hybrid objective function that minimizes the TAC and checks products specifications simultaneously carries out optimization. The results demonstrate the capability of complex distillation sequences in process improvements, reducing costs and saving energy in NGL fractionation plants resulting in reductions of up to 3.5% in annual operating costs, which is very significant bearing in mind the huge NGL fractionation scale of operation.

A fully coupled fluid-solid three-dimensional model based on cohesive zone method is presented to simulate the interactions between hydraulic fracture (HF) and natural fracture (NF). Unknown interaction mechanisms of the crossing, arresting and bypassing are revealed. The interactions between HF and NF in three dimensions are shown to be more complex than in two dimensions. Numerical results demonstrate that vertical stress contrast plays a significant effect on the fracture geometry for single and multiple stages fracturing treatments. Multiple fractures propagation of different fracturing sequences under the influence of NFs distribution and in-situ stresses is investigated. For Texas-two step fracturing method, the propagation of interior HF in horizontal direction will be terminated by stress perturbation in closely spaced HFs. A method to weaken severe mechanical interactions and enlarge the separations of NFs is proposed. The findings provide some new insights for stimulation design to create fracture network.

This study presents the experimental and numerical evaluation of tertiary-CO2 flooding (CF) using reservoir and outcrop composite chalk at reservoir conditions. We were able to reproduce the results of composite core flooding experiment and our findings show the considerable effect of rock properties and wettability on the tertiary-CO2 flooding. In our experiments, we place the composite reservoir chalk core vertically in the core-holder with the total length of 45 cm and average diameter of 3.74 cm. The composite core consists of six core plugs of 7.5 cm each and include a centralized axial hole that represents the single fracture with the diameter of 0.6 cm. The whole core plugs are sampled from the reservoir formation in North-Sea-Chalk-Field (NSCF) and are saturated with NSCF stock tank oil (STO) and synthetic connate water. Once reservoir conditions are established, the sea water is injected from the bottom of the core holder and the STO is produced from the top. After no additional produced oil is observed, water flooding (WF) is stopped and CO2 is then injected from the top and the hydrocarbon streams are produced from the bottom of the fracture. The whole core flooding is operated at constant reservoir conditions of 258 bara and 110 ºC, representative of NSCF reservoir conditions. Two different chalks, one from the reservoir formation (Exp-3C) and the other from outcrop (Exp-2C) are used in this study to investigate the effect of wettability during tertiary CO2 flooding. A comprehensive compositional numerical simulation with a tuned equation of state (EOS) was developed to model the experiments. Best match for the WF experiments was achieved mainly through tuning the oil-water capillary pressure and reducing the oil relative permeability data. Although the final tuned capillary pressure and relative permeability data were different, we observe similar water saturation at the end WF experiments for Exp-3C and Exp-2C.The tertiary CF lab results were reproduced by numerical model mainly by tuning the multi-component diffusion coefficients. The produced water during CF was successfully modelled by taking the hysteresis effect into account in water-oil capillary pressure data. We also observe that although the final water saturation at the end of WF is comparable, performance of tertiary CO2 flooding for each experiment is different. The reservoir chalk experiment shows higher oil recovery during the tertiary CO2 injection.

Fracture networks in coalbed reservoirs serve as the primary gas pathway and thus determine the gas production potential for coalbed methane (CBM) recovery. However, the characterization of the fracture network is extremely challenging due to the complexity of both the induced and natural fracture system. A microseismic event analysis can be used to locate the fracturing, and determine the orientation, length, complexity, and temporal growth of the induced fracture by using the focal mechanism. In this study, the fracture system of a coal-bearing formation covering 1.2 km2 in the Luan mining area of China is probed via passive microseismic imaging. Focal mechanisms of individual events are used to characterize the gas production potential for the 10 CBM wells in this area. Fracture reactivation modes are of three types - strike slip, dip slip, and extensional modes – with strike slip the most common followed by dip slip and then extensional type as the least likely. In addition, the location of different types of fractures are different, which indicates the difference of the in-situ stress regime. The 10 CBM wells were hydraulically stimulated in December 2017 then dewatered and allowed to produce for 14 months. We show that the microseismic data have a general positive correlation with gas production with a few exceptions - the higher the event count, the higher the gas production. This result is a best embodiment of the mutual control of reservoir fractures, stress regime, permeable and gas production in CBM development. We suggest passive microseismic imaging as an effective technique in evaluating the potential for gas production.

Multi-stage fracturing of horizontal wells to recover shale gas has attracted substantial renewed interest in the physical and mechanical characteristics of shale. The mechanical characteristics, typically the strong anisotropy, significantly affect the nucleation and propagation of hydraulic fractures, as the nucleation mechanisms and propagation pathways primarily depend on the interaction between the actual in situ stress conditions and the anisotropic mechanical properties. However, there remains a lack of effective experimental data on the mechanical properties of the rock matrix and bedding planes. To investigate the mechanical properties, a series of tests, including Brazilian, direct shear and three-point-bending (TPB) tests, were performed on variously shaped Longmaxi shale samples in distinct bedding orientations relative to the loading directions. The results showed that the tensile strength, cohesion, internal friction angle and mode-I fracture toughness of the bedding planes are 4.713 MPa, 8.93 MPa, 31.216° and 0.566 MPa·m1/2, respectively, which are significantly lower than the rock matrix, corresponding to values of 13.164 MPa, 16.175 MPa, 36.222° and 0.957 MPa·m1/2, respectively. This finding demonstrated that the bedding layers are weakness planes on tensile strength, shear strength and fracture toughness in a quantitative manner. However, the values for the rock matrix and Arrester orientation are generally very similar; hence, the mechanical parameters of the rock matrix, especially the fracture toughness and tensile strength, can be approximated by the values determined in the Arrester orientation. For fractures propagating in the direction normal or oblique to bedding, a complex fracture geometry with tortuous propagation pathways is usually generated by bedding cracking and/or fracture deviation towards the bedding-parallel orientation. The mechanical characteristics of the bedding layers play a vitally important part in shale gas development, including the fracture-initiation pressure (FIP) prediction, borehole stability analysis, hydraulic fracture propagation pathways, and complex fracture network generation.

Outcrop and core observations show that discontinuities are well developed in unconventional reservoirs. Presence of these discontinuities can have beneficial or detrimental effect on hydraulic fracturing behaviors. Previous studies summarized several interaction modes between hydraulic fracture and a single-weak plane based on a large number of laboratory tests and theoretical analysis. In some cases, the reservoir contains a considerable number of parallel weak planes that make the fracture propagation more complex. The interaction mechanisms between hydraulic fracture and parallel weak planes remain unclear. In the present study, a bonded-particle model (BPM) coupling fluid-mechanical is used to study the driven force of these interactions. Moment tensor inversion is used to analyze the failure mechanisms of those weak planes. The results indicate that as the number of weak plane increases, the interaction mode becomes complex. Several modes (e.g. arrested, arrested with offsetting, crossing, and crossing with offsetting) are observed in the multiple parallel weak planes contained models. The interaction mode is closely related to a stress concentration around the intersection area between hydraulic fracture and weak planes. The arrested or crossing with offsetting mode is mainly caused by the shear slippage of the weak plane through moment tensor inversion analysis.

This study proposes a transient gas-liquid-solid multiphase flow model for gas kick during MPD, considering the effect of dynamic wellhead back-pressure, temperature field, and velocity relation of different phases. Based on this model, the gas kick control strategies and outflow response characteristics during MPD are thoroughly investigated. The simulated results reveal that, with the upward migration and expansion of the invading gas, the hydrostatic pressure and frictional pressure in the annulus changes accordingly, resulting in a non-linear relationship between the wellbore back-pressure and the bottom-hole pressure. Moreover, the effects of an under-balance pressure at the bottom hole, the pressure balance relationship between the formation and the bottom hole, the kick detection level, the well depth on the wellhead back-pressure control and the response behaviors of the outlet flow rate are discussed. The results of this investigation can provide engineering guidance for MPD to address the issue of gas kick.

Venturi is one of the most commonly used flowmeters for measuring gas-liquid flow at the natural gas wellhead. A better understanding of the flow mechanism and the causes for pressure drop is necessary for the establishment of the measurement model. In this study, a one-dimensional pressure drop model between the two pressure taps of Venturi has been proposed for vertical annular-mist flow. The relations between the liquid film, entrainment rate, void fraction, and velocity slip have been revealed when the annular flow went through Venturi. We consider the gas acceleration in the throat of Venturi. The proportions of various pressure drops are given. The experimental results show that the relative errors of the pressure drop have a confidence probability of 98.5% within 10%. The predicted results are also in good agreement with the on-site data. It proves that the new pressure drop model has good performance.

Gas hydrate is a crystalline compound made of water and mainly of gases methane and carbon dioxide under specific conditions of pressure and temperature. Increasing worldwide petroleum exploitation in deep waters, where these conditions are encountered, favours the precipitation of gas hydrate in seafloor pipelines, resulting in partial or total obstruction of petroleum flow. Brazil’s largest petroleum reserves of the pre-salt interval, for example, are located in ultra-deep waters (> 1500 m) and may have a gas composition of up to 80% of CO2. Huge investments are necessary to inhibit the formation of gas hydrate and to assure petroleum flow in pipelines. Here we present the results of the synthesis of new organic compounds obtained from L-Threonine, which show a high potential to be used as CO2 hydrate inhibitors. This characteristic is related to the increase carbon chain in each molecule (higher hydrophobicity) leading to a reduction on CO2 solubility in water. In addition to that, our study also shows the occurrence of the “salting out effect” and reduced water activity coefficient.

To understand the fluid-shale interactions and the mechanisms that produce cracks during the retention of the fracturing fluid. This paper details experiments performed in shale particle immersion and core plug imbibition, and it investigates changes in fluid properties during a shale well shut-in period at the Longmaxi formation in Southeast Chongqing, China. The results indicate that soluble salts in a shale formation are dissolved during fracturing fluid retention in a shale reservoir, causing the salinity and pH of the fracturing fluid to increase significantly and eventually resulting in the formation of a weakly alkaline solution. Furthermore, fracturing fluid retention significantly reduces the mechanical strength of shale. In the experiments, the elasticity modulus and Poisson’s ratio of the shale decreased by 85% and 54%, respectively, which was immersed in the fracturing fluid for seven days. Two types of forces, repulsive hydration and electric double-layer repulsion, are shown to be the main factors that lead to the formation of a large number of microcracks. The repulsive hydration pressure force is in a short range, operating over only a few nanometers, while the electric double-layer repulsion force operates over a long distance and can act between clay mineral slices. As the concentration of the solution increases, the double-layer repulsive force increases, though the range over which it acts becomes shorter. The increase in the OH- concentration of the solution also accelerates crack propagation. These conclusions are conducive to developing reasonable rules for fracturing fluid flowback in shale gas wells.

Reservoir characterization of thermogenic gas hydrates have so far been limited to seismic mapping, well log interpretation, and lab scale experiments, with numerical modeling mostly focusing on gas production from hydrates. What is unknown however is the hydrate saturation at the sub-seismic and interwell scales. This study uses numerical simulations for predictive modeling of hydrate formation and geobody distribution using TOUGH+Hydrate. Results indicate that the flow of gas is buoyancy and advection dominated, and affects hydrate formation rate and saturation distribution. Geological controls such as fault angles, stratigraphic thickness, permeabilities and permeability anisotropies and contrasts with adjacent formations significantly impact distribution of hydrate saturation, and geobody shape. Hydrates form in both high and low porosity rocks. While full-scale reservoir modeling is contingent upon the availability of detailed reservoir and fluid data, this study underscores the importance of using numerical modeling tools for quantitative predictive modeling and reservoir characterization of hydrate reservoirs for better resource estimation, well-placement decisions, reservoir management and production planning.

Gas extraction in destressed coal seams is one of the effective measures to improve degasification efficiency and reduce gas disasters in deep coal seams. In order to reveal the influence of damage zone induced by stress relief on gas extraction of slotted boreholes, a multi-field model incorporating stress, damage, gas diffusion and gas flow in heterogeneous coal was established. By using this model, the stress relief induced coal damage around slotted boreholes was simulated with the example of Guhanshan Coal Mine. Besides, the effects of homogeneity coefficient, Langmuir volumetric strain constant and overburden stress on coal seam damage and gas extraction were analyzed. The following results were drawn: The rationality and engineering applicability of the model were verified by comparing the numerical simulated results with the field test data in Guhanshan Coal Mine. The stress relief induced coal damage zone around slotted boreholes rises gradually with the increase of the homogeneity coefficient and overburden stress, and the coal seam permeability rises gradually with the increase of Langmuir volumetric strain constant and the decrease of overburden stress. The reasonable borehole distance is determined to be 8 m which is obtained based on the variations of total gas production and gas pressure with different borehole distances. Therefore, reasonable borehole distances under varying conditions can be determined by integrating different geological conditions of coalmines with the proposed simulation method. The simulation results can provide theoretical guidance for the layout of stress relief boreholes and the improvement of gas extraction efficiency in the future.

In order to meet the increasing demand of natural gas worldwide, the development of large-scale and environmentally friendly submerged combustion vaporizer (SCV) burners, which can vaporize massive liquefied natural gas (LNG), is strongly desired. One of good solutions to reduce nitrogen oxides emissions is the installation of water spray system into the burner. In this study, a large-scale SCV burner equipped with a water spray system is developed by numerical simulations of cold flows and experiments of small-scale SCV burners, and the characteristics of the nitric oxide (NO) and carbon monoxide (CO) emissions are investigated. In addition, Large Eddy Simulations (LESs) of turbulent combustion fields generated by the large-scale SCV burner are performed, and the validity is assessed by comparing with the experiments. As a combustion model, the five-dimensional non-adiabatic flamelet progress variable (5D-NA-FPV) approach, which can take into account the effects of various heat losses including latent heats by the evaporation of sprayed water, conduction, and radiation of hot burned gases, is newly introduced. The experimental results show that NO emissions for a large-scale SCV burner decrease to less than 50 ppm corrected at O2 = 5% as the water spray amount increases, but that CO emissions increase due to the heat loss by water evaporation. Also, the LES/5D-NA-FPV adequately captures the effects of the water spray amount on the NO and CO reactions, which suggests that it could be a powerful tool for an optimal design of large-scale SCV burners.

To evaluate the crack initiation and propagation behavior during pulsating hydraulic fracturing (PHF) process, lab tests on PHF of coal samples were performed by using a self-developed true triaxial testing system. The results demonstrate that: the crack could be better developed under pulsating hydraulic load. And as PF increase, the initiation pressure decreases firstly, then increases and reached minimum when the PF is 4Hz. Besides, the changes of acoustic emission (AE) behavior can be divided into four stages: quiet phase, acceleration phase, lifting phase and resting phase. And the accumulated energy of acceleration and lifting phase increased respectively about 4.81 and 17.21 times (average) in comparison with quiet phase. Our results also show that the proportion of minor AE events and great AE events were higher in acceleration phase, and lifting and resting phase respectively. And when the PF is 4Hz, the proportion of smaller AE events is larger.

Pressure dropping control is one of the key technologies for coalbed methane surface development. Low-field nuclear magnetic resonance (LF-NMR) technology was adopted to study methane diffusion and pore dynamics of various pressure drop procedures. Calibration, isothermal desorption, pressure step-down desorption, and pore dynamic experiments were conducted with high-rank coal samples. The results include the following four aspects. 1) Calibration experiments of methane content and LF-NMR T2 spectrum in confined cylindrical coal samples show that adsorbed, porous medium-confined and bulk methane can be distinguished and quantitatively calculated using LF-NMR technology. 2) According to the desorption equation fitting using LF-NMR technology, ultimate desorption volume at different stages of depressurization is calculated and the overall desorption process has been divided into inefficiency, slow, rapid, and sensitive stage. 3) LF-NMR desorption experiments with constant confine pressure show that cumulative desorption volume of two and three step-down pressure reductions are higher than that of one-step and uniform pressure reductions. Synchronisation of rapid or sensitive desorption stage and large effective diffusion coefficient (De) are keys to achieve a high cumulative gas production. 4) De varies significantly along with the variation of equilibrium gas pressure at the inefficiency and slow desorption stage, which is mainly affected by the surface coverage. Stress compression and matrix shrinkage coupling influence the pore structure at rapid and sensitive stage, thereby on De. The different desorption results using various depressurization schemes are the reason that pore deformation behaviour causing dynamic variation of the diffusion process. The above results can provide a basis for optimising the drainage work of coalbed methane.

There is a growing body of evidence that gas situated within the pores of nanoporous materials may not have the same equation of state (pressure, volume, and temperature, PVT) properties as macroscopic free gas. However, there is limited experimental measurement of in-situ fluid properties for gases taken up by nanoporous shales. In this work, we use a gas injection porosimetry approach to measure the gas storage capacity of four different North American shales (Bakken, Marcellus, Haynesville, and Mancos) and in-situ gas density for a few different hydrocarbon and noble gases. We find the porosity measured with helium to be reasonable between 5% and 16.4%. However, when using other gases such as methane, argon, and ethylene, the equivalent porosity estimations are extremely high, with the highest measured value being 309% for ethylene gas in a Marcellus shale sample. Such extreme results raise questions on the validity of the underlying assumptions of the porosimetry equations, in particular, the description of gas density within shale nanopores with macroscopic density. The experimentally measured density of in-situ gas is found to be up to 28 times higher than the theoretically estimated one at the equilibrium PVT conditions. This in-situ densification of gas is independently verified using X-ray CT imaging on one of the samples – the Marcellus. The underlying mechanism for gas densification could be explained by adsorption, in which case the proportion of adsorbed gas is estimated to be between 12% and 96% for the various gas-sample pairs. Surface area measurements show that a monolayer of adsorbed gas can only account for 27% to 42% of the adsorbed gas. This calls into question the commonly assumed Langmuir monolayer model of adsorption, and indicates that gas densification within shale nanopores can be attributed to a multilayer adsorption mechanism and/or other unidentified mechanisms that require further study.

The successful production trial in Ignik Sikumi located in Alaska North Slope showed the significant potential of applying the CH4-CO2 replacement method to natural hydrate production. The comparison on the mechanical behaviors of CH4 and CO2 hydrate-bearing sediments is the key factor in assessing the geomechanical stability of hydrate reservoirs before and after the CH4-CO2 replacement. Large amount of hydrates have been discovered in silty reservoirs. However, studies focusing on the mechanical behaviors of CH4 and CO2 hydrate-bearing silty sediments are rare to date. This paper reports the results of a comparative analysis of the mechanical behaviors, including the isotropic consolidation behaviors and stress-strain relationships of CH4 and CO2 hydrate-bearing silty sediments. The results show that the CH4 and CO2 hydrate-bearing silty sediments present similar compression and swelling indexes. Various hydrate saturation shows differential effects on the strength and stiffness of CH4 and CO2 hydrate-bearing silty sediments. In addition, CO2 hydrate-bearing silty sediments displays weaker contraction behaviors than that of CH4 hydrate-bearing silty sediments.

The study on the permeability of methane hydrates in fine quartz sands contributes to the accurate prediction of gas/water production and can effectively express the characteristics of fluid migration. In this study, the water effective permeability (kw) of methane hydrates in three fine quartz sands containing various hydrate saturation (SH) were carried out under steady water injection and production. Experimental results indicated that the effect of particle sizes on kw was significant even though the difference in the average particle size of three fine quartz sands was relatively limited. The differential pressure of the hydrate sediments was completely recorded during permeability measurement, and the stabilization and decomposition process of hydrate could be clearly represented by differential pressure. The kw increased rapidly with the decomposition of hydrate and the appearance of flow channel in the hydrate sediments. Hydrate dissociation process was recorded in situ by Cold Field Emission Scanning Electron Microscopy (CFE–SEM), the dissociation of hydrates in pore space was clearly visible, and the hydrates in pore space were first dissociated into many small regions. A new reduction model between water relative permeability (krw) and SH was proposed on the basis of experimental results, and the experimental data fitted well with hydrates occupying pore centers. The effect of particle sizes on krw was eliminated, the Archie saturation exponent n tended to increase with the increase of SH, and n was recommended between 20 and 30.

Pipeline inspection gauge (PIG) applied for dewatering and cleaning operation frequently suffers from blockage risk, especially in elbow section. Definitive solution to the blockage problems does not currently exist in literature. In this research, the concept of traveling ability is introduced to describe the performance of blockage avoidance, and it is defined by three influencing factors: sealing performance, which is expressed by the leak area ratio of the PIG cups; driving performance, which is evaluated by the necessary driving pressure, and collision possibility, which is described by the minimum distance between the PIG head or tail and the pipeline inner wall during pigging. The 3D finite element model (FEM) is adopted in this work and is verified by a custom–built experimental setup. Then, the FE model of the PIG running in elbow is established, and the effect of friction coefficients on the influencing factors is discussed. The results indicate that the traveling ability of the PIG will be obviously weakened under the condition with the higher friction coefficients. When the friction coefficient increases from 0.1 to 0.5, the sealing performance decreases by 27.7%; the driving performance decreases by 80% and the collision possibility increases by 94%. The researches of this work are beneficial for improving the PIG designment and avoiding the blockage risk.

Water-injected technique was once considered as an effective way to solve the gas-induced problems in the mining industry, but has been stagnant due to its unstable conditions. A systematic knowledge on the micro-mechanism of water injection into coal seams is beneficial to better understand water-based techniques. However, the laboratory investigations considering the engineering background of water injection as well as the pore morphology of targeted coal seam have been rarely studied. In this paper, characterization of pore morphology and the effect of injected water on gas desorption characteristic were carried out using pore structure analyzers (N2 adsorption and mercury intrusion methods), high-resolution scanning electron microscopy and a home-made instrument (water-injecting desorption test). The pore morphology results from pore size distribution, fractal dimension, pore shape and connectivity indicate that the essential configuration of pore structure is well-developed larger pores containing abundant smaller pores with extensive distribution of constricted pores that are inaccessible to fluid migration. The influence of pore morphology on desorption process may be attributed to the microporous constrictions with non-effective pores, which are geometrically interpreted by pore blocking mechanism. The desorption test results show the total desorption volumes and initial effective diffusion coefficient have the reduce rates of 26.65% and 38% with the moisture content increasing from dry to 1.8% while have a little change as moisture increases (1.8%−11.2%), demonstrating the obstruction of water molecule on gas desorption pathway and the existence of extremity moisture content. Water injection has a remarkable effect on the average desorption rates in the initial period of 10 min; however, the ultimate desorption volume of 0.81 mL/g with higher moisture content of 11.2% is not sensitive to adsorption equilibrium pressure. Moreover, combined with the mature water-injected technique in the actual coal seam, a conceptual design was summarized to consider the effect of the constricted pore morphology on the interactions of "water-gas-coal", which may demonstrate the micro-mechanism of gas migration in the far water-injected coal seam. Meanwhile, in the near water injection zone, due to more energetic gas molecules forming gas bubble in the aqueous condition, nucleation appears to be imagined to explain the pressure-insensitive phenomenon. These findings are of great guiding significance to the theoretical studies and field applications of actual water-injected coal seam.

In relationship with the potential application of nanoparticle (NP) foam technology in CO2 enhanced oil recovery (EOR) and greenhouse geological storage in tight reservoirs, the rheological properties of the NP-stabilized CO2 foam with surfactant (α-olefin sulfonate-AOS) solution were experimentally investigated. The foam was treated as a power-law non-Newtonian fluid, and the relationships between the shear stress and shear rate as well as the apparent viscosity of foam and foam quality were determined under influential parameters including the NP additive, internal gas type, salinity, and oil presence. Critical foam quality values in the range 91–96% were observed for the NP-stabilized foam, whereas no such values were obtained for the corresponding AOS-CO2 foam. The results show the NP-AOS-CO2 foam has lower viscosity compared to the NP-AOS-N2 foam, and the solution salinity decreases the viscosity of the NP-AOS-CO2 foam, whereas oil presence has no obvious effect on the foam viscosity.

Permeability significantly affects the production of shale oil and shale gas, and shale microstructures characterized by pore and fracture spaces innately affect the permeability. However, the quantitative characterization of permeability related to pore and facture spaces is not fully understood. This work aims to propose 3D spatial fracture-pore fractal dimensions to predict shale permeability and their effects on fluid flow behaviors. First, triaxial compressive stress and X-ray CT imaging tests are conducted on shale samples to establish fractural models. The digital surface roughness segmentation (DSRS) method is then proposed to obtain the fracture-pore microstructures. Next, spatial fractal dimensions of self-similarity microstructures are proposed to predict the microstructural permeability. Finally, two-phase fluid flows are simulated to study the hydrocarbon flow behaviors in fractural microstructures using the level set method. The results show that the average relative errors between the microstructural spatial dimensions and theoretical fractal dimensions are all less than 3%, highlighting the accuracy of the proposed method. The numerical results for permeability are very close to the analytical solutions, in which fracture permeability is almost 100 times the order of magnitude of the pore structure permeability in the nanoscale pore shale, and the facture and pore structure permeabilities both increase with increasing spatial fractal dimension. The changes of fluid flow behaviors are similar to the permeability variations, and the fluid phase fraction increases with increasing fractal dimension.

Due to the large resource potential of tight sandstone gas, the Upper Paleozoic strata of the Linxing area has become an important exploration target in the Ordos Basin. In this study, by analyzing the distribution characteristics of tight sandstone gas (TSG), the relations of source-reservoir assemblages were divided into three types containing interbedded with source rocks (Type I), adjacent to source rocks (Type II) and far from source rocks (Type III). Five factors were identified to control their genesis, migration, and accumulation. Firstly, the gas generation intensity of source rocks is larger than 10×108 m3/km2, which indicated that source rocks could provide gas to the reservoirs. The gas generation centers of source rocks control the distribution of TSG reservoirs, while the areas among the hydrocarbon generation centers were the enrichment zones for TSG. Secondly, under the condition of generally tight reservoirs, the sandstone reservoir which has good quality (high porosity, high permeability) provided the favorable accumulation condition for TSG. Thirdly, the study area experienced one stage of gas charging mainly from the late early Jurassic to late early Cretaceous, which was 178 to 100Ma. Continuous gas generation and expulsion led to the sustained accumulation of TSG. Fourthly, with the general development of overpressure in the Upper Paleozoic strata, gas expansion force as the main power drive the natural gas to migrate into the reservoirs of type I and type II during the accumulation period. Finally, the development of faults and fractures provided a migration path for natural gas charging in the reservoir of type III vertically. The existence of microcracks can improve the petrophysical properties of tight reservoirs. Under the joint control of these factors, the reservoir of Type I is the most favorable exploration target.

Natural oil-gas-bearing rock formation generally contains a large number of discontinuities, which have a large influence on the cracking process inside the rock under high water pressure condition. Hence, a good understanding of the propagation and coalescence of fluid-driven cracks is important to improve the oil and gas exploitation efficiency. This study numerically investigates the fluid-driven crack propagation process in rock specimen possessing two pre-existing flaws using a fluid coupled discrete element method. The micro-parameters are first calibrated against the mechanical properties of the Lac du Bonnet granite. The numerical specimen models are generated by installing two flaws with different ligament angles, ligament length, and flaw angles. The injection test with a constant rate is then conducted to study the propagation and coalescence patterns in these pre-cracked numerical models. The numerical results show that a relatively large ligament angle is better to accelerate the crack coalescence in the specimen with two paralleled flaws. Numerical model possessing two pre-existing flaws with a small ligament length is inclined to traverse the ligament area. Overall, the results in this study reveal that the pre-existing flaws inside the rock have a significant effect on the fluid-driven cracking process and much attention should be paid to the crack propagation behavior when many discontinuities are associated in the oil-gas-bearing rock formation.

Fractures have a significant effect on the mechanical properties of coal and rock, and fracture structure is complex. Combinined with indoor physical tests and corresponding numerical simulation tests, the relationships among rock mechanics parameters, cleat complexity, and wave velocity were analysed. Based on the conversion relationships between coal and rock cleat complexity and coal seam fissure complexity, four calculation models of rock mechanics parameters based on coal bed fracture complexity and wave velocity were established. The results indicate that coal with different cleat density, quantity, or angle has noticeable fractal characteristics, and cleat fractal dimension can be used to characterize the cleat complexity. The fractal dimension of deep lateral resistivity curve can characterize the fracture complexity of coal beds. There is a good linear positive correlation between cleat fractal dimension and fractal dimension of deep lateral resistivity logging curve. Velocity and cleat complexity have no significant correlation, and the attenuation coefficient increases with the increase in cleat complexity. Uniaxial compressive strength of coal and rock decrease with the increase in cleat complexity and is more sensitive to the change in cleat angle than to cleat density and cleat quantity changes. Young's modulus of coal and rock decrease with the increase in cleat complexity and is more sensitive to the change in cleat quantity than cleat density or angle changes. Poisson’s ratio of coal increase with the increase in cleat complexity and is more sensitive to the change in cleat quantity than cleat density or angle changes. The tensile strength of coal and rock increases with the increase in cleat complexity and is more sensitive to the change of cleat quantity than cleat density and angle changes. Centralized distribution of cleats has more significant influence on the uniaxial compressive strength, Young's modulus, Poisson’s ratio, and tensile strength of coal than an even distribution. The calculation models of rock mechanics parameters based on fracture complexity are suitable for the study area. The method of combining the fracture complexity of coal beds with acoustic wave velocity to calculate rock mechanical parameters are feasible.

Gas hydrate pore habits significantly impact the pore-scale structure within hydrate-bearing sediments, and thus, play a central role in the physical property evolution. Characterization and quantification of the effective pore space within hydrate-bearing sediments at different hydrate saturations have not been well offered. This study performs random simulations of hydrate nucleation and growth in quartzitic sands to understand effects of hydrate saturation and hydrate pore habits on fractal characteristics of the effective pore space. Normalized pore-size fractal dimension and normalized maximal pore diameter are characterized and found to decrease with increasing hydrate saturation. In order to predict hydrate saturation dependent pore fractal characteristics, theoretical and empirical models are proposed and further extended to give implications to hydraulic, mechanical, and electrical properties of hydrate-bearing sediments during hydrate dissociation. Implications include that hydrate dissociation facilitates the absolute permeability and the electrical conductivity, and enhances first and then reduces the saturation exponent of Archie’s law; hydrate dissociation also lowers the capillary pressure, and this promotes relative permeability to gas but inhibits relative permeability to water even the water saturation remains as a constant; shear strength of unsaturated hydrate-bearing sediments drops down due to the decreasing capillary pressure as hydrate dissociation. These implications all meet with conclusions in previous literatures.

Fracture identification using conventional logs is a cost effective way of identifying fracture zones in reservoirs. However, there are challenging problems including complex well log responses and small amount of labelled data available from cores or image logs, making it difficult to build a prediction model with a good generalization capability. To address these problems, a semi-supervised learning method termed Laplacian support vector machine (LapSVM) is introduced in this work, which is a combination of the supervised kernel method and the unsupervised clustering method. LapSVM inherits SVM’s capability of handling nonlinear problems and overcomes partially the issue of limited labelled data by using the unsupervised clustering technique with the help of abundant well log information. To examine the effectiveness of LapSVM for fracture identification in tight reservoirs, a dataset from the tight sandstones of the Ordos Basin in China is used. Both statistical and geological evaluations indicate that LapSVM outperforms other three nonlinear SVM methods tested. It has been demonstrated that LapSVM can provide an accurate and effective means for the identification of fracture zones in tight reservoirs.

This paper studies the axial load transfer along the drill-strings in deviated wells by developing a finite element model based on the Euler beam theory and the augmentation Lagrangian contact algorithms. The model can simulate the entire drill-strings showing nonlinear contact model between drill-strings and casing. Special attention is given to the axial load loss, the pipe-casing contact force distribution and the slender pipe deformation. The efficacy of the proposed model is validated experimentally using a packer releasing procedure. Various drill-string factors, such as deviation angle, dogleg severity, hook load magnitude and buckling configurations, are considered for evaluating the efficiency of axial load transfer. Our analysis shows that the dogleg severity has a significant influence on the transfer, and the helical buckling of the drill-strings due to excessive loading could make it worse. This study provides a theoretical understanding of the variation of the contact force and the axial load transfer for the drill-strings in deviated wells. It can be used to better understand the working condition of downhole and guide field drilling.

We propose a radial differential pressure decay (RDPD) method to measure the apparent permeability in shale matrix. The experimental design extends the upper limit of pressure in conventional methods. The experimental error caused by the dispersiveness of sample size and the irregularity of particle shape is reduced by utilizing precisely prepared micro-plug samples with highly uniform diameters, and the reliability of the result is improved substantially. The late-time approximate solution for cylindrical samples based on the experimental configuration is presented for data interpretation and verified with the numerical simulation. The experiments based on shale matrix samples are demonstrated. The results reveal that the RDPD method can capture a differential pressure decay curve precisely, which can be closely fitted through the late-time solution. It also has great potential for various tight porous media. This method sets the stage for the accurate investigation of gas transport mechanisms in tight porous media in future work.

In the current study, the effect of tetra-n-butyl ammonium bromide (TBAB) and cyclohexane mixture on CH4 semi-clathrate hydrate formation was studied. Semi-clathrate dissociation conditions for CH4 + TBAB + cyclohexane+ water were investigated at different concentrations of TBAB (0.05, 0.10, and 0.15) mass fraction in the presence of cyclohexane at the pressure and temperature ranges of 1 - 8 MPa and 275.1 - 295 K, respectively. In addition, a thermodynamic model was suggested to predict the phase equilibria of our system, which is divided into four phases, where the van der Waals–Platteeuw Solid Solution Theory has been used to predict the hydrate phase. For gas phase, The SRK equation of state was applied. For oil phase, the cyclohexane activity coefficient in the organic phase was calculated by the non-random two-liquid model (NRTL). Finally, to determine the activity coefficient of the electrolyte species in the aqueous phase, the semi-empirical electrolyte NRTL (eNRTL) activity model was used. The results showed that the proposed model has an acceptable agreement with the experimental semi-clathrate hydrate dissociation data with an approximately average absolute relative deviation of 5.4%.

Natural gas hydrate in marine sediments and permafrost areas is considered as an important potential energy source. Since hydrate dissociation will reduce the stability of gas hydrate-bearing sediments (GHBS) and may cause wellbore failures and geological disasters during gas production, it is necessary to reveal the mechanical behavior of GHBS for the safe exploitation of natural gas hydrate. This paper proposes a geomechanical constitutive model of GHBS within the multishear bounding surface framework. Following the slip theory of plasticity, a constitutive formulation is obtained by splitting the macro constitutive response of sediments into a macro volume response and a series of micro shear responses in spatial distributions related to virtual microshear structures. Each microshear structure describes micro shear and dilatancy responses in three orthogonal orientations. A micro stress–strain relationship and a micro stress–dilatancy relationship are established for each orientation of the microshear structure. The model comprehensively describes the consolidation, hardening, softening, dilatation, collapse, and non-coaxial characteristics of gas hydrate-bearing sediments by introducing the multishear concept, state parameter, evolution law of hydrate bonding and debonding, and collapse strain caused by hydrate dissociation. The effectiveness of the model is confirmed by simulating the available published laboratory tests on the samples of synthetic and natural GHBS under different pore pressures, temperatures, initial void ratios, hydrate saturations, and initial effective confining stresses.

Aqueous phase trapping (APT) that causes vital damage has not attracted attention in production yet. In this paper, the APT damage is evaluated based on imbibition and backflow experiments via studying six factors (imbibition pressure difference, liquid types, initial permeability and porosity, initial water saturation, clay minerals and natural crack). Experimental results show that the imbibition pressure difference is positively correlated with APT damage, and the damage extent is still medium (Permeability Damage Ratio (PDR) is 0.32) even for underbalanced conditions. Compared to the 3%KCl solution and 0.01% scale inhibitor solution, the 0.1% foam fluid presents the weakest damage (PDR is only 0.14) because of its lower interfacial tension (σ=22.1mN/m) and higher contact angle (θ=20.5°). A lower initial water saturation can result in more serious APT damage. The APT damage increases with the declination of permeability, but the increase of porosity. The content of kaolinite is positively correlated with the damage extent and plays a significant role because of the largest adsorption capacity compared to other studied clay minerals (maximum adsorption volume and specific surface area of kaolinite are 120.28cm3/g and 24.63m2/g, respectively). The APT damage extent of natural crack is lower than that of matrix pores (PDRs are 0.10 and 0.32 for the natural crack and matrix pores, respectively). However, the natural crack can strengthen APT damage in matrix pores because the crack face provides a larger imbibition area for the matrix pores. Three unfavorable engineering responses can be induced by APT damage in production, which are the reduction of production flow rate, increase of residual liquid in wellbore and the elevation of the pressure difference between casing and tubing, respectively. This paper provides new insights for the APT damage and presents a great significance for the diagnostics and control of APT damage for tight sandstone gas reservoirs.

As a new resource, NGH (Natural gas hydrate) has aroused wide concern around the world, and it is instructive to use mechanics experimental technique to research the strength and deformation characteristics of hydrates and their sediments. In this research, firstly the triaxial compression tests had been conducted under various conditions, and the influence of sand contents on the mechanical characteristics of sediments containing methane hydrates was researched. The results showed that the stress-strain type was affected by quartz sand content, and the failure strength increased as sand content rose. The confining pressure, temperature and strain rate also had a remarkable influence on the failure strength of samples. Moreover, according to Mohr-Coulomb failure criterion, the mathematical relationships between the inner friction angle, the cohesion force and the sand content were also set up, and it was found that the inner friction angle was greatly changed by the sand content of the sediment matrices.

This study aimed to investigate the influence of cyclic hot/cold shock on the change in the permeability of coal sample under different temperature gradients. The low permeability of Chinese coal seams has always hindered the exploitation and use of coalbed methane resources. In the present study, bituminous coal produced from the Xutuan Mine was selected as the test coal sample. The hot/cold shock experiments of a number of multicycle treatments under the same temperature gradient and different temperature gradients, for single-cycle processing times, were carried out by heating of samples in a drying oven, followed by a low-temperature treatment using liquid nitrogen. The fixed-point photography and nitrogen percolation tests were carried out on the coal samples at various stages of exposure. The results showed that the cracking effect of coal was proportional to the number and the temperature gradient of the cycle hot/cold shock treatment. After two treatments, the maximum increment of the permeability of coal samples reached 1129.79%. The permeability increment of coal sample with heating temperature of 100°C is 2.89-6.44 times that of coal sample with 20°C. This paper used a quadratic function to fit and discuss the seepage results of coal samples. The influence of hot/cold shock treatment on the Klinkenberg effect of nitrogen flow in coal was also analyzed.

Fracture closures have direct effects on the void ratio distributions of proppant particles filled in hydraulic fractures and then influence the fluid pathways of coalbed methane (CBM) within those fractures. In this paper, a numerical simulation method was used to investigate the void ratio distributions of proppant-filled fracture under different compression amounts of 0%, 5%, 10%, 15% and 20%. The results showed that the void ratio distributions had exhibited an inverted “U” shape, with a trend of decreasing first, followed by remaining constant, and then increasing from the bottom to the top of the fracture, which could be divided into three zones: bottom loose zone (BLZ), middle compaction zone (MCZ) and top loose zone (TLZ). At the same compression, the small coordination numbers in the BLZ and TLZ had accounted for a larger proportion, and the average coordination numbers and void ratios in the two zones were 0.8-0.9 and 1.3 times than those in the MCZ, respectively. In addition, the proportion of the large coordination numbers in the different layers had increased, and the layered average coordination numbers had increased linearly, which had resulted in the layered void ratios decreasing linearly with the increases in the compression amounts.

This study provided direct evidences for the evolution of the growth modes, morphology, lateral growth rate and the mass transfer channels during the thickening growth of hydrates on gas (CH4 – C3H8) bubble suspended in water with inhibitors. Uniform and dense hydrate film was observed on the bubble and at the gas-liquid planar interface during thickening growth in absence of inhibitor (Growth Mode 1), but the film was heterogeneous when low concentration kinetic inhibitor was added (Growth Mode 2) and even not able to form when high concentration thermodynamic inhibitor was added (Growth Mode 3). In the Growth Mode 2, the adsorption of inhibitor on the frontier of the hydrate film destroyed the continuity of the hydrate growth and decreased the lateral growth rate of the hydrate film on the bubble. Moreover, the heterogeneousness resulted in numerous cross-shaped or circular craters on the film, which served a new mass transfer channel and accelerated the initial thickening growth of the hydrate. In the Growth Mode 3, the evolution of the hydrate morphology was correlated to the viscosity of the hydrate formed in the reaction systems and the spreading ability of the aqueous solution on the reactor wall. To the best of our knowledge, this was the first study on the microscopic effects of inhibitors on the kinetics of the hydrate growth on gas bubbles, which is of added value for better understanding the hydrate evolution process in the hydrate inhibition scenarios with gas bubbles.

Oriented perforation steering fracturing technology with the purpose of enlarging fracture extension scale and increasing reservoir reconstruction volume is a common method for stimulating and reforming low permeability oil and gas reservoirs. Accurate prediction of the fracture deflection trajectory is the key to successful implementation of this technology. Based on the relevant theories of elastic fracture mechanics and the microelement method, a prediction model for the fracture deflection trajectory of steering fracturing is established by comprehensively considering the influences of horizontal in situ stress difference, perforation parameters, injection parameters and formation mechanics parameters. The model completes the cyclic iteration calculation between the minor increment and the propagation direction angle of the crack tip using the calculation program compiled by Visual Basic and realizes the simulation of the dynamic propagation of steering fractures. Taking the tight sandstone reservoir of the Shihezi Formation in Linxing block as an example, the differences between the hydraulic fracture trajectory calculated by this model and the extended finite element model and the micro-seismic monitoring values in the fracturing field were compared, and the influence of various factors on the deflection distance of the hydraulic fracture trajectory were studied. The results show that, compared with the XFEM model, the hydraulic fracture trajectory simulated by this model has better matching with the micro-seismic monitoring results. The deflection distance decreases negatively logarithmically with increasing horizontal in situ stress difference and increases linearly with increasing fracturing fluid displacement and viscosity. The larger the perforated angle, the longer the deflection distance, and the length of perforation has little effect on the deflection distance. The model and conclusions are significant to further understand oriented perforation steering fracturing.

CO2 Water-Alternating-Gas injection (CO2-WAG) is still a challenging task to simulate and predict accurately, due to the complex interaction of CO2/oil phase behaviour, 3-phase flow and the heterogeneity of the porous medium. In this paper, we focus specifically on the regime of viscous fingering flow in CO2-WAG in heterogeneous systems because of its importance in elucidating this complex interaction. This work presents a detailed simulation study of both immiscible and near-miscible CO2-WAG and continuous CO2 displacements with unfavourable mobility ratios for 1D and 2D systems. 2D heterogeneous permeability fields were generated as Correlated Random Fields (CRF) with specified degrees of heterogeneity (permeability range, described by the Dykstra-Parsons coefficients, VDP) and structures (defined through the dimensionless correlation range, RL=λ/L). Our central aim is to improve the modelling of CO2 displacement in the transition from immiscible to miscible flows in CO2-WAG processes. To do so, two key physical mechanisms that occur during near-Miscible WAG (nMWAG) processes have been studied in detail, namely compositional effects (denoted as Mechanism 1, MCE) and low-interfacial-tension (IFT) film flow effects (denoted as Mechanism 2, MIFT). The low IFT effects in MIFT manifest themselves in an increased mobility of oil phase due to enhanced film formation and flow processes. This latter mechanism (MIFT) is modelled as an increased oil relative permeability using different well-known models (Betté and Coats) parameterized by the gas/oil IFT (σgo), calculated in the simulation from the compositional PVT model via a built-in correlation (the McLeod-Sugden equation, in this case). A range of various combinations of oil-stripping effects (MCE) and IFT effects (MIFT) has been tested to evaluate the potential impact of each mechanism on the flow behaviour such as the local displacement efficiency and the ultimate oil recovery. Oil bypassed by viscous fingering/local heterogeneity, can be efficiently recovered by WAG in the cases where both MCE and MIFT are taken into account (as opposed to either mechanism being considered alone). We also show that the way these two distinct but related mechanisms (MCE and MIFT) operate in near miscible conditions cannot be observed in (i) a simple 1D system such as a slim tube experiment, or (ii) in a heterogeneous system under continuous CO2 injection. Using tracer analysis in our simulations, we demonstrate that a major recovery mechanism in near-miscible WAG displacement is viscous crossflow between non-preferential (bypassed) flow-paths and preferential flow-paths (i.e. between the viscous fingers). Due to the significance of IFT effects (the MIFT mechanism), we also present comparative results from two of the IFT-dependent relative permeability models (Betté and Coats) showing the impact of each model on the simulation of the near-miscible WAG flow behaviour.

The co-existence of gas, water, and monoethylene glycol (MEG) is common in produced fluids of high-pressure gas wells. Accurate predictions on the temperature changes during the choking process are essential for the design and operation of the choke valve. This paper presents an efficient multiphase isenthalpic flash method based on the cubic-plus-association equation of state (CPA EOS) to calculate the choke temperatures. In comparison with the traditional isenthalpic flash algorithm, this new method accounts for the self- and cross-association between polar water and MEG molecules, yielding more accurate enthalpy calculation results and multiphase component distributions for fluids containing water and MEG. The proposed model is validated by field test data with pressures from 8.68 MPa to 119.3 MPa. The average absolute deviations between the calculated choke temperatures and measured values are less than 1.6 °C even for vapor-liquid-aqueous three-phase mixtures at various pressures. Moreover, case studies show that accounting for the association between polar water/MEG molecules contributes to accurate predictions on choke temperatures. At high pressures, the CPA EOS tends to give higher choke temperatures in comparison with those calculated based on the traditional SRK-Peneloux EOS. In contrast, the CPA EOS tends to yield lower temperatures at low pressures.

Gas void fraction plays a significant role in determination of several multiphase flow parameters. Good insight of its behaviour coupled with accurate prediction is imperative for design of efficient equipment which has the potential to translate to higher production rates in the petroleum industry. Against the background of the prevalence of higher viscous and imminent application of highly viscous liquids in the petroleum industry, air-water and air-low viscous liquid mixtures dominate gas void fraction research in vertical pipes. In this work, gas-liquid (μl=100−7000mPas) mixtures are used to investigate the behaviour of gas void fraction in vertical pipes. The influence of superficial phase velocities and liquid viscosity are observed. Further, a combined database consisting of experimental and the reported data of Schmidt et al. (2008) is employed to evaluate the predictions of 100 existing correlations. The results indicate that the Hibiki and Ishii (2003) and Bestion (1990) correlations are the overall best and second-best performing correlations. In the absence good performing correlations for churn and annular flows, two correlations each, based on drift flux and slip ratio, are developed respectively. Predictions from these correlations show good agreement with the database and comparable performance with the overall best correlations.

The widely distributed microfractures play an important role in shale gas production. However, limited studies focus on gas flow behavior in microfractures, and ignore the complex transport mechanisms, leading to a large error for gas permeability evaluation. In this work, a newly dynamic apparent permeability (AP) model, coupling poromechanics, sorption-induced strain, and gas slippage, has been proposed to effectively reveal the gas flow mechanisms through microfractures of shale. Specifically, a dynamic aperture is innovatively incorporated into the Navier-Stokes (N-S) equation using the second-order slip boundary condition to calculate the gas velocity and volume flux in single microfracture. Based on that, the gas transport model for microfracture networks considering the distributions of aperture and tortuosity is derived using the fractal theory. The newly developed model is verified well with experimental data and network simulation. Results indicate that the gas conductance highly depends on the structure of microfracture networks (i.e., the maximum aperture and fractal dimensions). There are three different AP evolutions under various boundary conditions (i.e., constant confining pressure (Pc), constant pore pressure (Pp), and constant effective stress (σeff)) resulting from the coupling transport mechanisms. The AP presents a similar shape of “V” at reservoir conditions (i.e., constantPc ), indicating the “negative contribution” of poromechanics at an early stage, and the “positive contribution” for both gas slippage and sorption-induced strain at the late stage should be underlined during gas production. Moreover, the “negative factor” of poromechanics is positively correlated with fracture compressibility coefficient but negatively associated with Biot’s coefficient at high pressures (>15 [MPa]). Increasing gas desorption capacity, fracture spacing, and internal swelling coefficient can enhance the “positive factor” of sorption-induced strain at low pressures (<15 [MPa]). This work provides a theoretical guidance to develop shale gas effectively.

In situ coalbeds are commonly inclined to one or multiple directions. The principal permeability tensor of an inclined coalbed may thus be non-coaxial to the uniaxial strain conditions, which are composed of two underlying assumptions: invariant vertical stress and confined horizontal boundaries . This paper developed a mathematical model to represent the principal permeability tensor of inclined coalbeds during pore pressure depletion under uniaxial strain conditions. This model adopted two geological properties to correlate the orientations of principal permeabilities and uniaxial strain conditions. One is the dip angle of inclined coalbeds, and the other is the pitch angle of cleats. The developed model was verified by field permeability data of the coalbeds in the San Juan Basin. Dip angle and pitch angle are the featured parameters of the developed model. This paper thus evaluated how the two angles influenced the magnitudes and evolution behaviors of principal permeabilities during pore pressure depletion under uniaxial strain conditions. The influences of dip angle and pitch angle on principal permeabilities can be attributed to their influences on the effective stresses normal to principal permeabilities. In order to upscale the principal permeability tensor for CBM recovery, this paper formulated a tensor-based index system for coal permeability evaluation and discussed how to use this index system in CBM recovery. The tensor-based index system is composed of three indexes: the principal permeability tensor, the equivalent permeability tensors coaxial to engineering issues, and the geometric mean (GM) permeability of the three principal permeabilities. The principal permeability tensor can be used to optimize the pattern of vertical CBM wells, to determine the optimal directions of lateral wells, and to predict the propagating orientation of hydraulic fractures. The equivalent permeability tensors coaxial to engineering issues can be implemented into numerical simulators to improve the precision when evaluating and predicting the production performance of CBM wells. The GM permeability represents the ensemble transport capacity of coalbeds and can be used as an indicator to represent the productivity of coalbed reservoirs. This paper also preliminarily discussed how to use the tensor-based index system in CBM recovery.

The modified Claus process is one of the major processes employed to convert hydrogen sulfide to elemental sulfur form acid gas. Kinetic models provide a better estimation of Claus reaction furnace effluents, however, kinetics of the coupled reactions are complex and depend upon the feed composition and conditions. Previous studies focused on fitting the kinetic parameters to better approximate the furnace effluents of the observed plants. In this work, a proposed kinetic model based on key global reactions is examined with different feed conditions and acid gas compositions without parameter fitting. Results of the proposed reduced model are compared with plant data, published data from lab-scale experiments, results of a detailed reaction mechanism and of a reduced kinetic model from the literature. Overall, a good agreement is found with each case for the prediction of the reaction furnace effluent temperature and compositions by the proposed model. The proposed model is computationally efficient for Computational Fluid Dynamics design, and optimization and control studies for Claus reaction furnaces.

Accurate acquisition of the coal seam permeability is of great significance to gas drainage. Field measurements are more reliable to determine the coal permeability than laboratory. The Radial Flow Permeability (RFP) measurement is the most used field measuring method to measure the coal seam permeability in Chinese coal mines. The theoretical basis of the RFP method is the single-porosity medium flow theory and the gas content in coal seams is described by a parabolic equation. However, coal is a typical dual-porosity medium. The simplifications may lead to nonnegligible influence on the accuracy of the RFP method, which are not well documented in the literature. In this paper, three different mathematical models of gas flow in coal seams are established. The field gas flow data was matched by these three models to prove the reliability of these models. Then the influences of single-porosity simplification and the parabolic gas content simplification on the accuracy of the RFP method were investigated by comparing the differences of gas flow rates and gas pressures. Results show that there are apparent deviations of gas flow rate and gas pressure between these three models. The precision of the RFP method rapidly reduces with the increase of cleat spacing, which is induced by the single-porosity simplification; the precision decreases with the increase of Langmuir volume, first increases and then declines with the increase of Langmuir pressure, which are led by the gas content simplification. Moreover, because of the imperfections of the numerical calculus in the derivation of the RFP method, the calculated value is always smaller than the true value of coal seam permeability.

The success rate of wellbore strengthening is relatively low in shales. Thus, a multi-field coupling wellbore strengthening model was developed by introducing the fully coupled thermo-chemo-poroelastic theory into wellbore strengthening physical model and the fracture element was added to study the closure process of isolated fracture in shales. The thermal and chemical effects were researched by observing the change of hoop stress and fracture aperture. Results show that thermal factor is crucial to wellbore strengthening. The preliminary treatment of wellbore strengthening is mainly controlled by adjusting thermal gradient, combined with a reasonable chemical gradient to inhibit hydration and swelling of clay. The anisotropic Young’s modulus and Poisson’s ratio are sensitive to hoop stress, the solute diffusion coefficient and permeability anisotropy have a minor effect on wellbore strengthening in low permeability formation. Compared with short fracture, the relatively longer fracture is beneficial for wellbore strengthening with bridging location near the fracture mouth.

Water-shale interaction remains an unsolved problem because of the complexity involved in the physical processes and the heterogeneity in the chemical composition and pore structure of rocks. By considering the basic physical properties of Longmaxi shale and performing a series of physical experiments representing the water-shale interaction, the relationship between the mineral composition and water-shale interaction, which is the process responsible for causing structural damage when water and shale interact, is analysed. The ion exchange of clay minerals occurs when shale makes contact with water and different kinds of cations experience different degrees of overflow in water-shale interaction. The charge of clay mineral changes during water-shale interaction, resulting in a change of the gravitational and repulsive forces between the particles. This leads to the passivation of the contour edges of clay minerals and changes in the mechanical properties. In a relatively short time, illite can produce a large hydration stress with a small expansion value, but the hydration speed of Na-montmorillonite (Na-MMT) is relatively slow. Uneven stress caused by the hydration of different clay minerals being soaked in different aqueous solutions can cause local stress concentration, further promote the expansion and increase of original micro-cracks in shale, and then appear as disordered macro-cracks. The macroscopic cracks also provide a channel for the continuous entry of working fluid. More water molecules enter the shale faster and make contact with clay particles, weakening the interaction and cementation between particles. Macroscopically, they are manifested as a decrease in the rock cohesion, internal friction angle, and compressive strength as well as a failure of the structural integrity. Different inorganic salt solutions have different inhibitory effects on reducing the hydration degrees of illite and Na-MMT. Therefore, the water-shale interaction of organic-rich shale is a process in which the microscopic damage of water to rock gradually evolves into macroscopic damage, and results in the local continuity loss of rock on the basis of surface hydration, ion hydration, and osmotic hydration of clay minerals. The higher the clay mineral content, the more likely that hydration will occur, resulting in more serious shale structure damage and a shorter time for damage to occur.

This work aims at assessing the cement integrity during CO2 injection by illustrating the relevance of injection operations to the failure modes of cement. A single-phase flow model for pure CO2 is developed to provide the temperature and pressure profiles in the well and solved by a finite difference method. With the downhole temperature predicted by the flow model as a boundary condition, we present a mechanical model to estimate the stress state in the well section under plane-strain conditions as well as the stress intensity factor at a radial crack tip. This study introduces four failure factors corresponding to shear compressive failure, radial cracking, interfacial debonding, and fracture propagation. Further parametric studies are implemented to reveal the effects of injection operations, including injection temperature, rate, and time, on the above failure factors. They indicate that the injection rate is the most influential factor for cement integrity. High injection rates could cause radial cracking and its propagation as well as interfacial debonding but reduce the risk of shear compressive failure. Low injection temperatures or a long injection time would undermine the tensile and interfacial strength and extend the radial crack in cement while they benefit on shear compressive strength. The results provide guidelines to optimize injection operations for long-term well integrity during CO2 injection.

Accurate prediction of the apparent permeability relies on deep understanding of unconventional gas transport mechanism in microscale. To achieve this goal, it is necessary to develop a controllable generation method for low porosity porous media with shale characteristics and explore advanced numerical method to study non-equilibrium phenomena in porous media. Based on the VORONOI tessellation and its topological relationship, a new method for the generation of controllable low porosity porous media is proposed. Two numerical simulation methods are adopted to study gas flow in VORONOI porous media systematically, including Multiple-Relaxation-Time Lattice Boltzmann Method for the intrinsic permeability and Linearized BGK equations via the Discrete Velocity Method for the apparent permeability influenced by the non-equilibrium effect. The results show that the effective porosity is the decisive factor affecting the intrinsic permeability. Porous media with low porosity and high tortuosity are more affected by the rarefied gas effect. The velocity contours show that velocity gradually increases in the throats perpendicular to the pressure gradient direction due to the rarefied throttling effect, which reduces the mass flow rate and apparent permeability significantly. Furthermore, by considering the influences of both porosity and tortuosity, a high-order apparent permeability correction model is proposed to fit porous media with different structures.

Carbon dioxide capture and storage is an efficient technology to reduce CO2. A candidate CO2 reservoir is a sub-seabed aquifer under the cap rock. However, there is a risk of CO2 leakage even though such probability is low. Once CO2 seeps at the seafloor, it forms bubbles/droplets and dissolves during their rise. The rising speed and the dissolution rate significantly depend on the bubble/droplet size. Interestingly, past observations of natural seepage of CO2 indicated that the bubble/droplet sizes were not very different, regardless of the seepage depths. In this study, a numerical method to simulate 3D solid-liquid-gas three-phase flow in the unconsolidated particle layers was developed and applied to elucidate the influences of gas flux, porosity, and particle size on the formations of gas channels and subsequent bubbles. The results suggested that the bubble size did not depend on the flux, but on the gas channel size at the level of gas flux expected in possible seepage events.

Recently, natural gas-fired power generation has attracted more attention as they have lower specific CO2 emissions and higher operational flexibility than coal- and oil-fired power generation. In state-of-the-art gas-fired oxy-combustion cycles, combustion takes place at elevated pressure, which enables high efficiency and competitive economic performance. This review presents a new insight by summarising the main challenges associated with these types of cycles and covers: (i) O2 production methods and purity; (ii) types of exhaust gas recirculation; and (iii) operating pressure. The techno-economic evaluation of the state-of-the-art gas-fired power cycles showed that although the Allam cycle is superior among other cycles with an efficiency of 55.1%, its highly affected levelised cost of electricity by interest rate adds more uncertainty to investment decisions. Importantly, the progress towards the next generation of gas-fired oxy-combustion cycles will require the development of less complex cycles with more efficient and less energy-intensive O2 production.

The porosity system in shale, which is a combination of organic-hosted pore and inorganic pore, plays a pivotal role in adsorbing methane for gas shale reservoir. In order to identify the porosity system with different properties and to give a quantitative characterization of pore structure for each individual porosity system, the dual liquid NMR method was conducted on six shale samples from the Lower Silurian Longmaxi Formation in the Southern Sichuan Basin. In shale reservoir, hydrophilic pore and oleophilic pore are respectively related to hydrophilic minerals and oleophilic organic matter. The NMR method detected the signal of water penetrating the water-wetting porosity system when saturated with water, which can reflect the pore size distribution (PSD) of hydrophilic porosity. The PSD of oleophilic pore can also be reflected when saturated with kerosene, vice versa. Generally, the NMR T2 spectra of sample saturated with DI water and kerosene, T2_water and T2_kerosene exhibit bimodal pattern, a dominant peak with a relaxation time range of 0.1-10 ms and a minor peak with a relaxation time ranging 10-100 ms. Generally, higher TOC content corresponds to large amounts of cumulative organic nanoscale pore. The organic-rich shale shows high content of organic-hosted pores and large amounts of inorganic pores which are validated by plane porosity statistics from the observation of SEM photographs. By correlating the two pore structure characterizing methods, the relaxivity of shale can be calculated to be 0.055-0.092 μm/ms when saturated with water. The quantitative estimation of organic-hosted porosity system and inorganic porosity system in the shale will provide a pivotal basis for elucidating gas occurrence states in shale.