Microscopic pore throat structures and water flooding in heterogeneous low-permeability sandstone reservoirs: A case study of the Jurassic Yan'an Formation in the Huanjiang area, Ordos Basin, Northern China

doi:10.1016/j.jseaes.2021.104903

Journal of Asian Earth Sciences

Volume 219, 1 October 2021, 104903

https://doi.org/10.1016/j.jseaes.2021.104903 Get rights and content

Highlights

•
The pore throat structure is classified based on inflection points of fractal curves.
•
The heterogeneity is caused by the varying proportions of micro-structure types.
•
The water flooding is mainly affected by the heterogeneity of pore throat structure.

Abstract

The complexity of the pore throat structures of heterogeneous low-permeability reservoirs causes uneven water flooding, resulting in the retention of a considerable amount of residual oil. Therefore, it is necessary to characterize pore throat structures and the effects of these structures on water flooding. Such structures and effects were investigated using scanning electron microscopy (SEM), cast thin sections (CTS), high-pressure mercury intrusion (HPMI) and microvisualization seepage experimentations. In the fractal dimension curves, there were two relatively close inflection points, corresponding to pore throat radii of approximately 3 µm and 0.02 µm. On this basis, the pore throat structures of all the samples were categorized into macrothroats, mesothroats and microthroats. All the samples contained these three types of pore throat structures. The heterogeneity of the pore throat structure was caused by the varying proportions of the different pore throat structure types. This was the first analysis of the effects of these varying proportions of pore throat structure types in one sample on water flooding. The influences of the proportions and pore throat radii of the different pore throat structures on water flooding were notable. Through this study, the effects of the pore throat structures on water flooding were found to be controlled by the proportions of the different pore throat structure types. This finding is of great significance to the efficient development of heterogeneous low-permeability reservoirs.

Graphical abstract

Introduction

Low-permeability sandstone reservoirs are important oil-producing reservoirs in the continental sedimentary basins of China. The permeability of these reservoirs is generally lower than 50 × 10⁻³ μm² (Li, 2016). Their main characteristics are small pore throats, high capillary pressures and complex pore throat structures (Zeng and Li, 1994). Influenced by various geological factors, such as sedimentation and diagenesis, low-permeability reservoirs have complex pore throat structures with notable microheterogeneity (Chen et al., 2019, Li et al., 2017a, Li et al., 2017b, Liu et al., 2018, Wang et al., 2017, Wang et al., 2020). The pore throat structure greatly affects the petrophysical and oil-bearing properties of a reservoir (Cui et al., 2019, Dou et al., 2018, Qiao et al., 2019, Wang et al., 2018, Zhou et al., 2016). The shape and connectivity of pores notably impact the porosity and permeability (Desbois et al., 2011). Understanding the pore throat structure is important in reservoir evaluation and efficient development (Schmitt et al., 2015, Xiao et al., 2016). At present, the low-permeability reservoirs in China are mostly developed by water flooding to supplement formation energy (LI et al., 2015, Li, 2016). During the process of water flooding, the pore throat structure exerts an important impact on the sweep efficiency of the injected water. Li et al. (2018) analyzed the effect of pore throat structure on water flooding and found that pore throats were the most important feature affecting water flooding. The larger the pore throats are, the better the water flooding effect. Microheterogeneity imposes a major effect on water flooding. The more homogeneous a reservoir is, the lower the remaining oil saturation (LI et al., 2018a, Li et al., 2018b, Sun et al., 2013), but quantitative evidence of this relationship is lacking.

The low-permeability sandstone reservoirs of the Jurassic Yan'an Formation are important exploration and development targets in the Ordos Basin, China (Shi et al., 2011). At present, related research on pore throat characteristics has focused on mainly the tight sandstone reservoirs of the Triassic Yanchang Formation, and research on low-permeability sandstone reservoirs of the Jurassic Yan'an Formation is limited.

Considering the current insufficient research on the pore throat structures of Jurassic low-permeability reservoirs in the Ordos Basin and the limited quantitative evidence regarding the effect of pore throat structure heterogeneity on water flooding, this study was conducted. A series of laboratory investigations, such as scanning electron microscopy (SEM), cast thin sections (CTS) and high-pressure mercury intrusion (HPMI) and microvisualization seepage experimentation, was carried out on low-permeability sandstone samples collected from the Jurassic Yan'an Formation of the Ordos Basin to analyze the pore throat structures and their influence on water flooding. The effects of the microscopic pore throat structure, especially heterogeneity, on water flooding were studied. This work is of great significance for further developing heterogeneous low-permeability sandstone reservoirs.

Section snippets

Geological background

The Ordos Basin is one of several petroliferous basins located in Northern China (Fig. 1a). The Huanjiang area is located in the central part of the Tianhuan Depression of the western Ordos Basin (Fig. 1b). The Jurassic Yan'an Formation reservoirs are typical low-permeability sandstone reservoirs, and the lower section of the Yan'an Formation is the main oil-bearing layer in the study area. The lithology of the reservoir is mainly characterized as fine sandstone and siltstone (Fig. 1c).

Experimental samples

In this study, samples were collected from 6 wells of the Jurassic Yan'an Formation reservoir in the Huanjiang area of the Ordos Basin. The samples were processed into thin sections and cylinders. The helium porosity and nitrogen permeability were measured. SEM (12 samples), CTS (12 samples), HPMI (8 samples) and microvisualization seepage (6 samples) analyses were performed.

Scanning electron microscopy (SEM)

Rock characteristics such as the interstitial material, pore throat structure and cementation type can be directly

Porosity and permeability

The helium porosity of the Jurassic Yan'an Formation reservoirs in the Huanjiang area of the Ordos Basin ranges from 8.01% to 17.42%, mainly measuring between 12% and 16%, with an average porosity of 14.41%. The nitrogen permeability ranges from 1.03 to 143.62 × 10⁻³ μm², with most values less than 50 × 10⁻³ μm² and an average permeability of 35.08 × 10⁻³ μm², which indicates low-permeability reservoirs (Li, 2016). Compared to the other reservoirs of the Ordos Basin, the Jurassic Yan'an

The heterogeneity of the pore throat structure

Pore throat structure heterogeneity is mainly reflected in two aspects: heterogeneity among samples and heterogeneity within a sample.

Conclusions

(1)
The pore throat structures of the low-permeability reservoirs of the Jurassic Yan'an Formation in the Huanjiang area of the Ordos Basin exhibit high heterogeneities. Based on their fractal characteristics, the pore throat structures of the reservoir samples are categorized into three types: macrothroats, mesothroats and microthroats.
(2)
The fundamental cause for the heterogeneity of the pore throat structure is the varying proportions of the three types of pore throat structures.
(3)
Pore throat

CRediT authorship contribution statement

Xinyu Zhong: Methodology, Formal analysis, Writing - original draft. Yushuang Zhu: Methodology, Writing - review & editing. Tao Jiao: Data curation. Zhao Qi: Data curation. Jianghua Luo: Investigation, Data curation, Formal analysis. Yuhang Xie: Writing - review & editing. Linyu Liu: Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was funded by the National Major Science and Technology Projects of China (2016ZX05046). This work was supported by the State Key Laboratory of Continental Dynamics and PetroChina Changqing Oilfield Company.

References (41)

L.M. Anovitz et al.
Diagenetic changes in macro- to nano-scale porosity in the St. Peter Sandstone: an (ultra) small angle neutron scattering and backscattered electron imaging analysis
Geochim. Cosmochim. Acta
(2013)
J. Chen et al.
Sedimentary and diagenetic controls on reservoir quality of low-porosity and low-permeability sandstone reservoirs in Chang101, upper triassic Yanchang Formation in the Shanbei area, Ordos Basin, China
Marine Petrol. Geol.
(2019)
J. Cui et al.
Oil-bearing heterogeneity and threshold of tight sandstone reservoirs: A case study on Triassic Chang7 member, Ordos Basin
Mar. Pet. Geol.
(2019)
G. Desbois et al.
High-resolution 3D fabric and porosity model in a tight gas sandstone reservoir: A new approach to investigate microstructures from mm- to nm-scale combining argon beam cross-sectioning and SEM imaging
J. Petrol. Sci. Eng.
(2011)
W. Dou et al.
Diagenetic heterogeneity, pore throats characteristic and their effects on reservoir quality of the Upper Triassic tight sandstones of Yanchang Formation in Ordos Basin, China
Mar. Pet. Geol.
(2018)
S. Dong et al.
A novel method for extracting information on pores from cast thin-section images
Comput. Geosci.
(2019)
A. Giri et al.
Fractal pore structure of sedimentary rocks: simulation in 2-D using a relaxed bidisperse ballistic deposition model
J. Appl. Geophys.
(2012)
H. Gao et al.
Comprehensive characterization of pore and throat system for tight sandstone reservoirs and associated permeability determination method using SEM, rate-controlled mercury and high pressure mercury
J. Petrol. Sci. Eng.
(2019)
P. Guo et al.
Simulation of multi-period tectonic stress fields and distribution prediction of tectonic fractures in tight gas reservoirs: A case study of the Tianhuan Depression in western Ordos Basin, China
Mar. Pet. Geol.
(2019)
R. Guo et al.
Fractal characteristics of pore-throat structure and permeability estimation of tight sandstone reservoirs: A case study of Chang 7 of the Upper Triassic Yanchang Formation in Longdong area, Ordos Basin, China
J. Petrol. Sci. Eng.
(2020)

J. Lai et al.

A review on pore structure characterization in tight sandstones

Earth Sci. Rev.

(2018)

P. Li et al.

Pore throat structure and fractal characteristics of tight oil sandstone: a case study in the Ordos Basin, China

J. Petrol. Sci. Eng.

(2017)

Z. Li et al.

Diagenetic alterations and reservoir heterogeneity within the depositional facies: A case study from distributary-channel belt sandstone of Upper Triassic Yanchang Formation reservoirs (Ordos Basin, China)

Mar. Pet. Geol.

(2017)

J. Li et al.

Effects of microscopic pore structure heterogeneity on the distribution and morphology of remaining oil

Petrol. Explorat. Develop.

(2018)

J. Li et al.

Effects of deposition and diagenesis on sandstone reservoir quality: A case study of Permian sandstones formed in a braided river sedimentary system, northern Ordos Basin, Northern China

J. Asian Earth Sci.

(2021)

Y. Liu et al.

Diagenetic constraints on the heterogeneity of tight sandstone reservoirs: A case study on the Upper Triassic Xujiahe Formation in the Sichuan Basin, southwest China

Mar. Pet. Geol.

(2018)

J. Qiao et al.

Heterogeneity of reservoir quality and gas accumulation in tight sandstone reservoirs revealed by pore structure characterization and physical simulation

Fuel

(2019)

G. Shi et al.

Late Cretaceous-Cenozoic exhumation of the Helanshan Mt Range, western Ordos fold-thrust belt, China: Insights from structural and apatite fission track analyses

J. Asian Earth Sci.

(2019)

M. Schmitt et al.

Characterization of Brazilian tight gas sandstones relating permeability and Angstrom-to micron-scale pore structures

J. Nat. Gas Sci. Eng.

(2015)

A. Sakhaee-Pour et al.

Fractal dimensions of shale

J. Nat. Gas Sci. Eng.

(2016)

Cited by (21)

Using digital cores and nuclear magnetic resonance to study pore-fracture structure and fluid mobility in tight volcanic rock reservoirs
2024, Journal of Asian Earth Sciences
Pore-fracture structure characterization and fluid mobility analysis are key to the effective development of tight reservoirs. However, it is difficult to accurately characterize the pore-fracture structure of tight volcanic rocks using a single method, and the mechanisms of fluid mobility are not fully understood. A combination of X-ray diffraction, casting thin section observations, scanning electron microscopy, high-pressure mercury injection, constant-speed mercury injection, nuclear magnetic resonance (NMR), X-ray computed tomography, and the advanced mathematical algorithms in the AVIZO visualization software was used to analyze the Permian tight volcanic rock in the ZhongGuai area of the Junggar Basin to investigate its overall pore-fracture structure characteristics. On this basis, NMR centrifugation experiments were conducted to monitor the fluid migration dynamics in the tight volcanic rocks, and the mobile fluid migration characteristics were studied based on the NMR T₂ spectra. The results show that the porosity and permeability of the samples are 0.2–18.7 % (avg. of 8.3 %) and 0.001–6.680 × 10⁻³μm² (avg. of 1.602 × 10⁻³μm²), respectively, and the various pore types are due to the differences in cementation and compaction. The distribution of the pore throats are mainly contiguous and isolated, and the connected pores are mainly distributed in enriched bands, which is due to the interconnection of gas pores, intergranular pores, and dissolution fractures; while the disconnected pores are mainly isolated, which is related to the development of inter-gravel dissolved pores and matrix dissolution pores. In addition, the contribution of the pore connectivity to seepage is greater than that at the pore scale. The pores in the tight volcanic rock have a wide range of sizes, from 3 nm to 120 μm, and are dominated by Gaussian and bimodal distribution patterns. The micron-scale pore radius in this area is mainly 4.47–31.56 μm. Also, the fractures can be divided into three types according to their occurrence and openings. They are mainly high-angle structural fractures and vertical fractures; the pore-fracture structures have strong heterogeneity, and the fractures play a larger role on the seepage of oil and gas. The connectivity of the pore throats in the tight matrix is poor, and the pore throat structure has a great influence on fluid mobility. Subsequently, the movable fluid saturation increases with increasing permeability, and the fractures and micropores have less flow resistance and are more conducive to water flow than small pores. This study provides new insights for the exploitation of similar tight reservoirs.
Experimental analysis of the pore structure, relative permeability, and water flooding characteristics of the Yan'an Formation sandstone, southwestern Ordos Basin
2023, Energy Geoscience
The oil and gas potential of the Yan'an Formation in the Ordos Basin has yet to be fully tapped. In this study, the pore structure, mobile fluid saturation, and water flooding micro-mechanism of the Yan'an Formation sandstone are systematically studied through the application of a series of rock physics and fluid experiments. The results show that there is a good positive correlation between porosity and permeability, and the reservoirs are divided into types Ⅰ, Ⅱ, and Ⅲ. Mercury injection tests show that the average pore throat radius of the oil-bearing reservoir ranges from 1 to 7 μm. The displacement pressure of the Yan'an Formation is also relatively low, and it decreases from 0.1 MPa to 0.01 MPa as the rock porosity increases from 11% to 18%. NMR tests show that small (diameter <0.5 μm) and medium pores (diameter ranging from 0.5 to 2.5 μm) are predominant in the reservoir. Different types of reservoirs have different characteristics of relative permeability curve. In addition, when the average oil recovery rate is less than 1 ml/min, the oil displacement efficiency increases faster. However, when the average oil recovery rate is between 1–3.5 ml/min, the oil displacement efficiency is maintained at around 27%–30%. Physical properties of the reservoir, pore-throat structure, experimental pressure difference, and pore volume injected — all have significant effects on oil displacement efficiency. For Type Ⅰ and Type Ⅱ reservoirs, the increase of the pore volume injected has a significant effect on oil displacement efficiency. However, for Type Ⅲ reservoirs, the change of pore volume injected has insignificant effect on oil displacement efficiency. This study provides a reference for the formulation of estimated ultimate recovery (EUR) measures for similar sandstone reservoirs.
Quantifying the relative contribution and evolution of pore types to shale reservoir space: Constraints from over-mature marine shale in the Sichuan Basin, SW China
2023, Journal of Asian Earth Sciences
Pore types significantly affect the pore properties of shale reservoirs, but their contribution to shale pore space in over-mature marine shale remains controversial because organic and inorganic pores fail to be quantitatively characterized. In this study, mercury intrusion porosimetry, low-temperature N₂ and CO₂ adsorption, and image analysis were conducted on over-mature marine shale in the Lower Silurian Longmaxi Formation to quantify the contribution of pore types to pore volume (PV) and surface area (SA). The full-scale pore size distribution of over-mature shale indicates that mesopores account for the largest PV, while macropores are developed in organic matter (OM)-poor shale. By analyzing the influence of multiple factors and heterogeneity of different pore types, OM pores, pyrite framboid pores, clay mineral pores, and dissolution pores are identified as dominant types in OM-poor/moderate/rich shale. The PV and SA of these four pore types are quantitatively characterized by extracting parameters of shale components and pores using image analysis. In OM-poor shale, clay mineral pores contribute the most to the PV and SA, followed by OM pores and dissolution pores, and finally pyrite framboid pores. In OM-rich/moderate shale, the contribution of OM pores to the PV and SA is dominant and increases with the increasing total organic carbon content. The evolution of pore types throughout the maturation sequence is revealed based on previous studies. Especially at the over-mature stage, clay mineral pores contribute the most to pore space in OM-poor shale, while OM pores make a major contribution to pore space in OM-rich/moderate shale.
A novel polymer gel with high-temperature and high-salinity resistance for conformance control in carbonate reservoirs
2022, Petroleum Science
Citation Excerpt :
It is estimated that 2.8 barrels of water are produced on average with each barrel of oil worldwide (Buciak et al., 2013; Al-Nakhli et al., 2016; Rassenfoss and Jacobs, 2017; Kumar et al., 2020). Most mature oilfields have entered the mid to late stages of water flooding, and dominant channels are formed due to the long-term water scour and the enhanced reservoir heterogeneity, yielding a significant decrease in sweep efficiency of the injected fluids (Hu et al., 2015; Lamas et al., 2021; Zhong et al., 2021). Therefore, how to effectively control water and increase oil has gradually become the mainstream topic in the field of oil exploitation.
Plugging agents have been widely used to enhance oil recovery in fractured-vuggy carbonate reservoirs. However, the harsh conditions of fractured-vuggy carbonate reservoirs yield a significant challenge in maintaining a long-term stabilization of plugging agents. In this work, we developed an anti-high-temperature and high-salinity polymer gel (APG) with excellent resistance to high temperature (140 °C) and ultra-high salinity (240000 mg/L). The rheology and microstructure of APG were characterized before and after gelation. Core plugging tests on fractured cubic cores were conducted to quantify the plugging performance of the gel system. Experimental results showed that the Sclerglucan and Cobalt (II) Chloride Hexahydrate filled the three-dimensional (3-D) network with various morphologies, providing extra protection to the cross-linking points of the 3D network structure of APG and thus, leading to a prolongation of the dehydration time. The dehydration rate of APG was only 5% within 30 days, and the strength of APG could be maintained at a rigid or near-rigid level over 150 days. Moreover, APG exhibited satisfactory shear and scour resistance. Core plugging tests showed that APG could achieve a plugging rate of 90% and demonstrate ignorable minor damage to the substrate. Our results indicate that APG can serve as a great candidate in channel plugging in fractured-vuggy carbonate reservoirs where fractures are fully developed.
Experimental study on CO<inf>2</inf>/Water flooding mechanism and oil recovery in ultralow - Permeability sandstone with online LF-NMR
2022, Energy
Citation Excerpt :
The multiphase flow, diffusion of water, oil and CO2, and formation response is directly related to the oil recovery efficiency [13–17]. The interaction between displaced oil, displacing agent (water, CO2 or sc-CO2) and heterogeneity formation, characterized by miscible, immiscible, hydrophilicity and lipophilicity, influences the successful implementation of CO2 geological storage and CO2/water-EOR [18–24]. The optimal formation condition created by artificial fracture and appropriate displacing agent promote the oil channel connectivity and displacement flooding efficiency [25–27].
The investigation of the mechanism of CO₂/water flooding in ultralow-permeability oil reservoir is critical to the enhanced oil recovery. In this study, a series of core flooding experiments in pore and pore-fracture cores subjected to different in situ stresses was conducted, and online LF-NMR technology was employed to dynamically monitor the multiphase flow and pore-fracture behavior. The results show that the percolation pore dominates the volumetric sweep efficiency, the capillary pressure caused by sandstone wettability controls the oil recovery in adsorption pore, and viscosity fingering and thief channeling mainly occurred in migration pore. The oil recovery is primarily contributed by the percolation pore and migration pore in CO₂ flooding, and all pores averagely contribute to the oil recovery in water flooding. In addition, the oil recovery in the pore core decreased with the increase of the in situ stress (15–35 MPa) in CO₂/water flooding. Compared with the increase of pore-fracture core oil recovery responding to the increased in situ stress in water flooding, the highest pore-fracture core oil recovery presented at 25 MPa followed by 35 MPa and 15 MPa in CO₂ flooding. The oil saturated in fracture and fracture-connected pore was firstly displaced, and the evolved heterogeneous morphology of the residual oil was characterized by the increased fractal dimension from 1.5 to 1.7 in CO₂ flooding and 1.78 to 1.82 in water flooding responding to the increased in situ stress. The pore core oil recovery rate in CO₂ flooding is about 10 times than that in water flooding. The peak value of oil recovery presented at 25 MPa for CO₂/water flooding, and the fracture influence is more obvious in water flooding. The findings provide significant guidance to the engineering practice of enhanced oil recovery in ultralow-permeability reservoir.
Response time of waterflooding in low-permeability reservoirs
2022, Unconventional Resources
The time at which the difference between the waterflooding development effect and the elastic development effect equals 1% of the elastic development effect refers to the waterflooding response time, which is an important index for evaluating the effect of waterflooding in low-permeability reservoirs. Taking the five-point well pattern as an example, the response time–determining method was first designed using numerical simulation technology. Subsequently, the effects of different reservoir properties and development parameters on the response time of waterflooding were analyzed. Finally, the degree of influence of each factor on the waterflooding response time was determined using the orthogonal design method. The results showed that the higher the permeability and injection-production pressure difference is, the lower the response time is. The greater the oil viscosity and well spacing are, the longer the response time is. Thus, permeability exerted the most substantial effect on response time.

View all citing articles on Scopus

View full text

Microscopic pore throat structures and water flooding in heterogeneous low-permeability sandstone reservoirs: A case study of the Jurassic Yan'an Formation in the Huanjiang area, Ordos Basin, Northern China

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Geological background

Experimental samples

Scanning electron microscopy (SEM)

Porosity and permeability

The heterogeneity of the pore throat structure

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

Geochim. Cosmochim. Acta

Marine Petrol. Geol.

Mar. Pet. Geol.

J. Petrol. Sci. Eng.

Mar. Pet. Geol.

Comput. Geosci.

J. Appl. Geophys.

J. Petrol. Sci. Eng.

Mar. Pet. Geol.

J. Petrol. Sci. Eng.

Earth Sci. Rev.

J. Petrol. Sci. Eng.

Mar. Pet. Geol.

Petrol. Explorat. Develop.

J. Asian Earth Sci.

Mar. Pet. Geol.

Fuel

J. Asian Earth Sci.

J. Nat. Gas Sci. Eng.

J. Nat. Gas Sci. Eng.