Evaluation of underground coal gas drainage performance: Mine site measurements and parametric sensitivity analysis

Highlights

• A mechanism-based model is established to describe gas desorption/diffusion/flow through coal around the drainage borehole.

• Effects of parametric sensitivity on drainage performance are evaluated by numerical simulations and mine site measurements.

• The control mechanism of gas drainage production and air leakage changes dynamically with gas pressure depletion.

• Sealing up coal wall around the borehole can linearly reduce drainage air leakage.

Abstract

Underground gas extraction from coal formations has triple-effects involving mining safety, low-carbon gas capture and greenhouse gas control. Air leakage around the drainage borehole is a serious problem that continuously affects gas drainage performance. In this study, mine site measurement of gas drainage data is firstly performed in coal mine, and then a mechanism-based model is proposed to theoretically describe gas desorption and diffusion and flow through coal around the drainage borehole. Further, the propose model is numerically solved and verified with borehole drainage data measured in mine site. Followed this, the validated mechanism-based model is implemented to conduct parametric studies. The results showed that: (a) as the fracture permeability increases from 3 × 10⁻²² m² to 3 × 10^-14 m², the air leakage flux increases from 7355 m³/d to 18,303 m³/d, and the gas concentration decreases from 46.9 % to 12.7 %, it indicates that changing the permeability around the borehole may be a wise strategy to control air leakage; (b) the coal matrix parameters (including permeability, sorption constant and radius of matrix) have a dynamic impact on gas drainage performance at the different stage of gas drainage. For example, the increment of methane production induced by increasing the sorption constants does not exceed 3.3 % at the drainage time ∼0.34 day; while the growth increases to more than 19.5 % at the drainage time ∼ 9.75 days; (c) at the initial stage of extraction gas production is mainly determined by the fracture flow. A higher permeability of coal fracture will incur more air leakage flux, reducing gas concentration in drainage borehole; (d) whereas, the matrix parameters dominate gas flow at a later stage. Increasing matrix permeability, sorption property or decreasing the matrix radius will enhance the gas exchange flux from the pore system of coal matrix to the fracture system, subsequently incurring a higher concentration/production of drainage gas. The simulated result and field tests demonstrates that sealing on the coal wall around the borehole can block a portion of air leakage paths and reduce air leakage linearly, which illuminates a more efficient strategy to minimize air leakage for underground gas extraction.

Introduction

Coalbed methane (CBM) is a kind of unconventional gases with high combustion efficiency and less carbon-intensive burning, and well suited to generate electricity (Moore, 2012; Wang and Liu, 2016; Fan and Liu, 2018). CBM drained from seam is also called coal seam gas, the original existence of which in coal seam is always considered as the first “killer’ to underground safety mining process due to its flammable and explosive characteristics (Wang et al., 2020; Liu et al., 2020a). From the perspective of environmental concerns, methane is a more potent greenhouse gas contributing climate change than CO₂ because of the greater radiative forcing produced per CH₄ molecule (Balcombe et al., 2018; Crow et al., 2019). Uncontrolled methane emissions involving in coal mining to the atmosphere will do irreversible damage to the earth climate. Therefore, the effective and efficient gas extraction from gassy coal is expected to contribute to three distinct aspects: capturing clean unconventional energy, reducing the gas disaster risk in underground coal exploitation and controlling greenhouse gas emissions associated with coal mining process (Wang et al., 2019a; Zheng et al., 2018).

With the advancement of CBM exploitation technique, CBM resource is becoming the candidate for relaxing the deficient of conventional natural gas resources (Tao et al., 2019; Liu et al., 2014). In China, underground gas drainage in coal mine is the main pattern to capture CBM resource, and gas production from underground drainage makes up about eighty percent of the total CBM production (Zhou et al., 2016). However, the rapid decline of the drained gas production and concentration in boreholes induced by drainage air leakage is a ‘bottleneck’ in the practice of coal mine CBM extraction. Also, low-efficiency extraction caused by air leakage problem will lead to insufficient degassing of coal within the prescribed pre-drainage time, and mining the high gassy coal will risk gas disaster, which is inconsistent with the requirements of coal mine safety regulations. Moreover, the air leakage will provide sufficient oxygen to coal and promote the oxidation of coal, which may result in coal spontaneous combustion. The low concentration (6∼20 %) drainage gas usually needs to be purified before further utilizing, which will inevitably increase the utilization cost (Lu et al., 2009; Zhou et al., 2013; Xia et al., 2014a). Therefore, the air leakage control and improvement of the drained gas concentration will be the sticking points in promoting the quality and efficiency of underground gas extraction, which is a basic trick to reduce the risk of gas explosion and gas outburst in the underground coal mining process and frame green coal industry by controlling greenhouse gas release.

The development of CBM extraction projects has attracted much attention in the past few decades, and those studies have been implemented on vast efforts that are deeply involved with coal-gas interactions, which is deeply involved in coal gas extraction. Bulk coal is commonly divided to fractures and matrixes due to the difference in pore size distribution (Warren and Root, 1963; Liu et al., 2016; Liu and Harpalani, 2013). Coal produces CBM using a multi-mechanism, which involves multi physical process. After the water in coal reservoir is depleted, gas first desorbs and transports in the pore system of coal matrix, matrix releases gas to fracture system, and then gas migrates in fractures and finally flows into the CBM well or drainage borehole (Liu et al., 2020b; Zhang et al., 2018; Wang et al., 2018). Bumb and Mckee (1988) built a gas transport model based on the assumption of continuous pressure equilibrium exists between coal matrix and fracture network, gas diffusion process was ignored, which is apparently contrary to the dual porosity of coal (Bumb and McKee, 1988). King and Ertekin (1989) used a dynamic diffusion coefficient to describe gas transport process through the pores within coal matrix and modeled gas migration in the dual porosity coal (King and Ertekin, 1989). The water flow in coal was honored by Zhao and Valliappan (1995), and they combined the gas flow and water flows in coalseam to characterize the two phase interactions during CBM production (Zhao and Valliappan, 1995). The Fick’s law was applied by Young (1998) to theoretically depict gas desorption and transport within the matrix pores, and he considered the methane released from matrixes as the gas source of gas flows in fracture system, and then built a gas flow model for the dual porosity reservoir (Young, 1998). The non-Darcy flow in coal cleats was studied by Guo et al. (2004), and they used a fluid-solid coupling model to simulate the gas transport and permeability dynamic behaviors during CBM depletion (Guo et al., 2004). Shi and Durucan (2005a),b divided the pores in coal matrix into two grades, and proposed a triple-porosity model to describe gas desorption and transport in the multi-scale pores/fractures in coal seam (Shi and Durucan, 2005a). Xia et al. (2014a),b coupled methane flow, air flow and the coal deformation induced by pore pressure change to estimate gas drainage performance in the pre-drained coal seam (Xia et al., 2014b). Zang and Wang (2016) studied the anisotropic characteristics of permeability in coal seam and numerically modeled the variation of gas production in response to permeability anisotropy for coal gas extraction (Zang and Wang, 2016). Liu et al. (2018a),b indicated the diffusive flow and Darcy flow in coal matrix can be integrated to a density-driven flow model and derived a modified dual-porosity model to evaluate in-seam gas drainage (Liu et al., 2018a).

In addition, some experimental work observed that the adsorption capability of coal is gas type-dependent, and the diffusivity of CH₄ and CO₂ and N₂ in the same coal are quite different, which initials theoretical studies on the multi-component gas flow behaviors through coal for CO₂-ECBM and carbon sequestration in coal reservoirs (Cui et al., 2004; Busch and Gensterblum, 2011). Wei et al. (2007) experimentally examined the counter-diffusion process in coal with CH₄ and CO₂ and built a multi-component gas desorption/diffusion/flow model to evaluate the CO₂ injection in coal to enhance CBM recovery (Wei et al., 2007). Connell and Detournay (2009) numerically investigated the coupled gas flow and geomechanical responses involved in the operation of CO₂ injection to coalseam for ECBM recovery (Connell and Detournay, 2009). Wang et al. (2012) incorporated the non-Darcy flow, gas diffusion flow and the gas depletion induced shrinking of coal matrix, and numerically modeled coal-gas interaction behaviors (Wang et al., 2012). Yin et al. (2017) studied the gas mass exchange from the coal matrixes to the fracture network, and numerically investigated the effect of injection pressures on gas flow flux in coal during CO₂-ECBM operations (Yin et al., 2017). Ren et al. (2017) developed a binary gas transport model to analyze the CH₄ and N₂ flow in the dual-porosity coal and estimated the effect of N₂ injection to coalseam on gas extraction performance (Ren et al., 2017). Ma et al. (2017) coupled multi-phase (gas and water) flow, multi-component (CO₂ and CH₄) diffusion, heat transfer process and the dynamic deformation in coal to investigate the thermal-hydrological-mechanical responses in the CO₂-ECBM operation (Ma et al., 2017).

To summarize, previous scholars continuously conducted studies for understanding gas (single or multiple components) transport behaviors through coal. However, few studies addressed the air flow in coal induced by air leakage during in-seam gas extraction, which may obtain the incorrect outcomes such as overestimation of methane production, and will inevitably mislead the drainage planning strategy and the optimization of gas control system. This study documents mine site measurements to obtain key data for in-seam borehole gas drainage including the gas production rate and gas concentration. Then, a mechanism-based model resolving the gas (methane and air) flow flux in coal is highlighted. Finally, the sensitivity studies on compositional gas flow behaviors are conducted to investigate the effects of intrinsic physical parameters of coalseam, including fracture flow and matrix flow parameters, on the air leakage flow flux and pure methane production. This work aims to estimate the underground gas drainage performance and its key influencing factors through the mine site measurements and numerical simulations, the output will provide theoretical guidance for in-situ gas drainage, which will be beneficial to mining-process safety and gas disaster control.