Abstract
The increasing demand on coal production has led to the gradually increase of mining depth and more high methane mines, which bring difficulties in terms of coalbed methane (CBM) extraction. Hydraulic fracturing is widely applied to improve the production of CBM, control mine gas, and prevent gas outbursts. It improves coal bed permeability and accelerate desorption and migration of CBM. Even though the impacts of hydraulic fracturing treatment on the coal reservoirs are rare, negative effects could not be totally ignored. To defend this defect, the presented work aims to study the influence of water filtration on coal body deformation and permeability evolution. For this purpose, a simulation based finite element method was developed to build a solid-fluid coupled two-phase flow model using commercial software (COMSOL Multiphysics 5.4). The model was verified using production data from a long strike borehole from Wangpo coal mine in Shanxi Province, China. Several simulation scenarios were designed to investigate the adverse impacts of hydraulic fracturing on gas flow behaviors. The mechanisms of both relative and intrinsic permeability evolutions were analyzed, and simulation results were presented. Results show that the intrinsic permeability of the fracture system increases in the water injection process. The impacts of water imitation were addressed that a critical time was observed beyond which water cannot go further and also a critical pressure exists above which the hydraulic pressure would impair the gas flow. Sensitivity analysis also showed that a suitable time and pressure combination could be observed to maximize gas extraction. This work provides an efficient approach to guide the coal bed methane exploitation and other unconventional gas reservoirs.
Highlights
Coal is one of the major energy resources worldwide, and the increasing demand on coal products led to the gradually increasing of mining depth [1]
Hydraulic fracturing technology plays a basic role in the exploitation of coalbed methane (CBM)
Several simulation scenarios were designed to investigate the adverse impacts of hydraulic fracturing on gas flow behaviors
Summary
Coal is one of the major energy resources worldwide, and the increasing demand on coal products led to the gradually increasing of mining depth [1]. Higher methane gas mines will be replacing the low methane mines, and new challenges will arise in the coal seam mining and in the coalbed methane (CBM) drainage. Hydraulic fracturing technology plays a basic role in the exploitation of CBM. It is widely used to improve the production of CBM by transforming a reservoir with low permeability and porosity [2]. Hydraulic fracturing of coal and rock mass uses high pressure pump to inject water or other fracturing fluids into the coal seam through the borehole. As the injection rate of liquid is Energies 2020, 13, 1151; doi:10.3390/en13051151 www.mdpi.com/journal/energies
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