Abstract
The extraction of low-permeability coalbed methane (CBM) has the dual significance of energy utilization and safe mining. Understanding hydraulic fracturing mechanism is vital to successful development of CBM. Therefore, it is important to improve the law of hydraulic fracture propagation in coal and rigorously study the influencing factors. In this paper, laboratory experiments and numerical simulation methods were used to investigate the hydraulic fracture propagation law of coal in coalbed methane reservoir with natural fractures. The results show that the maximum and minimum horizontal in situ stress and the difference in stress significantly affect the direction of crack propagation. The elastic modulus of coal, the mechanical properties of natural fractures, and the injection rate can affect the fracture length, fracture width, and the amount of fracturing fluid injected. To ensure the effectiveness of hydraulic fracturing, a reservoir environment with a certain horizontal stress difference under specific reservoir conditions can ensure the increase of fractured reservoir and the controllability of fracture expansion direction. In order to increase the volume of fractured reservoir and fracture length, the pumping speed of fracturing fluid should not be too high. The existence of stress shadow effect causes the hydraulic fracture to propagate along the main fracture track, where the branch fracture cannot extend too far. Complex fractures are the main hydraulic fracture typology in coalbed methane reservoir with natural fractures. The results can provide a benchmark for optimal design of hydraulic fracturing in coalbed methane reservoirs.
Highlights
Coalbed methane (CBM) is a valuable resource, and it can affect the safe production of coal
We observed the change of stress during hydraulic fracture generation and expansion by simulating hydraulic fracturing with horizontal stress difference HSD = 9 MPa
Understanding the law of hydraulic fracture propagation in low-permeability CBM reservoirs with natural fractures is essential for evaluating the complexity of hydraulic fractures and improving the efficiency of CBM development
Summary
Coalbed methane (CBM) is a valuable resource, and it can affect the safe production of coal. China’s CBM reserves are very large, and the amount of such resources at the depth of 0-2000 meters has reached 3:68 × 1012 m3. One of the key issues restricting the development of CBM in China is that the permeability is generally very low. The coal reservoir fracture system is the main channel for gas flow that controls the permeability characteristics of the coal reservoir [1]. Through hydraulic fracturing to increase permeability, hydraulic fracture can conduct the original natural fractures in coal to a certain extent, which can greatly increase the production of coalbed methane. The mutual coupling relationship between hydraulic fractures and natural fractures has become a research hotspot in recent years [2, 3]
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