Efficient and simple simulation method for matching stress‐permeability test results

Abstract

AbstractIn the process of pressure‐relief mining in high gas coal seams and hydraulic fracturing of deep coalbed methane, a large number of irregular fractures are generated inside the coal, which becomes the main path for gas migration. The stress sensitivity of fractures directly affects the gas migration characteristics in the coal. For better matching stress‐permeability test results, a fluid–solid coupling simulation method with the joint stiffness changing method (JSCM), which accommodates fracture (or joint) stiffness through the stress acting on the fracture, is proposed. The stress sensitivity of permeability under different stress states was matched in a simulation and compared with a laboratory experiment. This method has the following advantages compared with the traditional joint stiffness fixing method (JSFM). (1) The JSCM does not need to set the residual joint aperture, which can match a wider range of stresses, so there is no upper limit in principle. (2) The JSCM can match the laboratory test results better than the JSFM. A parameter‐determining method for the JSCM is given for the flow test results. Based on the JSCM, the flow in an irregular fracture under different confining and axial stresses was determined. The results show that the occurrence of segmental fractures with different dip angles in persistent fractures determines the stress sensitivity of the fracture flow. With the increase in axial stress, the fracture aperture of the horizontal section is reduced, causing the decrease in horizontal section seepage capacity. It determines the whole flow of the persistent fracture. While with the increase in confining stress, the fracture aperture of vertical section is greatly reduced and causing flow rate of the whole fracture reducing greatly.