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

Abstract

In 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.

Dewatering Optimization of Coal Seam Gas Production Wells: A Review and Future Perspectives

“…It has been reported that the reservoir permeability and production rate decrease with the stress sensitivity coefficient for both coal seams and shale reservoirs, and the stress sensitivity coefficient reduces with the increasing effective stress. ,,, In other words, the increase of effective stress results in compaction of cleats, making cleats more resistant to compaction force, and the stress sensitivity coefficient drops, as proven by a number of experimental data (e.g., Figure ). ,− Studies have also revealed that (i) temperature and moisture content can enhance the stress sensitivity, (ii) the stress sensitivity decreases with the pore/fracture aperture and can be affected by fracture morphology, ,− and (iii) the stress sensitivity coefficient show strong anisotropy with a higher value typically in the horizontal direction. ,, Zhang et al proposed that the new fractures generated from shear and tensile failure in coal exert an evident impact on stress sensitivity, which matched with the results obtained by Tan and Konietzky . It is clear that the stress sensitivity coefficient constantly varies during production, so it is vital to quantify its variation to reliably estimate coal permeability evolution during the dewatering as well as gas production stages.…”

Using discrete element numerical simulation to determine effect of persistent fracture morphology on permeability stress sensitivity

“…Compared with individual vertical or horizontal fractures, there is a difference in the stress sensitivity when inclined segments (parts) exist on persistent fractures 27–29 . According to Xie et al., 30,31 vertical and horizontal stresses change differently when deep coal seam mining is subjected to high stress and high gas pressure.…”

“…For example, to ensure that the fracture aperture is not negative, the residual fracture aperture of the fracture was set in discrete element method (DEM) simulations. This weakens the simulation effect in a state of high stress, especially for fractures with a dip angle 27 . Moreover, the simulation parameters should be optimally selected by considering laboratory data.…”

“…[24][25][26] Compared with individual vertical or horizontal fractures, there is a difference in the stress sensitivity when inclined segments (parts) exist on persistent fractures. [27][28][29] According to Xie et al, 30,31 differently when deep coal seam mining is subjected to high stress and high gas pressure. To ensure the safe mining of coal seams and to realize the utilization of gas resources, the simultaneous extraction of both coal and gas is necessary.…”

“…This weakens the simulation effect in a state of high stress, especially for fractures with a dip angle. 27 Moreover, the simulation parameters should be optimally selected by considering laboratory data.…”