Numerical simulation of deep CBM hydraulic slotting pressure relief and desorption and permeability enhancement based on the MPM

Deep coalbed methane reservoirs are characterized by low permeability, high reservoir pressure, and high ground stress, leading to difficulties in the desorption and transport of CBM and low extraction efficiency. The water jet research team at Chongqing University proposes directional well + hydraulic slotting as a new idea to enhance deep CBM extraction, but it is necessary to carry out quantitative research on its effect on pressure relief, desorption, and permeability enhancement. Numerical simulation is a main method to study this problem, but traditional numerical methods such as FEM and DEM cannot effectively solve the complex problems of large deformation,large displacement, continuous change of inner boundary, and contact caused by coal slotting. In order to overcome the abovementioned difficulties, the numerical model of elastoplastic damage based on the material point method is built and simulates the influence of hydraulic slotting on the pressure relief, desorption, and permeability enhancement of deep CBM at Linxing. The following conclusions are obtained:① The numerical model of elastoplastic damage established based on the material point method can effectively simulate the large deformation, large displacement, and internal boundary contact problems of the coal seam caused by the hydraulic slotting. ② The direction perpendicular to the slotting presents a wide range of stress drops, with the extent of the stress dip coinciding with the extent of the plastic zone and the zone of slow stress drop corresponding to the region of small elastic deformation. ③ The distribution characteristics of the desorption and permeability areas are consistent with the stress drop area, and the permeability in the elastic and plastic area is 1-5 times and 10-20 times of the original permeability, respectively, with a single slotting contributing to the desorption of hundreds to thousands of cubic meters of CBM. ④ The width, length, and angle of the slotting are linearly correlated with the area of the increased permeability and the amount of desorbed gas. Based on the above research results, a parallel horizontal well staggered multislotting desorption technology is put forward based on directional well + hydraulic slotting, which provides a new idea and research means for the efficient development of deep CBM.