Numerical simulation of deep CBM hydraulic slotting pressure relief and desorption and permeability enhancement based on the MPM
ZHOU Lei,PENG Yu,LU Yiyu,XIA Binwei
深部煤层气储层具有渗透率低、储层压力大和地应力高等特点，导致煤层气解吸、运移困 难，开采效率低。 定向井+水力割缝是一种强化深部煤层气开采的新思路，但有必要对其卸压解吸 增渗效果开展量化研究。 数值仿真是研究该问题的主要手段，但传统有限元、离散元等数值方法无 法有效解决煤岩割缝引起的大变形、大位移、内边界持续变化和接触的复杂问题。 为克服上述难 题，基于物质点法建立了弹塑性损伤数值模型，模拟了水力割缝缝槽对临兴深部煤层气卸压解吸和 增透的影响，得到:1 基于物质点法建立的弹塑性损伤数值模型能有效计算水力割缝缝槽引起煤 层大变形、大位移和内边界接触问题;2 垂直于割缝方向呈现大范围应力下降，应力骤降区的范围 和塑性区域的范围重合，应力缓慢下降区则对应弹性小变形区域;3 卸压解吸气和增渗区域分布 特征与应力下降区分布特征一致，在弹性区和塑性区渗透率分别为原始渗透率的 1 ~ 5 倍和 10 ~ 20 倍，单个割缝促使煤层气解吸气量达数百至上千方;4 割缝宽度、长度、角度和增透区域面积以及 解吸气量基本呈线性相关，割缝宽度影响最大，其次是割缝长度，割缝角度影响最小。 通过上述研 究成果，在定向井+水力割缝基础之上提出了平行水平井错位多割缝卸压解吸增透技术，为深部煤 层气的高效开发提供了新思路和研究手段。
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 abovementioned 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 multislotting 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.
deep coalbed methane;hydraulic slotting;pressure relief and permeability enhancement;material point method