横向切槽几何形状对煤岩水力裂缝扩展影响分析

Analysis of transverse groove geometry on extension effect of hydraulic fracture in coal and rock

为解决煤层渗透率低、抽采量小、抽采率低等特点的煤层气抽采难题,针对红庆河煤矿3-1煤层,考虑用试错法调整模型的细观参数来匹配煤样的宏观性质,建立了基于颗粒离散元程序的煤岩水力压裂流固耦合模型,采用颗粒粘结模型及光滑节理模型模拟煤岩结构特征,研究了条形、三角形和正方形预制横向切槽对煤岩起裂难易程度和裂缝扩展形态的影响规律。模拟结果表明,预制横向切槽可驱使其附近裂缝沿着切槽定向起裂、扩展,在控制相同的水压且围压比为1.0等变量的条件下,条形切槽的煤岩水力压裂裂缝条数最少但平均缝长最长,正方形切槽的煤岩水力压裂裂缝条数最多但平均缝长最短;综合对比下,正方形的横向切槽比条形槽和三角形槽更易形成复杂的缝网、增大煤层有效改造体积,3种横向切槽均能减小煤岩的起裂难度,横向切槽的尖端角越多,起裂难度越小;煤岩水力压裂中距离横向切槽孔较近部分的破坏形式主要为张拉破坏,裂缝较为平直且不出现分叉,距离横向切槽孔较远部分开始出现剪切破坏。

In order to solve the coalbed methane extraction problems characterized by low permeability of coal seam,small extraction volume and low extraction rate,for the 3-1 coal seam of Hongqinghe Coal Mine,the fine view parameters of the model were considered to match the macroscopic properties of coal samples by trial-and-error method,a flow-solid coupling model of hydraulic fracturing of coal rock based on particle discrete element program was established,the particle bonding model and smooth joint model were used to simulate the structural characteristics of coal and rock,and the influence of prefabricated transverse grooves in strips,triangles,and squares on the difficulty of coal and rock cracking and the morphology of crack propagation was studied.The simulation results show that:the prefabricated transverse grooves can drive the fracture initiation and expansion along the grooves,and under the conditions of controlling the same water pressure and the surrounding pressure ratio of 1.0,the number of fractures in the coal and rock is the lowest but the average fracture length is the longest in the strip grooves,and the number of fractures in the coal and rock is the highest but the average fracture length is the shortest in the square grooves;under the comprehensive comparison,the square transverse grooves are easier to form a complex seam network and increase the effective transformation volume of the coal seam than the strip grooves and triangular grooves,and all three transverse grooves can reduce the fracture initiation difficulty of the coal and rock,and the more the tip angle of the transverse grooves,the less the fracture initiation difficulty;the damage form in the part of the coal and rock hydraulic fracturing near to the transverse groove hole is mainly tension damage,and the fracture is relatively straight and does not show bifurcation,and the part farther from the transverse groove hole starts to show shear damage.