煤层气井压裂缝壁面稳定性研究

Study on hydraulic fracture wall stability of CBM well

为了揭示煤层气压裂井在排采过程中裂缝壁面剪切失稳产生煤粉的机理,根据叠加原理,采用弹性力学和渗流力学理论,构建了排采过程中压裂缝的总应力场计算模型。基于D-P剪切破坏准则,提出了一种计算井底流动压力安全窗的方法,以有效避免煤岩压裂缝壁面失稳。研究结果表明:井底流动压力越小,压裂缝壁面净压力越大,在压裂缝壁面附近产生的诱导应力也越大;另外,井底流动压力越小,由于孔弹效应影响,形成的诱导应力也越大。在排采过程中,保证井底流动压力在安全窗口内,可以避免压裂缝壁面剪切失稳形成煤粉。

In order to demonstrate the coal pulverization mechanism caused by the shear instability of coal cracks wall in coal seams during gas drainage,elasticity mechanics coupled with percolation theory were used to construct the total principal stress in the formation surrounding the hydraulic fracture during discharging process. Based on the D-P shear failure criterion,a method for calculating the bottom hole flowing pressure safety window is proposed,which can effectively avoid the instability of the coal-rock fracture wall.The results show that lowering of bottom hole flowing pressure can increase the net stress around fracture,which can lead to higher induced stress perturbation around fracture wall.In addition,due to poroelastic effect,the lower the bottom hole flowing pressure is,and induced stress is also greater.During the drainage process,the bottom hole flowing pressure is ensured within the safety window,which can avoid the formation of pulverized coal due to shear failure of the fracture wall.