基于煤系渗透率各向异性测试的水平井穿层压裂效果模拟

应用自主研发的真三轴渗透仪,开展了煤样及顶、底板岩样的三轴渗透率测定。基于实验结果,通过有限元三维数值模拟方法,研究了渗透率各向异性对深部煤层顶板水平井穿层压裂裂缝扩展的影响。模拟结果表明:①真三轴渗透仪可以实现三轴差异化应力加载,突破了常规拟三轴渗透仪无法分别加载水平最大主应力和水平最小主应力的瓶颈,可以使实验条件与地层条件更为接近。②高阶无烟煤与顶板粉砂岩的真三轴渗透率测试结果都呈现出明显的各向异性特征。无论是高阶无烟煤,还是顶板粉砂岩,垂直层理方向的渗透率都小于平行层理方向者。对于无烟煤,平行层理方向的渗透率也存在明显差别,垂直端割理方向的渗透率大于垂直面割理方向渗透率值。③考虑渗透率各向异性模型与均质模型相比,压裂模拟过程中,破裂压力和施工压力增高,孔隙压力叠加程度增大;模拟最终时刻的整体裂缝形态更加短宽,且缝高增大。

Effect simulation of horizontal well fracturing through strata based on coal seam permeability anisotropy test

The independently-developed true-triaxial permeameter was applied to test the triaxial permeabilities of coal samples, roof and floor rock samples. Based on experiment results, finite element 3D numerical simulation method was adopted to study the influences of permeability anisotropy on the horizon-crossing fracturing fracture propagation of horizontal wells at deep coal seam roof. The simulation results have indicated that: (1) The true-triaxial permeameter can realize triaxial differential stress loading, thus breaking through the bottleneck of conventional triaxial permeameter unable to load maximum and minimum primary horizontal stress respectively, and making experimental conditions be more consistent with formation conditions. (2) The true-triaxial permeability test results of high-rank anthracite and roof siltstone present significant anisotropy characteristics. Both permeabilities in the direction perpendicular to beddings are smaller than those in the direction parallel to beddings. For anthracite, significant differences also exist in the permeability in the direction parallel to beddings, while the permeability in cleat direction at vertical end is greater than that on vertical plane. (3) Compared with homogeneous model, failure pressure and operation pressure are increased during the fracturing simulation process of permeability anisotropy model, as well as the superposition degree of pore pressure; the overall fracture morphology is shorter and wider with fracture height increasing at the termination of simulation.