基于物质点法的深部煤层水射流破岩成孔机制和影响因素

Mechanism and influencing factors of rock breaking and pore-forming by water jet in deep coal seam based on the MPM

我国深部煤层气资源丰富,但深部煤层渗透率极低,高应力−压力环境下深部煤层气解吸困难,导致产量低下。水平井水射流冲孔造穴是一种大范围卸压增透煤层的新技术,但在深部赋存环境下其破岩成孔机制还有待研究。由于深部煤层赋存于高地应力、高应力差的地质环境,室内物理模拟实验难以开展。因此,首先基于黏弹塑性理论和物质点法建立了水射流冲孔破岩数值模型,该模型集成拉格朗日和欧拉算法的优势,能有效模拟射流破岩过程中的动量交换、煤岩大变形、破岩、流体剥蚀携岩反排等全过程。然后,通过该数值模型开展了深部煤层水射流破岩成孔机制和影响因素研究,得到结论如下:①深部煤层以高地应力荷载作用造成的剪切破坏为破岩的主导机制,射流主要起到冲蚀和携岩反排的作用,而无应力荷载时以水射流碰撞煤岩产生的应力波造成张拉破坏为破岩的主导机制,由于煤岩塑性较强,水楔效应不明显。②深部煤层高地应力荷载作用加速了破岩过程,冲出煤量比无应力荷载时高,同时应力荷载促使煤体向开孔方向移动,造成空腔体积减小,但塑性损伤区较无应力荷载时明显增大。③冲出煤量、塑性损伤区面积都随射流时间增加而增加,但增幅逐渐变缓。冲出煤量、塑性损伤区面积随入射角度的增加整体呈下降趋势,射流角度越小,水射流冲孔破岩效果越好,射流垂直入射并不是破岩的最佳方式。④冲出煤量、塑性损伤区面积随应力差、煤岩开孔宽度的增加而增加,但应力差的增加对深部煤层射流破岩的促进作用不大。煤层开孔宽度不宜过高,适当增加开孔宽度可以提高射流破岩效率。

China has a wealth of abundant coalbed methane(CBM)resources at great depths.However,the low permeability of these deep coal seams coupled with the challenges posed by high-stress pressure environments,hampers efficient gas extraction.Water jet punching and cavitation in horizontal wells is a new technology for relieving pressure and enhancing the permeability of coal seam over a wide area.Nevertheless,the mechanisms underlying rock breaking and pore formation in deep environment remain to be studied.Because the deep coal seam exists in the geological environment of high ground stress and high stress difference,it is difficult to carry out the physical simulation experiment in the laboratory.Consequently,a numerical model of water jet punching and rock breaking is established based on the viscoelasticplastic theory and material point method.The model integrates the advantages of the Lagrange and Euler algorithms,proficiently simulating the whole process of momentum exchange,large deformation of coal,rock breaking,fluid erosion and rock carrying and reverse drainage throughout the process of jet rock breaking.Subsequently,based on this numerical model,the mechanism and influencing factors of rock breaking and pore-forming by water jet in deep coal seams are studied,and the conclusions are as follows:①the shear failure caused by high in-situ stress load in deep coal seams is the leading mechanism of rock breaking,while jet mainly plays the role of erosion and rock carrying and reverse drainage.The tensile failure caused by stress wave generated by water jet collision with coal rock is the leading mechanism of rock breaking when there is no stress load.Due to the strong plasticity of coal rock,water wedge effect is not obvious.②The high in-situ stress load accelerates the rock breaking process of deep coal seams.The amount of rushed-out coal under high in-situ stress load is higher than that without stress load.Meanwhile,the stress load causes the coal body to move towards the opening direction,resulting in a decrease in cavity volume,but the plastic damage zone is significantly increased compared with that without stress load.③The amount of rushed-out coal and the area of plastic damage zone increase with the increase of jet time,but the increase rate gradually slows down.The amount of rushed-out coal and the area of plastic damage zone show a downward trend with the increase of incident angle.The smaller the jet angle,the better the effect of water jet punching and rock breaking.The vertical incidence of jet is not the best way to break rock.④The amount of rushed-out coal and the area of plastic damage area increase with the increase of stress difference and the width of coal-rock opening,but the increase of stress difference has little effect on jet rock breaking in deep coal seams.The width of coal seam opening should not be too high,and the rock breaking efficiency of jet can be improved by increasing the width of the opening hole appropriately.