TOUGH-FLAC^(3D)热流固耦合模拟煤储层水力压裂过程

Investigation of hydraulic fracturing process in coal reservoir by a coupled thermo-hydro-mechanical simulator TOUGH-FLAC^(3D)

以沁水盆地南部安泽区块煤层气储层为例,将TOUGH与FLAC3D搭接耦合来设计模拟,以实现对煤储层水力压裂热-流-固耦合的分析,进而获得在多场控制下的裂缝扩展与穿层规律。研究结果表明:水力裂缝沿着先存裂缝方向扩展更容易发生于二者以较小逼近角相交时。当逼近角较大时,水力裂缝会沿着最大主应力方向扩展。在垂向平面上,由于煤岩与顶底板岩石力学性质差异显著且煤层微裂隙发育,压力在煤层当中传播速度快于在顶底板中,导致在煤层中靠近顶底板的交界面处,形成两个相对高压区域。同时埋深较深的煤层由于原地应力较大,相比于浅层煤层,更易发生塑性变形。深部煤层由于压裂液滤失系数较小且垂向应力较大造成的流压积累,是顶底板被压穿的主要原因。煤样物理实验和压裂曲线分析的结果验证了此次模拟结论的准确性。

A coal-bed methane reservoir of the Anze Block,southern Qinshui Basin,is the chosen study area. The research developed a simulator linking TOUGH with FLAC3 Dto analyze thermo-hydro-mechanical coupling during hydraulic fracturing in the coal reservoir,and then to reveal the rules of fracture propagation and bed-penetrating under the constraint of multiple fields. The results show that hydraulic fractures that propagate along the pre-existing natural fracture＇s direction,tend to occur when the two fractures intersect with a small approaching angle; otherwise,hydraulic fractures will propagate along the direction of the maximum horizontal principal stress. Due to the distinctive difference of rock mechanical properties between coalbed and its roof and floor beds,and high density of micro-fractures in the coalbed,the pressure in the coalbed propagates faster than that in the roof and floor rocks seen from vertical profiles,which results in two relatively higher pressure zones near the boundaries from coalbed to roof and coalbed to floor rocks respectively. In addition,compared with the shallowly buried coalbed,the deeply buried one is subjected to plastic deformation under an existing bigger in-situ pressure. The major reason for roof/floor fracture penetration is the occurrence of flow pressure accumulation,caused by the relative small leak-off rate of fracturing fluid and relatively bigger vertical stress than that of the shallow bed. The results of sampled coal experiments and fracturing curve analysis verify the accuracy of the simulation.