中文摘要：

水力压裂可显著提高页岩气等致密储层岩体的渗透性以增加油气产量,然而受多因素影响,水力压裂形成缝网结构的机理和压裂优化设计一直是研究的焦点和难点。本研究基于渗流-应力-破坏耦合计算模拟方法,对不同水力加载条件下的非均质储层水力压裂过程进行了模拟和对比研究。研究结果表明：水力压裂过程中起始注水压力和增量大小对水力压裂缝网扩展和改造区域形态有着显著的影响。当起始注水压力小于等于模型材料体抗拉强度,并缓慢增压致裂时,压裂过程可近似视为稳态应力-破坏-渗流耦合作用过程的不同阶段,这种情况下仅在压裂井孔周围形成两组对称式的伞状水力裂缝带。当对模型体施加高于模型材料体破裂压力的注水压力时,相当于对压裂孔快速施加高动水压力,水力裂缝沿压裂孔全方位迅速萌生并快速扩展,当注水压力值高于破裂压力一定幅值时,压裂改造可形成围绕压裂井全方位的放射状裂缝网络,使压裂储层得以最大范围改造。在拟静力和拟动力两种加载条件下,不同水岩相互作用机理是造成不同水力加载条件出现不同缝网结构的力学机制,而对于实际的页岩气储层改造,压裂产生围绕压裂井全方位放射状的缝网结构则是一种最优的体积压裂改造。

英文摘要：

Many studies focus on the characteristics and mechanisms of the hydraulic fracturing because there are lots of factors influence the fracturing process and results. In this paper,effects of different initial hydraulic pressures on hydraulic fracturing are numerically investigated with a finite element method. It considers the heterogeneous of rocks through Weibull distribution of mechanical and strengthen parameters in a coupled seep-stress-failure model. The simulations reveal that when the initial hydraulic pressure is less than or equal to the tensile strength of the rock specimens and increases with a low growth rate,similar to the static loading method,hydraulic fractures can be initiated and propagated at a comfortable speed,which eventually forms two groups ofumbrella-shaped fracture network. However,when the initial hydraulic pressure is greater than the breakdown pressure of the rock specimens,similar to the dynamic loading method,hydraulic fractures can initiated and propagated rapidly in all direction of the hole,which produces a radial fracture network. The processes and characteristics of hydraulic fractures reflect different water-rock interaction mechanisms under different loading modes. As for the reservoir fracturing improvement,the radial fracture network is the optimum result because it creates the maximum fracture surface area for migration and desorption of the shale gas.