页岩气藏网状裂缝系统的岩石断裂动力学

Rock fracture kinetics of the fracture mesh system in shale gas reservoirs

基于岩石断裂动力学理论,研究页岩气藏压裂网状裂缝的形成机理及天然闭合裂缝的激活机理。研究发现,水力裂缝在沟通天然裂缝后,需要在左右两端同时满足裂缝转向条件,才能形成复杂的网状裂缝,存在形成网状裂缝的最小排量(临界排量);临界排量随倾角增加而增加,倾角等于90°时,临界排量达最大值且为常数;倾角小于90°时,临界排量随裂缝与井筒夹角增加先减小后增加,垂直井筒方向的裂缝临界排量最小;天然裂缝长度越大、地层弹性模量越低需要的临界排量越高。天然裂缝面不吻合且缝面粗糙,水力裂缝开启天然裂缝后天然裂缝面上剪切应力释放使得缝面滑移,从而提高导流能力。裂缝倾角很大或很小时,对裂缝激活不利;30°～60°倾角裂缝激活效果最佳;弹性模量增加对裂缝滑移起抑制作用,泊松比变化对滑移量影响很小。

The formation mechanism of fracture mesh and the mechanism of activating natural closed fractures during shale fracturing are studied based on rock fracture kinetics. It is found that steering conditions should be satisfied at both left and right ends of a natural fracture to form intensive fracture mesh after the hydraulic fracture reaches the natural fracture; there exists a minimum critical pump rate （critical pump rate） to form intensive fracture mesh. The critical pump rate increases as the inclination of natural fracture increases and reaches a maximum value （constant） when the inclination of natural fracture is 90°.When the inclination of natural fracture is less than 90°, the critical pump rate first decreases and then increases as the angle between horizontal wellbore and natural fracture increases; the critical pump rate reaches a minimum value for the natural fractures perpendicular to the wellbore. The critical pump rate increases as natural fracture length increases and rock elastic modulus decreases. It is also found that natural fracture surface is rough and not fitting, so when hydraulic fractures reopen existing natural fractures, the release of the original shear stress in the fracture surface would lead to the fracture surface slippage, which would greatly enhance the flow conductivity of the natural fracture. Very small or very large inclination of natural fracture is not conducive to fracture activation, best activating result can be reached at 300-60° inclination. Rise in elastic modulus inhibits fracture surface slippage and Poisson＇s ratio has little effect on fracture surface slippage.