多裂缝“应力阴影”的有限元数值模拟

“应力阴影”效应对压裂人工裂缝扩展具有重要影响,目前对多缝内压力变化对应力干扰影响的研究较少。以弹性力学虚功原理为基础,利用有限元法自行编制程序模拟得出,在给定裂缝尺寸、缝内流压等条件下最优裂缝间距为140 m,模拟了最优裂缝间距的应力阴影分布状态,两裂缝尖端处主方向变化较大;在两缝之间存在特定的区域主应力值较小,并根据缝内压力反求出在排量约为4.8 m3/min下,可利用此区域的“应力阴影”促使两主裂缝沟通,形成复杂缝网。最后分析了各条裂缝内的流压比值及每条裂缝流压等因素对应力阴影的影响规律,可通过多级交替改变排量、液体黏度及加人工转向剂暂堵裂缝等措施实现多缝内流压变化,控制两缝流压比值在0.5~2.0区间内,增强缝间应力干扰,促进水力压裂过程中复杂缝网的形成,因此细致研究复杂裂缝间压力分布对“应力阴影”的分析非常关键。

Numerical Simulation of Stress Shadow Effect of Multi-fractures via Finite Element Methods

It is of great importance for stress shadow effect to influence the propagation of artificial fracture,but little research has been done on how fluid pressure affects stress interference between multi-fractures.Based on principles of virtual work in the elasticity theory,finite element code is successfully implemented to simulate the stress shadow effect.With the optimal fracture space equal to 140 meters according to given conditions,such as fracture geometry and fluid pressure,the stress shadow distribution in the case of the optimal fracture space is numerically simulated.At their fracture tips,there is a large change of the main direction of the principle stress.Between these two fractures,there is a certain region where the value of principle stress is smaller.The inversion-calculated injection rate is 4.8m3/min according to the given value of fluid pressure.At the level of this pump rate,the stress shadow effect on this area can be used to connect the two fractures,and then generate a complex fracture network.Finally,the impact of different factors,such as fluid pressure and ratio of fluid pressure of two fractures on stress shadow,are respectively analyzed.Some useful measures,such as multi-stage alternating injection rate,viscosity fracturing fluid,adding artificial diverting agent to block the fracture,are recommended to enhance the stress interference between multi-fractures and control the ratio of fluid pressure in the range of 0.5~2.0.Therefore,it is the key for analysis of stress shadow to have a further and detailed study on the fluid pressure distribution between these complex multi-fractures.