塔河油田重复酸压前应力场数值模拟分析

Numerical simulation analysis of the stress field before repeated acid-fracturing in Tahe Oilfield

近年来暂堵转向重复酸压已成为塔河碳酸盐岩油藏重复改造的主要手段,但现场对堵前应力场分布认识不足,限制了工艺选井选层及重复酸压时机的选择。为明确暂堵前孔隙压力及应力场变化规律,基于ABAQUS扩展有限元建立了井筒及缝周应力场分布的流-固耦合模型,对油井的初次压裂及生产进行了模拟,分析了多因素对裂缝及生产引起诱导应力的影响规律。结果表明:人工缝及生产产生的诱导应力使原始地应力场明显变化,地应力差沿缝长方向以井口为对称点呈对称分布,在井口及裂缝两端呈“应力奇异性”,距缝壁越远,诱导应力差越低;较大的初次压裂缝长、弹性模量,较小的原始水平地应力差、泊松比、油井产量及生产时间均有利于暂堵重复酸压施工;塔河储层高应力差特点导致仅依靠人工裂缝诱导难实现转向,必须配合适当的转向工艺。此研究结果为今后塔河老井重复改造设计、工艺选井选层及重复酸压时机的选择提供了直观可视的分析方法,具有一定的现场实用价值。

In recent years,the temporarily plugging-converting and repeated acid-fracturing technology has become the main mean of the repeat treatment in Tahe carbonate oil reservoir.However,the stress field distribution before the temporary plugging is not fully recognized on site,which limits the selections of the well-layer and re-fracturing opportunity.In order to clarify the variation laws of the pore pressure and stress field before the temporary plugging,the mechanical model of the stress field induced by the in-situ stress field,artificial fracture and production activities were deduced.And moreover,based on ABAQUS extended finite element(XFEM),the fluid-solid coupling model of the stress field distribution around the wellbore and fracture was established.The initial fracturing and production of the oil well were simulated,and the laws of the multiple factors affecting the induced stress caused by the fracture and production were analyzed.The results indicate that the induced stress caused by the artificial fracture and production changes the original stress field obviously,the stress difference distributes symmetrically along the fracture length direction with the wellhead as the symmetrical point,and presents the“stress singularity”at the wellhead and two ends of the fracture.The farther away from the crack wall,the lower the induced stress difference will be.The larger initial fracture length and elastic modulus,as well as the small initial horizontal stress difference,Poisson's ratio,oil well production and production time are conducive to the temporary plugging and repeated acid-fracturing operations.The high stress difference in Tahe reservoir results in the difficulty to realize the conversion and steering only by the artificial fracture induction,so the proper steering technology must be matched.The study achievements provide an intuitive and visual analyzing method for the design of the repeated reformation,the selection of well and layer along with the re-fracturing opportunity,which have a certain practical value in the field.