非均匀应力场影响下的裂缝扩展模拟及投球暂堵优化

Simulation of fracture propagation and optimization of ball-sealer in-stage diversion under the effect of heterogeneous stress field

水平井投球暂堵是实现致密油气藏压裂段均匀改造的关键技术,但目前针对压裂施工中实施不同投球暂堵工艺措施后多簇裂缝扩展形态的研究成果却较少,致使现场投球暂堵工艺措施的制订缺少相关的理论支撑,影响了该工艺在压裂施工现场的应用效果。为此,基于边界元方法建立了水平井"井筒-孔眼-裂缝扩展"全耦合模型,提出了暂堵球分配的计算方法,进而模拟在初始非均匀应力场条件下进行段内暂堵转向时暂堵球的投球数量、投球时机、投球次数及其对多簇裂缝扩展的影响。研究结果表明:(1)压裂过程中,射孔孔眼摩阻的限流作用会平衡诱导应力干扰带来的进液量差异,使得各簇裂缝中流体流动受到的阻力差距缩小;(2)当考虑初始应力场非均匀分布的影响后,各簇裂缝进液量发生明显的变化,高应力区域甚至出现不进液的无效射孔簇,而实施投球后,无效射孔簇会产生新的裂缝;(3)当初始最小水平主应力差(△σh)超过3 MPa时,在施工中期适当增加投球数量(大于单段总射孔数的一半),或者在中前期进行投球暂堵(包括在中前期分批次投球),有利于降低各簇裂缝的非均匀扩展程度;(4)当初始△σh低于2MPa时,应减少投球数量或在施工中后期投球,否则会加剧各簇裂缝的非均匀扩展。

Ball-sealer in-stage diversion in horizontal wells is a key technique to realize the uniform stimulation of fractured sections in tight oil and gas reservoirs.So far,however,there are fewer research results on the propagation morphologies of multi-cluster fractures after the implementation of different ball-sealer in-stage diversion processes during the fracturing treatment,which results in less theoretical support for the preparation of field ball-sealer in-stage diversion process and measures and impacts its application effects in the fracturing field.To deal with this situation,this paper established a fully coupled"wellbore-perforation-fracture propagation"model of horizontal wells on the basis of the boundary element method.Then,a method for calculating the allocation of ball sealers was proposed.Finally,the number of ball sealers,diversion time and number of diversions during the intra-stage temporary plugging and diversion under the condition of initial heterogeneous stress field and their effects on the propagation of multi-cluster fractures were simulated.And the following research results were obtained.First,the flow restriction of perforation friction can counterbalance the intake difference caused by the induced stress interference so that the friction difference applied on the fluid flow in each fracture cluster is reduced.Second,when the effect of the heterogeneous distribution of the initial stress field is taken into consideration,the liquid intake of each fracture cluster changes greatly and even ineffective perforation clusters without liquid incoming appear in the high-stress region.And after the ball is injected,new fractures are initiated from the ineffective perforation clusters.Third,when the initial minimum horizontal principal stress difference(△σh)is higher than 3 MPa,it is beneficial to reduce the non-uniform propagation of each fracture cluster by increasing the number of ball sealers appropriately in the middle stage of the construction(over half of the total perforations of each stage)or carrying out temporary plugging in the early-middle stage(including ball injection in batches in the early-middle stage).Fourth,when△σh is lower than 2 MPa,it is necessary to reduce the number of ball sealers or inject balls in the middle-late stage,or the non-uniform propagation of each fracture cluster will be aggravated.