基于扩展有限元的水力压裂缝间干扰及裂缝形态分析

为了研究水平井分段多簇压裂裂缝扩展过程中缝间干扰及其对裂缝形态的影响，在考虑裂缝流和压裂液滤失的情况下，利用扩展有限元方法，分别对不同裂缝间距、不同位置组合的裂缝在同时和分时压裂过程中，水平主应力、裂缝长度、加砂前裂缝宽度以及裂缝延伸方向的变化进行数值模拟分析。通过对比发现：在裂缝扩展过程中，由压裂液作用而产生的诱导应力使得裂缝两侧椭圆型区域内的水平主应力发生转向，其单侧作用距离约为1.5倍的缝长，而裂缝尖端应力方向不受影响；2条裂缝同时起裂时，2条对称裂缝延伸方向呈“相斥型”偏转，偏转角度随着裂缝间距增大而减小，2条交错裂缝延伸方向呈“相吸型”偏转，加砂前缝宽较单一裂缝扩展时窄；分时起裂的2条裂缝，后起裂裂缝的延伸方向随着裂缝间距增加先呈现“相吸型”后出现“相斥型”，且后期裂缝的延伸缝长会受到抑制，当2条裂缝间距较近时，后起裂裂缝会严重影响前起裂裂缝的加砂前缝宽甚至会使其闭合，但其缝长会有所增加，交错裂缝两翼会出现严重的非对称扩展的情况。

Analysis of stress interference and geometry of hydraulic fractures based on the extended finite element method

In order to study the stress interference between fractures and its effect on the fracture geometry during the multi-fracturing in horizontal wells,taking the fracture flow and fracturing fluid loss into consideration,variations of the horizontal in-situ stress and the length,the width before sand adding and the extension direction of fractures under different fracture spacing and positions at the same and different fracture initiation times were analyzed by numerical simulation based on extended infinite element method.Results show that the direction of the horizontal in-situ stress in the ellipsoidal regions around the fracture is changed by the induced stress caused by the fracturing fluid pressure.The unilateral distance of the effected region is approximately 1.5 times the length of the fracture.The direction of the horizontal stress on the tip of fractures is not effected.When two cracks initiated simultaneously,the extension direction of the two symmetrical cracks is “repulsive”,and the deflection angle decreases with the increase of the fracture spacing.The extension direction of two staggered fractures is “inter-attracting” and the width before sand adding is narrower than that of a single fracture.When two fractures initiate at different times,the extension direction of the latter fracture firstly is “inter-attracting” and then becomes “repulsive” with the increase of the fracture spacing and its length is inhibited by the former one.The width of the former fracture before sand adding is affected seriously by the latter one while its length becomes longer and there is a seriously asymmetrical expansion on both wings of the staggered fractures.