由于老井经历长期注采,区域地层压力分布在空间上呈现非均匀变化,在后续的重复压裂过程中亟需耦合考虑这种非均匀应力演化。为此,以长庆油田W区块长6储层为例,基于快速拉格朗日差分法建立了垂直井丛长期注采下的地应力演化模型,并将模拟得到的地应力场导入基于离散格子方法的水力压裂数值模型中,预测了重复压裂裂缝扩展,实现了地应力场演化—水力裂缝扩展的一体化模拟。结果表明:(1)WJ井生产后,井周围孔隙压力下降约4 MPa,并且两个水平主应力发生类似的同步降低,但变化幅度更小(约2.5 MPa),这表明生产将导致井周围地层的总应力降低但有效应力增加;(2)水力裂缝倾向于朝地层压力衰竭区优势扩展,在现场建议采取工程措施如重复压裂前缓慢注液或关井以使衰竭区的地层压力上升,避免或减少重复压裂裂缝扩展至压力衰竭区;(3)随着注入时间增加,重复压裂裂缝由以面积增长为主转为以宽度增长为主,说明重复压裂应该选取短时间、大排量的策略,一方面可在短时间内实现最大化的储层改造,另一方面增大排量也可促进多簇裂缝均衡扩展。
Before refracturing, due to the long-term injection and production of old wells, the distribution of regional formation pressure shows non-uniform variations. It is urgent to coupling consider this non-uniform stress evolution in the subsequent refracturing. For this purpose, taking the Chang-6 reservoir in the W block of Changqing Oilfield as an example, an in-situ stress evolution model under long-term injection and production of vertical wells is established using the Fast Lagrangian Analysis of Continua. The simulated stress field is then imported into a hydrofracture numerical model based on the discrete lattice method for modeling fracture propagation of refracturing, achieving an integrated simulation of in-situ stress evolution and hydraulic fracturing evolution. The results show that: (1) After the production of well WJ, the pore pressure around this well decreases by about 4 MPa, and the two horizontal principal stresses experience a similar synchronous reduction, but the decreasing magnitude is only about 2.5 MPa. This indicates that production will result in a decrease in the total stress but an increase in the effective stress. (2) Hydraulic fractures tend to propagate towards the depleted area preferentially. The engineering measures, such as slowly injecting fluid or shutting in before refracturing to increase the formation pressure in the depleted area, are recommended on site, thereby avoiding or reducing refracturing fractures extending into these areas. (3) As the injection time increases, the primary growth of refracturing fractures transitions from area expansion to width expansion, indicating that a short-duration, high-volume refracturing should be adopted. On the one hand, maximizing reservoir transformation can be achieved in a short time, on the other hand, increasing the injection rate can promote the even expansion of multiple fractures.
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