致密砂岩油藏体积压裂簇间距优化新方法

为了增大致密砂岩油藏体积压裂改造体积,需要对水平井分段压裂射孔簇间距这一关键参数进行优化。一般基于诱导应力分析进而优化簇间距的方法,存在无法与实际泵注程序有效结合的问题,也就不能有针对性的优化簇间距。基于有限元方法,借助ABAQUS软件,建立转向裂缝起裂扩展模型,以天然裂缝开启准则和临界转向簇间距为优化标准,结合实际泵注程序,综合考虑诱导应力和缝内净压力的耦合效应,形成了簇间距优化方法。研究认为,在诱导应力的作用下,存在一个可使主应力发生反转的临界转向簇间距,且随着施工排量的增加,该临界转向簇间距也越大;当簇间距取该临界值时,储集层水平主应力较为接近,主裂缝沟通天然裂缝起裂转向,最易形成复杂裂缝,这可作为优化簇间距的一个标准。现场应用表明,在依据该方法优化的簇间距下,主裂缝可沟通天然裂缝起裂转向,增大改造体积,改善致密砂岩油藏体积压裂效果。

A New Method for Cluster Spacing Optimization During Volumetric Fracturing in Tight Sandstone Oil Reservoirs

To increase the stimulated reservoir volume during volumetric fracturing in tight sandstone reservoirs, perforation cluster spacing of staged fracturing in horizontal wells needs to be optimized. The traditional method based on the induced stress analysis can’t be effectively combined with the actual pump program, and also can’t optimize the cluster spacing more precisely. A crack initiation model is established with ABAQUS software based on the finite element method. Taking the natural fracture opening criterion and critical cluster spacing for fracture reorientation as the optimization standards and combining with the actual pump program, a cluster spacing optimization method is developed by considering the coupling effect of induced stress and net fracture pressure. It is considered that under the effect of induce stress, there is a critical cluster spacing for fracture reorientation which can make the principle stress inverse between clusters, and the critical cluster spacing for fracture reorientation becomes larger with the increase of slurry rate. When the critical value is taken as the cluster spacing, the two principal stresses are close to each other, the main cracks link up with natural fractures and complex fractures form, which can be used as a standard to optimize the cluster spacing. Field application shows that with the optimized cluster spacing obtained from this method, the main cracks can connect with natural fractures, increase stimulated reservoir volume and improve the volumetric fracturing effect in tight sandstone reservoirs.