基于诱导应力的页岩气“井工厂”压后效果评价

INDUCED STRESS BASED EVALUATION METHOD FOR FRACTURING IN SHALE GAS WELLS

水力压裂是页岩气高效开发的必要措施,页岩气压后效果评价是提高页岩气压裂水平的关键。文章以涪陵一页岩气井平台为例,提出了一种针对页岩气压裂后应力场分析的压裂效果评价新方法。根据现场微地震实时监测结果获得了3口井压后裂缝情况,在此基础上,采用有限元模拟软件建立了3口井压裂后应力场模拟有限元模拟模型。重点分析了压裂后最大、最小水平主应力场变化,缝长和间距对压后诱导应力场的影响,分析发现,裂缝条数增加、裂缝长度不均匀将极大地影响压后诱导应力场;长裂缝将抑制中间短裂缝的扩展;各条裂缝缝长差异不大时有利于裂缝扩展;段间距对裂缝中心产生的诱导应力场影响较小;裂缝尖端诱导应力为压应力时裂缝延伸受限,SRV相对较小、压后产量较低。

Hydraulic fracturing is a necessary measure for the efficient development of shale gas.Post-frac evaluation is the key to improve hydraulic fracturing in shale.Taking a shale gas well platform at Fuling as an example, a new method for the evaluation of post-frac results is described here in this paper, which is based on the analysis of stress field.By the real-time microseismic monitoring, hydraulic fracturing results in three wells were obtained.Finite element simulation software was used to establish a finite element simulation model for the post-fracturing stress field of the three wells.The changes in the maximum and minimum horizontal principal stress field after fracturing, the influence of the length and spacing of the cracks on the induced stress field after fracturing were analyzed.It is found that the increase in the crack number and the uneven hydraulic fracture length will greatly affect the induced post-frac stress field.The long fractures will restrain the expansion of the short fractures in between.When the length difference between fractures is small, it is beneficial to fractures’ propagation.The spacing between stages has little effect on the induced stress field.When induced stress at the fracture tip is compressive stress, the crack extension is limited, SRV is relatively small, and the yield after fracturing is rather low.The post-frac induced stress field analysis results presented in this paper have good consistency with microseismic monitoring and yielding test results, it further enriches post-frac evaluation technology for shale.And it can provide guidance for shale gas hydraulic fracturing design optimization.