多分支缝水力压裂近井筒起裂机制研究

Fracturing mechanisms in near-well zones during hydraulic fracturing with multi-branch fractures

针对多分支缝水力压裂井筒起裂效果差的问题,基于弹性力学理论,考虑初次压裂诱导应力对近井地带应力场的作用,建立多分支缝起裂角及起裂压力的力学计算模型。在此模型的基础上,计算并分析了多分支缝起裂角的影响因素和多分支缝的延伸情况。研究结果表明:在低水平应力差及高闭合应力条件下,形成分支缝的起裂角越大,分支缝水力压裂效果越好;在低水平应力差,初始裂缝缝长越短的条件下,分支缝偏转程度高,更易重新定向。根据力学模型计算7口井分支缝的起裂压力,并对比现场施工起裂压力,验证了模型的准确性。该研究成果为多分支缝水力压裂工艺提供了理论基础,对优化分支缝压裂施工具有一定的指导意义。

With regard to poor fracturing performances in hydraulic fracturing involving multi-branch fractures, me- chanical computational models for initiation angles and initiation pressures of multi-branch fracture have been con- structed on bases of elastic mechanics theories and with consideration to impacts of induced stress in initial fractu- ring on near-well stress field. By using the newly-developed models, influencing factors for initiation angles of multi-branch fractures, together with extension of multi-branch fractures have been calculated and analyzed. Re- search results show ： under low differential horizontal stresses and high fracture closure stresses, higher initiation an- gles in formation of branch fractures may lead to better hydraulic fracturing performances of such fractures ; Under low differential horizontal stresses and shorter initial fractures, branch fractures may deviate significantly and conse- quently may experience reorientation easily. By using the mechanical model, initiation pressures for branch fractures in 7 wells have been calculated. In addition, reliability of these models have been verified by comparing with initia- tion pressures measured on site. Relevant research results may provide reliable theoretical foundation for hydraulic fracturing with multi-branch fractures. In addition, these results may provide valuable guidance for optimization of fracturing operations involving multiple branch fractures.