深层页岩气水力裂缝起裂与扩展试验及压裂优化设计

受地质构造、成岩作用等多方面因素影响，深层页岩的层理发育程度、脆性指数、岩石力学特性、最小水平主应力梯度及水平应力差都与中深层页岩有明显差异，使人工裂缝起裂压力更高，裂缝复杂程度更低，从而极大地影响了深层页岩气地层的体积压裂设计和安全施工。为此，利用大尺寸岩样，模拟研究了深层页岩气地层的水平应力差、压裂流体黏度、施工排量等地层和工艺参数及缝内暂堵措施对人工裂缝的起裂与扩展特征的影响规律。研究发现，裂缝起裂与扩展特性受层理胶结强弱、水平应力差及前置液黏度等因素影响较大，压裂裂缝容易沿层理起裂导致早期憋压超压，从而使施工失败，高应力差条件下裂缝扩展形态相对简单，前置中黏压裂液、缝内暂堵等措施有利于裂缝多次破裂、产生次生裂缝使裂缝复杂化。在此基础上，提出了密切割分段、短簇距射孔、组合液体及变排量施工等压裂优化设计方案，现场应用后深层页岩气产量获得了重要突破。 

Hydraulic Fracture Initiation and Extending Tests in Deep Shale Gas Formations and Fracturing Design Optimization

Due to geological structure, diagenesis and other factors, deep shale presents different characteristics compared with that in medium-deep formations in terms of bedding development degree, brittleness index, rock mechanical characteristics, in-situ stress gradient and horizontal stress difference. Taken together, theseresult in higher fracture initiation pressure and less complicated fractures geometry and greatly affect fracturing volume design and operation safety in deep shale gas formations. Experimental study on the initiation and expansion characteristics of artificial fractures was conducted. A large cubic rock sample (300 mm×300 mm×300 mm) was used to investigate the influential effects of horizontal stress difference, viscosity of fracturing fluid and pumping flow rate, and the temporary blocking within fractures in hydraulic fracturing. The investigation showed that fracture initiation and propagation are largely affected by those factors as strength of bedding cementation, horizontal stress difference and pad viscosity. Fractures are prone to initiate along bedding planes, resulting in early overpressure and operation failure. Fracture growth pattern is relatively simple under high stress difference, but measures such as using medium-level viscous fracturing fluid to temporarily block flow within fractures can help the generation of multiple fractures and secondary fractures for more complex fracture networks. On this basis, the design optimization of fracturing that incorporates techniques such as densely subdivided stages, short cluster perforations, fluids combination and variable flow rate operation were advanced, and an important breakthrough was made in deep shale gas production after the field application of the optimized design features.