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.