Shale gas development by hydraulic fracturing in Southern China highlights the features of high fracture pressure and of serious difficulties in proppant placement due to deep burial (over 4000 m). This scenario differs from that in North America.
To solve the problems described above, conventional triaxial tests were conducted on the properties of rock mechanics under a simulated high temperature and high pressure reservoir environment. A full-size 3-D finite element method (FEM) is used to predict the fracture pressure of deep shale formation, taking both the rock deformation behavior of ultra-deep shale and the influence of fracturing fluid filtration into account. Effective engineering approaches are also presented that focus on reducing the friction of fracturing fluid and on the efficiency of acid pretreatment.
The triaxial compressive test results indicate that under the condition of high temperature and increasing confining pressure, the plasticity of the rock is strengthened and the nonlinear characteristic will inhibit fracture initiation. The correctness and reliability of the FEM model are confirmed through a comparison of the FEM model calculation result with the field data. Moreover, preliminary proposals are made for perforation parameter optimization based on this model. An optimal perforation phase angle of 60° and a perforation density of 18 perforations/m are considered appropriate parameters. A long perforation tunnel (more than 1 m) mitigates the state that the majority of the perforation component endures pressure stress. Finally, both laboratory test and field application results show that a refined slick water system design can generate a friction reducing rate of no less than 75% and that a new acid system can lower pressure by 10–15 MPa before fracturing. As a result, fracture pressure is reduced.
This work presents a method of addressing the challenges arising from the abnormal hydraulic fracture pressure on ultra-deep shale, including the causes determined during laboratory tests and the treatments generalized from FEM results and engineering approaches. These solutions are a necessary step toward reducing fracture pressure in deep shale gas development.