The pumping rate is low due to higher in-situ stress and initiation pressure, so that the fracture width and the corresponding proppant concentration are all limited.
Due to high differential stress between maximum horizontal stress minimum, the hydraulic fracture tend to be single fracture instead of network fractures. Meanwhile, the overburden pressure is maximal in most cases which may cause un-open of horizontal bedding fissures.
Rock ductility is increased due to higher temperature and pore pressure in matrix, which may cause difficulties in fracture initiation and propagation.
The conductivity within multiple scale fractures are all decreased quickly due to the higher in-situ stress magnitude. In this case, the higher stress magnitude and the larger crash ratio and embedment of proppant caused the reduction of fracture conductivity. In some extreme cases, the above mentioned conductivity may even decrease to near zero so that the ESRV is also accordingly decreased largely in this circumstance.
In many cases, high angle natural fractures may exist with horizontal bedding fissures synchronously, especially in the south of the structure in many shale gas blocks, so that the initiation and propagation mechanisms of hydraulic fractures are very complicated and poor understood. In this case, if the pumping rate and the viscosity of fracturing fluid are not adaptable, the fracture height may be out of control, and the ESRV may not be available as expected.
There are tremendous shale gas resources with depth of beyond 3500m in Sichuan basin, China. The tectonic stress is very high which causes an abnormally in-situ stress in this case, so that it brings challenges during horizontal staged fracturing, such as high breakdown pressure, low fracture complexity, low proppant concentration and weight, as well as less sufficient effective stimulated reservoir volume (ESRV) as expected. The research in this paper can improve the above situation. For achieving a better production in the shale formation, a new method of hydraulic fracturing technique was developed, including optimized perforation simulation, engineering solution for multiple scale fractures opening, the optimized propped method of multiple scale fractures, fracture height controlling. The simulation results demonstrated that the improving of fracture length, width and height is impressive which is 36%, 19% and 20%, respectively, after adopting 6 perforations in fixed plane perforation compared with 20 perforations in spiral perforation. In addition, the breakdown pressure was found to have an impressive decline after simulation by varied fluid viscosity from 200 mPa. s to 2.5 mPa. s. After adopting a combination of above work, the optimized design method has been applied in two wells for shale gas exploration.
