Study of the relationship between fractures and highly productive shale gas zones, Longmaxi Formation, Jiaoshiba area in eastern Sichuan

Shale fractures are an important factor controlling shale gas enrichment and high-productivity zones in the Longmaxi Formation, Jiaoshiba area in eastern Sichuan. Drilling results have, however, shown that the shale fracture density does not have a straightforward correlation with shale gas productivity. Based on logging data, drilling and seismic data, the relationship between shale fracture and shale gas accumulation is investigated by integrating the results of experiments and geophysical methods. The following conclusions have been drawn：（1） Tracer diffusion tests indicate that zones of fracture act as favorable channels for shale gas migration and high-angle fractures promote gas accumulation.（2） Based on the result of azimuthal anisotropy prediction, a fracture system with anisotropy strength values between 1 and 1.15 represents a moderate development of high-angle fractures, which is considered to be favorable for shale gas accumulation and high productivity, while fracture systems with anisotropy strength values larger than 1.15 indicate over-development of shale fracture, which may result in the destruction of the shale reservoir preservation conditions.

Production analysis in shale gas reservoirs based on fracturing-enhanced permeability areas

Hydraulic fracturing has been widely applied in shale gas exploitation because it improves the permeability of the rock matrix.Fracturing stimulation parameters such as the pumping rate, the fracturing sequence, and the fracture spacing significantly influence the distribution of the stimulated reservoir volume(SRV). In this research, we built a numerical model that incorporates the hydraulic fracturing process and predicts gas production. The simulation of fracture propagation is based on the extended finite element method(XFEM), which helps to calculate aspects of the fractures and the SRV; we imported the results into a production analysis model as the initial conditions for production prediction. Using the model, we investigated the effects of some key parameters such as rock cohesion, fracture spacing, pumping rate, and fracturing sequence on the shale gas production.Our results proved that the SRV was distributed in the vicinity of the main fractures, and the SRVs were connected between the fractures in a small fracture spacing. We obtained optimal spacing by analyzing the production increment. High pumping-rate treatment greatly changes the in-situ stress around the hydraulic fractures and enlarges the field of SRV. Simultaneous fracturing treatment improves the flow conductivity of formation more than sequential fracturing. This study provides insights into the hydraulic fracturing design for economical production.