RESEARCH ON THE GEOSTRESS CHANGE OF SHALE RESERVOIR VOLUME FRACTURING

Shale reservoirs have ultra-low porosity and permeability. Without creating complex fracture network, there will be no economical production. Volume fracturing is the primary treatment for shale reservoirs stimulation. The geostress and the di erential stress between the two horizontal principal stresses control the creation of fracture networks. Previous researches have shown that: (1) The small di erential stress between the two horizontal principal stresses eases the development of fracture network and vice versa; (2) The reservoirs geostress can be changed by the creation of hydraulic fractures. The numerical model about volume fracturing in shale gas reservoirs was constructed in which the pore pressure elements were used to simulate the behavior of porous media and the pore pressure cohesive elements were adopted to catch the characters of hydraulic fractures. The Texas Two-Step fracturing method was simulated in the model. The reservoirs stress distribution and change during the process of fracturing were obtained. The simulation results and the theoretical calculation are fit very well. The results of the simulation show that: (1) The generation of the hydraulic fractures reduces the stresses anisotropy; (2) The complex fracture networks could be created by means of the Texas Two-Step fracturing method. For the situation that the initial difference of the two horizontal principal stresses is large and the fracture networks could not be created by adopting the Texas Two-Step fracturing method, the triple and quartic stress "resonance" fracturing model were proposed. The reservoirs stress distribution and change during the process of fracturing were obtained. The area of fracture network was marked. The simulation results show that the triple and quartic stress "resonance" fracturing methods are e ective methods for the high stresses difference shale gas reservoirs.