Abstract:

Fracture is a key factor for the enrichment and high production of shale gas. Compared with other types of reservoirs, shale reservoir is characterized by a relatively strong ductility and multitypes of structural and nonstructural fractures such as high dipangle tensionshear fracture, low dipangle slip fracture and bedding fracture. According to the characteristics of shale reservoirs and fractures from the perspective of geological origin, a method of tectonic stress field simulation and fracture distribution prediction was put forward and effectively applied in the Lower Silurian Longmaxi shale in the southeastern Chongqing area, the emphasis of which is the establishment of accurate geological, mechanical and mathematical Models. By the use of uniaxial and triaxial compression and acoustic emission tests, the paleostress, presentday stress and mechanical data were obtained to conduct a numerical simulation of the tectonic stress field. The rationality of the established models was examined and calibrated by the comparison of the simulation results and actual geological data. For the particular existence of tension and shear fractures in shale, Griffith and MohrCoulomb criteria were respectively used to calculate the tension and shear rupture rates. According to the proportions of the tension and shear fractures, a comprehensive rupture rate was introduced. By the combination of the comprehensive rupture rate and other dominant controlling factors of fractures, such as total organic carbon (TOC) and brittle mineral contents, the fracture developmental coefficient was proposed as an ultimate evaluating index to characterize the developmental degree and distribution of fractures. This method not only provides a new technical approach for the optimization of ‘sweet spot’, but also has important reference value for the design of horizontal wells and fracturing programs.

Key words: southeast Chongqing area, Longmaxi Formation, shale reservoir, tectonic stress field simulation, fracture prediction