Evaluation method of compressibility of shale gas reservoir in consideration of the influence of fracture toughness

Compressibility is an important basis for measuring the response of shale gas reservoirs to hydraulic fracturing. Currently, the brittleness index commonly used for shale compressibility has been evaluated. However, in practice, there are problems in which the brittleness index is similar and the actual fracturing effect is far different. In order to make up for the lack of brittleness index for the evaluation of compressibility of shale gas reservoirs, this study incorporated critical mechanical energy release rate in consideration of the influence of fracture toughness, and defined the average critical mechanical energy release rate of the integrated modes I and II of fracture toughness of rock fracture development. Finally, combined with the average critical mechanical energy release rate and brittleness index, the compressibility evaluation model of shale gas reservoir with fracture development index as the quantitative index was established. The elasticity modulus, Poisson's ratio, modes I and II of fracture toughness were calculated based on the logging data of well 48-2HF in Longmaxi Formation in Jiaoshiba. According to the constructed fracture development index model, the variation of the compressibility of shale reservoirs with the burial depth was obtained. Combined with on-site microseismic data, it is verified that the model could effectively predict the change trend of the compressibility of shale gas reservoir with burial depth, and provide a reliable method for optimization of shale reservoir fracturing interval in the actual development of shale gas.