Upscaling of hydro-mechanical coupling in shale matrix based on homogenization theory

Key words:  shale gas  hydro-mechanical coupling  equivalent continuous medium  homogenization theory  upscaling

Abstract:

In shale gas reservoirs, gas is mainly stored in shale matrix. Therefore, to establish a hydro-mechanical coupling (HM) model in shale is important for the macroscopic HM simulation of shale gas production. At microscopic scale, shale matrix is usually composed of organic and inorganic matters. The mechanical properties of these two media are quite different, and so that both gas storage type and transport mechanisms are different in the two media. Thus it is necessary to develop different microscale models to accurately describe the HM process of gas in shale matrix. However, the microscale models cannot be straightly applied to macroscale simulation due to huge calculation time and data storage requirement. In this study, an efficient upscaling method based on the homogenization theory was proposed for the HM process in shale matrix, which can accurately represent the microscale characteristics of the organic and inorganic matters in macroscale simulations. Firstly, the shale matrix is assumed as a heterogeneous porous elastic medium composed of organic and inorganic matters, and the microscale HM models were developed according to different storage type and transport mechanism of real gas in these two media. Then, the microscale HM models were homogenized to obtain the equivalent macroscopic HM models for the shale matrix, along with the definition and calculation method of equivalent parameters in the micro and macro models. Lastly, the accuracy and application of the proposed method were verified via several case studies, and the influence of the mechanical properties, the content and distribution of organic matters on the macro HM simulation in shale gas production were also analyzed. The results indicate that the cumulative gas production increases with the decrease of the elastic modulus of the organic matter, and it correlates positively with the connectivity of the inorganic matter. Besides, at early period, gas production decreases with the increase of the total organic content, but it will increase at late period of the process.