A multiple porosity media model for multi-fractured horizontal wells in shale gas reservoirs

Abstract

Shale reservoirs are becoming more important in worldwide oil and gas production because of the use of multi-fractured horizontal wells (MFHWs). Shale reservoirs have abundant kerogen and nanoscale pores, making it crucial to consider the influence of gas flow on these reservoirs; the Darcy formula cannot accurately describe the gas flow in shale reservoirs. Because shale gas reservoirs are composed of media with multiple pore sizes (including porous kerogen, the inorganic matrix, and natural fractures), it is necessary to consider the effects of slippage during viscous flow, Knudsen diffusion, surface diffusion and gas adsorption and desorption. Continuity equations for porous kerogen, the inorganic matrix and natural fractures are given to describe the mechanism of flow through multiple pore sizes in shale gas reservoirs using apparent permeability. Given that MFHWs are reservoir stimulated in shale gas reservoirs, we present an analytic multiple porosity media model (MPM model) to describe gas flow from kerogen to MFHWs. Solutions for the MPM model were derived in the Laplace transform domain and were validated by actual production data. The MPM model solutions are in good agreement with those from the classic dual-porosity trilinear flow model. The results show that porous kerogen, Knudsen diffusion and the effects of slippage during viscous flow in the inorganic matrix significantly influence production and should not be neglected in models. The pressure transient behavior and the production rate derivative were used to analyze the flow regime without considering wellbore storage and the skin effect. The type curves show that the fluid flow in MFHWs in shale gas reservoirs can be divided into six main flow periods. Finally, we performed sensitivity studies to quantify the key parameters that affect well performance. The model proposed in this study is more comprehensive, considering not only the reservoir-stimulated characteristics of MFHWs but also the mechanisms of transport through media at multiple scales. Thus, this model is useful for performance analysis during shale gas reservoir development.