Diffusion and Flow Mechanisms of Shale Gas through Matrix Pores and Gas Production Forecasting

Abstract The transport mechanism of gas moving through matrix pores is the bottleneck of conquering the difficulties in shale gas development. The matrix pores can be divided into organic and inorganic matrix pores. The transport mechanism of shale gas in organic and inorganic matrix pores is different. However, the present gas transport model only focused on the gas transport in organic matrix pores, in addition, the impact of organic and inorganic mass ratio has been largely neglected by shale gas transport models in the literature, leading to an unclear recognition of shale gas production discipline and large derivation between prediction results by the present models and actual performance of shale gas wells. In this paper, both the pore size distribution and water distribution in shale matrix pores are investigated. Furthermore, a new diffusion-slippage-flow model in combination with the gas transport mechanism is proposed. Also, the organic content effect is considered and the range of Knudsen number is quantified. Finally, a gas production model based on this gas transport mechanism is derived and employed to reveal the discipline of shale gas production. The preliminary results illustrate that Knudsen diffusion is not suitable for shale gas reservoirs. This is because Knudsen number is generally less than 10, especially for such shale gas reservoirs with higher initial reservoir pressure. Gas moving through shale matrix pores to fractures is mainly divided into two forms: in organic matrix pores, both slip effect and transition diffusion mechanism are dominant; in inorganic matrix pores, the gas-water two-phase flow controls the gas transport mechanism because of the presence of water in these pores. The efforts of this work will provide a more accurate technique for forecasting shale gas production, and also give some insights into scientific evidence to the rational development of shale gas reservoirs.