Real Gas Effect on Gas Slippage Phenomenon in Shale

The past decade has seen the rapid development of shale gas across the world, as the record-breaking success and on-going surge of commercial shale gas production in such unconventional reservoirs pose a tremendous potential to meet the global energy supply. However, questions have been raised about the intricate gas transport mechanisms in the shale matrix, of which the gas slippage phenomenon is one of the key mechanisms for enhancing the fluid transport capacity and, therefore, the overall gas production. Given that shale reservoirs are often naturally deposited in the deep underground formations at high pressure and temperature conditions (much deeper than most typical conventional deposits), the real gas effect cannot be ignored as gas properties may vary significantly under such conditions. The purpose of this study is thus to investigate the real gas effect on the gas slippage phenomenon in shale by taking into account the gas compressibility factor (Z) and Knudsen number (Kn). This study begins with a specific determination of Z for natural gas at various pressures and temperatures under the real gas effect, followed by several calculations of the gas molecular mean free path at in-situ conditions. Following this, the real gas effect on gas slippage phenomenon in shale is specifically analyzed by examining the change in Knudsen number. Also discussed are the permeability deviation from Darcy flux (non-Darcy flow) due to the combination of gas slippage and real gas effect and the specific range of pressure and pore size for gas slippage phenomenon in shale reservoirs. The results show that the gas molecular mean free path generally increases with decreasing pressure, especially at relatively low pressures (< 20 MPa). And, increasing temperature will cause the gas molecular mean free path to rise, also at low pressures. Knudsen number of an ideal gas is greater than that of a real gas; while lower than that of a real gas as pressure continues to rise. That is, the real gas effect suppresses the gas slippage phenomenon at low pressures, while enhancing it at high pressures. Also, Darcy’s law starts deviating when Kn > 0.01 and becomes invalid at high Knudsen numbers, and this deviation increases with decreasing pore size. No matter how pore size varies, this deviation increases with decreasing pressure, meaning that the gas slippage effect is significant at low pressures. Finally, slip flow dominates in the various gas transport mechanisms given the typical range of pressure and pore size in shale reservoirs (1 MPa < P < 80 MPa; 3 nm < d < 3000 nm). Gas transport in shale is predominantly controlled by the slippage effect that mostly occurs in micro- or meso-pores (10 to 200 nm). Moreover, considering the real gas effect would improve the accuracy for determining the specific pressure range of the gas slippage phenomenon in shale.