Non-linear gas transport inside an ultra-tight Longmaxi shale core under thermal stimulation conditions

Gas adsorption/desorption and non-linear mass transport in ultra-tight shale were very sensitive to temperature and they become even more intricate as temperature changed. In this study, a self-similarity mathematical model was developed to simulate the gas decline process under thermal stimulation conditions. This model not only incorporated slip and free molecular flow but also gas adsorption/desorption. Besides, a canister test was conducted to study the effect of heating temperature on gas production rates of a fresh shale core. The experiment results showed that gas production rate was increased by raising the heating temperature. Free gas is the main source for the gas flow under 55 degrees C, while adsorbed gas is the main source under 110 degrees C. Lastly, the declined trend of simulated gas production rate followed a power law of t(-0.509) + t(-0.736) at 55 degrees C and t(-0.169) + t(-1.828) under at 110 degrees C, which matched the experimental data well. From the scaling data, it was suggested that free molecular diffusion was the main gas transport form inside the shale both under 55 degrees C-110 degrees C. Our experimental results and theoretic model showed that higher heating temperature could promote the gas production rate significantly, and the model can well predict the non-linear gas transport process. (C) 2019 Elsevier Ltd. All rights reserved.