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

Abstract

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 °C, while adsorbed gas is the main source under 110 °C. Lastly, the declined trend of simulated gas production rate followed a power law of t−0.509+t−0.736 at 55 °C and t−0.169+t−1.828 under at 110 °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 °C–110 °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.