Flow transportation inside shale samples during low temperature combustion: The effect of desorption and pyrolysis

Abstract

Flow transportation inside shale formations is a complex process mainly including slip flow, transition flow, and free molecular diffusion. Recently, using combustion to promote gas transportation inside shale formations for gas extraction has been studied. In this study, the shale samples were combusted at 280 °C and 400 °C respectively, to obtain transient pressure variation behaviors inside the shale samples. The effects of organic matter pyrolysis and adsorbed gas desorption on internal flow transportation were investigated. Moreover, a mathematic scaling power law was used to predict the pressure rising and declining processes. It was found that the pressure exhibited similar trend under 280 °C and 400 °C. Shale internal pressure built up quickly and reached peak value after combustion began. Then the pressure declined close to ambient pressure within a short period. Under 400 °C, the pressure of inner and outer holes were close to each other; the peak pressure could rise up to 7000 Pa. Both the gas generated from the thermal cracking of kerogen and gas desorbed from the pore surface contributed to pressure rising. However, at lower combustion temperature (280 °C), the rising of the pressure is mainly owing to the desorption gas from the surface to the pore volume. The peak pressure was only 780 Pa, much lower than that under 400 °C. By selecting an appropriate exponential m in the power law of scaling pressure, pressure rising and declining processes inside shale samples could be well simulated.