Fine-grained sedimentary rocks, such as mudstones and shales, contain abundant nanometer- to micrometer-sized pores. These narrow pores create intense fluid-rock interaction that may lead to complicated fluid storage and transport process. Concerns about the accurate evaluation of gas content and diffusion kinetics have led to many experimental studies about gas sorption on shales. However, data on high-temperature high-pressure sorption isotherms of shales are still scare. In particular, the burial depth of the Paleozoic shales in the Upper Yangtze region of China is mostly in a range of 2000–4000 m, which indicates that the temperature and pressure of shale reservoirs are in the range of 60–120 °C and 20–40 MPa. Experimental techniques employed in obtaining sorption data have to be optimized and at the same time the measuring conditions have to be extended to in-situ conditions of deep shales while many published sorption data are limited to moderate pressures and temperatures.
In this work, high-temperature high-pressure sorption data for methane on shales from Sichuan Basin have been obtained at 30–120°C and pressures up to 25 MPa using a specially designed two-temperature-zone manometric setup. Dubinin-Polanyi potential theory was modified to extend to supercritical gas sorption over wide temperature and pressure ranges. A modified adsorption potential method was proposed to calculate the characteristic curves for supercritical gas sorption, and then a rigorous function from the supercritical Dubinin-Astakhov equation was also developed to describe the modified characteristic curve. Furthermore, the physical meaning of characteristic curve has been elucidated by comparing characteristic curves of different kinds of shales and clay minerals.
The measured excess sorption isotherms of shales follow the physisorption trend of decreasing amounts of methane adsorbed with increasing temperature. Characteristic curves of methane on shales at 30–120°C were calculated using a new expression of adsorption potential. It is found that if the thermal expansion of adsorbed phase is considered, these modified characteristic curves are temperature-invariant. The characteristic curve equation is capable to predict methane sorption at other temperatures based on the easily tested isotherm at room temperature. Using the sorption isotherm and characteristic curve equation at 30°C, the predicted isotherms at 120°C agree well with experimental data. The modified characteristic curves comprehensively characterize the available pore space for sorption and the affinity of methane molecules. The later stage of the modified characteristic curves (limited adsorption volume) is mainly controlled by the available pore space provided by organic matter and clay minerals. The limiting adsorption volume of shales in the gas window is larger than shales in the oil window with the same TOC content. The initial stage of the characteristic curves reflects the affinity of methane molecules for sorption on organic matter. According to the characteristic curves, shales in the gas window show higher affinity than shale in the oil window and clay minerals, though the clay minerals may provide comparable adsorption volume. The sorption characteristic energy shows a parabolic-like shape with a minimum approximately around Req =1.1%, which are related with the evolution of porosity of shales.
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