Prediction of high-pressure adsorption of CH4 and CO2 in shale

Abstract

The study of CH4 and CO2 adsorption behavior in shale under high-pressure is critical for the estimation of gas content and CO2 sequestration in deep shale gas reservoirs. However, it is very difficult for the adsorption experiment pressure to reach the high pressure of the deep shale gas reservoirs. This paper aims to study the feasibility of Langmuir model, supercritical Dubinin-Redushckevich (SDR) model and Simplified Local-Density (SLD) model to predict the high-pressure adsorption based on the low-pressure adsorption data. The results showed that the Langmuir model, SDR model and SLD model can fit the adsorption isotherm of CH4 and CO2 in shale well. In the ranges of pressure (0-12 MPa) and temperature (308.15-328.15 K), the adsorption phase densities obtained by the Langmuir model and the SDR model are fixed values, but the adsorption phase density obtained by the SLD model is a function of temperature and pressure, which makes the SLD model more predictive. The high-pressure adsorption data in four literature sources were used to compare the prediction capabilities of the Langmuir model, the SDR model and the SLD model, and average absolute percent error (%AAD) predicted are 11.56%, 6.15%, and 3.05%, respectively. When predicting the absolute adsorption at higher pressures, it is unreasonable to use a fixed adsorption phase density. Compared with the SLD model, the Langmuir and SDR models slightly underestimated the CH4 absolute adsorption capacity and seriously overestimated the CO2 absolute adsorption capacity. The results can help evaluate shale gas production and the potential for CO2 sequestration.