Adsorption kinetics and diffusion modeling of CH4 and CO2 in Indian shales

The concept of increased gas (methane) recovery with simultaneous CO2 sequestration in unconventional reservoirs like gas shales has been studied in the past few years. Diffusion is the main transport mechanism in shale gas reservoir. Understanding the methane and CO2 diffusion properties of shales and its modeling is important for planning successful methane recovery and CO2 sequestration in these reservoirs. Methane and CO2 adsorption kinetic studies were carried out at four different pressure steps (in the range of 3.5-8.9 MPa for CH4 and 2-6 MPa for CO2) to investigate their diffusion behavior on two shale samples (Pakur and Salanpur) from Damodar Valley Basin, India. The sorption kinetic data was modeled using unipore model and modified unipore model (MM). The unipore model is giving good match with experimental sorption kinetics data up to a fractional uptake of 62-92% for methane and 62-88% for CO2. The MM model is giving very good match up to the fractional uptake of 78-99% for methane and 77-98% for CO2. It was observed that the MM model is giving better fit than that of unipore model for both gases for the entire pressure range. Thus it can be suggested that MM model is a better model to represent the diffusion of methane and CO2 in shales.