Shale gas reservoirs have a high total organic content (TOC) and are composed of a lot of microspores, which result in a high content of adsorbed gas. Laboratory and theoretical calculations show that the adsorption potential of CO2 in shale is higher than that of CH4. In other words, the shales prefer adsorbing CO2 to CH4. Therefore, during CO2 injection, the adsorbed CH4 is released by CO2 adsorption, even in a high reservoir pressure. Several models have been studied to describe the pure and multicomponent adsorption on shale. The Langmuir and extended Langmuir models are usually applied in reservoir simulators, because other models are more complex and not applicable to be coupled into a simulator. In this work, a simulation study is carried out to investigate the effects of gas adsorption on primary recovery and CO2 enhanced recovery processes.
Dual permeability, logarithmically spaced, locally refined grids are implemented to model natural and hydraulic fractures and to capture the sensitive changes of multicomponent adsorption. Reservoir pressure variation is coupled with a geomechanical module that updates porosity, permeability and fracture conductivity simultaneously at each time step. A multicomponent mixture on the basis of lab measured adsorption properties of the Eagle Ford shale are implemented into a reservoir simulator. Both primary recovery and CO2 huff-n-puff processes are investigated.
The simulation results show multicomponent adsorption behaviors of extended Langmuir model can slightly increase the well performance in the primary recovery. However, the adsorption behavior is more complex during CO2 injection processes. This study highlights the effect of multicomponent adsorption on gas production during CO2 cycling, and provides an optimal enhanced recovery strategy for shale gas reservoir.