New Considerations of Shale Gas CO2-EGR from Molecular Simulation

Abstract Following North America, shale gas has been proved to be a success in China, considering the commercial exploitation of shale gas in Sichuan province in China, and a continuous increase in the past few years and a good momentum in the next few years. This mainly attributes to the horizontal drilling and multistage hydraulic fracturing technologies and huge capital investment. However, we need to face that even using these advanced technologies, a sharp decrease of the production during the development still exists in all shale gas wells, due to the tightness of the shale matrix and the pressure drop in the fracture network. According to the successful experience of enhanced coalbed methane recovery all over the world, CO2 injection shows a promising future in EGR (enhanced gas recovery) process for shale reservoirs. In the meanwhile, CO2 can be simultaneously sealed underground to reduce pollution. Therefore, in order to guide the engineering process, it is of great importance to reveal the CO2 and CH4 absorption behaviors at microscopic scale,but the corresponding mechanism is as yet unclear. This is because the competitive adsorption process of CO2 and CH4 underground can be affected by a lot of parameters, such as pore size, temperature, pressure, CO2 concentration, etc., and laboratory experiment is usually difficult to carry out, due to limited laboratory temperature and pressure conditions, and difficulty in separating porous OM from shale, etc. In this paper, we employ molecular simulation of CO2-CH4 competitive adsorption behavior in organic matter (OM) nano pores. Using Longmaxi shale as sample, OM pore structure was first characterized using FIB-SEM and scanning ion microscope, and pore-size distribution was studied using N2 adsorption. Then a simplified pillar-layer model was used to study CH4 adsorption behavior and competitive adsorption effect between CO2 and CH4, using grand canonical Monte Carlo (GCMC) method. Results indicate that nanopores with good connectivity widely exist in OM, offering important storage space for absorbed shale gas. The amount of adsorbed CH4 can increase with lower temperature and increased pressure, and overpressure will significantly increase the amount of CH4 adsorbed underground; CO2 shows high competitive adsorption ability; CO2/CH4 selectivity coefficient decreases dramatically with increasing temperature or pressure, or both, and it corresponds to deeper burial depth. CO2 EGR during shale gas exploration will be more efficient, if it is conducted after the pressure drops to a certain degree.