Molecular Simulation of Adsorption and Diffusion Behavior of CO2 in Bakken Nano-Porous Media for Enhanced Oil Recovery Assessment

Abstract

ABSTRACT: The use of molecular simulation to quantify gas adsorption and diffusion is needed to study the CO2 Enhanced Oil Recovery (EOR) efficiency. Favoring CO2-Oil interaction over CO2-Rock interaction depends on the strength of the interaction bond between CO2 and nanopores surface. In this study, we explored the effect of pore size and reservoir temperature on CO2 adsorption and diffusion behavior in Bakken. This comprehensive study covers the characterization of Bakken CO2-EOR using grand canonical Monte Carlo and molecular dynamic simulations. The results indicated that increasing reservoir temperature would reduce the amount of adsorbed CO2 and the adsorption kinetics of CO2 into Bakken nanopores follows a first-order Langmuir model. CO2 adsorption in Bakken nanopores was strong at low temperatures and large pore sizes. Also, high CO2 injection pressures (e.g., 55MPa) led to strong diffusion of CO2 into the nanopores. This is undesirable as high CO2 adsorption and diffusion into the walls of the nanopores would cause molecular loss of the injected CO2, consequently lower EOR efficiency. The insights gained from this work might help improve theoretical understanding of the micro behaviors of CO2 in Bakken reservoir nanopores and give valuable recommendations in CO2-EOR. 1. INTRODUCTION AND BACKGROUND In 2021, North Dakoda contributed to US oil production as second-ranked with production over 1MMbscf. Although using advanced technologies of hydraulic fracturing in unconventional reservoirs such as Bakken made higher oil production possible, the recovered production oil is only 2% of the Bakken original oil in place (OOIP). Also, maintaining production is quite challenging in unconventional reservoirs. Thus researchers are exploring the flow mechanisms to sustain oil production, including developing several enhanced oil recovery (EOR) methods such as gas injection and CO2 EOR for improving the recovery factors over the past few decades. Based on the experimental and numerical studies, one of the potential methods with a promising indication of oil incremental is CO2 EOR (Jia et al., 2018; Thakur, 2019). During a CO2 injection process, oil recovery factor can increase as a result of CO2-oil interaction. Injected gas can swell the oil and reduce viscosity leading to higher oil mobility. Also, the underground injection of captured CO2 from combustion stations has the benefits of reducing greenhouse gas emissions which makes CO2 injection a valuable process. (S. Wang, 2016). On the other hand, CO2 injection processes are not always in favor of increasing oil recovery due to the adsorption of CO2 in rock. Moreover, there is always a gap between field and simulation-predicted oil recovery (Jia et al., 2018). Therefore, it is important to investigate the effect of some common assumptions in tight reservoirs such as the ignorance of the diffusion flow mechanism and adsorption to improve oil recovery prediction.