CO2 Sorption and Storage Associated with EOR Operations in Tight Shales

Abstract Advancement of carbon storage during enhanced oil recovery (EOR) in ultra-tight shale formations requires a thorough understanding of carbon dioxide (CO2) sorption behaviors at subsurface-relevant conditions. In this study we examine the sorption (kinetics and isotherms) and storage of CO2 during cyclic Huff and Puff (HnP) processes, to facilitate the design and optimization of pilots and larger-scale operations. Combined experimental and modeling efforts, including pressure-pulse decay (PPD) measurements and cyclic HnP experiments on a shale core from the Eagle Ford formation, alongside molecular simulations for the building blocks of shale, were employed to delineate the behaviors of the shale/hydrocarbon/CO2 system. To facilitate the interpretation of the cyclic CO2 HnP experiment, PPD measurements were performed for loading and unloading processes at pressures up to 4,500 psia. The experiments were analyzed in terms of the kinetics and equilibrium sorption behavior of CO2, providing new insight into the related hysteretic behaviors. Furthermore, competitive CO2 sorption in the presence of hydrocarbons on shale-related materials was examined using molecular simulation. The experimental observations demonstrate significant hysteresis between adsorption and desorption, and that the shale core retains more than 10% of the sorbed CO2 after the core pressure is reduced below atmospheric conditions for a duration exceeding one month. Molecular modeling and simulation demonstrate that CO2 dominates in the competitive sorption process on the surfaces of shale, to effectively replace hydrocarbons during EOR processes. Insights from experiments and molecular modeling were used to guide the formulation of a mathematical framework that integrates sorption characteristics in HnP process calculations. The enhanced modeling framework is demonstrated to facilitate the interpretation of the pressure response during soaking, and production cycles in the HnP experiments, providing for an improved understanding of CO2 storage in tight shale formations during CO2-EOR processes. The results and analysis provided demonstrate that sorption kinetics and hysteresis for CO2, that have generally been overlooked in many previous efforts, can significantly impact a) the design of CO2-EOR processes in terms of CO2 required per cycle, and b) the evaluation of the related CO2 storage potential in tight shale formations. The results and observations presented here, can readily be integrated into existing practical workflows to promote a more realistic design and optimization of CO2 storage during EOR in unconventional formations.