The Potential of N2 as Cushion Gas on Dispersion Coefficient of SuperCritical CO2 in Sandstone Core Plug During Enhanced Gas Recovery by CO2 Injection

Abstract

Abstract Despite the several kinds of literature on the dispersion behaviour of supercritical CO2 in CH4 at conditions relevant to enhanced gas recovery (EGR), studies have so far limited in keeping this parameter as low as possible. This study aims to highlight, experimentally, to determine the effect of N2 as cushion gas on the dispersion coefficient in consolidated sandstones core plug under reservoir conditions applicable to EGR. A laboratory core flooding experiment was carried out to simulate a detailed process of an unsteady state methane displacement in Bandera gray and Bentheimer core plugs at reservoir conditions of 40oC temperature, 1500 psig of pressure, the optimum injection rate of 0.4ml/min, and at varying N2 cushion volumes (8-36 cm3). Further experimental runs were carried out to investigate the effect of high CO2 injection rates (0.6-1.2 ml/min) on the longitudinal coefficient of dispersion as it plumes transverses into the core plugs during the EGR process. The coefficient of longitudinal dispersion declines with raises in cushion gas volume, hence the higher the amount of N2 cushion volume the less the dispersion of CO2 into CH4. This is due to the high shielding barrier inhibited by nitrogen, making it difficult for the CO2 to dispersed itself and mixed with the nascent natural gas resulting in delayed breakthrough as it plumes transverses into the CH4 during the displacement process. The inclusion of N2 as cushion gas prior to CO2 injection recorded the highest decline at 36cm3 cushion volume, presenting a 48 and 28% reduction in longitudinal dispersion coefficient for Bandera and Bentheimer core samples respectively, compared to that of conventional/traditional CO2 injection (with zero cushion volume). Recording lower nascent CO2-CH4 mixing resulting in less natural gas contamination and more storage volumes. Also, a reverse phenomenon was observed when the CO2 injection rate was increased from 0.6-1.2 ml/min due to the high diffusion rate of CO2 at higher interstitial velocities, resulting in a rapid increase in dispersion coeffient and indirectly high widespread contamination of the remaining natural gas.