Enhanced gas recovery by CO2 injection: Influence of monovalent and divalent brines and their concentrations on CO2 dispersion in porous media

Abstract

This study aims to experimentally investigate the roles of different brine types and concentrations on the longitudinal dispersion coefficient (KL) during enhanced gas recovery by CO2 injection. Core flooding process was used to simulate the displacement of CH4 by supercritical CO2 in a Buff Berea core sample at a pressure and temperature of 1400 psig and 50 °C respectively, and a CO2 injection rate of 0.3 ml/min. Individual NaCl, KCl, CaCl2 and MgCl2 solutions were prepared as test brines with ionic strengths (IS) of 1M, 2M, and 3M. The results revealed that, at lower IS of 1M, MgCl2 and CaCl2 brines had the lowest KL while the monovalent brines showed relatively higher KL. Divalent brines showed a higher degree of salting out effects at higher concentrations resulting in higher KL. The salting and drying out effects of divalent brines were responsible for higher CH4 recovery at 2M IS as CH4 comes out of solution. A hyperbolic-type relationship exists between the two properties (KL and IS), where KL decreases from 0 to 1M IS, and then increases sharply at IS > 1M – this behaviour is most pronounced in the divalent brines. Lowest contamination of the recovered CH4 was found to be between formation water salinities of 5–15 wt%, regardless of salt type, during EGR by CO2 injection and sequestration. This study will not only present new knowledge on EGR process but will also provide an avenue for establishing a screening criterion based on formation water salinity for effective EGR process. This is a first experimental investigation which establishes the relationship between salt types and concentration and the KL in porous media.