Application of Gas for Mobility Control in Chemical EOR in Problematic Carbonate Reservoirs

Abstract

Abstract The use of gas for mobility control in chemical enhanced oil recovery is an attractive option to substitute polymer in conditions where its application is not feasible. Especially in carbonate reservoirs, which generally have low permeability and contains vugs and fractures, the use of polymer may result in the loss of permeability and benefits of chemical conformance control. In such conditions, gas is a good alternative. The direct dependence of dispersed gas mobility on rock permeability reduces the likelihood of gas plugging oil-rich low permeable rock matrix in carbonate reservoirs. The mobility of both gas and liquid phases in fractures can be controlled by changing fractional gas and liquid rates in such a manner that the two phases retains their flow characteristics. A series of high performance surfactant-gas corefloods were performed on different core samples having very low to medium permeability values. In each experiment, foamed chemical slug was chased by foamed drive. The main objective of the research work was to determine mobility control characteristic of dispersed flow by evaluating pressure response and oil recovery data obtained from corefloods. All corefloods resulted in oil recoveries above 75% of residual oil to waterflood. No early surfactant and gas breakthrough was observed, indicating absence of viscous fingering and presence of effective mobility control. The scaled pressure gradients remained within the practical limits observed in field EOR applications. Particularly, low pressure gradients observed at relatively high fluid rates indicates a degree of in-situ foaming for favorable injectivity, thus highlighting the potential use of gas in low permeability reservoirs. The beneficial effect of trapped surfactant mobilization on foaming, with the decrease in salinity, was observed in the form of increased foam strength during the drive injection phase. The experimental study provides an insight into the effective application of gas in chemical floods in problematic carbonate formations. Substituting polymer with gas increases the applicability of chemical EOR methods in different types of reservoirs.