Gel Systems for Controlling CO2 Mobility in Carbon Dioxide Miscible Flooding

Abstract

AbstractCarbon dioxide miscible flooding is widely used in carbonate reservoirs in West Texas and has potential application to carbonate reservoirs in other areas of the country including Oklahoma, central, and southwest Kansas. One of the major difficulties of this process is the CO2 early breakthrough because of its unfavorable mobility ratio and because of the heterogeneous nature of this type of reservoir. Injection of alternating slugs of water and carbon dioxide, known as the WAG process, is the remedy most widely applied to control the early breakthrough, but its success has not been universal. Conformance control using gels has not been widely attempted. Laboratory research efforts at the University of Kansas have produced promising in situ gelation techniques aimed at this application.Three in situ gel systems have been developed and tested in laboratory cores to control the movement of supercritical CO2 in matrix rock. Two systems are based on a new biopolymer, termed KUSP1, which is soluble in alkaline solutions above pH 10.8, but forms firm gels when the pH is reduced to 10.8 or below. The third gel system uses the reaction of sulfomethylated resorcinol and formaldehyde to form a gel. This paper describes the behavior of these gel systems at 32.2-41°C in Berea sandstone cores (initial permeability ~ 70 – 700 md) when exposed to supercritical carbon dioxide and brine after in situ gelation.KUSP1 gel systems were studied using two different methods of inducing in situ gelation. In the first method, gelation was accomplished by injecting CO2 at low pressure into the Berea sandstone core saturated by alkaline polymer solution. Permeability reduction to the brine and CO2 in the range of 80% was achieved. Stability of the gel was tested in the presence of supercritical CO2. When supercritical CO2 was used to induce in situ gelation, the same degree of permeability reduction was achieved. The gel remained stable after the injection of many pore volumes of supercritical CO2. The second method of initiating in situ gelation involved the use of an ester. Hydrolysis of the ester, monoethylphthalate, in the alkaline polymer solution caused the pH to drop to levels where in situ gelation occurred. The permeability of the treated core to supercritical carbon dioxide was about 1 md which was equivalent to a permeability reduction of 95-97 % of the initial brine permeability.The third gel system, based on the reaction of sulfomethylated resorcinol and formaldehyde, termed SMRF, was gelled in situ and contacted with both brine and supercritical CO2. Permeabilities to carbon dioxide on the order of 1 md or less were observed. This permeability is equivalent to a reduction of about 99% in the initial brine permeability. Reduced permeabilities were maintained after injecting many pore volumes of supercritical CO2 and brine.