Gel Systems for Controlling CO2 Mobility in Carbon Dioxide Miscible Flooding

Summary Conformance control for carbon dioxide miscible flooding using gel 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 were developed and tested in laboratory cores. Two systems are based on a new biopolymer, termed KUSP1, and the third gel system uses the reaction of sulfomethylated resorcinol and formaldehyde to form a gel. 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 CO2The 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 (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. Introduction Carbon dioxide miscible flooding is one of the most important tertiary oil recovery techniques employed in the United States. However, the process experiences major difficulties in field application because of reservoir heterogeneity due to high permeability contrast. CO2 tends to finger through the high permeability zones and bypass the oil. Early CO2 production occurs with increased recycling and other operating costs. Different methods have been investigated for improving the overall efficiency of the CO2 flooding process. In almost all these methods, attempts have been made to achieve a favorable mobility ratio by affecting the CO2 relative permeability. Examples of these methods are:water alternating gas (WAG) process,1carbon dioxide-foam process,2 andviscosified carbon dioxide process.3 Another technology which is under study is permeability reduction by in-depth placement of polymer gels. The objective of this research is to reduce the permeability in permeable zones of the reservoir. Reduction of matrix permeability in the CO2 process has been studied by other investigators.4,5 No systems were found that gave satisfactory permeability reduction when exposed to prolonged injection of CO2. Three new in-situ gel systems developed and tested in our laboratory are described in this paper. Two of these systems are based on a biopolymer termed KUSP1.6,7 The third system is based on a modification of a previously reported organic crosslinking system. Experiment The experimental program consisted of gelling each polymer system in a 1 ft Berea core which was mounted in a core holder and determining the permeability of the treated rock to brine and carbon dioxide at supercritical conditions. Five separate tests were conducted. Dispersion tests were run in some tests to estimate the pore volume contacted by the injected fluids after treatment with a gelled polymer system. Equipment and Materials Experimental Apparatus. Fig. 1 is a schematic presentation of the experimental apparatus used in this work. An ISCO syringe pump was used for injecting CO2 brine, and gel solutions into the core. All the experiments were conducted at constant rate. The effluent of the core was collected by a fraction sample collector for further analysis. A TEMCO high-pressure core holder equipped with pressure ports was used. The rubber sleeve was filled with water and the injection pressure was kept at 500 psi below the sleeve pressure because higher sleeve pressures caused the rubber sleeve around the pressure taps to deform and seal off the pressure ports. One ft Berea cores, 2 in. in diameter, were used in all experiments. Pressure ports were located such that the core was divided into four sections. The first and fourth sections were 5 cm in length and sections two and three were 10 cm long. The pressure difference for each section and the overall pressure difference were measured by pressure transducers and recorded via a computer-based data gathering system. The apparatus was placed in an air bath in which the temperature of the core and the injected fluids was kept constant. The pressure of the core was maintained by a TEMCO back-pressure regulator connected to a cylinder containing nitrogen at high pressure. The back pressure was maintained at 1200 psi. Details of the experimental setup are presented elsewhere.8 Gels Produced from KUSP1. KUSP1 is an acronym for a biopolymer developed at the University of Kansas. The polymer is a ?-1,3-polyglucan and is produced by fermentation of a bacterium known as Alcaligenes faecalis and certain species of Agrobacterium.6 The polymer grows on the surface of the bacteria. During the fermentation process, the polymer laden bacteria aggregate and settle out from the growth medium. Polymer is extracted from the bacteria by suspension in dilute alkali. Neutralization of the alkaline polymer solution produces a hydrogel. The gelation process is reversible and the hydrogels are stable at high temperatures in neutral solutions. The polymer degrades in alkaline solution with time and at elevated temperatures.