Impact of Surfactant on the Retention of CO2 and Methane in Carbonate Reservoirs

01.Jan.2018

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Enhanced gas recovery methods such as foamed CO2 are recommended for depleted gas reservoirs. Viscoelastic surfactant (VES) is a form of a surfactant used for forming CO2 foam. In this study, the impact of VES on CH4 and CO2 retention and adsorption in calcite rock samples was studied. Crushed samples of Indiana limestone rocks of average particle size (125 to 250 micron) were used in the static adsorption experiments. To study the effect of VES on CH4 and CO2 adsorption, 50% of the crushed samples were conditioned in a solution of NaCl of 0.1 vol% surfactant. X-ray diffractions shows that the rock samples are almost 100% calcite. The gas adsorption experiments were performed at different temperatures; namely; 50, 100 and 150oC and at a pressure of 45 bars. At 50oC, the plain calcite samples adsorbed more CH4 and CO2 compared to that with the treated with VES. However, at 100 and 150oC the plain sample the adsorbed CH4 and CO2 was much less than that in the case of the treated sample. This means that, at higher temperatures (100 and 150oC), VES enhanced the adsorption of both CO2 and CH4 on the rock surface. Thermodynamic investigations showed that, the process of gas adsorption in the plain samples was exothermic with ΔHads of -13.5 and -16.7 KJ/mol for CO2 and CH4, respectively and at 50oC the adsorption was spontaneous. In contrast, at higher temperatures (100 and 150oC), the adsorption of CO2 and CH4 is spontaneous and endothermic with corresponding ΔHads of 36.2 and 60.1 kJ/mol. The adsorption of CO2 was three times that of CH4 because of the high affinity of calcite to CO2. Freundlich isotherm was found to be the best model that can fit the experimental data of calcite with both CO2 and CH4 at different temperatures. Dynamic adsorption experiments were carried out using gas coreflooding experiments with the same calcite cores used in the static adsorption experiments. The results of this study showed that, carbonate rock samples conditioned in the surfactant solution have great adsorption potential for CO2 and are excellent candidates for CO2 sequestration. However, surfactant promoted high CH4 adsorption at 100 and 150oC, and this will reduce the natural gas recovery. In contrast, using viscoelastic surfactant at low temperature reservoirs (50oC), reduced CH4 adsorption by blocking the active adsorption sites in the carbonate rock samples and this will increase the gas recovery.

https://www.researchgate.net/publication/323763500_Impact_of_Surfactant_on_the_Retention_of_CO2_and_Methane_in_Carbonate_Reservoirs