The effect of water-soluble polymers on water-alternating CO2 performance in porous media

Abstract

In order to supply the demand of oil, enhanced oil recovery (EOR) techniques are nowadays widely applied in the oil recovery industry to maximize reservoir sweep efficiencies. CO2-EOR method has been largely used in the petroleum industry for several decades, which can contribute to the reduction of global greenhouse gas emission through CO2 sequestration in geological formations besides the elevation of oil recovery levels. Conventionally, CO2 is implemented as a continuous flooding scheme. But CO2 flooding is prone to unfavorable displacements, viscous fingering, gravity segregation and early breakthrough due to the high mobility ratio, low viscosity and low density of CO2. A method to overcome these problems is the alternation of water and CO2 slugs. This method, better known as water alternating gas (WAG) injection, is widely applied through the field. But despite the successes of the WAG injection scheme, the method still suffers from a relatively poor mobility ratio and gravity override. In this experimental study, the performance of the polymer assisted water alternating gas (PA-WAG) EOR method is examined. This method makes use of the high viscosity of the polymer solution in combination with immiscible CO2, to improve the improve the mobility ratio, volumetric sweep efficiency as with that the ultimate recovery of the WAG process. For the polymer, a specially modified polymer is considered, which uses the ATBS (2-acrylamido-terbutylsufonic acid) addition to HPAM in order to improve the stability of the polymer under reservoir conditions. In this thesis, the displacement and recovery performance of the ATBS-modified HPAM polymer is studied via a series of core-flood experiments and the stability is tested via polymer aging in the presence of CO2. The results of this work
have shown that the addition of the ATBS-modified HPAM polymer to the water in the WAG method increased the recovery by 10% over the normal WAG injection, by improving the mobility ratio and displacement efficiency. By implementing the PA-WAG on a carbonate rock saturated with crude oil, a base line experiment with regarding reservoir simulated recovery was established, with a recovery of 76%. The stability test under the presence of CO2 showed that the dissolved CO2 impacts the polymer viscosity. After the initial decrease in apparent viscosity to 54% of the original viscosity, the ATBSmodified HPAM polymer was able to recover its viscosity to 84% of its original apparent viscosity after 29 days of aging. These results were achieved under full saturation of the polymer solution with CO2 . In addition, the impact of CO2 at PA-WAG flooding CO2 concentrations was evaluated, which showed now degradation with the exposure to CO2. The study concludes that the ATBS-modified HPAM polymer performs excellently in the PA-WAG method for enhanced recovery, significantly outperforming the WAG injection method.