(398o) Differential Permeability Reduction of CO2 and Water By Polymer Gel in Sandstone Rocks during Wag Process

Polymer gels have been widely applied to control reservoir conformance and improve sweep efficiency for CO₂ flooding reservoirs. The plugging performance of polymer gels on water and CO₂ have been reported separately; however, no research has been carried out about how the cycles of CO₂ and water affect the plugging efficiency of gels on CO₂ during a WAG (water alternated with CO₂) process. The objective of this study is to investigate whether a polymer gel has a different effect on water and CO₂ flow and how the cycles of water and CO₂ affect the plugging performance during the WAG process. Core flooding experiments were carried out using four sandstone cores with different permeability varied from 100 to 1200 md. After determining the permeability of a core using brine and CO₂, the Cr(III)-acetate-HPAM gelant was injected into the core until injection pressure reached stable and the cores were shut in for 24 hours to allow gelation. After that, two cycles of water and CO₂ were injected to determine the effect of gel on the plugging efficiency of water and CO₂ at different injection rates. Results showed that the polymer gel is a selective plugging material that can reduce the permeability to water much more than that to CO2 during the first flooding cycle. The resistance to CO2 was only 5% of that to brine. The selective plugging in low permeability cores was more prominent than that in high permeability cores. The cycles had a significant effect to polymer gel plugging performance. With increased cycles, the resistance to both CO₂ and brine significantly decreased. The resistance of the polymer gel to CO₂ became negligible only after two cycles of water and CO2 was injected for low permeability cores. This paper will be the first time to report that a polymer gel has much less plugging efficiency to CO2 than that to water. The WAG cycles could significantly decreased the plugging efficiency of a polymer gel to both water and CO2. The results will help the field engineers to understand why most polymer gel treatments are not very effective or the effective term is very limited. The paper will provide a guidance about how to better select a plugging agent for a WAG process.