Impact of pore-pressure on the recovery efficiency from CO2-methane displacement experiments

The intricate behavior of carbon dioxide at super critical conditions is considered to influence the ultimate recovery of additional hydrocarbon reserves through the application of CO₂-enhanced gas recovery projects. At the more extreme reservoir conditions of pressure of 41.37 MPa and temperature 433.15 K, CO₂ exhibits a greater viscosity and density if compared to methane. This variation is approximately a factor of three, which is in favor of the CO₂-natural gas displacement. In contrast, at near super critical conditions of pressure of 7.39 MPa and temperature 304.15 K, the CO₂ physical property (viscosity and density) is slightly superior to methane´s. Therefore excessive mixing is anticipated if CO₂ is injected to displace natural gas, particularly at well abandonment pressure. Because mixing in such a system is a diffusion-like process, which definitely depends on the physical properties of the displacing and displaced phases and the heterogeneity of the medium. The aim of this paper was to experimentally investigate the consequence of pore pressure on the recovery efficiency and multiphase gas flow in a reservoirs porous medium. For this purpose a long reservoir core plug of 1 foot length was prepared. The displacement tests were carried out at five different pore pressures ranging from 10.34-40.68 MPa. Temperature of 433.15 K and displacement rate of 10 cm/h maintained through out the tests. Results determined that improved recovery by approximately 40 % can be obtained by applying the highest pore pressure in comparison to the test conducted at low pore pressure. This confirmed that the greater difference in physical properties in a gas-gas system will result in improved recovery and limited mixing. The effects of pore pressure on the relative permeability for both phases are highlighted.