Experimental Investigation of Supercritical CO2 Injection for Enhanced Gas Recovery in Tight Gas Reservoir

Tight gas reservoirs always have low natural gas recovery due to poor physical properties. However, there is still no effective means to enhance gas recovery (EGR) in tight gas reservoir. In this paper, experimental approaches were carried out to study the supercritical CO₂ injection for EGR in tight gas reservoirs. Phase behavior investigation was performed to indicate the property difference between supercritical CO₂ and CH₄ under reservoir condition. Results show that supercritical CO₂ has significantly higher density and viscosity than CH₄ under reservoir condition. Gravity differentiation and near-piston displacement can be achieved in case of supercritical CO₂ injection and thus the displacement efficiency can be improved. Gas adsorption tests show that the adsorption capacity of supercritical CO₂ in tight sandstone is more than 60% higher than that of CH4. The lower the permeability is, the stronger the adsorption capacity of supercritical CO₂ become, which means supercritical CO₂ can replace CH₄ in tight cores and EGR through competitive adsorption. Diffusion simulation show that supercritical CO₂ shows weak diffusion capacity and slow diffusion process in CH₄, which indicates that supercritical CO₂ and CH₄ are not easy to mix and it is good for displacement and EGR. On the basis of these fundamental tests, long-core experiments of supercritical CO₂ injection for EGR in tight gas reservoir were carried out using natural cores from DS tight gas reservoir. Results indicate that supercritical CO₂ injection can improve gas recovery by 13.5% on the basis of gas depletion. Furthermore, the influencing factors of supercritical CO₂ injection in tight cores were experimentally investigated. Results demonstrate that supercritical CO₂ injection have better EGR effect in case of lower permeability, higher water saturation and greater dip angle.