Abstract
The dispersion coefficient is a key physical parameter for enhanced gas recovery (EGR) and reflects the degree of mixing between the displacing-fluid CO2 and natural gas. However, no comprehensive overview of existing research on EGR dispersion characteristics has been conducted to date. In this review article, previous experimental and simulation studies are summarized to provide an overview of the current research progress and, hence, to elucidate the limitations of EGR and identify directions for future research. The literature analysis reveals that temperature and flow rate promote CO2-CH4 dispersion, whereas pressure and residual-water salinity inhibit dispersion under supercritical conditions. The CO2-CH4 dispersion coefficient is smaller in porous media with high permeability. However, on the core scale, the effects of residual water on the CO2 dispersion and breakthrough time are controversial and further verification is required. To date, only limited research has been conducted on the effects of impurities under supercritical conditions, such as considering CO2 containing N2, and CH4 containing CO2, N2 and ethane. Existing pilot EGR trials are insufficient to provide standardized field operation instructions, for example, with regard to the wellhead layout. Field-scale simulations have verified the feasibility of EGR technology and economy. However, factors such as the permeability heterogeneity, initial water saturation distribution, connate water salinity, gas-reservoir non-isothermal conditions, and vertical stratification should be considered in simulations of actual gas reservoirs. More in-depth experimental and simulation-based investigations of EGR should be performed on various scales to accurately assess the dispersion characteristics and natural-gas recovery efficiency under gas-reservoir conditions.
NSTL