Enhanced oil recovery via dissolution of low molecular weight PDMS in CO₂ during immiscible gas injection in matrix-fracture system

Oil recovery in natural fracture reservoirs can be improved by reducing the capillary force during the gravity drainage process in matrix-fracture system. In the current study, a modified gas was generated via dissolution of low molecular weight poly(dimethyl siloxane) (PDMS, average M-w 3780 g/mol, 10000 - 50000 ppm) in CO2 to enhance oil recovery at 50 degrees C or 70 degrees C and 3000 psi. Cloud point measurements indicated that PDMS dissolved in CO2 at pressures less than 3000 psi, which was below the minimum miscibility pressure (MMP) of similar to 3600 psi at 70 degrees C determined with the vanishing interfacial technique. The solution density increased from 669.6 kg/m(3) for pure CO2 to 780.3 kg/m(3) for CO2/PDMS (50000 ppm) at 70 degrees C and 3000 psi, thereby reducing the density difference between the CO2-rich fluid and crude oil (rho(o)=865.2 kg/m(3)) in matrix-fracture system. Also, CO2-oil interfacial tension (IFT) was reduced from 65 to 15 dyn/cm via the dissolution of 50000 ppm PDMS in CO2. Therefore, it led to the CO2 diffusion coefficient increased in both reservoir-fluid saturated porous media and bulk oil scenarios 3-4 fold and 4-7 fold respectively in comparison to pure CO2 injection. These PDMS-induced changes reduced the capillary effects in gas-invaded zone, which consequently led to an increase in oil recovery of 25% points (from 31% to 56%) compared to pure CO2 injection in the case of capillary continuity during gravity drainage tests. For the field-scale, because the PDMS can dissolve in CO2 at pressures and temperatures which are commensurate with CO2 EOR, it is a promising CO2-philic chemical for increasing CO2 density and improving CO2 diffusion coefficient in gas invaded zone compared to conventional CO2 injection.