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

Abstract

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 Mw 3780 g/mol, 10000 – 50000 ppm) in CO2 to enhance oil recovery at 50 °C or 70 °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 ∼3600 psi at 70 °C determined with the vanishing interfacial technique. The solution density increased from 669.6 kg/m3 for pure CO2 to 780.3 kg/m3 for CO2/PDMS (50000 ppm) at 70 °C and 3000 psi, thereby reducing the density difference between the CO2-rich fluid and crude oil (ρo=865.2 kg/m3) 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.