Determination of the optimum solvent bank size requirement during CO₂ flooding

Abstract

This laboratory investigation studied the efficiency of water and of nitrogen as CO(,2) slug propellants, determined the optimum CO(,2) slug size required for flooding, and the effect of horizontal permeability variation on oil recovery during CO(,2) miscible flooding in horizontal consolidated Berea sandstone cores. The interaction between carbon dioxide and dolomite rock during CO(,2) flooding in carbonate oil reservoirs was also investigated using dolomite cores. Water was found to be a more efficient CO(,2) slug propellant than nitrogen in the range of pressure and temperature studied. (110(DEGREES)F (LESSTHEQ) T (LESSTHEQ) 120(DEGREES)F, 1500 psig (LESSTHEQ) flooding pressure (LESSTHEQ) 2650 psig). While the efficiency of nitrogen as the CO(,2) slug propellant increased with the flooding pressure, that of water was almost unaffected by the flooding pressure. A 0.3 HPV CO(,2) slug was found to be the optimum solvent bank size required for miscible flooding in the cores used. The total oil recoveries from a core with variable horizontal permeability were a little lower than those from an homogeneous core at similar flooding conditions (crude oil, pressure and temperature). The heterogeneous core contained zones with horizontal permeabilities greater than or equal to the permeability of the homogeneous core. The total oil recoveries were found to be dependent on the flooding temperature--they decreased as the flooding temperature was increased. Studies on CO(,2) - dolomite rock interaction showed that CO(,2) in the presence of water dissolved part of the rock and increased the rock permeability at high pressures. The amount of rock dissolved and hence the enhancement of the rock's permeability increased with the flooding pressure. Some of the dissolved carbonates from the cores were precipitated in the second dolomite cores connected in a series with the first cores due to pressure reduction across the second cores. This was manifested as a reduction in the permeabilities of the second cores. Carbonate precipitation in the second cores and hence, the decrease in the permeabilities, due to pressure reduction increased with pressure drawdown across the cores.