GAS DISPLACEMENT EFFICIENCY FOR A LOW PERMEABILITY CARBONATE FIELD

Miscible gas injection schemes are potentially attractive methods of improving oil recovery because they can result in lower residual oil saturations than water flooding alone. Gas injection may also access regions of attic oil bypassed by water flooding. Where there is no route to market associated gas, and environmental considerations prevent flaring, gas injection may also provide a means of managing excess gas early in field life. To achieve a horizontal displacement with gas injection, water may be injected in alternating slugs (WAG) to help control the high mobility of the gas. Prediction of miscible gas injection performance, and comparison with water flooding, requires good quality relative permeability data. This paper describes the methods used, and presents the results of miscible gas displacement experiments which were performed at reservoir temperature and pressure using reservoir core material that was characterised and conditioned to provide initial oil saturations typical of reservoir conditions. Experiments were undertaken to investigate secondary miscible gas flooding, secondary water flood followed by tertiary miscible gas flooding and water alternating gas (WAG) floods. The experiments were undertaken on a 52 cm long carbonate composite with an effective oil permeability of 1.66 mD. Test conditions for gas flooding were investigated just above the Minimum Miscibility Pressure (MMP) at reservoir temperature. The recovery from secondary miscible gas flooding, as one would expect, was high and resulted in very low residual oil saturation (Sorm). For the core material studied, water flood recovery (before tertiary gas flooding) was also good with high break-through recovery of oil. Tertiary miscible gas flooding resulted in additional oil recovery of 14%HCPV. Additional oil recoveries from two secondary WAG experiments (compared to secondary water flood) showed that recovery was proportional to the size of the initial gas slug. The measured laboratory data reduces the uncertainty in the planned field development options of this prolific reservoir.