Experimental Investigation On CO₂ Miscible Drive In Ultra-low Permeability Reservoirs

Longhupao Gaotaizi Reservoir, mainly consisting of argillaceous siltstone, is the ultra-low permeability in which the average porosity and air permeability are 13.5% and 0.7810-3μm2 respectively. The reservoir has such features as the low starting pressure, the poor water intake ability, quick breakthrough when injecting water and the low displacement efficiency. In order to develop the reservoir reasonably and effectively, the paper carries on the feasibility of CO2 miscible drive on the basis of the sufficient survey about low permeability features and the low permeability condition at home and abroad when injecting gas.Based on the prediction of both the Minimum Miscibility Pressures of CO2 and the crude oil of Gaotaizi reservoir, the MMP is examined to be 22.4MPa by means of slim tube method. The MMP is higher than formation pressure (18.2MPa) and lower than formation fracture pressure(33.7MPa) while it could reach close CO2 miscible drive under the formation pressure. Through CO2 miscible drive experiment, a series of parameters such as recovery efficiency,oil/gas exchange ratio,gas-oil ratio and gas-injection ability are obtained under various kinds of pressure. The result indicates that as CO2-injection pressure gets higher, the break-through time,the recovery when gas makes breakthrough,final recovery efficiency,oil/gas exchange ratio and CO2-injection ability are all enhanced, among which the gas-oil ratio is higher after the breakthrough of gas, and the CO2 drive efficiency of low permeability core is more effective than the high permeability core. When the pressure reaches 29MPa, the pressure increases, the increasing amplitude of recovery is quite small and oil/gas exchange ratio is lowered. Some physics and chemistry phenomenon such as flow profile,viscous fingering,water blocking and mechanism of CO2 drive are researched through theoretical analysis and reality calculation.The recovery efficiency of CO2 miscible drive is higher than water flooding, and the injection ability of CO2 is far higher than the water ,so the Reservoir of Gaotaizi is fit for CO2 drive...