Soaking effect on miscible CO2 flooding in a tight sandstone formation

Abstract

In this paper, the phase behavior and viscosity reduction of CO2-saturated light oil were experimentally studied. The saturation pressures (Psat), gas–oil ratios (GORs), and oil-swelling factors (SFs) of four light oil–CO2 systems with different light oil and CO2 compositions were measured by using a PVT system. The corresponding viscosities of CO2-saturated light oils were measured by using a capillary viscometer. It was found that Psat, GOR, and oil SF were increased respectively in the ranges of 4.97–8.44MPa, 59.0–149.5cm3 CO2/cm3 oil, and 1.14–1.34 when CO2 concentration in the light oil–CO2 system was increased in the range of 38.94–60.46mol.%. Accordingly, the respective viscosities of CO2-saturated light oils with 38.94 and 60.46mol.% CO2 concentrations were reduced to lower than 16% and 10% of the dead light oil viscosity at the same reservoir temperature. Furthermore, a total of six miscible CO2 coreflood tests were conducted to study CO2-soaking effect on miscible CO2 flooding in a tight sandstone formation. It was found that in the coreflood tests without CO2 soaking, a large amount of oil was produced in the first pore volume (PV) of CO2 injection, whereas only a low oil recovery factor (RF) of 0.77–6.10% was obtained in the second PV of CO2 injection. In the coreflood tests with CO2 soaking, the residual light oil and reservoir brine in the composite sandstone reservoir core plugs were soaked with the remaining CO2 for 24h after the first PV of CO2 injection. A high oil RF of 11.05–14.74% was achieved in the second PV of CO2 injection. Among the six CO2 coreflood tests, pre-waterflooding plus CO2 tertiary flooding with CO2 soaking resulted in the highest oil RF, which was attributed to the mobility-control effect of pre-waterflooding and the subsequent CO2-soaking effect.