Oil production enhancement, asphaltene precipitation and permeability damage during CO2-SAG flooding of multi-layer sandstone reservoirs

The process of CO2-SAG flooding involves conventional miscible CO2 flooding until breakthrough (BT), followed by a period of CO2 soaking or shut-in, and then a continuation of the miscible CO2 flooding. The SAG process provides different improvements in the oil recovery for different positions of each layer in multilayer reservoirs, and has different effects on the distribution of pore throat blocking and adsorption of asphaltene to mineral surfaces. In this paper, both miscible CO2-SAG and conventional CO2 flooding experiments have been carried out at reservoir conditions and on multi-layer systems composed of 3 long cores each with increasing porosities and permeabilities, which were connected in parallel. After CO2-SAG flooding oil recovery factors (RF) of the low, medium and high permeability cores were 7.7%, 8.3%, and 7.6% higher compared to the RFs after CO2 flooding, respectively. The respective fractional oil production (FOP) of each long core was 10.6%, 27.7%, and 61.6% after CO2-SAG flooding, with less difference between each long core than for CO2 flooding. After CO2 flooding, the permeability of the high permeability core at the injection end dropped by 24.5–25.8%, which is 5.5–14.3% higher than the value at the outlet. The permeability decrease due to CO2-SAG flooding was 0.7–9.7% higher than that due to CO2 flooding, and the distribution of permeability decline was more homogeneous. The contribution of the total permeability decrease attributable to asphaltene particle blockage due to CO2 flooding was 84.7–62.7%, 5.2–10.1% higher than that due to CO2-SAG flooding, gradually decreasing along the flow direction. Complex two-phase flow of oil and gas is more likely to cause pore throat blockage instead of causing the adsorption of asphaltene precipitation.