Mechanism of CO2 enhanced oil recovery in shale reservoirs

Combined with NMR, core experiment, slim-tube tests, nano-CT and oil composition analysis, the mechanism of CO2 enhanced oil recovery had been studied. CO2 flooding under supercritical state could achieve higher oil recovery. In the process of crude oil displaced by supercritical CO2, the average oil recovery was 46.98% at low displacement pressures and 73.35% at high displacement pressures. The permeability of cores after CO2 flooding was only 28%–64% of those before flooding. As to the expelled oil, the contents of asphaltenes and non-hydrocarbons decreased, and saturated hydrocarbons of above C25 were absent in some samples, indicating that they had been retained in cores as demonstrated by CT and NMR experiments. In slim-tube tests, the heavy components of oil were expelled when the pressure increased to 30 MPa. There was a reasonable bottom hole pressure (BHP) below which the heavy components driven out from the far-well zone would deposit in the near-well zone, and when the pressure was too high, the nonhydrocarbon detention may cause block. The smaller throat and worse physical properties the porous media had, the higher displacement pressure would be required to achieve a good oil displacement efficiency. The increase in displacement pressure or time of interaction between oil and CO2 could effectively enhance oil recovery.