Optimization technique for CO2-EOR miscibility management in an oil reservoir

1. A method of recovering crude oil from a reservoir, the method comprising:

determining a depletion pressure for providing a lights-depleted crude oil comprising a reduced amount of light ends including methane, nitrogen, carbon dioxide (CO 2 ), or a combination thereof and having a CO 2 multi contact Minimum Miscibility Pressure (CO 2 -MMP) below a CO 2 -MMP of a native crude oil, wherein the native crude oil is crude oil extracted from the reservoir prior to primary oil recovery therefrom;

depleting the pressure of the reservoir from an initial reservoir pressure to the determined depletion pressure, thus providing the lights-depleted crude oil;

repressurizing the reservoir to an operating pressure for recovering the lights-depleted crude oil from the reservoir via carbon dioxide (CO 2 ) injection;

injecting CO 2 into the reservoir via at least one injection well; and

recovering at least a portion of the lights-depleted crude oil from the reservoir via at least one production well.

2. The method of claim 1 , wherein the depletion pressure is a pressure below which the CO 2 multi contact Minimum Miscibility Pressure (CO 2 -MMP) does not further decrease during CO 2 -MMP experiments on the native crude oil.

3. The method of claim 1 , wherein the operating pressure is less than the CO 2 -MMP for the native crude oil.

4. The method of claim 3 , wherein the operating pressure is 15-35% less than the CO 2 -MMP for the native crude oil.

5. The method of claim 3 , wherein the operating pressure is below the bubble point pressure of the native crude oil, wherein the bubble point pressure is the pressure at which a first bubble of gas is liberated from the liquid phase at the reservoir temperature.

6. The method of claim 5 , wherein a bubble point pressure of the lights-depleted crude oil is 25-45% less than the bubble point pressure of the native crude oil.

7. The method of claim 1 , wherein determining the depletion pressure comprises:

(i) determining the CO 2 -MMP for a sample having a composition of the native crude oil;

(ii) pressure depleting a sample having a composition of the native crude oil to a test pressure;

(iii) removing excess gas from the pressure depleted sample to provide a lights-depleted crude oil sample;

(iv) measuring the CO 2 -MMP for the lights-depleted crude oil sample at the test pressure;

(v) repeating (ii) through (iv) for a plurality of test pressures; and

(vi) utilizing the CO 2 -MMP values for the lights-depleted crude oil samples produced at the corresponding test pressures to determine the depletion pressure.

8. The method of claim 7 , wherein (vi) further comprises determining the depletion pressure as the test pressure at which the CO 2 -MMP for the lights-depleted crude oil sample is a minimum.

9. The method of claim 8 , wherein determining the depletion pressure as the test pressure at which the CO 2 -MMP for the lights-depleted crude oil sample is a minimum further comprises plotting the CO 2 -MMP as a function of the test pressure for each of the samples.

10. The method of claim 1 , wherein the lights-depleted crude oil comprises a mole fraction of light ends that is 10-30% less than a mole fraction of light ends in the native crude oil.

11. The method of claim 1 , wherein the lights-depleted crude oil comprises a mole fraction of intermediate components that is 10-30% greater than a mole fraction of intermediate components in the native crude oil, wherein the intermediate components have molecular weights (MWs) between that of ethane (C2) and hexane (C6).

12. A method of recovering crude oil from a reservoir via carbon dioxide (CO 2 ) injection, the method comprising:

determining a depletion pressure by:

(i) determining a CO 2 multi contact Minimum Miscibility Pressure (CO 2 -MMP) for a sample having a composition of a native crude oil, wherein the native crude oil is crude oil extracted from the reservoir prior to primary oil recovery therefrom;

(ii) pressure depleting a sample having a composition of the native crude oil to a test pressure;

(iii) removing excess gas from the pressure depleted sample to provide a lights-depleted crude oil sample;

(iv) measuring the CO 2 -MMP for the lights-depleted crude oil sample at the test pressure;

(v) repeating (ii) through (iv) for a plurality of test pressures; and

(vi) determining the depletion pressure as the test pressure at which a lowest CO 2 -MMP is obtained,

depleting a pressure in the reservoir from an initial reservoir pressure to a pressure greater than or equal to the depletion pressure, whereby light components are removed from the reservoir to provide a lights-depleted crude oil in the reservoir, wherein light components comprise methane (CH 4 ), nitrogen (N 2 ), CO 2 , or a combination thereof; and

producing at least a portion of the lights-depleted crude oil from the reservoir via CO 2 injection at an operating pressure of greater than or equal to the CO 2 -MMP of the lights-depleted crude oil.

13. The method of claim 12 , wherein the producing is effected at an operating pressure less than the CO 2 -MMP for the native crude oil.

14. The method of claim 13 , wherein the producing is effected at an operating pressure that is 15-35% less than the CO 2 -MMP for the native crude oil.

15. The method of claim 12 , wherein the producing is effected at a pressure less than a bubble point pressure of the native crude oil, wherein the bubble point pressure is the pressure at which a first bubble of gas is liberated from the liquid phase at the reservoir temperature.

16. The method of claim 12 , wherein a bubble point pressure of the lights-depleted crude oil is 25-35% less than the bubble point pressure of the native crude oil.

17. The method of claim 12 , wherein the bubble point pressure of the native crude oil is greater than or equal to the CO 2 -MMP of the native crude oil.

18. The method of claim 12 , wherein the CO 2 -MMP of the native crude oil is greater than the initial reservoir pressure.

19. A method of recovering crude oil from a reservoir via carbon dioxide (CO 2 ) injection, the method comprising:

injecting CO 2 into the reservoir at an operating pressure of less than a CO 2 multi contact Minimum Miscibility Pressure (CO 2 -MMP) of a native crude oil, wherein the native crude oil is crude oil extracted from the reservoir prior to primary oil recovery therefrom; and

depleting the reservoir of light ends to provide a lights-depleted crude oil in the reservoir prior to injecting the CO 2 into the reservoir, wherein the light ends comprise methane (CH 4 ), nitrogen (N 2 ), CO 2 , or a combination thereof.

20. The method of claim 19 , wherein depleting the reservoir of the light ends increases a mole fraction of intermediate components in the lights-depleted crude oil by 10-30% and decreases a mole fraction of light ends in the lights-depleted crude oil by 10-30%, wherein the light ends comprise methane (CH 4 ), nitrogen (N 2 ), CO 2 or a combination thereof, and wherein the intermediate components include components having a molecular weights (MW) between that of ethane (C2) and hexane (C6).

21. The method of claim 19 , wherein the operating pressure is 30-40% less than the CO 2 -MMP for the native crude oil.

22. The method of claim 19 , wherein the operating pressure is below the bubble point pressure of the native crude oil, wherein the bubble point pressure is the pressure at which a first bubble of gas is liberated from the liquid phase at the reservoir temperature.

23. The method of claim 22 , wherein the operating pressure is 15-25% less than the bubble point pressure of the native crude oil.