Design of CO2 Floods in Reservoirs Impacted by Strong and Limited Aquifers (in the US Gulf Coast Region, and in Many Other Parts of the World)

January 8th, 2018

In the US Gulf Coast region, and in many other parts of the world, some of the largest oil reservoirs have strong waterdrive aquifers which assist in maintaining reservoir pressure during primary production. While these aquifers are somewhat infinite, they are insufficient for maintaining pressure at original 'discovery' levels and often the pressure drops below MMP (Minimum Miscibility Pressure). While good oil recoveries have been seen by conducting CO2 floods below the MMP, it is desirable to pressure up the reservoir as high as possible (above MMP) to achieve maximum oil recovery. A&A consultants have experienced this in actual field demonstrations where reservoir pressure was raised by water injection (SPE #144961-PP- 'Large Scale CO2 Flood Begins Along Texas Gulf Coast', Presented at 2011 SPE Enhanced Oil Recovery Conference in Kuala Lumpur, Malaysia, 19-20 July, 2011). While doing so, it was found that large volumes of water leakoff into the aquifer during the re-pressurization process, and this can go on for many years at rates as high as 200,000 - 300,000 BWPD in the applicable fields. The design was effective in raising reservoir pressure and reducing the volume of CO2 purchase, but the cost of water injection was expensive and often required large volumes of make-up water which had to be produced from other water bearing sands. In the case described, a nearby producing offset field also experienced reduced aquifer pressure, thus compounding the problem. All this must be considered while designing CO2 floods in reservoirs impacted by strong and limited acquifers in the US Gulf Coast region, and in many other parts of the world.

A reservoir type which is often overlooked due to its low reservoir pressure, at the time of CO2 flood, is the depletion drive reservoir. The depletion drive reservoirs contain higher oil saturations than waterdrive reservoirs and can be pressured-up with CO2. The salt dome fields in the US Gulf Coast region are ideal candidates for CO2 flooding as many of the small fault blocks had limited aquifers, and therefore had low oil recoveries during primary production. These small fault blocks were also poor waterflood candidates due to the inability to establish good patterns to improve sweep. Continuous CO2 injection, on the other hand, offers some clear advantages for maximizing oil recovery from these isolated fault blocks. The reservoir can be pressured-up with CO2 and then the wells flowed back while recycling the CO2 to maintain reservoir pressure. The CO2 moves to the boundaries of the fault block and contacts more oil than what could be achieved by water injection. The key parameters for successful CO2 flooding of these reservoirs are good fault seal, and  ability to pressure-up the reservoir. Material balance is used during the re-pressurization process to determine pore volume impacted by injection, and if CO2 leaks into the adjacent fault block this can be accounted for. 

A&A consultants have screened numerous oil fields in the US Gulf Coast region for reservoirs amenable to successful CO2 flooding, and we can assist your company with screening/ scoping/ feasibility studies, and CO2 flood project design studies.

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