The Effect of methane and fluid geometry on CO₂ enhanced oil recovery

Abstract

CO₂ Enhanced Oil Recovery (EOR) is a process that involves injecting large volumes of carbon dioxide as a supercritical fluid into hydrocarbon reservoirs in order to recover hydrocarbons that are not mobilized during primary or secondary production. Some of the injected CO₂ is produced with the produced hydrocarbons and then recycled by reinjection into the reservoir. Most CO₂ floods performed for EOR are miscible, which means the fluids mix to form a homogeneous mixture under a specific set of conditions. For a typical oil field, miscible floods are more efficient in recovering oil than immiscible floods. When recycled CO₂ includes a high percentage of methane, miscibility is significantly reduced. For a typical oil field, miscible floods are more efficient in recovering oil than immiscible floods. Calculations from produced fluid data base shows that at 18 mole percent methane, 28 percent of offshore oil reservoirs became immiscible (Ogbaubau, 2015). The effect was more pronounced in nearshore fields. In this study, I assessed the fluid distribution in a study area to determine if methane production can be avoided by strategic completion of wells to avoid high methane areas. High Island 10L, High Island 24L and ST TR 60S were selected due to availability of structural data. Using seismic, well log interpretation, and production data it was found that, of the wells evaluated, 94 percent had solution gas drive. A number of economic solutions to the problem were postulated; these included a methane separation facility, changes to CO₂ recycling, cutting CO₂ with another gas, and accepting immiscible flood conditions. The following equation was developed to estimate the increased cost for miscible CO₂ enhanced oil recovery: General Additional Costs of CO₂ Enhanced Oil Recovery = (Cost of CO₂ Recycling Plant + Cost of Pipelines + Cost of CO₂ to Offset Methane Immiscibility + Transportation costs + O&M costs + Pipeline Operation Costs) - (Value of Storage Tax Credit), Where: Cost of CO₂ to Offset Methane Immiscibility = (cost of CO₂/ton * tons of CO₂ needed to offset Methane), Cost of Pipelines= (cost of pipeline construction per mile * mumber of miles), Value of Storage Tax Credit = ($35/ton of CO₂ stored Tax Credit*tons of stored CO₂). The equations parameters were then used to create a table showing how the economic solutions might affect the cost of CO₂ enhanced oil recovery.