Experimental Study of Blending CO₂ with Triethyl Citrate for Mitigating Gravity Override During Reservoir Flooding

Challenges during CO₂ flooding of oil reservoirs are mobility control and gravity override caused, respectively, by lower viscosity and density of CO₂ compared with reservoir fluids. Blocking agents such as foam, cross-linked polymers and gels have been tested. However, such techniques are more effective in preventing fingering and blocking thief zones than reducing gravity override. Moreover, no foaming agent has been identified to withstand the high temperatures and salinities of some reservoirs. This experimental study investigates the use of triethyl citrate (TEC) as a densifying agent to blend with supercritical CO₂ (scCO₂) to minimize gravity override. High-permeability, Indiana limestone core samples saturated with either dead or live light crude oils at irreducible water saturations were flooded vertically upward at 24.132 MPa and 100 °C with about one pore volume of either pure scCO₂ or a scCO₂–TEC blend of 0.15 mol fraction (0.525 mass fraction) TEC. At breakthrough, scCO₂ flooding of dead oil recovered 14.82% of the initial oil in place, while the blend flood recovered 31.99%. At 1 PV, the scCO₂ flood recovered 60.14%, while the blend flood recovered 72.67%. The live-oil-saturated core samples produced similar results but with generally lower oil recoveries. At 1 PV of fluid injected, all floods reduced the oil saturation to levels lower than the residual oil saturation achieved with water flooding. With TEC’s low solubility in both crude oil and water, blending scCO₂ with TEC is, thus, a technically viable method to mitigate gravity override in CO₂–EOR process.