Experimental Study of Blending CO_2 with Triethyl Citrate for Mitigating Gravity Override During Reservoir Flooding

摘要

Challenges during CO_2 flooding of oil reservoirs are mobility control and gravity override caused, respectively, by lowerviscosity and density of CO_2 compared with reservoir fluids. Blocking agents such as foam, cross-linked polymers andgels have been tested. However, such techniques are more effective in preventing fingering and blocking thief zones thanreducing gravity override. Moreover, no foaming agent has been identified to withstand the high temperatures and salinitiesof some reservoirs. This experimental study investigates the use of triethyl citrate (TEC) as a densifying agent to blend withsupercritical CO_2 (scCO_2) to minimize gravity override. High-permeability, Indiana limestone core samples saturated witheither dead or live light crude oils at irreducible water saturations were flooded vertically upward at 24.132 MPa and 100 °Cwith about one pore volume of either pure scCO_2 or a scCO_2–TEC blend of 0.15 mol fraction (0.525 mass fraction) TEC.At breakthrough, scCO_2 flooding of dead oil recovered 14.82% of the initial oil in place, while the blend flood recovered31.99%. At 1 PV, the scCO_2 flood recovered 60.14%, while the blend flood recovered 72.67%. The live-oil-saturated coresamples produced similar results but with generally lower oil recoveries. At 1 PV of fluid injected, all floods reduced the oilsaturation to levels lower than the residual oil saturation achieved with water flooding. With TEC’s low solubility in bothcrude oil and water, blending scCO_2 with TEC is, thus, a technically viable method to mitigate gravity override in CO_2–EORprocess.