Optimization of CO₂ WAG processes in a selected carbonate reservoir - An experimental approach

Miscible gas flooding using carbon dioxide is currently being investigated as a possible EOR process for a number of United Arab Emirates (UAE) reservoirs. It has high potential to improve oil recovery in addition to possibly utilizing most of the carbon dioxide emissions from industrial sources. The major factors affecting implementation of CO² floods are the availability of CO² at economic prices (generally within 2 - 3 $/MSCF) and the net utilization ratio of CO² per barrel of additional oil recovered. Typical net utilization of CO₂ for well-designed floods vary from field to field but on average has been estimated at 5.5 MSCF CO₂ per additional barrel of oil in a US EOR overview study by Broome et al.1 and between 4-6 MSCF/barrel by a more recent study by Jeschke et al.². At other fields it might be as high as 15 MSCF/barrel or more. Minimizing net utilization requires controlling the high mobility ratio for miscible gas injection which causes lower sweep due to gas channeling and by-passing of the oil in the reservoir. To control the mobility ratio, the Water-Alternating CO₂-Gas (WAG) technique is proposed by injecting alternately small solvent [CO₂] and water slugs. The slug of water reduces the speed of the solvent and solvent fingering thus improving the mobility ratio of the injected fluids to fluids in place. The objective of this work is to experimentally assess the recovery of oil with CO₂ injection in a selected UAE carbonate reservoir. Two types of CO₂-flooding experiments were conducted, continuous miscible CO₂ injection and CO²-WAG injection using a specialized experimental rig. The effects of changing the CO₂-Water ratio and WAG timing on the overall performance of the flood were investigated. All laboratory tests were conducted under controlled conditions of pressure and temperature corresponding to field conditions. Results of this laboratory investigation reveal a general trend of improved oil recovery with increased volume of CO₂ inside core samples during the flood process. The observed ultimate oil recoveries range from 52 percent with continuous water injection to 72 percent of the original oil in place with continuous CO₂ injection over the full period of the experiment with recoveries of the CO₂-WAG floods falling in between. The optimum CO₂-WAG ratio was found to occur at 1:2. The outcomes of this work should contribute to our understanding of WAG CO ₂ floods for the UAE reservoirs and supports the ongoing R&D efforts made by the operating oil companies in the UAE towards application of CO₂-WAG floods