A Comparison Simulation Study of GDWS-AGD EOR Process Performance Using CO2/APG Alternatives in South Rumaila Oil Field, Iraq

Abstract

Abstract To date, feasibility studies of the Gas and Downhole Water Sink-Assisted Gravity Drainage (GDWS-AGD) process for the South Rumaila oil field have considered using Carbon Dioxide gas injection to enhance oil recovery. As availibility of CO2 is limited and its cost considerable it might be also feasible to use natural gasAssociated Produced Gas (APG), that has been recently tested as an alternative solvent to enhance oil recovery. The paper reports on a simulated comparison of the APG or CO2 application in the South Rumaila oil field. The comparison metrics include field recovery factor, water cut in oil and water producers, Gas Oil Ratio in oil producers, cumulative gas injection, and average saturation. In the study, the GDWS-AGD process installation comprises 20 vertical APG injection wells completed at the top of the reservoir to build a gas cap in the oil pay zone. In addition, eleven horizontal oil-producing wells are placed at the bottom of the oil pay zone with six horizontal water drainage (sink) wells below the oil-water contact (OWC). The two horizontal leg installation may be made from a vertical well with 7-casing dual-completed (from two kick-off points) in the oil payzone and in the bottom water (below OWC) with two horizontal well legs and the two 2-3/8 inch tubings in each well. In a dual-tubing design of the process the two horizontal well legs produce independently. If only one tubing is used production from the water sink well is hydraulically isolated inside the vertical well by a packer. In either design, the water sink well is operated with a submersible pump. In this study, the GDWS-AGD process with APG is considered for the upper sandstone member/South Rumaila Oil Field, located in Iraq to improve oil recovery. The Rumaila field has an infinite acting-aquifer with very strong edge water drive. In the GDWS-AGD, the bottom water drainage would not only reduce water cut and water cresting, but would also significantly reduce the reservoir pressure, resulting in improving gas injectivity by significantly reducing the need for high injection pressure and gas solubility will be also reduced in the immiscible mode. Significant improvement with the GDWS-AGD process - oil recovery increased from 76.5% by CO2 to 82.5% by APG and water cut was readily controlled resulting in more rapid reduction with APG (from 95% to less than 5%) than that with CO2 in all horizontal oil producers. The results show that the use of APG alternative to CO2 for the GDWS-AGD process not only improves water-cresting and controls water cut, but also enhances gas injectivity and significantly improves oil recovery.