Gas injection has been widely used for IOR/EOR processes in oil reservoirs. Unlike the conventional modes of continuous gas injection (CGI) and water –alternative gas (WAG), the Gas Assisted Gravity Drainage (GAGD) process takes advantage of the natural segregation of reservoir fluids to provide gravity stable oil displacement. It has been proved that GAGD Process results in better sweep efficiency and higher microscopic displacement to recover the bypassed oil from un-swept reservoir regions. Therefore, CO2 was considered in this research for immiscible injection in the main pay/upper sandstone formation in South Rumaila oil field located in Iraq through the GAGD process application. This field, with a 60-year production history, has 40 production wells and is surrounded by an infiniteacting edge water aquifer from the east and the west flanks. Since the east flank is much less effective than the west one, 20 injection wells have been drilled at the east flank over the last 35 years to maintain the reservoir pressure. The formation depth is 10350 ft. sub-sea with a maximum vertical oil column of 350 ft.
The GAGD process was adopted here using compositional reservoir simulation and PVT modeling to increase oil recovery. The GAGD process consists of placing a horizontal producer near the bottom of the payzone and injecting gas through existing vertical wells that have been used in prior waterfloods. As the injected gas rises to the top to form a gas zone, oil and water drain down to the horizontal producer. The location of horizontal wells is slightly above the oil-water contact.
In the reservoir modeling, different reservoir and fluid properties have been investigated their effect on the flow response to implement sensitivity analysis, history matching through Design of Experiments. The operational design parameters of production/injection wells were considered to determine the optimal future reservoir performance through the GAGD process. Among many prediction scenarios, the GAGD process led to significant recovery incremental, especially in early future production years in comparison with the base case of no injection and 10,000 barrels of water injection per well in the same injection wells.