Lessons Learned from the Field-Scale Simulation of the Gas-Assisted Gravity Drainage GAGD Process in Heterogeneous Sandstone Oil Reservoirs

摘要

Unlike the 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 gravitystable oil displacement and improve oil recovery. In the GAGD process, the gas is injected through vertical wells to formulate a gas cap to allow oil and water drain down to the horizontal producer (s). Therefore, the GAGD process was implemented through immiscible and miscible injection modes to improve oil recovery in a sector of the main pay/upper sandstone member in the South Rumaila oil field, located in Iraq. A high-resolution Multiple-Point Geostatistics-based reservoir characterization was reconstructed to model the lithofacies and petrophysical properties in order to provide the most realistic geological environment for the GAGD process simulation. After upscaling the geostatistical models, EOScompositional reservoir simulation was built to evaluate the GAGD process and test its effectiveness to improve oil recovery. Next, a notable history matching was obtained with respect to the oil rates, cumulative production, water injection rate, and cumulative injection for the entire field and all the wells. Then, 20 vertical injectors and 11 horizontal producers were installed for CO2 injection and oil production, respectively. The reservoir has 12 layers and the gas injection wells are placed at the crest of the reservoir through the first two top layers to formulate a gas cap. The horizontal production wells were installed in the fifth, sixth, seventh, and eighth layer of high oil saturation. The 2nd, third, and fourth layers were left as transition zones to achieve the gravity drainage. The four bottom layers were left with no injection/ production activates as they fully flooded with water.