Dual Ring Injection Fence Technology: A Novel Method to Optimally Co-Develop Hydrocarbon Reservoirs with Free Gas Caps Over Producing Oil Zones

Abstract

Abstract To develop oil fields with significant gas caps, some operators have utilized a water injection fence to be able to co-develop the gas cap and the oil leg. ADNOC is preparing to co-develop the gas cap and oil legs of many of its giant oil reservoirs by drilling horizontal water injection wells at the current gas oil contact, separating the gas cap from the oil leg. This allows co-production of both the gas cap and the oil leg. However, the use of a water injection fence can lead to a loss in hydrocarbon recovery (oil, gas and condensate) when water invades both the oil leg and the gas cap. Furthermore, injection of water into the gas oil contact zone could result in gas producers with high water production which could ultimately lead to the cessation of production from the gas production wells if the water in the production tubing cannot be lifted to surface. The Dual Ring Injection Fence Technology (DRIFT) provides a method to achieve high hydrocarbon recovery with much lower produced water handling. DRIFT also allows for the option to inject large volumes of CO2 into the reservoir to further improve gas, condensate and oil recovery and to sequester CO2 in the subsurface, leading to a very low CO2 footprint. Fundamental to DRIFT is a second set of horizontal inner ring gas injection fence wells (IRGIF wells) that offset the IRWIF wells. The IRGIF are placed in the lower flank of the gas cap, above the GOC contact. Gas production wells are drilled into the crest of the gas cap. Lean gas is injected into the IRGIF wells and gas is produced gas from the crestal horizontal gas production wells. To test and validate the method, a history matched numerical simulation model of a real giant oil field was used to explore the performance of DRIFT compared to the current field development plan which utilized only the IRWIF concept. Sensitivities were run comparing the placement of the injection rings relative to each other, both in terms of true vertical depth and lateral offsets. It was found that the Dual Ring Injection Fence Technology improves oil, gas and condensate recovery compared to the method of a single ring of water injection wells. By placing the second ring of gas injection wells down structure and producing from the crest, water short circuiting is greatly mitigated. However, it is noted that when water invades the gas cap, a substantial volume of gas is trapped in the water phase, since the residual gas saturation to water for this system is as high as 30%. If the gas is hydrocarbon, this represents a substantial value loss. To improve the recovery of hydrocarbon gas, the use of different sacrificial gases was examined. By injection of a gas with little economic value (such as CO2, N2, H2S or other), the sacrificial gas displaces the hydrocarbon gas before the invasion of water. When the water does invade, the gas that is trapped is of low economic value. Thus, there is a remarkable improvement in the net hydrocarbon gas recovered from the gas cap. Furthermore, if CO2 is the sacrificial injection gas, there is substantial environmental benefit as the greenhouse gas is injected into the reservoir for disposal and storage instead of being released into the atmosphere. In summary, DRIFT is a step change improvement over the current method of a single ring of water injection wells. Dual Ring Injection Fence Technology enables an increase in the recovery of oil, gas and condensate and a decrease in water short circuiting compared to a standard water injection fence method. This process leads to a lower carbon-intensity barrel, particularly when CO2 is used as the injectant in the inner ring of gas injectors.