Autonomous Gas Shut-Off in Gas Oil Gravity Drainage Reservoir, Sultanate of Oman

Abstract

Abstract FH is one of the largest oil producing assets in the Sultanate of Oman. With more than 30 years of production, the current recovery mechanism from the naturally fractured carbonates is relying 80% on the gas oil gravity drainage (GOGD) process whereby produced formation gas is re-injected into the reservoir for pressure maintenance. Optimizing the GOGD process and hence maximizing oil production is the main objective in FH WRFM strategy. Controlling the formation gas production from subsurface and surface gas capacity handling are the main challenges to unlock reservoir oil potential. Down hole gas shut-off technology has been proposed in 2018 to control high GOR (+1000) producers and minimize formation gas production for more efficient GOGD process. The technology provides a mean for blocking formation gas through autonomous valve compartments installed within a production liner in the open hole section of the horizontal wells. The independent compartments are positioned against the open fractures and the inflow from the fractures to the open hole is controlled autonomously based on the fracture GOR. The valves in each compartment is designed to close against low viscosity fluid (gas) and allow high viscosity (liquids) to flow into the liner. Screening process has identified up to 80 candidates for the technology with a potential gain of up to 6% of the total field production. Formation GOR, well uptime and lateral fracture interference are the top criteria for candidate selection. The full field deployment started in July 2018 after a six months trial period with a target to complete the full scope by 2022. 90% reduction in formation gas has been observed in the wells where gas shut-off technology have been installed. A sustainable oil gain up to 50% has been realized after the gas reduction from the first 14 candidates. In addition to that, a significant improvement in the well uptime from 40% to 80% has also been recognized due to low producing GOR after the technology deployment. The technology has also enabled the re-instatement of long term closed-in wells due to high producing GOR. The originality of the technology lies on the ability of the downhole inflow control valves to react to the gas autonomously and successfully shut off the gas contribution and the same time allow oil to flow.