Condensate Field Debottlenecking (Das Island, Abu Dhabi)
Problem and Field Background: The Das Island onshore oil field was operating at only a fraction of estimated throughput capacity because of a surface production bottleneck. EnSys Yocum was contracted to devise solution(s) to the production constraint in the flowline system.
The crude from the field was a light oil/condensate (50˚ API crude and gas-oil ratio of 3500 scf/stb). An 18 mile subsea flowline from Das Island to Abu Dhabi delivered approximately 70,000 bpd oil (water cut < 5 percent) through an 18 inch flowline. In the subsea, a significant incline starts to occur about a mile from onshore Abu Dhabi after almost 17 miles of little elevation change. Then, upon reaching onshore Abu Dhabi, the flowline passes over a 250 foot cliff which is almost a 90 degree incline. The 1st stage gas oil separator is located approximately 150 feet after the cliff.
Analysis: EnSys Yocum employed PRODSIM (production facilities simulation) and GOSPSIM (gas-oil separator simulation) to accurately simulate the system and subsequently devise solutions to improve the oil field’s performance. An accuracy of 9 percent was achieved for oil, water and gas rates as compared to test separator results. PRODSIM also accurately predicted the flow regime at successive increments throughout the system. PRODSIM indicated that the flowline was operating in stratified flow for the first ~17 miles until the significant elevation change begins as the flowline approaches onshore Abu Dhabi. The flowline problems are compounded by the sharp elevation change of the 250 foot onshore cliff. After travelling over the steep cliff, the production is operating in a highly unstable flow regime (slug flow).
Because sizable slugs were formed in the flowline during the steep incline and the 1st stage gas-oil separator was located only 150 feet after the cliff, the separator was effectively being swamped with slugs. This bottleneck was limiting the total system throughput to only 60 percent of capacity. GOSPSIM estimated the sizes of the gas bubbles were six times the length of the liquid slugs and that 11-13 percent of gas was entrained in the liquid slugs.
Solution: EnSys Yocum proposed (3) solutions to alleviate the system bottleneck and provided simulations to verify each solution:
(1) After the 18 inch pipeline reaches shore (directly before the steep incline), utilize concentric piping by placing a 12 inch line inside of the 18 inch line. In this way, flow would occur both up the annulus and center flowline depending on the level of throughput. This would break up the flow and result in slugs not forming as flow traveled over the cliff.
(2) Create a manifold onshore directly before the steep incline where the 18 inch flowline would be divided into (3) 8 inch flowlines to travel over the cliff.
(3) Alter the control valve setting located after the cliff and before the 1st stage gas-oil separator—reduce the lift from 70 percent (~10 psi pressure loss) to 25 percent (~200 psi pressure loss). The high pressure drop would effectively burst slugs of this type.
After preventing slug flow from entering the separator, the total oil delivered by the system increased to slightly over 110,000 bpd, i.e. by nearly 60%.