Oversized Gas-oil Separator Design Project in Duri Field (Sumatra), Indonesia
Background: Caltex and Texaco selected EnSys Yocum to evaluate installation options for separation facilities in their 106,000 bpd Duri (Sumatra), Indonesia oil field. A major obstacle to separation was the high viscosity oil-water emulsion. This problem was further compounded by sand content exceeding 2 volume percent.
The summary fluid characteristics were:
- Crude Oil: 19.0˚ API Gravity
- Water cut: 50% by volume
- GOR: 115 scf/bbl
- Viscosity: 50-70 centipoise (@ 228˚F and 55 psig)
The design feasibility for a conventional separator was compared against that of a gas-liquid cylindrical cyclone (GLCC) separator. The GLCC installation is described in detail in an Engineering Technology Conference on Energy (ETCE) 2000 joint paper co-authored by Jack Marrelli of Texaco.
Detailed Analysis: A primary challenge was to reduce the separator gas-carry-under (GCU) corresponding to a target gas volume fraction (GVF) of 0.005. This target permitted the accurate measurement of the oil production by a meter/cut probe. A series of GOSPSIM cases were run to reflect (i) fundamentally different designs such as horizontal vs. vertical separators and (ii) variations in separator size, internals and assumed liquid droplet and gas bubble particle diameters. For the most part, we evaluated horizontal separator designs because of separation efficiency. A drawback to this is that vertical separators are better suited to sand removal.
EnSys Yocum GOSPSIM (gas-oil separator simulation) is a gas-oil-water-sand separation simulator, providing throughput capacity and effluent quality analysis for single or multi-stage separation (including predictions for gas-carry-under and liquid-carry-over, oil-in-water and water-in-oil as a function of separator inlet conditions and internal components). A wide range of separator internal options are available, including vortex tubes. As stated in the ETCE paper, the EnSys Yocum GOSPSIM model was selected by Texaco/Caltex because of its combination of extensive field-proven history and theoretical correlations.
Because of the high liquid phase viscosity in the degassing separator, the size of the vessel was determined by the residence time of the liquid phase required to allow the gas bubbles trapped in the liquid phase to rise to the gas/liquid interface. In gravity settling the gas rise velocities are reduced by the viscous drag forces operating against their buoyancy. GOSPSIM simulations showed that degassing the emulsion required 12 –14 minutes residence times, requiring a large diameter separator vessel outside of normal codes and standards.
Source: Methods for Optimal Matching of Separation and Metering Facilities for Performance, Cost, and Size (Figure 10)
At a higher produced oil GVF target of 0.125 fraction, a 6 foot diameter separator equipped with perforated baffles would have been feasible. In order to approach the 0.005 GVF target though, a 14 foot diameter horizontal separator would have been required to meet the GVF target, which would have overtaxed the capability of the nearest harbor. Also, the ability of a horizontal separator to process large sand volumes would have presented significant problems and additional costs.
The alternative to a conventional separator was a GLCC separator. We evaluated the viability of a GLCC separator based on a University of Tulsa design compared to the results of a series of GOSPSIM case study simulation runs.
Final Design Recommendation: As indicated by GOSPSIM case study simulation results, we came to the conclusion that a conventional separator adequately designed to separate the multiphase flow was simply not deliverable to the Duri location because of its large size (outside normal design standards at 14 feet in diameter). We presented this finding to Caltex and Texaco management and recommended the installation of GLCC separator. This recommendation was adopted, successfully installed, and subsequently operated at the terminus of the Duri, Indonesia field gathering system. Methods for Optimal Matching of Separation and Metering Facilities for Performance, Cost, and Size