Case Study I: Surface line and gas-oil separator optimization for a Venezuelan heavy oil field (Cabimas)
Engagement Background and Field Overview: The Venezuelan Cabimas oil field was originally developed by Shell Oil in 1917 and had declined over the years to a reservoir pressure of only 300 psig, with the facility operating at 30 psig in the gathering trunklines and flowing into booster stations at 1-3 psig. Because of the low reservoir pressures, the wells are produced with sucker rod pumps and a few electric submersible pumps. EnSys Yocum was engaged by Preussag Energie, operator and joint owner with PDVSA, and in conjunction with the established engineering company Tecnoconsult (Caracas office), to construct a design for a multiphase flow gathering system capable of producing the field while avoiding cease-flow conditions, to size the separators, manifolds, pumps and compressor, and to design a control system capable of maintaining stable oil and gas production. As shown by the data below, Cabimas crude oil is heavy and viscous with high sand content—this proved to be a key consideration throughout our analysis and design.
Degrees API: 20˚ API
Viscosity@100 deg F: 42 centipoises
Associated/casing head gases (GOR): 560 – 1100 scf/bbl
Water cut: 10%
Sand content: 1% by volume
Analytical Framework: EnSys Yocum applied PRODSIM (production facilities simulation) and GOSPSIM (gas-oil separator simulation) to assess the operational boundaries of the production system. PRODSIM is a rigorous, multi-phase, steady-state, pressure temperature flow (PTQ) simulator designed for and proven against hundreds of oil and gas field production systems and GOSPSIM 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). A wide range of separator internal options are available, including vortex tubes.
Overview of findings via EnSys Yocum PRODSIM and GOSPSIM:
• Field characterized by low pressures, low Froude numbers, and low flow rates
• Computed liquid and gas slug frequency and size probability calculations based largely on the work of Brill and established sand settling correlations were employed
• Slug flow mitigation, adequate separator facilities to handle slug surges, and sand handling in the stublines are key considerations to optimally designing oil and gas production facilities
• Extensive rehabilitation work would significantly improve production in the gathering systems and multiphase flow lines.
In-depth analysis: EnSys Yocum PRODSIM, conducted across a range of flow rates, indicated that the multiphase well connecting lines and trunklines operated principally in stratified and wavy flow regimes, where the effect of viscosity is somewhat muted. This was a key consideration because Cabimas was a heavy, viscous crude with high sand content.
A minimum flow scenario of 6,000 barrels oil per day revealed operable flow conditions for two of the multiphase gathering systems reflecting sufficient GOR and pipeline velocity. However, a third gathering system leg predicted cease flow in the pipeline with pulsating/unstable flow at more than one location corresponding to the lowest GOR portion of the Cabimas flow system. Manifold calculations were made to size the manifolds to avoid slug flow and low Froude Numbers consistent with homogeneous flow while minimizing sand deposition. The maximum flow case of 23,000 barrels oil per day was limited by a maximum pipeline velocity of 60 feet/sec. and the gas compressor was sized accordingly.
A comprehensive system for process control and instrumentation was also developed for the Cabimas to ensure stable operation consistent with the facilities’ physical equipment. Firstly, parallel inlet control valves were sized for the slug catchers. These valves allowed the unloaded transient slugs to flow into the slug catcher without shock waves and controlled transient slugs with properly sized separator/slug catcher control valves. A gas back-pressure regulator was specified to open the vent valve to flare excess gas when necessary. As a result, the multiphase trunklines could be unloaded successfully using the correct opening and closing sequence for the gas and liquid outlet valves during the unloading period.
The first stage separator/slug catcher was designed considering the liquid level and residence time, the maximum slug volume, and a gas residence time for moderately foamy crude oils. In addition to the steady state flow pump, a second, parallel pump was installed downstream.
As the liquid level rose due to the arrival of a slug, the controller opened the valve to the second pump. The two pumps could pump out the predicted mean slug in about nine minutes to restore the separator to mid-level. The separator/slug catcher was designed to operate at 3 – 4 psig and 90 ˚F and separate a mixture of Cabimas heavy crude with the aforementioned characteristics.
Recommendations and Final System Design: EnSys Yocum recommended that the first stage separator/slug catchers be sized as a 6 x 30 foot horizontal vessel with a vortex breaker and high-gravity vortex tubes be installed to break the foam. The high-gravity separation can separate the liquid in gases down to 5 – 10 microns particle size and reduce the need for elaborate separator internals. The vortex tubes have also proven to be effective in removing sand from the oil phase. The gas phase is provided with adequate volume in the slug catcher for liquid phase elimination and a back-pressure regulator is recommended to maintain vessel pressure. In order to achieve best performance, EnSys Yocum recommended an internal vortex breaker designed especially to prevent gas breakthrough at low liquid level operation and an inlet diverter plate designed to act as a primary separator stage, minimize small gas bubbles formed and minimize gas-carry-under.
The pressure flow calculations indicated that a rapid shut off of the liquid control valve at the flow station will generate a surge wave traveling back along the line toward the pump stations. Based on the liquid flow rates in the 6 inch line section, the initial surge magnitude was estimated at 240 psig with a 20-30 psig surge pressure at the first pump. In order to handle the surge at the flow station when the valve closes, the controls automatically open a valve to a relief tank that captures and dissipates the surge. A radio or wireless relay signal can be set up at the surge generating point to inform the pump stations that the surge is coming. The pumps would then automatically trip off, sending a negative surge wave down the line to cancel the arriving surge waves in order to protect the flowlines and pumps. It was recommended that a pressure recording instrument be installed 1000-2000 feet upstream of the pump station to signal the trip off of the pump to produce the negative wave when required. This to be installed along with a backup telephone line that signals the pump to trip off if activated by a surge of sufficient magnitude to reach the pump.
The final EnSys Yocum system design was comprised of three trunklines, a 30 inch trunkline handling 267 wells and two 24 inch diameter trunklines handling 50 wells each, with each trunkline terminating in a 6 by 30 foot separator/ slugcatcher operating at 3-4 psig, followed by a horizontal 12 by 40 foot second stage holding vessel operating at atmospheric pressure and sized so that the wells could continue to flow while the pumps and compressors were down for repair upstream or due to a power outage. The manifolds before the slug catchers operate at 3 psig and required large 24 inch diameter piping. The combined trunkline design of 13,000 bpd of oil flowed into pump booster stations and into a 10 inch 14 mile flowline terminating at a Lake Maracaibo oil refinery, with a parallel 8 inch line sized to handle a design gas flow of 10.5 mmscfd. Gas diverted from the upstream separators provided the electric distribution system with sufficient fuel to power the secondary recovery well equipment.