Water Management Study for Unocal in Serang, Indonesia
Field Background: EnSys Yocum was contracted by Unocal to assess current field operations and recommend system modifications to increase capacity and meet water disposal requirements for the Serang, Indonesia offshore facility. In the Serang field, 21 offshore oil wells produced a total of 63,000 barrels/day of oil and water.
The summary fluid characteristics were:
- Crude Oil: 42˚ API Gravity
- Water cut: 60 volume percent initially; predicted to rise to 65 percent in 3 years
- Crude oil GOR: 400 scf/bbl plus free reservoir gas
- Viscosity: 8 centipoise (@ 81˚F and 171 psig) at the onshore separator
The flowing fluid (oil plus water) traveled 29 miles in a 12 inch line from an offshore platform to three stages of onshore gas-oil separation. After the water, gas, and oil was separated in the onshore gas-oil separator train, the water was kept in a holding tank for three days. After this time, the ~0.5% oil still in the water was skimmed off the top. The oil was then fed back into the distribution system and the water was disposed of into the seabed by injecting it through long underwater lines. The water was injected deep into the seabed away from the fish life and, as a precautionary measure, the underwater lines had dispersers to reduce droplet size.
Because production was forecast to rise from 63,000 b/d currently to 90,000 barrels/day within the next 3 years, Unocal wanted to determine the way to most effectively accommodate this extra throughput. One consideration was to develop a system by which water could be separated out at the offshore platform, and in doing so, avoid the transportation of tens of thousands of barrels/day of water through the 29 mile line to shore. In recommending a facility design to handle forecast increases in oil production (and water cut), two principal design options were considered:
- Separate the water at the offshore platform first stage separator and dispose of the water into the sea at this location
- Upside: Maximize oil production by transporting only oil to shore (thus alleviating the system bottlenecked which was the 12 inch diameter pipeline to shore).
- Downside: Risk of potentially impacting marine life through disposal of water with entrained oil. If separator failed to perform, operations would have to be shut-in to avoid risk of disposing of water containing oil. The regulatory authorities could also shut down operations if they were deemed hazardous to the environment.
- Continue to transport all liquid flow (oil plus water) to shore for onshore separation
- Upside: Reduced risk from a regulatory/environmental standpoint because this avoids releasing water with a significant oil fraction into the sea
- Downside: Throughput inherently limited on the 12 inch pipeline to shore
Analysis: Firstly, EnSys Yocum used its GOSPSIM (gas-oil separation simulation) and PRODSIM (production facilities simulation) models to evaluate current system operations. The results from our modeling closely matched test separator results and were thus validated by this data. The validation included a crosscheck of separator effluent oil-in-water and water-in-oil at each stage of separation. Then, extending the simulation to the projected total flowrate (oil plus water) of 90,000 bpd (65% water cut) being pumped from the Serang offshore platform, we identified several major constraints on the system.
The high pressure separator located on the offshore platform was currently operating as only as a de-gasser. This offshore separator would not be effective in sufficient oil/water separation to discharge water into the sea. At this higher rate the water content of the oil increases to 2.2 percent and the oil in the water to 0.6 percent. By contrast, the maximum allowable threshold for oil-in-water was 10 parts per million per environmental regulations.
With respect to the offshore high pressure separation, although the gas residence time decreases to 6 seconds, the horizontal gas velocity is below critical, but above mesh pad velocity limits. The installation of separator vanes is recommended to reduce liquid carryover in the gas. The additional flow from the offshore separator through the 29 mile pipeline to shore requires a separator pressure in excess of 690 psig. With 90,000 bpd pumped from the Serang platform, simulation results indicate that a pressure of 1000 to 1100 psig would be required to flow through the 29 mile subsea flowline, indicating the need for a booster pump to be installed on the offshore platform.
The onshore separation facilities are not undersized and can be upgraded to effectively dehydrate the increase to 90,000 barrels/day. However, an electrostatic coalescer and steam coil heater should be installed in the intermediate pressure separator to break an emulsion, predicted to form half-way along the underwater pipeline. Additionally, EnSys Yocum PRODSIM predicted slug flow forming approximately 15 miles from shore as gas begins to evolve from the crude oil. The intermediate pressure stage horizontal liquid velocities were predicted to increase to 0.18 feet per second indicating that short-circuiting could occur. Therefore, the installation of baffles and a water wash section of the intermediate stage separator could be investigated to improve the quality of the discharged water.
Possible options to produce acceptable water discharge from the Serang platform separator and to increase overall field production include:
- Install a Cyclonic Separator Upstream of the Platform High Pressure Separator:
Pending detailed study, A Portatest vortex tube or equivalent cyclonic separator could be installed upstream of the high pressure offshore platform separator. This reduces load on the high pressure separator by routing the pre-separated gas to the gas line and compression, and the oil to the oil line pumps, where the remaining water-in-oil is removed onshore. With the reduced oil flow into the high pressure separator remaining in place and operating the separator with the liquid level at 80-85%, the liquid residence time in the high pressure separator will increase by approximately three-fold. By installing an advanced design hydro-cyclone followed by aeration to meet disposed water oil content requirements, disposal of the water at sea would be enabled. This was presented to Unocal as the preferred option.
We note that the company had previously placed a trial conventional hydro-cyclone on the platform but failed to meet disposal water standards because of the high concentration of sub 10-50 micron oil droplet sizes, rendering the hydro-cyclone ineffective. Our prediction, with respect to the oil droplet size distribution in the oil-in-water, is that a conventional hydro-cyclone would fail to reduce the oil content of the water to the required 10 ppm level.
- “Piggy-back” an Additional Separator on top of the Existing one
An additional separator could be “piggy-backed” on the existing separator, enabling production to flow through both separators. This was done in the North Sea to conserve space on the offshore platform. It also reduces the loss of production that occurs during replacement. The new separator should be sized in conjunction with the existing separator to reduce horizontal velocities to 0.05 feet per second. However, if considering this as an option, the platform weight capacity should be checked. GOSPSIM performed a platform weight calculation to determine if the platform should be reinforced, which was required for this option.
In conclusion, EnSys Yocum GOSPSIM and PRODSIM results indicated that, in its current configuration, the 29 mile flowline (12 inches in diameter) would not be able to meet the targeted production increase to 90,000 barrels/day. However, installing sufficient gas-oil separation facilities offshore would not be a straightforward undertaking; rather, several options had to be considered in order to meet the 10 parts per million oil-in-water specification for water disposal into the deep sea, away from marine life, while allowing for adequate capacity to handle future increments of production from the Serang field.