Smart-Fluid-Processing System Reduces Footprint, Improves Separation Efficiency
An important function of topside facilities is efficient processing of fluids from flowing wells to maintain optimum hydrocarbon production. To add processing flexibility when addressing a changing fluid composition from commingled production wells, and to remove the bottleneck in topside processing capacity, a chemically enhanced, smart compact separation system has been developed. The new separation system is based on the centrifugal separation principle.
This paper describes a two- and three-phase compact chemically enhanced separation system that can achieve the necessary hydrocarbon quality without sacrificing footprint. The system has been field-tested for reliability. Separation efficiency will be further optimized by injecting appropriate chemical demulsifier agents and testing different dosage levels to determine the final operating envelope of the equipment and to optimize the chemical usage on the basis of well locations and sales-oil quality.
There are two versions of this system. The first is a liquid/liquid unit that uses centrifugal forces to separate the fluids using liquid-density differential. An increased specific-gravity difference between the two incoming fluids produces greater separation efficiency. Initial mechanical performance of the system was observed during a series of laboratory tests and a field trial conducted in Oklahoma. The first version of the separator is optimal for light crudes and high-salinity water, and it can handle solids because of its relatively open internal configuration and its abrasion-resistant bearing and seal design.
The second version incorporates the gas handler that is currently used downhole, attached to electrical submersible pumps (ESPs) to separate gas from well fluids. This version uses axial flow and centrifugal forces in multiple stages to separate the gas from the well fluids. It offers gas/oil/water or three-phase separation. This unit is still in the development stage. Both versions of this centrifugal separation system reduce the separation train footprint by eliminating or reducing vessel size, the number of liquid- and gas-holding vessels, and the residence time required to achieve the hydrocarbon quality needed for export or sales.
Computational Fluid Dynamics (CFD) Analysis
The geometry considered for CFD modeling has been simplified by removing the outer stationary cylinder used for pressure balancing. A separate study has revealed that the flow communication between rotating and stationary drums has a negligible effect on overall performance of the separator. The diameter of the oil-outlet tube is designed for specific water-cut ranges to improve separation efficiency. There is a 30 to 50% range, a 50 to 70% range, and a 70 to 95% range. The final design of this piece will be detachable to create a flexible design.
A grid with 1,778,586 hexahedral elements is used to discretize the computational geometry. Fig. 1 illustrates the mesh. The computational domain has been divided into five subdomains to accommodate the rotating and stationary domains. The mesh-quality requirements were carefully chosen, such as minimum angles of the hexahedral elements, growth rate, variation in volume between adjacent elements, and mesh refinement near the flow-separation region to capture the physics. The CFD models and their results are detailed in the complete paper.
Multiphase Flow Model. A two-fluid (Euler/Euler) multiphase model with a heterogenous velocity field and a homogenous turbulence field is selected, with water being the continuous phase and oil being the dispersed phase when water-volume fraction is greater than 0.5. Oil is considered the dispersed phase, with a fixed droplet diameter. The velocity fields of each phase are coupled by interphase drag terms.
Laboratory Testing Setup and Results
The separation system was laboratory-tested in a continuous flow loop using city water and a synthetic oil with a specific gravity of 0.82. The overall dimensions of the separator are 5.5 ft in length and 4 in. in diameter. The laboratory setup includes a water and oil mixture tank, a centrifugal pump to circulate the fluids, and the centrifugal separation system mounted on a surface pumping system with a 15-hp motor. The separator only requires a 5-hp motor, but a 15-hp motor was the smallest unit that could be purchased for testing. Two outlet-control valves were tested at different positions to determine optimal separation efficiency on the basis of inlet-mixture fluid conditions.
Different tests were conducted in the laboratory to determine the optimal separation efficiency of the two-phase liquid/liquid centrifugal separation system. The first test campaign used a flow rate range of 800 to 1,200 B/D to determine separator capacity. The second campaign varied the separator rotational speeds between 500 and 3,500 rev/min to determine optimal speed. The third campaign varied the inlet water cut to determine the operating envelope of the separator, which ranged between 60 and 99% of the inlet water cut. An additional campaign involved changing the control-valve position to determine the setting of the outlet control valves to achieve optimal separation efficiency and determine controllability of the unit on the basis of inlet conditions. The results from this initial controllability testing will be used to develop the final, optimized control algorithms that will enable the system to be dynamic and controlled with changing inlet-flow conditions.
Laboratory tests for a centrifugal separation system at a speed of 2,000 rev/min were concluded with an inlet-flow rate of 800 B/D. The separator showed an average residual oil-in-water separation efficiency of greater than 99% for the inlet-flow rates and water cuts tested. At 800 B/D, the separator achieved a treated water discharge quality of 150-ppm residual oil for the lower 60% water cut. All tests proved to be below 400-ppm residual oil in the discharge water. Water content in the product oil stream is still being determined in the laboratory, and is showing promising results. The final basic sediment and water (BS&W) levels must be determined during a field trial that incorporates chemical injection to optimize fluid separation. Chemical injection is not possible in the current laboratory setup because of the recirculation of the loop. Demulsifier injection in the field should produce even lower oil-in-water levels for the treated water stream, and achieve costumer requirements for BS&W levels.
Field-Application-Test Setup and Results
The two-phase liquid/liquid centrifugal separation system was field-tested at a wellsite in Oklahoma that was operated by a local company. The main purpose of the field trial was to determine separator integrity, mechanical reliability, and dynamic stability with produced fluids. In addition, the field trial helped determine the separation efficiency with produced water and crude oil. The average flow rate of the well was approximately 500 B/D, and the average inlet water cut ranged between 90 and 95%. The total dissolved solids (TDS) were approximately 229,000 mg/L for the produced water, which provided sufficiently severe operating conditions to test the reliability of the dynamic components of the separator, such as bearings and seals.
The separation system was placed between two free-water knockout tanks in the field to avoid flow interruptions and slug flow. The separator ran continuously in the field for a period of 50 days. The bearing and seal design proved to be reliable, with no mechanical failures running high TDS water and light crude oil. For the duration of the field trial, the separation system maintained an average vibration of 2.2 mm/sec peak. This average vibration level proved the mechanical integrity and dynamic stability of the design and the durability of the rotating components for long-term continuous use. The separation efficiency of the separator with produced fluids achieved a water discharge quality of 340-ppm residual oil with an inlet water cut of 90%. This first field trial was successful in determining the mechanical reliability of the design, the dynamic stability of the rotating equipment, and the separation efficiency of the unit with produced fluids in a typical well environment.
This system will reduce the footprint of separation trains and improve separation efficiency for changing well profiles by functioning dynamically with the changing flow conditions. The control algorithm of the system is in development currently, but the transfer-function foundation models to develop the algorithm are in place for different water-cut ranges and speeds. The separation efficiency of this centrifugal unit is governed by the density difference between the well fluids and will be best used in wells with reasonable oil density. This unique technology provides turndown capacity for wells that range in flow rate between 10:1 in flow capacities and have a wide water-cut range of 30 to 99%.
The first version of this technology is a liquid/liquid separator, which has been tested in the laboratory, yielding a clean water stream with water-in-oil levels of less than 400 ppm for all flow rates tested. The unit was also field-tested for mechanical reliability and separation efficiency. The overall treated water in the field had an effluent water stream containing residual oil of less than 400 ppm. The solids in the field trial remained in the water stream and did not affect the bearings or mechanical integrity of the unit. The second version has an integrated gas separator, making it a three-phase separator that can be used in wells that have gas volume fraction range of 10 to 50%. This unit is in the mechanical design phase and has not yet been tested in the laboratory or field.
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