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Designing Efficient Downhole Gas Separators with ANSYS CFD

Downhole gas separators are complex devices in terms of flow physics. ANSYS Fluent can meet the challenges of this difficult application with appropriate modeling capabilities and expedite the design of efficient gas separators.

The oil produced from wells in productive oil fields is always accompanied by a free gas fraction, which forms a two-phase (gas – oil) fluid mixture. Depending on the characteristics of the well and of the oil produced, the pressure in the reservoir may not be sufficient to raise the production fluids up to the surface at the desired flow rate. These wells are equipped with downhole pumps to enhance production. Free gas entering a downhole pump reduces the pumping efficiency. When the volumetric gas fraction at pump inlets exceeds a certain percentage, gas lock and cavitation can occur. At this point, fluid suction is reduced to zero, causing frequent pump shutdown and significantly increasing operation costs.

To reduce downhole pump failure and maximize volumetric pumping efficiency, downhole mechanical liquid-gas separators (or simply gas separators) are used at the bottom of oil producing wells to separate gas from crude oil. Separation minimizes the entry of free gas into downhole pumps.

Mechanical gas separators are classified as either static or dynamic. Both types exploit the density difference between the heavier liquid oil phase and the lighter natural gas phase. Static gas separators are designed with tortuous flow paths. As the gas–oil mixture traverses these flow paths, denser oil separates from the lighter gas due to gravity and/or centrifugal forces. Dynamic gas separators, on the other hand, employ rotating elements such as impellers to impart energy to the mixture to separate the gas from the liquid. Static gas separators are mostly used for low gas fractions up to ~45 percent (or simply 0.45 as normalized in the range 0 to 1) gas volume fraction. For higher gas volume fractions, dynamic separators are used.

Obviously, efficient gas separators will have a great impact on the downhole pump performance and oil well operation costs. The performance of a gas separator is characterized by the separation efficiency, defined as the ratio of the total gas mass flow rate summed over the intended or designed gas outlets to the total gas mass flow rate at the inlet of the separator.

Experimentation and testing in production wells are prohibitively expensive, if not impossible. To characterize the gas separator performance, manufacturers traditionally rely heavily on land-based physical experiments using an air–water mixture in place of the natural gas–crude oil mixture. While more practical, these land-based air–water experiments are still expensive and time-consuming. To reduce costs and save time, many companies have turned to CFD simulation as an indispensable engineering tool for designing efficient gas separators. With CFD simulation, they can perform parametric and optimization studies on geometry and operation variables such as intake flow rate, gas volume fraction, and impeller RPM in the case of a dynamic separator.

However, an accurate simulation of downhole gas separators requires reliable and appropriate CFD modeling capabilities. First and foremost, CFD simulations of a gas separator need advanced multiphase models. Not all multiphase models are suitable for gas separator analysis. When a gas separator is in operation, the gas-liquid mixture enters the separator intake section with the free gas present as dispersed bubbles in the oil. As the mixture flows through the separator, strong centrifugal body forces and, to a small extent, the gravitational force act on the mixture, separating gas from liquid due to density difference. The mixture changes from a dispersed flow regime to a sharp stratified flow regime with gas and liquid phases clearly delineated.

Most multiphase models are formulated to simulate either the sharp regime or the dispersed regime, but not both. The volume of fluid (VOF) model is numerically formulated for the sharp regime, while the Eulerian-Eulerian and the mixture models are intended for the dispersed regime. To appropriately capture both sharp and dispersed regimes in a single flow model requires a special kind of multiphase model. The Eulerian-Eulerian multifluid VOF model in ANSYS Fluent is the appropriate model to address applications with both sharp and dispersed multiphase flow regimes. As the name suggests, this model combines aspects of the Eulerian-Eulerian model for treating the dispersed flow regime and the VOF model for the sharp regime.

In addition to using an appropriate multiphase model, the solver must be robust enough to include interactions such as the strong body forces mentioned above, as well as turbulence and rotating elements such as impellers. Furthermore, it must solve the complex 3D model in a very efficient manner (i.e., with great parallel scalability). ANSYS Fluent meets all these stringent requirements.

(As a side note, today in the oil and gas industry the geometry of every gas separator is a highly guarded, trademarked IP. Every separator bears design signatures unique to its manufacturer. Truly, “everybody knows everybody else” when it comes to identifying gas separator design. It is impossible for a third party to showcase a gas separator, especially the dynamic type, in the public domain without legal hazards.)

To demonstrate, let’s consider a mocked-up static gas separator with spiral internal flow paths (Figure 1). The air–water mixture enters the inlet at 0.5 m/s with 30 percent air volume fraction.

Figure 1. Gas separator geometry (flow volume only).

A transient simulation of 4 seconds flow time (Figure 2) is conducted using the appropriate Eulerian-Eulerian multifluid VOF multiphase model in ANSYS Fluent.

Figure 2. Animation showing flow in the water volume fraction. Red (volume fraction = 1) indicates the space is totally filled with water; blue (volume fraction = 0) indicates all air.

Due to the centrifugal force induced when flowing through the spiral paths, the air–water mixture starts to separate and transit from a dispersed regime to a sharp regime. The heavier water phase is flung to the outer perimeter of the pipe, as expected. An air core forms at the center near the outlet section. In a real downhole operation, the liquid at the outlets is directed toward the intake of a multistaged pump section for artificial lifting to the surface; the gas is vented to the annular space between the downhole pipe and the well.

As discussed earlier, the separation efficiency is an important design objective. Figure 3 shows the mass flow rates for water and air monitored at their respective intended outlets. After about 1.5 seconds, judging from the outlet flow rates and the volume fraction contours, the flow more or less exhibits stable periodicity. The separation efficiency fluctuates between 98 – 99 percent. This is just for one set of flow conditions such as inlet velocity and inlet gas volume fraction. However, it demonstrates all the important considerations when choosing the appropriate models for an accurate and successful gas separator simulation.

Figure 3. Left: water mass flow rate at the intended water outlets. Right: air mass flow rate at the intended air outlet. With this mocked-up geometry, after about 1.5 seconds the separation efficiency fluctuates between 98 – 99 percent.

In reality, engineers have to optimize for a range of operating flow conditions, gas volume fractions and so on. Even though the flow physics in a gas separator is very complex, ANSYS Fluent has the appropriate advanced models, along with reliable and efficient solver technologies for this application. Coupling these advanced modeling capabilities with automatic persistent parametric updates in ANSYS Workbench and optimization capabilities in ANSYS DesignXplorer, engineers can design efficient gas separators much faster and at lower cost than ever before.