Rocky-Fluent Coupling For Flow Assurance Applications: Simulating The Liquid-Solid Flow For Scaling In Downhole Environments

Guaranteeing maximum output production for oil and gas wells is a top concern in the petroleum industry. The deposition of solids (e.g. inorganic salts, paraffin waxes, asphaltenes, hydrates) blocks flow passageways and cause disastrous production losses. How much encrustation each well is susceptible to depends on many factors, including the mineralogy of the geological substrate. For instance, the water flooding process creates the needed conditions for barium sulfate formation, and the presence of dissolved carbon dioxide may synthetize calcium carbonate.

Depending on the thermodynamics conditions, the dissolved inorganic salts precipitate, creating a two-phase system consisting of a fluid phase with solid particles (crystals) dispersed within. The precipitated crystals interact continuously with the fluid flow as well as with other crystals, triggering effects related to the adhesive forces, like formation of particle agglomerates and the subsequent deposition on surfaces (scale formation). The numerical study of the two-phase liquid-solid flow is a research topic at the Research Center for Rheology and Non-Newtonian Fluids (CERNN) at the Federal University of Technology – Parana (UTFPR). CERNN-UTFPR has been using Rocky DEM to simulate particulate flows in which the adhesion force plays a major role, as in fouling formation.

Rocky DEM is a powerful software for simulating particulate systems via Discrete Element Method (DEM). A number of features provide reproducibility for engineering applications, such as:
• choosing among a variety of models for computation of forces due to collision, friction, and adhesion;
• setting a particle granulometry as an input for the release of particles;
• creating customizable output variables;
coupling with ANSYS Fluent for CFD applications.

One interesting feature of the Rocky-Fluent interface is the simulation of the interaction between fluid flow and particles.

In the two-way coupling mode, an Euler-Euler approach is employed for the fluid flow, resulting in a fluid and a particulate phase. The information exchange is constant, with ANSYS Fluent interacting with Rocky DEM to obtain information about the particulate phase, such as the position and the velocity of each particle, to compute the velocity field and the solid volumetric phase field. Also, ANSYS Fluent provides Rocky DEM information about the fluid flow, allowing the computation of fluid-particle interaction forces such as drag. The simulation set-up is user friendly, with Rocky DEM being compatible with .caz and .gz files. One major advantage is the ability to allocate the simulation at the GPU for Rocky DEM and at the CPU for ANSYS Fluent.

Characterizing scaling phenomena is complex, with a number of parameters controlling the inter-particle mechanisms, like the magnitude of the adhesive force. The dynamic tube block test, known as DTB, gives insight about the deposition and growth of the solids layers, registering the pressure drop over time as a supersaturated ionic solution is pumped through a capillary tube.

The Rocky–Fluent coupling simulates the DTB, analyzing the influence of the variation of the adhesion force magnitude, denoted by the adhesive force ratio with the particle weight, fadh [-]. Results are presented in Figure 1, with fadh affecting the formation of particle agglomerates, influencing the particle transport pattern.

Influence of the adhesive force over the particle transport pattern in the DTB test.
Figure 1 – Influence of the adhesive force over the particle transport pattern in the DTB test.

The results in terms of the dimensionless pressure P [-] over the dimensionless time τ [-], obtained with the fluid flow simulation in ANSYS Fluent, are shown in Figure 2. The reference pressure pref [Pa] stands for the monophasic fluid flow, and tref [s] is the simulation time. The pressure P increases because of the adhesive force that creates particle agglomerates on the walls of the capillary tube. Therefore, the Rocky–Fluent two-way coupling is effective for simulating the liquid-solid flow for applications—inorganic scaling, for instance—in which the adhesion force plays a major role.

Dimensionless pressure P response over the dimensionless time τ as a function of the magnitude of the adhesion force fadh in the DTB test.
Figure 2 – Dimensionless pressure P response over the dimensionless time τ as a function of the magnitude of the adhesion force fadh in the DTB test.

Identifying fouling hot spots is advantageous in designing reliable equipment to handle oil production in environments subjected to scale formation.

Industry experience has proven that the sliding sleeve valves, known as SSV, are damaged by inorganic salts deposition. The main purpose of the SSV is to cease communication between the annular region and the production tubing. The valve consists of an outer sleeve with circular holes and a grooved inner sleeve. Aligning the holes with the grooves sets the valve to the open position.

The geometry of the SSV is notably complex, since the fluid has to accelerate to flow though the holes and the grooves’ constrictions. The geometry is shown from an isometric perspective in Figure 3, indicating the main parts of the valve assembly, such as the outer sleeve with the circular holes, the inner sleeve with the grooves, and the production tubing. The fluid enters the domain through the upper and lower annular surface, flows through the holes, through the grooves, and finally flows upward through the production tubing. The particles are released from two surfaces situated in the annular region.

Isometric view of the sliding sleeve valve assembly.
Figure 3 – Isometric view of the sliding sleeve valve assembly.

The particles are drained from the annular region in each hole-groove configuration, as shown in Figure 4, eventually touching the valve’s surface and remaining adhered. The particles may also agglomerate with adhered particles, creating a plug that hinders the flow. Furthermore, considering that in real-world conditions, the crystal size would span over a wide range, a particle diameter distribution is set for simulation in Rocky DEM. Results in terms of diameter distribution are presented in the details in Figure 4. Notably, the Rocky–Fluent coupling computes the drag over particles of distinct diameters.

Particle adhesion on the SSV surfaces as a function of the particle velocity and the particle diameter.
Figure 4 – Particle adhesion on the SSV surfaces as a function of the particle velocity and the particle diameter.

A top view of the SSV is displayed in Figure 5, with the details showing the particles’ agglomerates built on the valve wall surface. The agglomerates may form in the annular region or in the production tubing as well, depending on the intensity of the adhesive forces.

Adhesion of particles agglomerates on the SSV surface (view from above).
Figure 5 – Adhesion of particles agglomerates on the SSV surface (view from above).

The fluid flow response for the solids scaling on the surface of the SSV is shown in Figure 6 as a function of the dimensionless pressure P [-] over the dimensionless time τ [-]. Again, pref [Pa] stands for the pressure of the monophasic fluid flow and tref [s] stands for the simulation time. The particles plug the holes and grooves, reducing the available surface for the flow. The fluid pressure shows an uptrend tendency to compensate for the pressure drop imposed by the particles. Such behavior is also illustrated in Figure 7 by the fluid velocity field zoomed in a hole-groove configuration. The flow accelerates from τ=0, in the absence of particles, up to the end of the simulation at τ=1.

Dimensionless pressure P response over the dimensionless time τ for the scaling on a SSV valve surface.
Figure 6 – Dimensionless pressure P response over the dimensionless time τ for the scaling on a SSV valve surface.
Particle deposition on the holes and grooves (transparency is applied in the outer and inner sleeves) and fluid flow velocity field.
Figure 7 – Particle deposition on the holes and grooves (transparency is applied in the outer and inner sleeves) and fluid flow velocity field.

Finally, the two-way coupling between Rocky and Fluent allows the simulation of the liquid-solid two-phase flow in complex geometries when the adhesion is paramount. The fluid flow simulation is able to capture the particle interactions like the adhesion force, consequently showing a pressure increase. The particles are influenced by the fluid flow as well, being subjected to fluid-particle interaction forces.


Vinicius Poletto

Research engineer of the Research Center for Rheology and Non-Newtonian Fluids (CERNN) at UTFPR

Vinicius Poletto holds a M.Sc. in Mechanical Engineering and B.E. (Mechanical) degree, from the Federal University of Technology – Parana (UTFPR). He is a research engineer at the Laboratory of Porous Media (LaMP) of the Research Center for Rheology and Non-Newtonian Fluids (CERNN) at UTFPR. Mr. Poletto has been working with CFD-DEM techniques aiming the simulation of the liquid-solid flow for applications in the oil and gas industry, like flow assurance, hydraulics of drilling wells and convection in porous media.

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