Applying CFD in the Analysis of Heavy-Oil Transportation in Curved Pipes Using Core-Flow Technique

S Conceição, A Lima*, T Andrade, S Neto, V Oliveira, K Angelim, L Rocha 1. Federal University of Campina Grande, Brazil. 2.State University of Paraíba, Brazil ABSTRACT Multiphase flow of oil, gas and water occurs in the petroleum industry from the reservoir to the processing units. The occurrence of heavy oils in the world is increasing significantly and points to the need for greater investment in the reservoirs exploitation and, consequently, to the development of new technologies for the production and transport of this oil. Therefore, it is interesting improve techniques to ensure an increase in energy efficiency in the transport of this oil. The core-flow technique is one of the most advantageous methods of lifting and transporting of oil. The core-flow technique does not alter the oil viscosity, but change the flow pattern and thus, reducing friction during heavy oil transportation. This flow pattern is characterized by a fine water pellicle that is formed close to the inner wall of the pipe, aging as lubricant of the oil flowing in the core of the pipe. In this sense, the objective of this paper is to study the isothermal flow of heavy oil in curved pipelines, employing the core-flow technique. A threedimensional, transient and isothermal mathematical model that considers the mixture and k-εε turbulence models to address the gas-water-heavy oil three-phase flow in the pipe was applied for analysis. Simulations with different flow patterns of the involved phases (oil-gas-water) have been done, in order to optimize the transport of heavy oils. Results of pressure and volumetric fraction distribution of the involved phases are presented and analyzed. It was verified that the oil core lubricated by a fine water layer flowing in the pipe considerably decreases pressure drop.


INTRODUCTION
The multiphase flow of oil, gas and water occurs in the oil industry from the reservoir to the processing units.The Brazilian scenario shows that as light oil reserves are scarce, the production of heavy hydrocarbons is the only possible alternative future.In this way, we are looking for technologies that will minimize possible problems due to the production of heavy oil.
Problems related to the fluid flow in pipes are easily found in engineering practice.Situations that seek information fluids flow through tubes, calculation of energy conversions, influences of variables such as viscosity, density and roughness are commonly found.
As regards to the transport of this type of hydrocarbon, it can be stated that as high viscosity ___________________________________ *Corresponding Author: antonio.gilson@ufcg.edu.br and density and a high pressure drop by friction that occurs during the flow are common situations.These conditions require an excessive workload of the equipment and hence a high energy demand, which becomes economically unviable the production of these oils.
Due to its unfavorable characteristics, transportation from the production areas to the processing and refining plants is the greatest difficulty encountered in the production of heavy oils.The most commonly used alternatives are trucking or heated piping.However, these methods are costly and apply only for short distances [1].Among the main techniques applicable to deepwater production are temperature control, flow control and dilution with light oil [2].
All possible ways of solving the problem of heavy oil transportation is related to the same principle, which is to control its physicochemical properties.Some other methods are also used, both for offshore and onshore fields: heating the oil with the injection of a heated fluid, or through electric heating; Generation of oil emulsions in water; Injection of water in pipe to form an "insulating" film (ring) that surrounds the oil (core-flow).
The core-flow technique consists of injecting water into the sides of the pipeline, at lower flow rates than the oil, in order to obtain an annular flow pattern, where the water flows as a continuous phase forming a film that surrounds the oil, avoiding contact of the oil with the duct walls consequently reducing the pressure drop by friction.Them, developing alternatives to ensure not only the efficiency of transport but also energy efficiency is currently one of the great challenges of the oil industry.
In view of the above this study aims to numerically study the three-phase flow, isothermal, and three-dimensional and transient flow in curved pipes via CFD.Highlight are related to multiphase modeling; evaluate the effect of the volumetric fraction on the hydrodynamic of the flow; to find an operating range in which the reduction of the pressure drop associated to a reduction of energy consumption is guaranteed; and finally evaluate the influence of temperature on water pumping.

HEAVY-OIL TRANSPORT TECHNIQUES
The main objective of the core-flow technique, according to Trevisan [1], is to obtain a considerable reduction in the pressure drop along the flow, since the viscosity of the heavy oil is the main disadvantage.In this way its influence is minimized, since the water acts as a lubricant.Figure 1 illustrates a duct in which it is possible to visualize the use of core-flow technique.
The core-flow study began decades ago where it was first cited by Russel et al. [3] referenced by Obregón [4].This method was devised by Isaacs and Speed in 1904, referred to U.S. Patent No. 759374, mentioning the ability to transport viscous products by means of water lubrication.It was subsequently tested in an experimental manner generating results that already pointed to a reduction of pressure loss in the flow line.
Several authors have studied heavy-oil transportation using the core-flow technique.From these works, in some studies have been carried out on the behavior of the annular flow pattern for water-oil two-phase flow [5,6].The presence of the gas during an oil production is constant.The gas, which was initially in solution without oil, is released forming different media during the flow.These patterns were identified experimentally as reported by Trevisan [1], Wegmann et al. [7] and Poesio et al. [8].However, the influence of the gas phase in the core-flow on pipelines and connections is still not well defined, and further studies are needed to ensure the efficiency of the technique in this physical situation.The tests of the related phenomena that emerged in connections were evaluated by Wang et al. [9], Andrade et al. [10], Andrade et al. [11], Andrade et al. [12], Andrade et al. [13] and Crivelaro et al. [14].Particularly with reference to reduction of pressure drop in flow, Brauner et al. [15] has reported a correlation applied to heavy-oil/water two-phase flow in an annular flow pattern.Vanegas [16] verified a reduction in pressure drop up to 93 times as compared to the singlephase oil flow, thus, a correlation was developed to predict the pressure drop in the flows.Similar results was reported by Bannwart [17].

The geometry and numerical mesh
The geometric domain was determined by the definition of points, curves, surfaces and solids describing the desired size and shape.In this way the duct under study was created computationally with a radius 15 cm, containing 3 m length before the curvature along the z axis, and 3 m after the curvature along the axis and.The curvature has a radius 20 cm with an angle 90 °. Figure 2 shows an overview of the geometry used.
From the geometry was possible to develop the structured computational mesh composed by 289552 hexahedral elements in the ANSYS ICEM CFD® module.Due to the complexity of the problem some regions, such as curvature, for example, needed further refinement, in order to obtain results that are closer and consistent with physical reality.Figure 3 illustrates the inlet and outlet regions of the mesh used.

Multiphase mathematical modeling 3.2.1 Governing equations
The mathematical conditions assumed refer to an Eulerian-Eulerian study with an inhomogeneous approach.These considerations make possible a study in which each phase has its flow field; thus, we have a set of solutions for each phase separately.The phases present in the flow are represented by the letters α, β, and γ.

Water Oil
• Momentum Conservation Equation: where is assumed that for the α phase, f is the volume fraction, ρ is the density, U � �⃗ . is the velocity vector, μ is the dynamic viscosity, p is the pressure, the term   �������⃗ describes the linear moment due to external body strength (gravitational force), while  ��⃗  describes the interfacial forces (drag force, lift force, virtual mass force, wall lubrication force and turbulent dispersion force at the interface), these forces act on the phase α due to the presence of other phases.
• Energy Conservation Equation: where ρ α , h α , λ  and T  represent the density, static enthalpy, thermal conductivity and temperature of phase α, respectively; S α is the external heat source; Q α represents the heat transfer to the α phase through the interfaces with other phases, and the term (Γ  + h  − Γ  + h  ) represents heat transfer induced for mass transfer.The heat transfer at the interface occurs due to thermal non-equilibrium through the interface of the phases.The total heat per unit volume transferred to the phase α due to the interaction with other phases and denoted Q α is given by: or yet The heat transfer rate   per unit time, through the interfacial area density, per unit volume   from the phase α to the phase β is given by: Equation ( 6) can also be written as follows: The heat transfer coefficient per unit volume,  ∝ (ℎ) , in equation ( 7) can be obtained by: The Nusselt Number is a quantity widely used to determine the heat transfer coefficient by convection ℎ  .This inference is based on dimensional analysis, which is used to determine parameters through similarity.Thus, to find the value of ℎ  , we use: where the Reynolds and Prandtl numbers are given as follows: where   is the heat capacity and   the thermal conductivity of the continuous phase.Them, the convective heat transfer coefficient is determinated as follows: The surface area per unit volume is calculated assuming that the phases are present as particles which may take various forms such as, for example, spherical defined by: where   is the volumetric fraction,   is the mean diameter of the particle.

Constitutive equations
For the core-flow, the water flows in a turbulent regime, thus, the K-ɛ model was chosen to calculate such effects.
where G a a is the turbulent energy generation within the α phase, C 1 and C 2 are empirical constants.Also in this equation, ε α corresponds to the turbulent energy dissipation rate of the phase α and K α the turbulent kinetic energy for the phase respectively, defined as follows: where l α is the spatial scale length, q α is the velocity scale and C μ is an empirical constant calculated by: where C α is also an empirical constant.The term μ α corresponds to the turbulent viscosity, defined by: where the constants used in the previous equations are: C 1 = 1.44,C 2 = 1.99,C μ = 0.09, σ ε = 1.3 and σ k = 1.0.

Pumping power
Some analyzes were performed based on the power required to pump the fluids involved in the flow, in order to understand aspects related to the energy efficiency of the system.To calculate the pumping power of the system it is necessary to use a system of equations in which the power due to pressure variation, as well as the volumetric flow rate of oil and gas phases as a whole are taken into account.Thus, the pumping power of the system   is given by: In a similar way, the contribution of the pumping power of both water   and oil and gas phases   , are given by: In relation to the water inlet, was considered the following values: In the outlet of the pipe was considered a pressure equal to 101.325 kPa.In the wall of the pipe we consider no-slip condition.At the begin of the process we consider that the pipe was full fill of water flowing with velocity 0.5 m/s.Table 1 shows an overview of the flow conditions for all simulated cases, where velocity (u) is given in m / s², temperature (T) in ° C, roughness (Ru) in m and volumetric fraction is dimensionless.

Thermal and physical properties of the fluids
Variables such as viscosity may vary greatly as the temperature changes.For example, in liquids as the temperature increases, the viscosity of the liquid is expected to decrease, whereas for the gases the reverse is the case.Viscosity of the oil is usually defined by means of correlations.Several viscosity correlations have been proposed, but none has been used as a standard method in the petroleum industry, since the composition of crude oil is complex and often undefined.Therefore, many methods of viscosity estimation are dependent on factors such as the API degree for light, medium, heavy, ultra-heavy oils and bitumen, and they are also linked to the region where they are produced.
In this study we used the correlation reported by Sattarin et al. [18] for oil and gas, as follows: where the value of α is given by where in the API value is between 10 and 52.5.
The data presented in Tables 2 and 3 report all the physical characteristics assumed for the fluids in all cases studied.These properties are extremely important in understanding the physical problem, and help in interpreting the solutions.
The gas used is chemically compounded by: CH 4 = 0.92; C 2 H 6 = 0.055; C 3 H 8 = 0.015; C 4 H 10 = 0.01.From these values, density and viscosity of the gas phase were calculated by the Ansys CFX software.In all three-phase cases water and oil were considered as continuous phases and the gas as dispersed phase having a bubble diameter equal to 3 mm.4 and 5 shows the pressure profile of the three-phase flow, it can be noted that a downer region of the curvature has a greater pressure than in the upper region of the curvature.This physical situation occurs because the fluids tend to continue in the same behavior, flowing in the horizontal direction causing, which provokes an increase in pressure and the oil core is pushed to a downer part of the curvature.

Volumetric fraction field
Figure 6 and 7 illustrate the volumetric fraction fields at the instant 15s of process.
From the analysis of the Figures 6 and 7, it is possible to visualize the water film formed near the duct wall at the beginning of the flow.However, this annular pattern does not remain stable until the end of the flow, and even almost collapses in the upper region of the curvature.In this way it is possible to affirm that in these conditions the core-flow becomes a non adequate method to be used.This is because in this region the flow is more complex due to the action of inertial, gravitational, centrifugal, and drag forces, among others.Therefore, annular flow can become a major problem, because as it is undone, the pressures become very high, causing problems in the piping.
In addition to the curved geometry of the duct, another factor that may contribute to the deconfiguration of the concentric annular profile is the presence of the gas phase, because it is less dense than other phases and tends to concentrate in the upper part of the duct, as can be seen in Figure 8.   Figure 9 shows that the water film does not remain concentric in the horizontal section near the curvature and also in the vertical straight section after the curvature.According to Bensakhria et al. [5], the radial position of the oil core in the annular flow depends solely on the ratio of the perimeter of contact between the tube wall and the fluid forming the core and the perimeter of the tube.This ratio in turn depends on the density difference between the fluids being transported and the water film as well as the amount of water injected.
Therefore, for operating with safety, a solution to this problem will be to operate with higher water flow rate, since altering the rheological properties of hydrocarbons would be a more complex task.
According to Andrade [19], an analysis of the stability of the perfect annular flow shows that this configuration is only stable for a very small set of conditions which typically do not occur in real applications but which is of interest for pipeline lubrication.

Fluid transport efficiency analysis
In addition to the efficiency of the transportation guaranteed by the core-flow technique, it was also sought to achieve energy efficiency during the flow, in order to optimized ways for oil transportation.As already seen, the pumping of water, oil and gas were done at different volumetric flow rates and, therefore, with different energy consumptions.
Two sets of cases were simulated where the pumping power required to flow with different water flow rates were analyzed, setting two different oil volumetric flow rates.With these same results, both pressure drop associated to core-flow utilization and the pumping power required for such flow, were analyzed.
Table 4 presents data for the first set of simulated cases.As can be seen, the flow velocity of the oil and gas has remained constant and thus the volumetric flow rate as well.The water velocity variation occurred every 4 m / s and the pipe was considered smooth.
As both the velocity and the water flow increase, the pressure drop associated with the flow is smaller.In this way, increasing the water injection rate leads to a significant reduction in the pressure drop.The graph illustrates in Figure 10 shows this decay more clearly.
With these data, through Equations 21 to 23, it was possible to calculate the pumping power of the water, the oil and thus the power of the system.For each case studied was associated reduction of the pressure drop with the energy efficiency and finally we found the interval in which the system can operate with minimum energy necessary to guarantee the fluid flow.The graph in Figure 10 shows the range of operation in which water, oil and gas can be transported efficiently and economically; this range is between water injection velocities ranging from 0.6 m/s to 1.8 m/s.Table 5 presents the values referring to the pumping power required for each case.By examining the pumping power of the system, it can be seen that for given water volumetric flow rate, the gains in pressure drop become constant, but the power required for fluid transportation begins to rise.
Still on the pumping power, it is observed that for the values corresponding to the oil pump there is a visible decay, clearly due to the reduction of the pressure drop.In this group of simulations, it can be stated that the ideal water injection rate should be less than 46% of the oil and gas flow rate; the water flow rate cannot exceed 0.007912458 m³/s.Thus, the best conditions for operating are around the range highlighted in Table 5.
The second group of simulations used the same physical assumptions of the previous group, changing only the oil velocity to 1.5 m/s and using a duct now with a roughness of 4.5 x 10-5 m.Higher pressure drop and consequently higher pumping power were evidently expected, both due to the higher oil flow and the influence of the roughness.In order to understand the behavior of the fluids under the effect of a higher temperature, four cases were run in which we tried to analyze what will be the best operating range for these configurations.Table 6 shows the values adopted for the flow.As the temperature increases, it is observed that the oil viscosity decreases, a fact which consequently facilitates the transport of heavy oil.Thus, when analyzing Figure 11, it can be seen that for low Uw/Uo ratios the pressure differential is around 31000 Pa, a lower value as compared with the cases where lower temperatures is used.The increase in temperature also influences the operating range, as the pressure drop becomes constant more rapidly, so the operating range is reached with lower water flow rate, as can be seen in Table 7.

CONCLUSIONS
Considering the simulations of oil and water single-phase flows, and heavy oil, water and gas three-phase flows, in curved ducts we can concluded that: a) There is a significant reduction in the pressure drop along the flow for the three-phase cases proving the efficiency of the transport of heavy oils using the core-flow technique; b) The maintenance of the water film along the flow becomes sensitive in regions where the geometry becomes complex allowing adverse situations such as variations of the velocity field and recirculation zones as in pipe with curvature.c) The ideal water volumetric flow rate must not be too much low or very high, because in the first case it does not form the annular profile, making the desired reduction of the pressure drop impossible.In the second, excessive volumetric flow rates can generate unnecessary energy consumption and even affect the stability of the annular profile.d) The presence of the gas directly influences the formation of the water film on the inner wall of the duct.Because it is a less dense fluid, it tends to concentrate in the upper parts of the duct, causing a greater misconfiguration of the water film in the upper region.e) There is an optimized operating range in terms of pumping power.This is directly related to the maximum capacity required for water injection.f) The increase in temperature reduces pressure drop and consequently provokes changes in the operating range.Thus, it can be reached with lower water volumetric flow rate.

Figure 1 .
Figure 1.Schema of the two-phase (oil and water) flow via core-flow

Figure 2 .
Figure 2. Geometry of the duct with 6 m length and 15 cm diameter

Figure 3 .
Figure 3. Numerical mesh with detail on the input and output region

22) 3 . 2 . 4 .
Initial and boundary conditionsWith the numerical mesh finished, a file was created to incorporate it into ANSYS CFX ® 15.1 Pre.It is possible to include the mathematical model adopted, fluid properties, type of flow regime, among others information.With the objective of analyzing the ideal volumetric flow rates for the flow in question was simulated three-phase cases in which the pertinent considerations for the oil inlet were:

Figure 4 .
Figure 4. Pressure profile in the longitudinal plane inside the pipe at t=15 s (Case 5).

Figure 8 .
Figure 8. Volumetric fraction of gas in the XY plane (Z = 0) at t = 15s

Figure 9 .
Figure 9. Volumetric fraction of the water at different transverse planes at t = 15s

Figure 10 .
Figure 10.Pressure drop as a function of the water velocity

Table 2 .
Thermo-physical properties of the fluids present

Table 3 .
Surface tensions between the involved phases

Table 4 .
Pressure drop for different inlet velocity

Table 5 .
Pumping power for different cases at t=15s

Table 6 .
Pressure drop for different inlet velocity of water (t=15 s)

Table 7 .
Pumping power for different cases at t=15s (T= 45 o C)