(28f) A Novel Core Annular Flow Inducing Device for Energy-Efficient Transportation of High Viscous Oil

Introduction

Heading from the start of 21^st century, the demand for crude oil has increased rapidly leading to a subsequent upsurge in its worldwide growth rate by 1.8%. The constantly increasing population and financial development have led to a major rise in the production of crude oil (over 80 million barrels per day), with an increasing proportion of heavy crude oil[1]. These oils exhibit high viscosity (>100cp) thereby requiring high pumping power for transportation[2]. Flow assurance during heavy crude transportation is also a challenge. To mitigate this, several cost-effective techniques have been adopted. One of them is transportation via the core annular flow (CAF) configuration. In this technique, the oil core is surrounded by an annular layer of water such that the pressure drop in the lubricated pipeline is almost close to the pressure drop required for flow of water only[3]. CAF also mandates the presence of a minimum inlet water fraction (conventionally termed as water cut) ranging between 10-30%. A higher water cut may lead to stable CAF, but does not necessarily ensure a decrease in pressure gradient. The increase in water cut additionally lowers the overall quantity of oil transported, also leading to separation issues.

Although core annular configuration has been known since long, it has not been widely applied in industries, a major challenge being to establish and stabilize CAF[3]. Past studies on oil-water flow patterns in pipelines have shown that core-annular flow is stable only for a narrow range of superficial oil and water velocities. Significant viscosity and density differences between the two fluids result in interfacial instabilities which disrupt CAF at even modest flow rates and cause stratification. This is particularly significant in horizontal pipelines, where buoyancy promotes oil core levitation[4]. A past study on the effect of bends and elbows have reported flow disruption due to centrifugal forces caused by change in the flow direction. The presence of fittings affects the oil-water interface, causing a reduction in energy saving[5]. The current state-of-the-art thus reveals that for commercial applications, CAF stability is a challenge except for density matched immiscible liquids. Re-establishment of core-annular flow downstream of fittings and maintaining the same is also an area which requires focus.

In the present study, a novel “CAF-inducing geometry” has been proposed to address these limitations. The purpose of the device is to ensure (i) establishment of stable CAF in horizontal pipelines, (ii) re-establishment of the disrupted flow after pipe fittings. The study also aims at forming a stable CAF with minimal water cut, ensuring a higher oil throughput. The effectiveness of the device is quantified from estimation of pressure drop and water cut, defined as the ratio of minimum water flowrate to the total flowrate necessary for stable CAF (Water cut = Water flowrate/ (Total oil and water flowrate)).

Experimental Setup

The experimental setup consists of a 3m long, 25mm diameter pipe (upstream section) with a gate valve positioned after 1m. The gate valve is selected as it is an inevitable fitting in pipelines. The “CAF-inducing geometry” is placed after the upstream pipe and is followed by an identical downstream pipe of 25mm diameter (Fig.1). Further, the gate valve causes flow disruption, and CAF is shown to be re-established following the proposed device.

The investigation is carried out using lubricating oil (density = 886.8 kg/m³, viscosity 0.314 Pa.s) and tap water (density = 1000 kg/m³, viscosity 0.001 Pa.s) as the test fluids. The superficial velocity of oil and water is kept within 1.02 m/s and 0.34 m/s respectively. Flow patterns at three sections, i.e., before valve (section I), between valve and “CAF inducing geometry” (section II), and after the “CAF inducing geometry” (section III) are recorded using a high-speed camera (Nikon, model Micro LC 320S). The pressure drop is measured by a differential pressure transducer (120STD type Honeywell Differential Pressure Transmitters with accuracy of ± 0.1%) and water cut is estimated from superficial velocities at the inception of CAF.

Prior to experiments, the gate valve is kept at 50% open position and the entire setup is filled with water. Subsequently, oil is introduced into the system at a given flowrate, and the water flowrate is decreased gradually to generate two-phase flow. The results are presented as flow pattern map and pressure gradient data. A few experiments are also performed with fully open gate valve and the efficacy of the device is established for both cases.

Results and Discussion

Flow pattern maps illustrate the observed flow patterns as a 2D plot with oil and water superficial velocity as the abscissa and ordinate respectively. Flow patterns in each of the three test sections is represented by three separate maps (Fig.2). Within the considered range of oil velocities (0.1 m/s - 1.0 m/s), the predominant flow patterns in Sections I and II are Stratified and CAF. However, beyond the valve (i.e., in Section II), most of the CAF transitions to stratified flow. In Section III, only CAF is observed.

Moreover, during experimentation two types of CAF are noted: thin and thick core annular flow. They are distinguished based on the thickness of the oil core (d) relative to the pipe diameter (D). The parameter d/D>0.5 indicates thick CAF, which is a preferred configuration for increased oil throughput and cost-effective oil-water separation.

A comparative analysis of flow patterns at the three sections reveals the following interesting features:

1. Gate valve disrupts CAF which is restored after the “CAF-inducing geometry”.
2. The distribution is thick CAF in Section III, while it is mostly thin CAF in section I, which implies the device effectiveness for higher oil throughput, regardless of the flow pattern upstream.
3. Thick CAF in Section III is more stable at higher oil flowrates. At low oil flowrates, interface mixing to some extent is observed.
4. The device facilitates a higher oil throughput, with a significantly lower water cut (6.25%) compared to section I & II where it is 16.7%.

Given that the primary objective of transporting viscous oil via the CAF configuration is to minimize pressure drop, the pressure gradient in both the sections II & III, are reported in terms of a pressure reduction factor (ratio of the pressure gradient for two-phase flow to pressure gradient for single-phase flow of oil only). The measurements show that at the higher velocity of 1 m/s, pressure reduction factor at Section III is 0.127. This improvement ensures an energy-efficient transportation with an energy saving of 87%.

Conclusion

In summary, we propose a device that can establish CAF at its downstream, irrespective of the upstream flow pattern during high viscous oil-water flow. The effectiveness of the proposed device is demonstrated for a partially opened (~50%) gate valve. The findings indicate that within the range of oil velocities (0.1 m/s - 1.0 m/s), the presence of gate valve exacerbates oil stratification. However, upon encountering the device, thick core annular flow forms in the downstream section. Notably, the minimum water cut for stable CAF is 16.7 % both before and after valve, while it is 6.25% downstream of CAF inducing geometry. Therefore, the device not only generates CAF at high throughput with a lower water cut, but provides an energy saving of 87% at an oil flowrate of 30 Lpm.

References

[1] D. D. Joseph, R. Bai, and K. P. Chen, “Core-Annular Flows,” pp. 65–90, 1997.

[2] R. Martínez-Palou et al., “Transportation of heavy and extra-heavy crude oil by pipeline: A review,” J. Pet. Sci. Eng., vol. 75, no. 3–4, pp. 274–282, 2011, doi: 10.1016/j.petrol.2010.11.020.

[3] A. Hart, “A review of technologies for transporting heavy crude oil and bitumen via pipelines,” J. Pet. Explor. Prod. Technol., vol. 4, no. 3, pp. 327–336, 2014, doi: 10.1007/s13202-013-0086-6.

[4] N. M. de A. Coelho et al., “Energy savings on heavy oil transportation through core annular flow pattern: An experimental approach,” Int. J. Multiph. Flow, vol. 122, p. 103127, 2020, doi: 10.1016/j.ijmultiphaseflow.2019.103127.

[5] F. Jiang, J. Chang, H. Huang, and J. Huang, “A Study of the Interface Fluctuation and Energy Saving of Oil–Water Annular Flow,” Energies, vol. 15, no. 6, 2022, doi: 10.3390/en15062123.