Energy savings on heavy oil transportation through core annular flow pattern: An experimental approach
Graphical abstract
Introduction
The current world economic scenario has its directions strongly oriented to the availability of oil and the expectation is that this context should be maintained in the medium term (International Energy Agency, 2013). However, light oil reserves which have dominated the exploration and production scenario since the beginning of the petrochemical industry are expected to deplete in the coming decades. As a result, it will be necessary to explore heavy oil fields in an economically viable way and in sufficient volume to meet the demands of the market.
Heavy oils account for about a third of the world's hydrocarbon reserves. They are mostly located in Alberta, Canada, as well as in the Orinoco belt, Venezuela (Martinez-Palou et al., 2011). However, the production of this oil still has little impact on the world market. In terms of Brazil, heavy oils make up about 11% of current production, especially in the reserves of Rio de Janeiro, Ceara and Espirito Santo States (ANP, 2019).
The production of highly viscous oil presents major technological challenges because of the great pump energy consumption, the high pressure drops and the environmental risks, especially in its transportation. One of the possible solutions transporting this good at lower pumping costs is to reduce the effects of viscosity by adding solvents, heat or diluting heavy oil with light oils. Nevertheless, these methods have several operational limitations, they are very expensive and are only economically feasible for short distances (Bensakhria et al., 2004; Martinez-Palou et al., 2011; Strazza et al., 2011b).
Among the potential techniques to overcome the heavy and ultra-heavy oil disadvantages, there is the core annular flow (CAF). CAF is characterized by the least amount of electric power consumption to pump oil (Bannwart, 2001). With this technique, water is carefully injected to the oil stream in order to perform an annular film along the pipe wall, encapsulating the core region where the oil flows. With this configuration, since the oil barely touches the pipe wall, the wall shear is comparable to the shear of a pure water flow under similar flow conditions (Ooms et al., 1983; Ghosh et al., 2009).
Isaacs and Speed (1904) were the pioneers to discuss the use of a core annular pattern for oil transportation. However, the first scientific papers were only published in the late 1950s by researches from the University of Alberta (Charles et al., 1961; Charles and Redberger, 1962; Russell et al., 1959; Russell and Charles, 1959). Since then, several experimental works have been aimed at identifying and describing the various possible oil-water flow patterns for different oil viscosities and pipe arrangements, as well as the pressure gradient reduction concerning the CAF pattern (Al-Wahaibi et al., 2014; Bai et al., 1992; Bannwart et al., 2004; Hanafizadeh et al., 2015; Loh and Premanadhan, 2016; Sotgia et al., 2008; Yusuf et al., 2012). However, according to Table 1 there are few studies concerned to oil with viscosities as high as 2750 mPa. s.
The flow patterns are the result of the interaction between the gravitational, inertial and interfacial tension present in the flow. Thus, the spatial arrangement of the phases in the flow is dependent on the physicochemical properties of the fluids and the characteristics of the installation. In the first case, is reported the superficial velocity of the phases, the volumetric fraction of the components, the difference in density and viscosity of the fluids and the wettability characteristics of the duct walls. In the second case, they refer to the position, the material (roughness), the diameter and the geometry of the pipe, besides the presence of hydraulic fittings (Angeli and Hewitt, 1998; Bannwart et al., 2004; Brauner, 2003; Hanafizadeh et al., 2015; Loh and Premanadhan, 2016). In view of this, flow pattern maps are specific to the operating conditions employed in each study. For instance, if the density difference between the fluids is too large, the CAF pattern is not observed. Therefore, the introduction of different observations and new experimental data to the literature is fundamental for the development of mathematical models capable of accurately predicting in real time flow patterns and pressure drops of the biphasic oil-water flow as well as estimating the reduction of the pump energy consumption in an extended scenario of practical situations.
Previous studies available in the literature describe theoretically and experimentally the benefits of transporting heavy oil with water, especially in CAF patterns (Arney et al., 1993; Grassi et al., 2008; Joseph et al., 1997; Rodriguez et al., 2009; Rovinsky et al., 1997). The energy savings provided by this technique are usually presented in terms of the pressure-gradient reduction factor (fΔP), which corresponds to the single-phase oil over the oil–water two phase (same oil flow rate) pressure gradient in a long straight test section. Furthermore, some works estimate the pump power reduction factor () using this basis (Table 2). However, the power reduction factor estimated through pressure drop refers only to friction issues and is usually based on straight section measurements. Hence, it does not consider the energy loss due to the pipe fittings, which is not within the same order of magnitude as that determined on straight sections.
To the best knowledge of the authors there is no experimental data concerning the overall power reduction factor of a CAF pumping facility, considering the impact of several hydraulic fittings, as valves, elbows, long bends, couplings and directional flow changes. According to Prada (1999) and Joseph (1997) oil fouling is more severe near these line irregularities and pumping stations, therefore, it would not be expected similar yield to the straight section. Although it represents more conservative numbers, they would be more likely to the ones that can be obtained in conventional industrial plants.
This paper presents a detailed follow-up of the flow patterns evolution in three sequential sections of tests (two horizontal sections separated by an intermediate vertical section). In addition, the influence of hydraulic fittings present in the experimental unit will be discussed, with emphasis on the 90° long bend. The energy coefficients of the heavy oil flow will also be reported using traditional monophasic oil pumping and biphasic oil–water pumping, especially in the CAF pattern, allowing quantitative evaluation in the attainable energy savings provided by the CAF technique.
Section snippets
Test facility
The experiments reported in this paper were performed at the Unit Operations Laboratory of Santa Cecilia University (UNISANTA) located in Santos, Brazil, where the test facility was designed, built and installed. The oil-water flow bench (Figs. 1 and 2) was fully constructed using transparent acrylic tanks and transparent clear PVC pipes for full visual monitoring. It is composed of one separation tank, one oil accumulation tank, and two cargo tanks, one for oil and one for water. A
Flow patterns
The nomenclature of the flow patterns, as well as the transitional boundaries are not a consensus among the various published works. In addition to the particularities regarding fluids and installation, the individual and subjective perception of the researcher is added. In the present work, the nomenclatures adopted were strongly inspired by the publication of Bai et al. (1992) and the descriptions of flows, although subjective, they are in agreement with the reports published by
Conclusions
The experimental studies have shown that the presence of hydraulic fittings in the test unit severely affects the energy gains attainable in transporting oil with water, especially in the CAF pattern which has the lowest pumping cost. When monitoring the flow, it was evident the appearance of oil fouling zones near the hydraulic accessories, what reveals the need of a higher amount of water to keep CAF structure. With this, flows with very high oil cuts (above 70%) were unfeasible.
The
Declaration of Competing Interest
We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
Acknowledgment
The authors thank Santa Cecilia University and University of Sao Paulo, especially their respective Unit Operations Laboratories for supporting this work. Sincere thanks are extended to the workshop technicians for all interesting discussions and vital help during the construction and assembly of the experimental bench.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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