Study of water wetting and water layer thickness in oil-water flow in horizontal pipes with different wettability
Introduction
Prediction of the phase wetting regime of internal pipe walls can be of paramount importance in industrial processes involving the flow of two immiscible liquids. For example, flow of liquid hydrocarbons and water is common in pipelines associated with oil production and transportation facilities. Contact between water and internal pipe walls can lead to serious corrosion problems when carbon steel is used (Kermani and Morshed, 2003, Pots et al., 2006, Smith and Joosten, 2006) as well as induce other problems, such as environmentally assisted cracking. This scenario is called water wetting (Cai et al., 2012, Pots et al., 2006). It is considered that under typical production conditions the hydrocarbon oil phase is not corrosive (Cai et al., 2012, Lotz et al., 1991). Since produced oils are generally less dense than produced water, the water tends to segregate and occupy the pipe bottom. However, if water is fully dispersed in oil (e.g., oil as continuous phase), water wetting can be avoided and corrosion occurrence becomes insignificant (Lotz et al., 1991). Full dispersion or entrainment of water into oil is only possible if the turbulent velocity fluctuations in the oil flow are sufficient to disrupt the water phase into droplets, keeping them suspended against gravity and preventing their accumulation and coalescence.
Appropriate knowledge of phase wetting can significantly reduce corrosion control costs as mitigation efforts can directly aim at the most critical pipeline areas where water wetting is likely to occur, as well as increase confidence in decisions taken to manage and ensure pipeline integrity. In this regard, several experimental studies have been performed to determine phase wetting regimes in oil-water pipe flow (Ayello et al., 2008, Cai et al., 2012, Kee et al., 2016, Paolinelli et al., 2017, Pots et al., 2006, Tang, 2011, Valle, 2000). Moreover, various efforts have been made on the quantification and modeling of water wetting phenomena (Cai et al., 2012, Pots et al., 2006, Pouraria et al., 2016, Tsahalis, 1977, Wicks and Fraser, 1975). Based on the information available in the literature, water wetting prediction has been suggested to be carried out using traditional models to predict the onset of liquid-liquid dispersed flow (NACE, 2008). These models, e.g., (Brauner, 2001, Torres et al., 2015, Trallero, 1995), assume that the flow is already dispersed and assess the balance between buoyancy forces and turbulent flow forces on dispersed phase droplets as criteria to determine if droplets will migrate towards the pipe bottom forming a separated fluid stream. Brauner (2001) also included an extra criterion as suggested by Barnea (1987), determining when dispersed phase droplets become excessively deformed and cannot be effectively dispersed. The aforementioned criteria only depend on fluids properties such as density, viscosity and interfacial tension; and the continuous phase turbulence intensity given by the flow rates of both fluids and the pipe geometry (e.g., diameter, inclination and internal roughness). However, the effect of the wettability of the pipe surface is not considered.
Surface wettability has been suggested to play a role on flow patterns in oil-water pipe flow. Charles et al. (1961) studied two-phase flows with water and oils of similar density and different viscosity in a transparent plastic pipe of 0.026 m internal diameter (ID). They found that the oil with higher wetting affinity with the pipe wall was more likely to develop flow patterns where the oil formed the continuous phase. Hasson et al. (1970) studied flows of water and oil with almost similar density in glass pipes with different wetting properties (hydrophilic and hydrophobic) and 0.012 m ID. They observed that the pipe wetting properties had a strong influence on the stability of annular flow patterns, favoring annular films of water and hydrocarbon on the hydrophilic and hydrophobic pipes, respectively. Nädler and Mewes (1997) studied oil-water flows in a horizontal 0.059 m ID acrylic pipe. They mentioned, based on the findings of other researchers (Efthimiadu and Moore, 1994, Joseph et al., 1984), that the formation and type of emulsions produced from two immiscible liquids is influenced by the wetting properties of the experimental equipment. Therefore, it was suggested that the use of polymeric pipe in their experiments could favor wetting by the oil phase. Angeli and Hewitt studied pressure gradients (Angeli and Hewitt, 1999) and flow patterns (Angeli and Hewitt, 2000b) in horizontal oil-water flows with stainless steel and acrylic pipes of 0.024 m ID. The wetting of the steel was characterized by water-in-oil and oil-in-water contact angles, and found to be either hydrophobic or hydrophilic depending on the conditioning of the surface, i.e., if it was previously oil or water wetted, respectively. Conversely, the acrylic pipe was preferentially wetted by oil in all cases (Angeli and Hewitt, 1999). Under this circumstance, the authors found that, in the acrylic pipe, oil tended to remain as the continuous phase over a wider range of flow conditions than in the steel pipe. They also pointed out that since acrylic pipes are widely used in experimental studies of liquid-liquid flows, care should be taken in applying the results of such experiments to practical cases where steel pipes are mostly used. Tang (2011) studied phase wetting in oil-water flows of different crude oils and a model oil in a 0.1 m ID flow loop with a carbon steel pipe section. He found that full entrainment of water in oil occurred at lower oil velocities for crude oil than for model oil. Moreover, he indicated that this behavior could not be fully explained by differences in the physicochemical properties of the oils such as density, viscosity and interfacial tension, and suggested the alteration of the wettability of the carbon steel pipe by contact with crude oil (e.g., from hydrophilic to hydrophobic) as an important factor. In this regard, it has been reported that compounds naturally present in crude oil, for example, aromatic hydrocarbons, nitrogen and sulfur containing compounds, and organic acids can adsorb onto carbon steel leading to hydrophobic surfaces (Aspenes et al., 2010, Ayello et al., 2013). Despite the fact that pipe surface wettability has been found to alter oil-water flow pattern transition boundaries, as far as the authors know, there are no systematic studies in the literature on this matter for carbon steel pipes, which are the preferable choice for economic reasons in the oil and gas industry.
The objective of this work is to study phase wetting, water layer thickness, and dispersed flow regime boundaries, in large scale oil-dominated oil-water horizontal flow in carbon steel and PVC pipes of similar internal diameter (0.1 m) and roughness, but different wettability. Flow tests using oil pre-wetted and water pre-wetted carbon steel pipe surfaces were performed to investigate the effect of surface wetting hysteresis (hydrophobic to hydrophilic) on phase wetting. Flow tests with PVC pipe were also performed to characterize phase wetting on a stable hydrophobic surface. The same oil and water fluids were used in all the experiments in order keep densities, viscosities and interfacial oil-water tension constant, while testing pipes with different wettability. Supplementary analyses of dispersed water droplet size and distribution, as well as water concentration at the pipe bottom, were also performed to better understand phase wetting results. It is demonstrated that pipe wettability plays a very important role on the phase wetting boundary (oil wet to water wet) in horizontal oil-water flow. The onset of water segregation is found to be different in hydrophilic and hydrophobic pipes. In addition, available hydrodynamic models are compared with experimental data and recommendations are provided.
Section snippets
Wettability measurements
Wettability tests of the selected pipe materials were carried out using a goniometer consisting of two main parts; a test cell vessel and an image capture system. The vessel is made of stainless steel and has two aligned circular openings of 0.05 m diameter on its sides with flat glass windows for visual examination of the internal fluids without distorting droplet images, and a holder to place test specimens. The image capture system is composed of a monochrome digital camera with specialized
Wettability of the employed pipe materials
Fig. 6 shows water-in-oil contact angles as a function of time measured on carbon steel surfaces pre-wetted by oil and water, and PVC surfaces. Oil pre-wetted carbon steel shows hydrophobicity (contact angle ∼ 145°) at the first second after contact with a water droplet. Then, contact angle gradually decreases becoming hydrophilic after about 1 min, reaching a value of about 75° after 10 min (Fig. 7a). On the other hand, water droplets spread very rapidly and collapse on water pre-wetted carbon
Conclusions
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The phase wetting regime (oil wet or water wet) and the onset of fully dispersed water-in-oil horizontal flow can be greatly affected by the water wettability of internal pipe surface. For example, the onset of fully dispersed flow occurs at significantly larger mixture velocities in a hydrophilic pipe (e.g., water pre-wetted carbon steel) than in a hydrophobic pipe (e.g., PVC).
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Carbon steel shows metastable hydrophobicity or extreme hydrophilicity according to the fluid that first contacts the
Acknowledgments
The authors want to acknowledge BP, ConocoPhillips, Enbridge, ExxonMobil, Petronas, Total and Shell for their financial support. Helpful discussion of Dr. Bert Pots and contribution of Ms. Taylor Gardner as well as assistance from laboratory engineers and technicians at the Institute for Corrosion and Multiphase Technology are also greatly appreciated.
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