Restart time correlation for core annular flow in pipeline lubrication of high-viscous oil

J Petrol Explor Prod Technol DOI 10.1007/s13202-016-0241-y ORIGINAL PAPER - PRODUCTION ENGINEERING Restart time correlation for core annular flow in pipeline lubrication of high-viscous oil Aniefiok Livinus1 • Hoi Yeung1 • Liyun Lao1 Received: 10 September 2015 / Accepted: 13 March 2016 Ó The Author(s) 2016. This article is published with open access at Springerlink.com Abstract One of the fundamental questions that must be addressed in the effective design and operation of pipeline lubrication of heavy oil is; ‘‘how much time will be needed to restart a blocked core annular flow (CAF) line after shutdown due to fouling or pump failures’’, if the pipe is to be cleaned using water only. In this work, laboratory results of shutdown and restart experiments of high-viscous oil conducted in a 5.5-m-long PVC horizontal pipe with internal diameter of 26 mm are first presented. A new correlation for the prediction of the restart time of a shutdown core annular flow line is then formulated. The predictive capabilities of the correlation are checked against measured restart time and pressure drop evolution data. Somewhat high but still reasonable predictions are obtained. The restart time correlation, together with the associated correlations formulated as well, can be of practical importance during the engineering design of highviscous oil pipeline transportation facility for predicting restart process. qw qo lw lo Usw Uso ho DPi DPðtÞ DPw L Dt t t VD V Keywords CAF  Heavy oil  Restart  Water assist flow  Pipeline transport Abbreviation Cw Water input fraction & Aniefiok Livinus ; Hoi Yeung Liyun Lao 1 Flow Process Assurance, School of Energy, Environment and Agrifood, Cranfield University, Cranfield, UK fw Re sw so si Si So Sw Ao Aw CAF WAF PVC Water density, kg/m3 Oil density, kg/m3 Water viscosity, kg m-1 s-1 Oil viscosity, kg m-1 s-1 Superficial velocity of water Superficial velocity of oil Oil holdup Initial pressure drop at inception of restart process, Pa Pressure drop evolution at time, t, during restart process, Pa Pressure drop of a single-phase water flow, Pa Length of blocked pipeline, m Restart time, s Time during the restart process, s Time when pressure drop evolution trend deviates from part a to part b, s Volume of fluid in blocked pipeline to be displaced, m3 Volume of water used during the restart process, m3 Frictional factor for single-phase water flow Reynolds number Shear stress of water, Pa Shear stress of oil, Pa Interfacial shear stress, Pa Wall-wetted perimeter of interface of oil and water, m Wall-wetted perimeter of water, m Wall-wetted perimeter of oil, m Area of oil phase in pipe, m2 Area of water phase in pipe, m2 Core annular flow Water assist flow Polyvinyl chloride 123 J Petrol Explor Prod Technol Z Y a-part of slope of transient pressure drop of the restart process, constant b-part of slope of transient pressure drop of the restart process, constant Introduction The world’s oil resources are majorly heavy and extra heavy viscous hydrocarbons; they make about 70 % of the world’s total oil resources of 9–13 trillion barrels. Considering the enormous world energy demand and the continuous decline of conventional oils, heavy oil promises to play a greater role in the future of the oil industry. Many countries are moving now to increase production, test new technologies and invest in pipeline facilities to ensure that these resources are being produced, transported and processed. Due to their very high viscosity, heavy crude oils cannot be transported with conventional pipelines and thus require additional treatments. Research articles by Saniere et al. 2004, Ghosh et al. 2008, Adewusi and Ogunsola 1993, Ngan et al. 2007 show that various methods, as presented in Fig. 1, of reducing the pressure drop have been studied; these include thermal method, addition of diluent, chemical and water assist. Of these, water assist flow (WAF), or core annular flow (CAF) as it is commonly called, seems to be the most environmentally friendly approach. The operation of oil production or transportation line in the core flow mode consists in injecting small amounts of water to create a lubrication layer around the viscous oil and avoid oil–wall contact. The resulting annular flow pattern reduces drastically the friction pressure gradient, allowing the oil to be pumped up to the surface at a flow rate similar to single-phase water flow (Vanegas and Jose 1999). However, there are some fundamental questions that must be addressed in the effective design and operation of pipeline lubrication of heavy oil as pointed out by Oliemans and Ooms (1986) and Strazza and Poesio (2012): ‘‘What is the velocity range for the co-injection of the oil and water into the pipeline?’’, ‘‘What pressure gradient has to be applied in order to restart the core annular?’’ and ‘‘What is the restart time needed after shut down?’’ Since the first successful recorded large-scale industrial pipeline lubrication of heavy oil, several studies have been dedicated to the CAF science and technology. A good number of experimental and theoretical studies have been carried out, models formulated to describe the flow pattern, and the specific range of velocity where this interesting flow regime is stable (Joseph et al. 1997). The pressure drop reduction for this flow regime has also been pointed out (Joseph et al. 1997; Brauner 2004; Bensakhria et al. 2004; Peysson et al. 2005). Stability of CAF has also been studied showing the interface between the annulus and the core (see Joseph and Rennardy 1993). The long-term stability of this technique requires minimization of fouling (i.e. oil adhesion) of the pipe wall, which causes a reduction in the useful diameter of the pipe (Ramos and Antonio 2001). With the vast studies on CAF, only very little attention has been paid to answering the questions pertaining to shutdown and restart of a CAF line. For transportation of large volumes of heavy crude oil, pipeline is known to be the most economic and feasible means. For efficient operation of a CAF line, it is expedient to maintain an uninterrupted steady and continuous flow. However, due to shutdown which may occur as a result of operational or emergency reasons, the CAF regime cannot be maintained. The water settles down on the bottom of the pipe while the oil floats to the upper part stratifying the flow pattern due to difference in density. Even after the initiation of a CAF, there are mechanisms that may be responsible for the transition back to a stratified flow, e.g. the fouling of the pipe wall; as this continues to prevail, a layer of oil grows at the wall reducing the internal diameter of the pipe and eventually blocks the line. A high pressure drop will occur. This has been experienced in the San Tomé 1-km test loop (see Fig. 2 as reported by Arney et al. 1996). The pressure drop increases from 25 to 175 psi. Fig. 1 Heavy oil transportation method Heavy crude transportaon m (...truncated)