energies Article Viscosity Loss and Hydraulic Pressure Drop on Multilayer Separate Polymer Injection in Concentric Dual-Tubing Yi Zhang 1, Jiexiang Wang 1,*, Peng Jia 2, Xiao Liu 1, Xuxu Zhang 1, Chang Liu 1 and Xiangwei Bai 1 1 School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China; [email protected] (Y.Z.); [email protected] (X.L.); [email protected] (X.Z.); [email protected] (C.L.); [email protected] (X.B.) 2 College of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao 266580, China; [email protected] * Correspondence: [email protected] Received: 13 February 2020; Accepted: 23 March 2020; Published: 2 April 2020 Abstract: Multilayer separate polymer injection in concentric dual-tubing is a special method for enhancing oil recovery in later development stage of the multilayer formation. During the injection process, heat exchange occurs among the inner tubing, tubing annulus and formation, making the thermal transfer process more complicated than traditional one. This work focuses on the polymer flowing characteristics during the multilayer separate polymer flooding injection process in the wellbore. A temperature–viscosity numerical model is derived to investigate the influencing factors on polymer dual-tubing injection process. Then, an estimate-correct method is introduced to derive the numerical solutions. Several influences have been discussed, including the axial temperature distribution, viscosity distribution, pressure drop, and flow pattern of polymer. Results show that under low injecting rates, below 5 m3/d, formation temperature will greatly decrease the polymer viscosity. When the injecting rates above 20 m3/d, the polymer just decreases 1–3 mPa s at the bottom · of well, which is really small. Additionally, the temperature distribution, the coefficient of friction under different injecting rates have been discussed. Generally, this method provides a new way to analyze thermal conductivity during the polymer injection process which is meaningful for polymer flooding in the oilfield application. Keywords: polymer flooding; multilayer dual-tubing injection; thermal-viscosity coupling; heat transfer 1. Introduction In the past decades, polymer flooding has gradually become an effective method to enhance oil recovery (EOR) [1–3]. In Daqing oil field, which is the largest oil field in China, polymer flooding has contributed to more than 10 % of enhanced oil recovery [4,5], and over ten million tons of crude oil has been produced every year by polymer flooding [6,7]. Nowadays, alkali-surfactant-polymer (ASP) combinational flooding system has become a promising direction of EOR technology [8–10]. It was proved that the ASP flooding has improved oil recovery by 17.2% and recovery increment of SP flooding can reach 14.3% [11]. Despite the existing challenges, many researchers have studied the advances and technological method for polymer flooding in heavy oil reservoir [12–14] and the field application shows that the polymer flooding of heavy oils could result in incremental recoveries ranging from 7.5 to 25% OOIP [15]. Moreover, due to high demand for energy, polymer flooding has been successfully applied in offshore oil production in China [16–19]. As the development of the oil field, the main objectives of polymer flooding turns into type II formation, which has strong vertical heterogeneity between layers, and the physical properties between the high-permeability Energies 2020, 13, 1637; doi:10.3390/en13071637 www.mdpi.com/journal/energies Energies 2020, 13, 1637 2 of 20 Energies 2020, 13, x FOR PEER REVIEW 2 of 21 Energies 2020, 13, x FOR PEER REVIEW 2 of 21 heterogeneityandheterogeneity low-permeability between between layerlayers, layers, di ffand erand greatly the the physical physical [20–22 pr]. properties Currently,operties between between under thethe the lowhigh-permeability high-permeability oil price, more and recoveryand low- low- permeabilityepermeabilityfficiency of polymerlayer layer differ differ flooding greatly greatly is[20–22]. 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Research Research on on dual-tubing dual-tubing injection injection of of multilayer multilayer separate separate polymer polymer flooding flooding is is meaningfulmeaningful to to field field application. application. Figure 1. Schematic diagram of allocator. FigureFigure 1. 1.Schematic Schematic diagram diagram of of allocator. allocator. Figure 2. Schematic diagram of dual-tubing injection. FigureFigure 2. 2.Schematic Schematic diagram diagram of of dual-tubing dual-tubing injection. injection. InIn polymer polymer flooding, flooding,flooding, polymers polymers are areare injected injected from from injecting injecting wells, wells, and andand then thenthen the the residual residual oil oil is is displaceddisplaced in inin the the the reservoir. reservoir. reservoir. As As such, As such, such, the the method method the method is is more more is complicated morecomplicated complicated than than conventional conventional than conventional water water injection injection water [24–26].injection[24–26]. Two Two [24– EOR26 EOR]. mechanisms Two mechanisms EOR mechanisms are are available available are for for available poly polymermer for flooding. flooding. polymer The The flooding. first first on onee is The is micro micro first displacement onedisplacement is micro efficiencydisplacementefficiency enhancement, enhancement, efficiency enhancement, and and the the other other and is is flooding the flooding other isvo vo floodinglumelume increment increment volumeincrement [27,28]. [27,28]. The The [27 effectivity, 28effectivity]. The e ffof ectivityof these these twotwo mechanisms mechanisms is is mainly mainly influenced influenced by by polymer polymer viscosity. viscosity. Viscosity Viscosity is is the the key key factor factor in in the the oil oil recovery.recovery. The The higher higher the the viscosity, viscosity, the the better better the the displacement displacement effect. effect. Studies Studies have have shown shown that that the the Energies 2020, 13, 1637 3 of 20 of these two mechanisms is mainly influenced by polymer viscosity. Viscosity is the key factor in the oil recovery. The higher the viscosity, the better the displacement effect. Studies have shown that the polymer solution is non-Newtonian power-law fluid, and its apparent viscosity is greatly influenced by shear stress and temperature [29–31]. The polymer is heated up by formation as the formation temperature increases with depth. Thus, the polymer viscosity decreases, and related friction and pressure drop will also change. The key to understanding the polymer viscosity change, hydraulic friction, and pressure loss during the dual-tubing injection process is to derive the variation of the polymer’s temperature in the wellbore.