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Multicomponent Shale Oil Flow in Real Kerogen Structures Via Molecular Dynamic Simulation

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Multicomponent Shale Oil Flow in Real Kerogen Structures Via Molecular Dynamic Simulation energies Article Multicomponent Shale Oil Flow in Real Kerogen Structures via Molecular Dynamic Simulation Jie Liu 1,2 , Yi Zhao 3, Yongfei Yang 1,2,* , Qingyan Mei 3, Shan Yang 3 and Chenchen Wang 4 1 Key Laboratory of Unconventional Oil and Gas Development, China University of Petroleum (East China), Ministry of Education, Qingdao 266580, China; [email protected] 2 Research Centre of Multiphase Flow in Porous Media, School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China 3 Exploration and Development Research Institute of PetroChina Southwest Oil and Gas Field Company, Chengdu 610041, China; [email protected] (Y.Z.); [email protected] (Q.M.); [email protected] (S.Y.) 4 Hubei Cooperative Innovation Centre of Unconventional Oil and Gas, Yangtze University, Wuhan 430100, China; [email protected] * Correspondence: [email protected] Received: 5 July 2020; Accepted: 22 July 2020; Published: 24 July 2020 Abstract: As an unconventional energy source, the development of shale oil plays a positive role in global energy, while shale oil is widespread in organic nanopores. Kerogen is the main organic matter component in shale and affects the flow behaviour in nanoscale-confined spaces. In this work, a molecular dynamic simulation was conducted to study the transport behaviour of shale oil within kerogen nanoslits. The segment fitting method was used to characterise the velocity and flow rate. The heterogeneous density distributions of shale oil and its different components were assessed, and the effects of different driving forces and temperatures on its flow behaviours were examined. Due to the scattering effect of the kerogen wall on high-speed fluid, the heavy components (asphaltene) increased in bulk phase regions, and the light components, such as methane, were concentrated in boundary layers. As the driving force increased, the velocity profile demonstrated plug flow in the bulk regions and a half-parabolic distribution in the boundary layers. Increasing the driving force facilitated the desorption of asphaltene on kerogen walls, but increasing the temperature had a negative impact on the flow velocity. Keywords: flow; shale oil; kerogen; molecular simulation 1. Introduction The development of shale oil has ameliorated global energy shortages, and many countries have launched programmes to investigate various development approaches [1–3]. However, the complex structure of nanopores in shale and the different components in shale oil hinder the further study of shale oil flow behaviours [4,5]. The sizes of pores in shale range from nanometres to micrometres, but experimental nanoscale studies are restricted to laboratory assessments [6–12]. Molecular dynamic (MD) simulations are often used to study the fluid behaviours within nano-confined spaces [13]. MD simulations are based on Newtonian mechanics and can be used to calculate a system’s thermodynamic quantities and other macroscopic properties. Shale is mainly composed of organic and inorganic matter [14–16]. The inorganic matter contains quartz and clay minerals. As a source of oil, kerogen is the main component of organic matter [17]. Thus, the flow behaviour of shale oil in realistic kerogen channels needs to be investigated. Energies 2020, 13, 3815; doi:10.3390/en13153815 www.mdpi.com/journal/energies Energies 2020, 13, 3815 2 of 12 Energies 2020, 13, x FOR PEER REVIEW 2 of 12 BousigeBousige et et al. al. [18] [18] produced produced a a realistic molecular kerogenkerogen modelmodel usingusing thethe generalised generalised phonon phonon densitiesdensities of of states. states. Ungerer Ungerer et et al. al. [19] [19] classified classified kerogen moleculesmolecules intointo fourfour typestypes ofof maturity maturity and and constructedconstructed corresponding corresponding structures. structures. It is didifficultfficult forfor shale shale oil oil to to flow flow in in a kerogena kerogen matrix matrix as aas result a result of ofkerogen’s kerogen’s dense dense matrix matrix structure structure [20], so[20], the so reasonable the reasonable construction construction of kerogen of channelskerogen ischannels necessary. is necessary.During the During kerogen the matrix kerogen generation matrix generation process, the process, insertion the of repulsiveinsertion dummyof repulsive particles dummy can produceparticles canporous produce models porous [21,22 models]. Perez [21, et al.22]. [8 ]Perez blended et al. a fluid[8] blended mixture a with fluid kerogen mixture monomers with kerogen and simulatedmonomers andthe simulated annealing the process, annealing and the process, fluid moleculesand the fluid gathered molecules as clusters. gathered Kerogen as clusters. monomers Kerogen form monomers organic formframeworks. organic frameworks. However, it However, is difficult it tois obtaindifficult clear to obtain transport clear characteristics transport characteristics in irregular in channels. irregular channels.Carbon nanotubesCarbon nanotubes (CNTs) and(CNTs) graphene and graphene slits are commonslits are common flow channel flow modelschannel [ 7models,23], but [7, the 23], flow but therate flow can rate be overestimated can be overestimated on ultra-smooth on ultra-smooth surfaces. surfaces. In multicomponent In multicompone fluids,nt heterogeneous fluids, heterogeneous density densitydistribution distribution characteristics characteristic cannots cannot be produced be produced on smooth on smooth surfaces. surfaces. Due to Due thedriving to the driving force’s eforce’sffect, effect,the velocity the velocity profile profil tendse tends to plug to inplug smooth in smooth channels channels rather rath thaner parabolas than parabolas on rough on wallsrough [ 24walls]. Thus, [24]. Thus,constructing constructing flow flow channels channels with with kerogen kerogen is a more is a more reasonable reasonable choice. choice. TheThe flow flow channel’s channel’s boundary boundary condition condition affects affects the slipslip length [[25].25]. Due toto thethe presencepresence of of slippage, slippage, Darcy’sDarcy’s law law is is invalid invalid in in nanoscale nanoscale fluid fluid flows flows [20, [20, 2626].]. TheThe slippageslippage eeffectffect cancan be be characterised characterised using using thethe slip slip length length concept, concept, which which is defined is defined as the as thedistance distance between between the solid the solid surface surface and the and point the pointwhere thewhere velocity the extrapolation velocity extrapolation is zero [27]. is zeroMartini [27 ].et Martinial. [27] found, et al. [at27 a] found,low driving at a lowforce, driving the movement force, the of onlymovement a few molecules of only a fewalong molecules a wall, called along a a “defect wall, called slip” a in “defect a nonlinear slip” in mode. a nonlinear At a high mode. driving At a highforce, alldriving of the force,fluid molecules all of the fluid adjacent molecules to the adjacent liquid–solid to the interface liquid–solid contribute interface to contribute“global slip.” to “global Conversely, slip.” onConversely, a rough kerogen on a rough wall, kerogen the boundary’s wall, the boundary’scavity can trap cavity some can molecules. trap some molecules. In addition, In fluid addition, molecules fluid impingemolecules protrusions impingeprotrusions on the interface, on the decelerating interface, decelerating the velocity the in velocity the boundary in the boundarylayer [28-30]. layer However, [28–30]. toHowever, the best toof theour best knowledge, of our knowledge, very few veryreports few in reports the literature in the literature discuss multicomponent discuss multicomponent shale oil slippage.shale oil slippage. InIn this this study, study, we we constructed constructed a a kerogen kerogen slit [[3311-33]–33] and used aa nonequilibriumnonequilibrium molecularmolecular dynamic dynamic (NEMD)(NEMD) simulation simulation to to analyse analyse the the flow flow behaviour behaviour of of shale oil. We alsoalso examinedexamined thethe eeffectsffectsof of the the drivingdriving force force and and temperature, temperature, and and the the simulation simulation re resultssults were were fitted fitted using using the the Poiseuille Poiseuille formula formula by dividingby dividing the thechannel channel into into two two sections. sections. We We calcul calculatedated the the flow flow rate rate using boundaryboundary layerslayers underunder multicomponentmulticomponent shale shale oil oil conditions. conditions. 2.2. Methodology Methodology 2.1.2.1. Molecular Molecular Models Models InIn this this study, study, type type Ⅱ II-C-C kerogen kerogen monomers monomers were were used used to tocons constructtruct realistic realistic organic organic slits slits because because the Ⅱthe-C kerogen II-C kerogen tended tended to be present to be present in organic-rich in organic-rich shales [19]. shales We [used19]. Wea simplified used a simplified shale oil component shale oil modelcomponent including model different including molecular different weight molecular models weight(C1, C4, models C8, C12, (C1, methylbenzene, C4, C8, C12, asphaltene)[8, methylbenzene, 33, 34].asphaltene) The details [8, 33and,34 ].structures The details of andthe kerogen structures molecules of the kerogen and shale molecules oil models and shaleare provided oil models in are the Supportingprovided in Information, the Supporting and Information,the model of andthe slit the system model ofis shown the slit in system Figure is 1. shown There inwere Figure 26,8991. There atoms
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