Dynamic asphaltene deposition control in pipe flow

Authors: Muhammad Ihtsham Hashmi B Ghosh

Publish Date: 2014/03/19

Volume: 5, Issue: 1, Pages: 99-108

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Abstract

Asphaltene–resin moieties in crude oil are found to carry residual surface electric charge which is characteristics to their colloidal structure asphaltene–resin ratio and system pH This research investigated the possibilities of controlling asphaltene deposition in oil well by applying static electrical potential taking advantage of their residual surface charge Laboratory experiments were conducted at static and also in dynamic condition constructing a dual flow loop setup equipped with precision pumping pressure recording and regulated DC power supply Neat and heptanediluted crude oil having inherent asphaltene deposition tendencies is used to investigate the influence of DC electrical potential on asphaltene deposition tendencies Real time deposition trend is interpreted through differential pressure buildup across the flow loops and also through quantitative estimation of deposited mass The results were encouraging showing up to 180  reduction in asphaltene deposition in the cathode loop and about 140  increase on the anode loop at an optimised potential of 60 V DC Further it was observed that higher the nheptane dilution higher is the effect of static potential in terms of arresting deposition Based on these optimistic results further studies and upscaling are planned considering the changes of pressure temperature and fluid composition in an oil well and looking at the possibilities of controlling asphaltene deposition by converting the well into a cathode along with a nearby sacrificing anode well applying optimum electrical potentialAsphaltenes in petroleum flow streams are often compared with lowdensity lipoproteins in blood stream both being responsible for flow restriction pressure surge and finally complete blockage resulting system shut down owing to phase separation and deposition on the flow lines In case of a petroleum production system this may lead to permeability damage of the reservoir pore matrix blocking production tubing surface flow lines and fouling of surface equipmentAsphaltene the major constitution of the residue fraction of crude petroleum is commonly known as polycondensed aromatic molecules of 4–10 rings Groenzin and Mullins 2000 with multiple alkyl side chains Heteroatomic NSO functional groups are the source of their polarity and amphoteric characteristics Goual and Firoozabadi 2002 and the coordination metals Fe Ni V are confined within porphyrin type moieties SPEIght 1998In general crude petroleum is a physical mixture of three major groups of compounds asphaltenes are the largest in molecular size resins fall in the mid range and the rest are the lighter aromatics and paraffins maltenes Asphaltenes and resins are polar while the maltene compounds are either nonpolar or mildly polar The interaction among these species strongly affects asphaltene precipitation from petroleum fluids Goual and Firoozabadi 2004Due to extremely wide molecular weight distribution and the ability to form molecular aggregates at different intensity at different chemical and physical condition only average chemical structures and properties of asphaltenes and resins could be established so far Stability of asphaltene in crude oil depends upon a variety of factors temperature pressure composition pH etc of which resin content in oil is attributed to have the greatest influence Andersen and SPEIght 2001 It is an established fact that petroleum resins with lesser aromaticity but higher polarity compared with asphaltene act as peptizing agents and help stabilizing asphaltene–resin colloids The mechanism behind this molecular association is believed to be due to opposite surface charge of resins and asphaltene moieties Taylor 1998 It is observed that destabilization of colloidal asphaltene in oil production systems may occur if the surface charge and the resulting attraction forces between adsorbed resins and asphaltenes are disturbed Leontaritis 1989 Alkafeef 2001 Later works of Fotland and Anfindsen 1996 and Behar et al 1998 on electrical conductivity of asphaltene helped in understanding their molecular association phenomena in solution phase while works of Igor and Aleksander 2010 on electrical conductivity of solid asphaltenes showed that asphaltenes in solid state behave like semiconductors conductivity increasing at higher temperature possibly due to structural phase transitionElectrokinetic electrophoretic and adsorption properties of asphaltenes are studied through potentiometric and electrokinetic measurements in great details on various crude oils which firmly established its chargebearing properties and streaming potential Kokal et al 1995 Leon et al 2000 Gonzalez et al 2003 ParraBarraza et al 2003 Asphaltenes in aqueous solutions are found to possess surface charge as a result of protonation and dissociation reactions of functional groups enabling electrophoretic mobility Janusz and Jablonski 2000 Solventinduced polarity is seen to reverse the electrophoretic mobility of destabilized asphaltene particles Dispersed in polar water media the particles presented a negative electrophoretic mobility whereas in toluene their mobility was positive When resins were present in the precipitating medium coprecipitation of resins occurred indicating a binding process of the resins on the nascent asphaltene particles ParraBarraza et al 2003Due to the dependency of several factors solvent temperature and influence of resins the nature of native surface charge on asphaltene is still not fully established Whereas some researchers claimed that asphaltene possess a predominantly positive charge in most nonaqueous dispersants James and Richard 1963 Lichaa and Herrera 1975 Leon et al 2000 Gonzalez et al 2003 some reported their net negative surface charges based on electrodeposition experiments with neat crude oil samples Idem and Ibrahim 2002 Nasser and Belhaj 2010 Belhaj and Khalifeh 2013 Also noteworthy is the work of Leontaritis and Mansoori 1987 who showed that the net negative charge on asphaltene colloidal surface is the prime cause of deposition on metal pipes due to the potential difference across them This resulted with the hypothesis that any effort in neutralising this charge may result in controlling asphaltene precipitation and deposition Yen 1994 Leon et al 2000 from their electrophoretic mobility studies with 27 crude oils established that the surface charges on asphaltene micelles were predominantly negative in crude oils and at low heptanes dilution but the surface charges were reversed at higher dilution The charge reversal was attributed to removal of positively charged peptizing resins Khvostichenko and Andersen 2009 investigated the charge reversal phenomenon under various conditions of dilution and found that application of a DC electric field can arrest asphaltenes aggregation or precipitation when diluted in aromatic solvent tolueneTaylor 1998 studied the quantum of electrodeposition of asphaltene and resins as a function of the electric field strength residence time dilution ratio and oil type It was observed that stable asphaltene ‘micelles’ in crude oils possess little net charge and are apparently not affected by the DC electric field However upon partial or total destabilization of micelles with nalkanes electrodeposition occurred depending on the crude oil characteristics alkane used and their relative dilution At lower dilutions partial destabilization the deposited material possesses a small net negative charge higher dilutions resulted in a positively charged deposit Although these findings are in contradiction with some of the observations described above they are consistent with the structural model for asphaltene micelles comprising negatively charged ‘peptizing’ resins surrounding a positively charged central asphaltene core The ambiguities existing on the surface charge of asphaltene and resins have been clarified by the work of Hashmi and Firoozabadi 2012 in which they have concluded that asphaltene colloids may possess heterogeneous surface charge Some particles may carry a small net positive charge that can deposit on cathode while others may carry a small net negative charge that can deposit on anode surface Metallic components pielectron clouds and acid–base functional groups on the periphery contribute to the surface charge which may vary from crude to crude depending on source maturation and migration history of the oil

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