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Petroleum 1 (2015) 139e145

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Original article

Solving asphaltene precipitation issue in vertical wells viaredesigning of production facilities

Mahdi Zeinali Hasanvand a, Mohammad Ali Ahmadi b, *, Reza Mosayebi Behbahani c

a Research Institute of Petroleum Industry (RIPI), Tehran, Iranb Department of Petroleum Engineering, Ahwaz Faculty of Petroleum Engineering, Petroleum University of Technology, Ahwaz, Iranc Department of Gas Engineering, Ahwaz Faculty of Petroleum Engineering, Petroleum University of Technology, Ahwaz, Iran

a r t i c l e i n f o

Article history:Received 3 May 2015Received in revised form1 July 2015Accepted 1 July 2015

Keywords:Asphaltene precipitationVertical wellChoke valveThermodynamic parametersWell column

* Corresponding author. Tel.: þ98 912 6364936.E-mail address: ahmadi6776@yahoo.com (M.A. AhPeer review under responsibility of Southwest Pe

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a b s t r a c t

Precipitation of heavy hydrocarbon components such as Wax and Asphaltenes are one of the mostchallenging issues in oil production processes. The associated complications extend from thereservoir to refineries and petrochemical plants. Precipitation is most destructive when the affectedareas are hard to reach, for example the wellbore of producing wells. This work demonstrates theeffect of adjusting choke valve sizes on thermodynamic parameters of fluid flowing in a verticalwell. Our simulation results revealed optimum choke valve sizes that could keep producing verticalwells away from Asphaltene precipitation. The results of this study were implemented on a well inDarquin Reservoir that had been experiencing asphaltene precipitation. Experimental analysis ofreservoir fluid, Asphaltene tests and thermodynamic simulations of well column were carried outand the most appropriate size of choke valve was determined. After replacing the well's originalchoke valve with the suggested choke valve, the Asphaltene precipitation problem diminished.

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1. Introduction

Asphaltenes and Waxes are the main heavy hydrocarbonprecipitations which cause the production companies toencounter with one of their most challenging problematic issuesduring crude oil production and subsequent related processes[1]. Although asphaltenes usually accompany with waxes inmost of petroleum reservoirs, their thermo dynamical behavior,chemical structure and precipitating procedures are absolutelydifferent. Waxes are normally soluble in middle alkanes, whichare laterals and without chains consisting 18 to 80 carbons, andstart crystallizing and precipitating due to temperaturedeclining.

madi).troleum University.

ier on behalf of KeAi

niversity. Production and hostcreativecommons.org/licenses/b

Based on some technical issues related to temperature inIran's reservoir, possibility of forming wax deposition in wellstring is almost rare, but asphaltenes which are included up to 1million carbons in their structures can be deposited because ofaggregation phenomena. While changing the temperature andcomposition of crude oil cause forming of asphaltene precipita-tion, decrease in pressure is the most important influence factor.During production of a well where the pressure and temperaturestarts simultaneously declining, the asphaltene molecule pre-cipitates and results in forming a sticky accumulation [2]. Theformed precipitation in oil production blocks the well and flowlines. Subsequently, valves, separators and the wellhead facilitiescan lose their sufficient performance as well [3]. Even infinites-imal amount of asphaltene causes the noticeable declining in theperformance of refinery and petrochemical units, catalysts andthe other additives. Therefore, great efforts have been put forthto segregate these materials as much as possible [4].

Increasing the oil viscosity, declining the rate of productionand enhancing the pressure losing gradient in flow lines can beresulted from the beginning of asphaltene precipitation period.While asphaltene precipitation is a consequence of thermodynamical instability. Based on previous research it can bededuced that there is an experimental relationship between the

ing by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an openy-nc-nd/4.0/).

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concentrations of asphaltenemolecules and the oil viscosity. Thisis because of forming a polymeric network of heavy hydrocar-bons and its fast development in the crude oil [5]. Organic pre-cipitations become very sticky and hard, if the aforementionedorganic precipitations include asphaltene precipitations. Thisphenomenon makes precipitation of asphaltene possible in alarge range of production related zone, from the porous media ofwell bore to the internal systems of refineries.

In petroleum upstream related industries, asphaltene pre-cipitations are treated in 2 ways: 1) inhibiting methods 2)removing methods. The second is divided into 6 subset methodswhich are: 1) adjusting production related processes 2) usingchemical materials 3) using external forces 4) using mechanicalmethods 5) thermal methods 6) using biotechnical methods.Nevertheless, researchers do believe that editing the productionrelated procedures the best solution according to the econom-ical, technical and environmental criteria. In fact, the othermethods (2e5) are normally considered when reconstructing/designing is not possible [6].

Generally, to solve the faced problems during the productionoperation, cheapest and the most stable method is the adjust-ment of production related processes. This methods is usedspecifically in petroleum production connected obstacles such asprecipitating, two-phase flow, decline of production rate and etc.It means that it must be tried as mush as possible to prevententering the produced oil to the thermo dynamical zoneasphaltene precipitation. Shear reduction, removing incompat-ible materials from the current of precipitator crude oil,decreasing the trend of pressure reduction in production facil-ities, preventing from themixture of precipitator and inhibitor ofcrude oil in production unit and neutralizing the electro statis-tical forces existing in pipes are the 6 ways of redesigning/reconstructing the production related processes [7].

To inhibit forming precipitations in the well string and byregarding the existing components within the crude oil, thethermo dynamical conditions of production related processesmust not overlap with the thermo dynamical zone of pipes,temperature figure and onset pressure of asphaltene forming [8].In the same conditions of production, if the figure of asphaltenein crude oil is blue, the oil will not enter to the asphalteneforming zone, from the beginning to the end of fluid movementin the well string. But, it is not a true story about the oil with theabout the oil with the red figure of asphaltene forming (SeeFig. 1). The production group of Arvandan Oil Company has putforwarded a method of adjusting the production related pro-cesses in Darquain light oilfield in order to prevent any asphal-tene precipitation. Firstly, they examine the thermo dynamicalbehavior of the supposed oil and then, plot its phase diagram

Fig. 1. Asphaltene Deposition Envelope (ADE) for two oil samples and the pro-duction range during the well.

based on pressure and temperature. After that, the relationshipamong the amount of deposition, pressure and temperature wascorrelated according to the plot of ADE through using the Win-Prop and PVTi software. The possibility of asphaltene formingwas analyzed through running a sensitivity analysis for pressure,temperature and different chock sizes, 22/64, 32/64 and 42/64inches.

2. Determining the thermodynamic behavior ofimplemented crude oils and asphaltene precipitations inDarquain oilfield

The Prosper software was used to meet the goal which wasselecting the best chock size by applying the material balancefor the produced precipitations through paying attention tothe production data of well No. 10 for the last 8 years. Applyingthe optimized chock size has resulted in blocking the pro-duction of asphaltene precipitations. Knowing the thermodynamical behavior of the asphaltene molecule and the sam-ple of the produced crude oil is an important principle tounderstand completely the problem of asphaltene precipita-tion in oil wells which is normally happened in both light andheavy oil [9]. One of the samples gained from the Darquainoilfield which is highly probable to precipitate asphaltene hasthe fluid properties of 37.5� API, 0.33 cp, 4285 psia and2115 ft3/STB for gravity, viscosity, bubble point pressure andsolution gas; respectively. Asphaltene concentration in thesupposed sample is 0.22 mol percent (2.69 weight percent). Toknow deeply the thermo dynamical behavior of the supposedsample a group of expansion testing phase were run whichdifferent parameters such as changing volume, associate gasand formation volume factor were tuned in PVTi software. Thegenerated results indicate the high level of accuracy existing inthe software (See Fig. 2). After concluding the reliability ofsimulated data, the phase diagram of the live oil sample wasplotted (See Fig. 3).

Because asphaltene precipitations exist in the supposed oilsample, determining asphaltene percent, asphaltene onsetpressure and the other hydrocarbon components such as thearomatic, the saturated, the resin, and asphaltene through SARAtest is inevitable. Observing the effect of temperature on pre-cipitation forming is themost prominent tip due to existence of 2opposite behavior about light and heavy oil [10]. Increasing thetemperature in light oil results in the reduction of depositionwhile it is somehow vice versa in heavy oil. Based on SARA data(Asphaltene 2.69%, Resin 3.29%, Aromatic 19.73% and Saturates74.28%) and plotting De-Bore figures, ratio of asphaltene to resin,colloidal instability index, asphaltene stability index, it wasconcluded that the oil samples is precipitation former. It hasbeen depicted in Fig. 4.

The filtration experiment for HPHT condition and the HighPressure Microscopy test were run to determine the quality andquantity effects of pressure and temperature on precipitationforming. The experiments reveal this fact that reducing thepressure until bubble point in isothermal condition causesincreasing the amount of asphaltene precipitation, butcontinuing the downward trend of pressure; the amount ofasphaltene deposition starts decreasing. Up and down points ofdeposition envelope are (2300 and 7300 psi) in 290 �F. In theother hand, reducing the temperature causes increasing theamount of produced deposition [11]. Fig. 5a shows lab outputs ofobserving the effect of temperature and pressure on asphalteneprecipitation forming.

Through overlapping the Figs. 3 and 5a, it becomes possible toplot the asphaltene deposition envelope which plays a leading

Fig. 2. The comparison between the experimental and simulated data of the supposed reservoir fluid.

M.Z. Hasanvand et al. / Petroleum 1 (2015) 139e145 141

role in production engineering. According to Fig. 5b, asphalteneprecipitation has an upper and downer onset pressure lineswhich forming happens in this zone. The maximum amount ofasphaltene precipitation happens around bubble point pressure,if move isothermally from the upper pressure towards the lowerone. Moreover, increasing temperature reduces the amount of

deposition although this effect is different in light and heavy oil.Hence, in order to have a certain rate of production, it must betried not to enter this zone, or at least not nearing the bubblepoint where the asphaltene precipitation is maximum. It mustalso be noticed that end of the route is thermo dynamicallysimilar to the conditions of chock.

Fig. 3. The phase diagram of the supposed live oil sample.

M.Z. Hasanvand et al. / Petroleum 1 (2015) 139e145142

3. Determining the thermodynamic changes of the fluid inthe well string

The supposed well has been drilled in one of layers of Dar-quin light oilfield which contains small amount of asphaltene

Fig. 4. De-Bore plots, Ratio of asphaltene to resin, colloid

(0.22%), but it is highly possible to precipitate. Since it has beendrilled in 2004, it has been blocked for 6 times due to asphal-tene precipitations. This problemmade the company to spend ahuge amount of money to solve it. Furthermore, the conven-tional solutions applied in this case have been very timeconsuming too. Also, the referred well was out of service for2.5 years. Technically, adjusting the chock size has alwaysbeen regarded as a solution which is a subset of editingthe production related processes, when the well string hasbeen blocked as a consequence of precipitations. Generally,increasing the chock size causes changing the thermo dynam-ical and production related parameters closing to the chock asthe following:

1. Production rate increases as a consequence of increasing thechock size.

2. Wellhead pressure decreases as a consequence of increasingthe chock size.

3. Wellhead temperature increases as a consequence ofincreasing the chock size.

al instability index, asphaltene stability index about.

Fig. 5. (a) Amount of asphaltene precipitation weight changing for the supposed oil sample based on temperature and pressure (b) ADE plot for each oil samples.

M.Z. Hasanvand et al. / Petroleum 1 (2015) 139e145 143

Thus, the velocity of fluid and its thermo dynamical condi-tions within the well string can be changed through adjustingthe chock size. In other words, for each different chock size, thefluid within the well string passes different thermo dynamicalroute from bottom hole to the wellhead and the time that thefluid remains in the well string changes too. (Less time is neededfor fluid to elevate for higher flow rate). Results of simulations,using the Prosper software, for productivity index and Gilbertfigures based on 3 different chock sizes, which have sequentiallybeen used in the reality by regarding the flow rates that havebeen close to the reality, have been depicted in Fig. 6a. Thereferred operationwas done to calculate the optimized flow rate.Flow rates of 2500, 3900 and 4800 STB/day have been calculatedfor 22/64, 32/62, 44/64 chock sizes respectively. But, as a realcase, flow rate has been observed as 1700, 2900 and 5400 STB/day. Because of this, real flow rates have been used to determinethe real thermo dynamical conditions of the well string. Prosperhas this ability to determine the changing temperature andpressure during the well string while it is also capable of clari-fying the type of fluid. For instance, the flow rate has shifted frombubbly to the slug in depth of 25, 1331 and 2031 for chock size of

22/64, 32 and 44 inches respectively. Changes of temperatureand pressure from the well bottom to the wellhead for relatedflow rates of each chock size have been depicted in Fig. 6b. Thesechanges indicate the thermo dynamical conditions that live oilpasses the well string during the production. Moreover, it cangraphically been deduced that pressure of well string decreasesby increasing the size of production valves, and oppositelytemperature increases which both of consequences are similar tothe operational results. Generated results from thermo dynam-ical simulation of well string are adapted with thermo dynamicalfigures of fluid behavior and asphaltene molecule to infer whathas happened in Darquin light oilfield.

4. Results and conclusions

The supposed well has been producing for 3 time intervalswith 3 different chock sizewhich have been 22/64, 32/64 and 44/64 inches that the first and second ones have resulted in wellblocking after 10 and 27 months, but about the third, it is 19months that it is properly working without any special problem.To analyze the pointed problem, well string related pressure and

Fig. 6. (a) Pressure and temperature profiles for each chock sizes in wellbore (b) pressure and temperature profiles for each chock sizes in wellbore.

Fig. 7. Changes of the formed precipitation in the well string.

M.Z. Hasanvand et al. / Petroleum 1 (2015) 139e145144

temperature data about all of 3 production technical conditionshave been adapted on both the phase diagram of reservoir fluidand the asphaltene molecule (Fig. 6b).

During using the chock size of 22/64 inch, the oil at thewellbore has the infinitesimal amount of asphaltene in theequilibrium conditions while it is going to reach its maximumlevel when it elevates to the wellhead, about 0.22 mol percent. Itcan be excused through paying attention to the low temperatureof the produced fluid and closing to the bubble point. (The mostunstable condition to precipitate) creates the best circumstancesfor precipitation forming and well blocking. As a result, theminimum amount of asphaltene precipitation is connected withthis chock size. About the chock size of 32/64 inch, it can beconcluded that at well bore it has 0.08 crude oil weight percentof asphaltene precipitation and at the well it has turned into0.14% that indicates the accumulation of asphaltene precipitationin the well string but less than the previous chock size and it isshown by 17 months of production period which is 7 monthslonger than the production time related to the smaller chock size.The chock size of 44/64 seems to be the best choice because it

M.Z. Hasanvand et al. / Petroleum 1 (2015) 139e145 145

has been calculated that at the wellbore, its asphaltene precipi-tation is about 0.04% and this number is going to become smallerthrough upcoming and it can instantly be concluded that noamount of asphaltene is produced in the well string. Passing the19 months of production without any changes in productionindexes proves this theory. It must be remarked that there is 2opposite force in the case of asphaltene precipitation. In fact,changes of thermo dynamical conditions in the well string is apositive force to precipitate the asphaltene on the surface of wellstring which is called the chemical potential, but the negativefactor is the surface tension created from the fluid movement inthe well bore which tries to separate the precipitation from thesurface. The negative force gets strengthened by the increasing offluid velocity [12]. The discussed velocity in the biggest chocksize is 3 and 2 times greater than the velocity of fluid in the chocksize of 22/64 and 32/64 respectively. The important tip to choosethe best chock size is considering the place that asphalteneprecipitates in the production processes. Normally, the fluidstarts flowing from the porous media of the reservoir to thepipelines after passing the well string wellhead facilities. Fromthe point of finding the most accessible place to precipitate, wellstring is not an appropriate area because of the alreadymentioned problems and costs. Regarding the same opinion canbe excused to reason that precipitating in the producing layer iseven more dangerous. Installing the chock size of 44/64 inchcauses precipitating 50% of asphaltenes in the producing layerand in sum about 0.1 mol percent of the oil producing remainedin the layer.

This conclusion is because of moving the fluid on the purpleline that at the beginning (Reservoir initial conditions) there isno amount of asphaltene, but at the end the amount of asphal-tene covers the line of 0.1 mol percent (Fig. 7). It is known thatabout the 50% of asphaltenes precipitate in the reservoir throughassociating with the material balance. The low velocity of fluidmovement, closing the conditions to the thermo dynamicalequilibrium, operating the porous media of the reservoir as afiltration unit and the huge volume of the reservoir rock putfacing with the probable problems of precipitating in the sup-posed layer off although the subsequent problems such asdecrease of porosity and the permeability of rock located aroundthe well bore. These can be taken as the further studies in thefuture.

5. Conclusions

Recently, a numerous number of researches have been doneto defeat the asphaltene precipitation associated problems[13e16]. In contrast with the conventional solutions, modernmethod have been proposed to edit the production relatedprocesses which are extremely environmentally friendly becauseof not using some toxic materials like BETX (Benzene, Ethylbenzene, Toluene, Xylene) [17]. The parameters of flow rate,pressure and temperature have been constant for the period of

19 months of production in addition to minimize the possibilityof precipitating the asphaltene in the well string of the supposedwell drilled in the Darquin light oilfield through adjusting theproduction related processes. Increasing the chock size decreasesthe produced precipitations and increases the flow rate whichmeans increasing the fluid velocity that helps to remove theasphaltene precipitations in the well string. In this method, theplace of precipitating in the route of production changes byaltering the thermo dynamical conditions of the well string. Inother words, this place moves from the well string to the porousmedia of the reservoir without paying enormous amount ofmoney and just through doing an exact research. It is highlysuggested that if the existence of asphaltene in a reservoir getsproved, calculations of asphaltene thermo dynamical compati-bility must be done in the well bore to choose efficiently valvesize and the tubing diameter.

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Solving asphaltene precipitation issue in vertical wells via redesigning of production facilities1. Introduction2. Determining the thermodynamic behavior of implemented crude oils and asphaltene precipitations in Darquain oilfield3. Determining the thermodynamic changes of the fluid in the well string4. Results and conclusions5. ConclusionsReferences