Asphalt Ene

38

ASPHALTENE PRECIPITATION IN HIGHGAS-OIL RATIO WELLS

Kokal is a reservoir fluid property specialist in SaudiAramco’s Research and Development Center in Dhahran.He is an expert in hydrocarbon phase behavior, crude oilemulsions and asphaltenes. Kokal has a BS degree fromthe Indian Institute of Technology, New Delhi, and aPhD degree from the University of Calgary, Canada, bothin chemical engineering. He has written more than 70technical papers and has authored the chapters on CrudeOil Emulsions and Reservoir Fluid Sampling, for the newrevised edition of the Society of Petroleum Engineers(SPE), SPE Petroleum Engineers Handbook (due in 2004).He is a registered professional engineer in Alberta,Canada and is a member of SPE. He has served on localand international levels on many SPE committees and isnow a technical editor for SPE’s Reservoir Evaluation andEngineering publication. His e-mail address [email protected].

Al-Ghamdi is a laboratory scientist working in SaudiAramco’s Research and Development Center in Dhahran.His research activities focus on emulsion studies anddemulsifier characterizations. Al-Ghamdi also investigatesasphaltene’s precipitation from crude oil and the mixingof hydrocarbons from deeper reservoirs. In addition, hestudies reservoir fluids. He holds a BS degree in industrialchemistry from King Fahad University of Petroleum andMinerals in Dhahran. Al-Ghamdi has authored and co-authored several technical papers. He is a member of theAmerican Chemical Society and SPE. His e-mail addressis [email protected].

Sunil Kokal

Abdullah Al-Ghamdi

Dimitrios Krinis

SAUDI ARAMCO JOURNAL OF TECHNOLOGY SPRING 2005

Krinis is a senior reservoir engineer working in a reser-voir management position with Saudi Aramco. He has par-ticipated in several oil and gas condensate field developmentprojects undergoing depletion, water injection or gas recy-cling in the North Sea, the Far East and the Middle Eastwith Marathon Oil and Shell. Krinis has a BS in mining andmetallurgical engineering (1984) from the NationalTechnical University of Athens, Greece, and a PhD inpetroleum engineering (1990) from Heriot-Watt Universityin Scotland. He is interested in hydrocarbon fluid phasebehavior, transient well testing, reservoir simulation andhorizontal/multilateral well completion and performance.His e-mail address is [email protected].

ABSTRACT

Asphaltene deposits have been observed in a number ofhigh gas-oil ratio (GOR) wells in North Ghawar. Eventhough the oil reservoir is undersaturated, two small gas-caps are present as a result of gas injection during the 1960sand 1970s. New development wells drilled recently to pro-duce oil and gas from the gas-cap areas have experiencedasphaltene deposition. The cause of precipitation is thestripping of the asphaltenes from the crude by the gas. Thispaper describes the results of an investigative study initiatedto determine the precipitation mechanism and ways to alle-viate the deposition problem. Asphaltene precipitationexperiments were conducted at reservoir conditions in aspecial pressure-volume-temperature (PVT) apparatus. Theeffect of GOR on asphaltene precipitation was determinedby titrating the reservoir oil with gas-cap gas. Bulk deposi-tion tests were also performed at different GORs with reser-voir fluids. The results demonstrate that the onset of asphal-tene precipitation occurs at relatively low GOR values.However, the amount of asphaltene precipitated at the onsetis negligible. Asphaltene precipitation and depositionincrease with increasing GORs. Asphaltene depositionenvelopes are provided for the reservoir oil as a function ofpressure and temperature. Guidelines are provided to allevi-ate the problem by controlling the GORs. Recipes for sol-vent treatment, including asphaltene dispersants, are alsodescribed in the paper.

INTRODUCTION

Ghawar Field is one of the major oil fields in Saudi Arabia.In the northern part of the field, some wells have experi-enced solid built-up in the wellbore. Analyses of solid sam-ples from several wells have shown the presence ofasphaltenes that may have precipitated during crude produc-tion and have started to deposit on the wellbore. The solid

39

Fig. 1. Wells with solid asphaltenes observed.

Fig. 2. Solids from a well.


deposition has been observed in high GOR wells. A locationmap of the wells is shown in fig. 1; photographs of the soliddeposit from one well are shown in fig. 2.

The Arab-D Reservoir of Ghawar Field contains anundersaturated light oil. The bubble point pressure is ~1,900psi at the reservoir temperature of 101.6°C (215°F), and theaverage GOR is ~570 scf/stb. The reservoir pressure at pres-ent is over 3,000 psi. In the 1960s and 1970s, the associatedgases from part of the field were injected back into the reser-voir at two locations due to the unavailability of gas process-ing facilities and to avoid excessive flaring. The injectedgases have formed two separate gas caps in the field (northand south gas-caps, fig. 1).

In recent years, oil production has started from these gas-cap regions. Due to the presence of the gas cap, some of thefree gas flows into the oil production wells, increasing theirtotal GORs. The coning or cresting of gas into the oil hascaused limited plugging in a few wells in the north andsouth gas-cap areas. The gas strips the oil of asphaltenes,which precipitate and deposit in the wellbores. Plugging ofthe wellbore by asphaltenes or organic deposits has thepotential to reduce productivity and cause productionimpairment. Furthermore, several more gas-cap wells are to

be drilled in the area, and their productivity may be impact-ed by the deposition tendency.

Fig. 3 shows the GOR for 11 wells in which asphaltenedeposits have been observed. The solid line shows the aver-age GOR for the entire field (~570 scf/stb). Except for onewell, the GOR for all wells is higher (in some cases substan-tially higher) than the average field GOR. The high GOR isa consequence of gas coning/cresting in the wells. The freegas strips the asphaltenes from the crude that deposits inthe wellbore.

One well was tagged over a period of time to ascertainthe buildup of asphaltenes. Fig. 4 shows the tag depths andindicates a loss of wellbore accessibility of ~61 meters(~200 ft), over 18 months. Recent results show that thebuild up has stabilized, and the asphaltenes may be draggedwith the oil to the gas-oil separating plant (GOSP). Otherwells are also being monitored and have shown somebuildup activity.

Asphaltenes comprise the heaviest and most polar frac-tion of crude oils (1 & 2). Asphaltenes exist in the form ofcolloidal dispersions and are stabilized in solution by resinsand aromatics that act as peptizing agents. Asphaltene pre-cipitation and deposition may occur deep inside the reser-voir, near the wellbore and/or in processing facilities (3).Asphaltene precipitation is a function of pressure, tempera-ture and live crude oil composition. Asphaltenes have a ten-dency to precipitate as the pressure is reduced, especiallynear the bubble point (however, precipitation can occureven at pressures higher than the bubble point, dependingon the crude).

Another important reason for precipitation is the strip-ping of crude oil by gas. When gas is added to the crude (asmay be happening during the production from the gas-capwells), the composition of the crude changes and may leadto precipitation. This is the same mechanism during de-asphalting of crude in a refinery where propane and butaneare used for stripping the asphaltenes. The precipitatedasphaltenes then deposit near, or in, the wellbore. This maylead to an increase in formation damage (skin) and subse-quently more precipitation.

This study was initiated to investigate the causes ofasphaltene precipitation and deposition in the high GORwells. If the stripping mechanism can be confirmed, thenappropriate stimulation programs can be designed toremove the solid deposits. Moreover, guidelines can be pro-vided to produce the oil wells at a certain maximum GORto keep the deposition under control.

This paper presents the extensive experimental work con-ducted and preliminary field data to address the issue. Thestudy includes a set of phase behavior experiments to deter-mine the conditions for the onset of asphaltene precipitation

40

Fig. 3. Gas-oil ratio of wells with asphaltene deposits.

Fig. 4. Tag depth for a well.


from live crude at different GORs. The extent (amount) ofasphaltene precipitation was also determined. From thesedata, asphaltene solubility in crude oil at different condi-tions (including different GORs) was determined. Solubilitytests with solvents were also conducted to determine thebest solvents for stimulation. Several commercial asphaltenedispersants were also evaluated, first with the dead crudeand with live oil, for their ability to disperse asphaltenes(i.e., reduce/eliminate their precipitation). These results arepresented in the paper.

EXPERIMENTAL INVESTIGATIONS

ASPHALTENE PRECIPITATION

Onset

A state-of-the-art solids detection system (SDS) was used tomeasure the onset of asphaltene precipitation from livecrude oils (4). It is a high-pressure, high-temperature appa-ratus and consists of a variable volume and a visual PVTcell retrofitted with fiber optic light transmission probes tomeasure the onset of organic solids precipitation concur-rently with fluid volumetric data. A schematic of the core isshown in fig. 5, and the principle of operation is depicted infig. 6. The SDS apparatus consists of a laser power source,a fiber optic bundle that carries the laser light into the PVTcell, the actual PVT cell containing the crude at pressureand temperature, another fiber optic bundle that carries thereceived laser light from the PVT cell, and a power meterthat measures the amount of light received (fig. 5). The PVTcell is mounted inside a temperature-controlled oven andhas an effective volume of 110 cc. The PVT cell is a win-dowed cell that permits visual observation of the oil insidethe cell. A variable-volume displacement pump controls thevolume and pressure of the fluid inside the cell. The PVTcell also has a specially designed magnetically-coupledimpeller mixer that provides powerful mixing and maintainsequilibrium in the fluid system. The PVT cell is designed foroperation at pressures of up to 10,000 psi and temperaturesup to 182°C (360°F).

Crude Oil and Gas Properties

The crude oil was sampled using a conventional bottomholesampler. The crude oil fluid composition is shown in table1. It has a bubble point pressure of ~1,900 psia at a reser-voir temperature of 101.6°C (215°F) and a GOR of 580scf/stb. The crude oil properties do not vary significantlyacross the area of interest. The dead crude has an asphal-tene content of ~3.0 wt%.

The composition of the injected gas (that was injected inthe 1960s and 1970s) is presented in table 1. This was the

associated gas from the crude after processing at the gas-oilseparating plant (GOSP). The gas used in the experimentswas prepared from the high-pressure production trap(HPPT) gas after flashing it at 1,300 psia and 23.8°C(75°F). The composition of the flashed gas (used in all theexperiments) is shown in table 1. The flashed gas matchesthe injected gas composition closely.

Procedures

The procedures for detecting the onset of asphaltene precip-itation from crude oils is described elsewhere (4). Exampleplots of precipitating and non-precipitating fluids are shownin fig. 6. For the non-precipitating fluid, the laser power (asmeasured by the power meter) increases monotonically

41

Fig. 5. Cross-section of the pressure-volume-temperature cell with laser lightsource and detector.

Fig. 6. Schematic of the principle of operation of the asphaltene precipitationcell.


(almost linearly) as the pressure drops to the bubble point.At this point, the evolving gas bubbles scatter light, and thetransmittance drops sharply. For the precipitating fluid thelaser power drops before the bubble point. The point atwhich the curve deviates from the straight line correspondsto the onset of asphaltene precipitation. This drop in laserpower is related to the precipitation of solids that scatterlight and cause the power of the transmitted light to deviatefrom the expected linear curve. The point determines theonset pressure for solids precipitation.

To determine the effect of the GOR on asphaltene precip-itation from live crude oil, pressurized gas was injected intothe PVT cell containing reservoir fluid at single-phase con-ditions. Approximately 30 cc of reservoir fluid were takenin the PVT cell, and a simple depressurization test(described above) was conducted at the reservoir tempera-ture of 101.6°C (215°F). Following the depressurizationtest, the sample was pressurized to 3,000 psi at 215°F andequilibrated at these conditions for 24 hours. Gas injectionwas started at a rate of ~1 cc/hr with continuous stirring.The results are described below.

Fig. 7 shows the results of depressurization tests for onewell. No onset of asphaltene precipitation was observedduring depressurization. Several experiments were conduct-ed with samples from other wells, and none of them

showed any asphaltene precipitation during depressuriza-tion. Some of the samples were taken through repeateddepressurization and pressurization steps without noticingany onset of precipitation. The results indicate that thecrude does not have a natural tendency to precipitate whenpressure is reduced.

The results of gas injection are presented in fig. 8. Thelaser light transmittance increases initially as the gas dilutesthe crude and then starts to decline after ~1.6 cc of gasinjection. This is the point of asphaltene onset and is shownclearly in the inset chart in fig. 8. As more gas is added, theamount of asphaltene precipitation increases, and the lighttransmittance declines monotonically. After 4 cc of gas areadded, the amount of light passing through the cell is notmeasurable. As more gas is added, the light transmittancestarts to increase (~6 cc), indicating a dilution effect or a re-dissolution effect. After ~7 cc of gas are added, the trans-mittance declines again as the bubble point is reached. Thegas addition was stopped after 9 cc. The results of gas injec-tion are re-plotted in terms of gas-oil ratio in fig. 9. Theresults indicate that the onset of asphaltene precipitationwill occur at a GOR value of ~625 scf/stb. This suggests

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Table 1. Crude Oil and Gas Properties

ComponentMole %

ReservoirFluid

Injects Gas(Actual)

Injected Gas(Experiment)

N2 0.14 0.41 0.34

CO2 5.89 12.30 12.62

H2S 1.82 1.91 2.49

C1 24.01 56.00 56.00

C2 9.79 17.45 16.23

C3 7.49 8.20 8.39

C4 4.92 2.64 2.86

C5 3.95 0.84 0.83

C6 3.14 0.25 0.25

C7+ 38.85 0.00 0.01

C7+MW 240

C7+SG 0.8652

BPP (psia)~1900 @

220°F

GOR (scf/stb)

Fig. 7. Onset of asphaltene precipitation with crude oil from a well without gas.

Fig. 8. Onset of asphaltene precipitation with crude from a well with gas.


that the crude is nearly saturated with asphaltenes and asmall amount of gas is enough to start the precipitation.This is in agreement with data from a nearby field (5),where precipitation occurs during depressurization. Similarbehavior was observed (not reported here) when the fluidfrom a nearby well was tested with the gas (similar proce-dures).

A depressurization test was conducted in another experi-ment in which 7 cc of gas were added to 30 cc of oil. Theresults are shown in fig. 10. The crude oil and gas mixturewas first pressurized to more than 6,000 psi. During depres-surization, the onset of asphaltene precipitation started at~4,500 psia. The bubble point (bubbles observed throughthe PVT cell) was ~3,000 psia. The results indicate thatsome of the precipitated asphaltenes (during gas addition)redissolved when the mixture was pressurized to 6,000 psia.

ASPHALTENE PRECIPITATION: BULK DEPOSITION

The experiments described previously show the onset ofasphaltene precipitation; they do not quantify the amountof asphaltenes precipitated. To quantify precipitation, a sep-

arate PVT cell was installed in the apparatus. It is essential-ly a high-pressure filtration apparatus and is referred to asthe bulk deposition apparatus. It consists of two floatingpiston pumps as shown in fig. 11 (schematic) and fig. 12(photograph). One of the cylinders is connected to a pump.The sample is taken in one of the cylinders and equilibratedfor ~48 hours. The sample is then transferred into the sec-ond empty cylinder through a filter assembly. The experi-ment is conducted at constant pressure and temperature,representing reservoir conditions. The deposited asphaltenesare trapped on the filter assembly that is removed at the endof the experiment and weighed. The amount of asphaltenesprecipitated is calculated in ppm, or as a percent of the totaloil charged.

Several bulk deposition experiments were conducted onwell samples at 3,000 psi and 101.6°C (215°F). The pressurewas selected to keep the samples in single phase. Sixty cc ofreservoir fluid were charged into one of the cylinders. Aknown amount of gas was then added to the sample. Theapparatus was rocked for 24 hours at these conditions. Thecrude oil was then filtered by transferring it into a secondcylinder. After all the sample had been transferred, the filterassembly was opened, and the filtered solids were weighed.A photograph of the filtered solids is shown in fig. 13. Fromthe weight of the filtered solids (mostly asphaltenes), theamount of precipitated asphaltenes was calculated. Fig. 14shows the amount precipitated in ppm as a function of gasadded (or GOR). Table 2 presents the data used in the plots.The results indicate that the amount of precipitate increaseswith an increasing GOR. While the amount of asphaltenesprecipitated is not significant per unit volume of oil, the totalamount can be significant over a period of time.

Another point to note is that not all the precipitatedasphaltenes will deposit. Some of the precipitatedasphaltenes will be carried with the oil to the GOSP.Moreover, some of the asphaltenes will re-dissolve as thepressure is reduced below the bubble point in the produc-tion strings. Complex hydrodynamics and deposition kinet-ics will determine how much asphaltene will deposit and

43

Fig. 9. Onset of asphaltene precipitation with crude from a well with gas.

Fig. 10. Onset of asphaltene precipitation with gas added.

Fig. 11. Schematic of a bulk asphaltene precipitation apparatus.


plug the wellbores. Some of this is being observed in theexisting wells.

ASPHALTENE SOLUBILITY TESTS

Several tests were conducted to determine the solubility of asphaltenes in solvents. Two solvents were selected for the tests: diesel and xylene. Diesel is readily available

and cheap. The solubility tests were conducted to select appropriate solvents for cleaning the pluggedasphaltenes in the wellbores.

A simple procedure was used to determine the solubilityof asphaltenes in these two solvents. The precipitated solids(fig. 2) were ground, and a fixed amount (5 g) was placedin a teabag. The teabag was then suspended in a solvent(100 mL) from a hook attached to a balance. The solventwas not stirred. The weight of the teabag (with theasphaltenes) was monitored. Using Archimedes’ principle,the loss in the weight of asphaltenes was calculated. Theresults are presented in fig. 15 for xylene, diesel, and mix-tures of xylene and diesel. The results show that diesel isnot a very good solvent, and less than 10 percent of theasphaltenes dissolved after soaking for three hours. On theother hand, complete solubility was attained with xylene in

44

Fig. 12. Photograph of bulk asphaltene precipitation apparatus.

Fig. 13. Bulk asphaltene precipitation showing precipitated asphaltenes.

Fig. 14. Bulk asphaltene precipitation at reservoir conditions.

Table 2. Bulk Deposition Test Results

GOR (scf/stb) 550 597 643 736 1250 1950

Amount oilcharged (cc)

60 60 60 60 60 60

Amount gascharged (cc)

0 2 4 8 30 60

Pressure (psi) 3000 3000 3000 3000 3000 3000

Temperature(F)

215 215 215 215 215 215

Amount pre-cipitated (mg)

17.9 33 58 62 81.1 132

Precipitatedasphaltenes(ppm)

403 739 1304 1396 1827 2973


under three hours. For cleaning purposes, xylene is a bettersolvent and a 50:50 (diesel:xylene) mixture was recom-mended for cleaning the wells.

ASPHALTENE DISPERSANT TESTS: DEAD CRUDE

One way to inhibit the asphaltenes from precipitating is byusing asphaltene dispersants or inhibitors (6, 7, & 8).Asphaltene inhibitors are specialized polymeric chemicals thatstabilize asphaltenes in the crude and prevent them from pre-cipitating. In their role, they act in the same manner as resinsand peptize asphaltenes. However, it is believed that theyhave a much stronger association with the asphaltenes, andhence, have a stronger peptizing effect than natural resins.Consequently, the inhibitors stabilize and disperse asphaltenesand prevent them from precipitating and flocculating.

Several commercial asphaltene dispersants were evaluatedfor their tendency to disperse asphaltenes in the crude. Thescreening test used for evaluation was the heptane precipita-tion test (6, 7, & 8). Six different commercial asphalteneinhibitors were selected for testing. Different amounts (inppm) of these inhibitors were first added to 50 mL of n-heptane and shaken. Five hundred microliters of the deadcrude oil was then injected into these tubes, shaken and leftto stand for a period of 24 hours. The amount ofasphaltenes precipitated at the bottom of the tubes wasmeasured and related to the base case where no inhibitorwas added. The results for one of the inhibitors are shownin fig. 16, and the results for all the inhibitors are presentedin fig. 17 as a function of inhibitor concentration. Theresults show that dispersant C-2 was one of the best andwas further used for live oil tests described below.

ASPHALTENE DISPERSANT TESTS: L IVE CRUDE

While the tests with the dead crude are good for screeninginhibitors, they do not provide an estimate of how effectivethe inhibitors will be at reservoir conditions. Based on the dis-persant selection process described above, the best dispersantC-2 was selected for testing with live crude oil. Four differentconcentrations were evaluated using the bulk deposition appa-ratus to estimate the effectiveness of the dispersant at 3,000psi and 215°F. Sixty cc of gas were added to 60 cc of crude oil(in each case). The concentrations used were 0, 10, 100 and1,000 ppm. A special technique was used to mix the disper-sant with the crude oil. The dispersants were diluted in xylene(a 10 percent solution), and aliquots of the required dosages(microliter volumes using appropriate syringes) were added tothe empty, clean cylinder (figs. 11 and 12) before crude charg-ing. The crude was then charged into the cylinder and equili-brated. The cylinder assembly was rocked for 48 hours at theset temperature and pressure before filtering (described earli-er). After filtering (transfer from one cylinder to the other), thecylinders were cooled down to room temperature, and the fil-

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Fig. 15. Solubility of asphaltenes in different solvents.Fig. 16. Asphaltene dispersant tests with dispersant C-2.

Fig. 17. Asphaltene dispersant tests.


ter assembly was opened, and the filtered solids were weighed.From these measurements, the amounts of precipitatedasphaltenes were calculated in ppm. The results are presentedin fig. 18. The results indicate that the dispersant was effec-tive, especially at the higher concentrations, in reducing theamount of precipitation.

WELLHEAD SAMPLE ANALYSES

As part of the investigative study, several wellhead samples(depressurized) were obtained from selected wells across thefield. These samples were analyzed for basic sediment andwater (BS&W), emulsions and saturates, resins, aromaticsand asphaltenes (SARA) analysis and density of the oil sam-ples. No free water was observed in any of the wellheadsamples. If water was present, it was in the form of a tightemulsion. The oil was separated using minimal demulsifier,and its density was measured. The results are shown in fig.19 and fall within a tight band of 0.875 g/cc at room tem-perature. A SARA analysis was also conducted on the sepa-rated oil and the asphaltene values are plotted in fig. 20.There was significant variation in the asphaltene values dueto contamination of the oil with emulsion (not completewater separation). The average value is ~3 wt% asphaltenein the dead crude. These tests were conducted to identifyany area anomalies.

BAILER AND SAND-BAILER SAMPLE ANALYSES

Bailer and sand-bailer samples were also obtained fromselected wells including all high GOR wells. A bailer sampleis collected from a specified depth, and the sand-bailer sam-ple is obtained from the bottom of the well. The bailer sam-pler is an open-cup sampler. BS&W, SARA (for separatedoil) and solid analyses were performed on the bailer andsand-bailer samples. The results are presented in tables 3

and 4. In 11 wells, black, solid asphaltenes were obtained.In several wells, only water and inorganic solids (from theformation) were obtained. The black solids were generallyfound in the highest GOR wells. The results clearly indicatethat asphaltenes are precipitating in the wells.

MODELING

A commercial PVT simulator (PVTsim by Calsep) was alsoused to simulate the asphaltene phase behavior. Therequired input for the simulator were the fluid compositions(table 1), the amount of asphaltenes in the crude oil andany minor adjustment or tuning to match the bubble pointpressure. The results are presented in fig. 20. The bubblepoint pressures are well-matched. The simulator was thenused to “predict” the phase boundaries for asphaltene pre-cipitation with injected gas. A few minor adjustments hadto be made to the amount of asphaltenes in the crude, but afairly good trend analysis was obtained with the PVT soft-ware. The use here highlights that, with a few experimentalpoints and minor tuning, such a package can be used todetermine the phase envelope for asphaltene precipitation.

Fig. 21 shows the results for the bulk deposition tests.The results are presented in terms of asphaltene wt% as apercent of the original asphaltenes in the crude. PVTsimpredicted that the amount of asphaltenes will declinebeyond a GOR of 1,000. Experimental results showed anopposite effect; asphaltene precipitation increased withGOR (fig. 14).

DISCUSSION

The results of this study clearly indicate that asphaltene pre-cipitation is a function of gas injection and increases withincreasing GOR. Asphaltene precipitation onset starts at rel-atively low GOR (~625 scf/bbl). The amount of asphalteneprecipitated is relatively small. Moreover, there is evidencethat some of the asphaltenes dissolve as the pressure is

46

Fig. 18. Effect of dispersant on bulk deposition tests at reservoir conditions.Fig. 19. Density of wellhead oil samples.


47

Table 3. Sand-Bailer Sample Data

Well No GOSPSampling

DateBS&W

(%)Density(g/cc)

SARA SampleType

Depth (ft) RemarksS A R A

1 1 24-Feb-01 97.0 sand-bailer 6892 10% solids, calcite/dolomite

2 1 26-Feb-01 0.0 0.8785 sand-bailer 6815 not enough sample

56 1 24-Feb-01 52.0 0.8826 44.14 40.13 12.79 2.94 sand-bailer 6915 small amount sample

218 1 21-Feb-01 0.0 ~98 sand-bailer 7050100% black solids,mostly asphaltenes


270 1 25-Feb-01 6.3 sand-bailer 6810


438 1 26-Feb-01 97.5 sand-bailer 7114 83% solids, mostly calcite

452 1 21-Feb-01 0.0 sand-bailer 6472

20 2 26-Feb-01 80.6 sand-bailer 7080



484 4 3-Mar-01 99.9 sand-bailer 7025 90% solids, calcite/dolomite

391 6 10-Mar-01 92.8 sand-bailer 7750 64% solids, calcite/dolomite

Table 4. Bailer Sample Data

Well No GOSPSampling

DateBS&W

(%)Density(g/cc)

SARA SampleType

Depth (ft) RemarksS A R A

439 1 20-Feb-01 0.0 bailer 6880 Empty bottle

454 1 20-Feb-01 99.0 bailer 7078 2% solids, calcite/dolomite

472 1 11-Sep-00 0.0 ~98 bailer 6772100% black solids,mostly asphaltenes

472 1 11-Sep-00 0.0 ~99 bailer 6772100% black solids,mostly asphaltenes

476 2 20-Feb-01 89.0 ~98 bailer 6398 51% solids, calcite/dolomite

397 4 21-Jul-01 0.0 ~98 bailer 6980100% black solids,mostly asphaltenes


469 4 20-Feb-01 0.0 ~98 bailer 6825100% black solids,mostly asphaltenes



257 6 21-Feb-01 87.0 0.8833 bailer 7020 44% solids, calcite/dolomite

477 09-Jul-01 0.0 ~98 bailer NA100% black solids,mostly asphaltenes

488 17-Jul-01 0.0 ~98 bailer 6847100% black solids,mostly asphaltenes


reduced below the bubble point. These effects have resultedin preventing severe asphaltene buildups in the wells underconsideration. Another factor that has kept the depositionunder control is the high flow rates in these wells that pre-vent the asphaltene flocculation process and “carry” theasphaltenes to downstream processing facilities. Some deposition is taking place in the wells as observed in thebailer samples.

PROPOSED FIELD TRIALS

The recommendation being made to production engineeringis to select a well and clean it with a solvent. Based onexperiments conducted, a 50:50 xylene and diesel mixture isrecommended for field trials. The solvent mixture will beallowed to soak for a minimum of 24 hours, and the wellswill be flowed back. Samples from the flow-back will beanalyzed to determine the effectiveness of the solventcleanout procedures, and recipes will be modified for futurecleanouts. While there are no plans at this stage to use dis-persants (for their cost), their use will be evaluated after afew wells have been cleaned and their performance meas-ured. The field trials will be described in a future paper.

Other wells in the area are being monitored for solidbuildup to evaluate whether or not the deposition is build-ing. Recent data indicate that the buildup in several wellshas “stabilized.” This could be due to the high flow ratesin the well and the complex hydrodynamics of fluid flowand depositional environment. Furthermore, no dramaticreduction in oil productivities has been observed in thewells being monitored.

CONCLUSIONS

The main conclusions of this study are:• Asphaltene deposition has been observed in several

high GOR wells. The precipitation has been linked togas coning/cresting in these wells.

• No significant productivity decline has been observedin the wells. However, solid buildup in some wells hasresulted in loss of wellbore accessibility of more than30 meters (100 ft).

• The gas from the gas-cap titrates the produced oil andcauses precipitation of asphaltenes that deposit in thewellbores.

• A commercially available PVT apparatus was used suc-cessfully to determine both the onset and amount ofasphaltene precipitation with gas titration.

• The data indicate that the precipitation onset occurs atrelatively low GORs. The amount of precipitation isvery small at onset conditions.

• The amount of asphaltene precipitation increases withincreasing gas volumes or GORs.

• A PVT simulator was partially successful in providingprecipitation trends. It can be a useful tool to estimateprecipitation envelopes and trends. However, it needsto be tuned with some experimental data.

• Simple tests are described to select a suitable solvent forasphaltenes.

• Several commercial asphaltene dispersants were alsoevaluated for their effectiveness in dispersingasphaltenes. One of the dispersants was used with livefluids, and the results are positive in terms of a reduc-tion in asphaltene precipitation at reservoir conditions.

• The presence of asphaltenes also makes the producedemulsions tight at the wellhead. This may lead to emul-sion problems at the wet crude handling facilities.

RECOMMENDATIONS

The main recommendations of this study are:• The oil wells should produce at as low a GOR as possi-

ble. This will reduce the amount of asphaltene precipi-tation and subsequent deposition.

• Continuous monitoring of asphaltene buildup in the

48

Fig. 20. Asphaltene content of wellhead oil samples.

Fig. 21. Asphaltene precipitation boundaries for a well fluid with injected gas.


wellbores should be maintained. • Clean wells with a solvent. Xylene is preferable. A

50:50 mixture of xylene and diesel is adequate provid-ed enough soaking time is allowed.

• Monitor the cleanout procedures to improve recipes forfuture cleanouts

• Investigate the use of asphaltene dispersants in severedeposition cases.

• Suspended asphaltenes in the crude may lead to emul-sion and other production problems at the wet crudehandling facilities and may necessitate increased demul-sifier dosages.

ACKNOWLEDGMENTS

The authors would like to acknowledge Saleh Al-Khudair,Nashi Al-Otaibi from Production Engineering and HishamNasr-El-Din for solvent cleaning and stimulation design.The onset and bulk deposition tests were performed byMohammad Al-Dokhi and Hussain Al-Hakeem. The bailersamples were analyzed by N.S. Meeranpillai andMohammad Al-Awaisi.

NOMENCLATURE

Gas-oil ratio GORGas oil separating plant GOSPHigh-pressure production trap HPPTPressure-volume-temperature PVTSolids detection system SDS

REFERENCES

(1) Kokal, S.L. and S.G. Sayegh, (1995) “Asphaltenes: TheCholesterol of Petroleum,” SPE 29787 paper presentedat the 6th Middle East Oil Show, Manama, Bahrain,March.

(2) Michell, D.L. and J.G. Speight, (1973). “The Solubilityof Asphaltenes in Hydrocarbon Solvents,” Fuel, vol 53,pp 149-152.

(3) Leontaritis, K.J. and G.A. Mansoori, (1989).“Asphaltene Deposition: A Comprehensive Descriptionof Problem Manifestations and ModelingApproaches,” SPE paper 18892 presented at the SPEProd. Operations Symposium held in Oklahoma City,Okla., March 13-14.

(4) Hammami, A., D. Chang-Yen, J.A. Nighswander andE. Strange, (1995). “An Experimental Study of theEffect of Paraffinic Solvents on the Onset and BulkPrecipitation of Asphaltenes,” Fuel Science & Tech.Int., Vol 13(9), pp 1167-1184.

(5) Kokal, S.L. et al., (2002). “Productivity Decline in OilWells Related to Asphaltene Precipitation andEmulsion Blocks,” SPE paper 77767 presented at theSPE Annual Technical Conference and Exhibition, SanAntonio, Texas, October.

(6) Stephenson, W.K., (1990). “Producing AsphaltenicCrude Oils: Problems and Solutions,” PetroleumEngineer Int., pp 24-32.

(7) Alleson, S.J., A. Hammami, H. Maeda and K. Ohno,(1996). “Control of Asphaltene Deposition LaboratoryScreening and Field Evaluation of AsphalteneInhibitors.”

(8) Yen, A., Y.R. Yin and S. Asomaning, (2001).“Evaluating Asphaltene Inhibitors: Laboratory Testsand Field Studies,” SPE paper 65376, Intl. Symposiumon Oilfield Chemistry, Houston, Texas.

49SAUDI ARAMCO JOURNAL OF TECHNOLOGY SPRING 2005

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