CIM 99-64

Advanced Acoustic Approachfor Reservoir Solids Problems/Effects

of Inhibitors on Solids Onset and EOS Modeling

A. Sivaraman, F.B. Thomas and D.B. BennionHycal Energy Research Laboratories Ltd.

Calgary, Alberta, Canada

This paper is to be presented at the SQIh Annual Technical Meeting of The Petroleum Society inCalgary, Alberta, Canada, June 14-18, 1999. Discussion of this paper is invited and may be presentedat the meeting if filed in writing with the technical program chairman prior to the conclusion of themeeting. This paper and any discussion filed will be considered for publication in CIM journals.Publication rights are reserved. This is a pre-print and is subject to correction.

ABSTRACT INTRODUCTION

The production. transport. and refining ofpetroleum and gas are constantly affected by thedeposition of unique phases such as wax, asphaltenes,diamondoids. hydrates and sulphur. The venue of theadvanced acoustic resonance approach has beensuccessfully tested to meet the challenges of solidsproblems in the new millennium. A mercury-freeadvanced acoustic resonance (AR) system, developedand tested at Hycal Energy Research Laboratories forreservoir applications. makes it possible to have fast,highly accurate, measuremenJs during depressurization(9000 psia to /2000 psia) runs on dark live oil atreservoir temperature to identify the onset ofasphaltenes precipitation and bubblepoint during thesame run. Black oil, a problem for visual and opticalmethods. is no barrier for this technology. Effects ofinhibitors on solid-liquid equilibrium have beeninvestigated at various concentrations andtemperatures. The results are presented and discussed.EOS modeling of the AR results have been presentedwith comparisons. There is good agreement betweenmeasured and modeled data.

Asphaltene precipitation from reservoir fluidscauses severe operational problems in the wellheadequipment, separators, tanks and surface equipmenr. Inoffshore production, the clean-up costs haveskyrocketed2. Asphaltenes are dark brown to black solidcompounds with no defmite melting point. Theydecompose while heating and leave a carbonaceousresidue. They are non-crystalline substances or mixturesof relatively high molecular weight fractions ofbitumenwith characteristics of strong aromatic polar substances.They are classified by the particular solvent (n-heptane,n-pentane) used to precipitate them3. Asphalteneprecipitation can be measured experimentally. Light-scattering techniques4 and cross-polarizationrnicroscopr have been used to determine the onset ofsolids precipitation experimentally. Poor signal to noiseratio limits the dynamic range of operation in the case ofN.I.R.

An advanced acoustic resonance system developedat Hycal has been successfully used in the detemlinationof solids precipitation onset in live reservoir fluids. The

system exploits the time evolution of the acousticresponse in fluids under variable and well-controlledconditions of pressure, volume and temperature6.7. Thesystem detects phase transitions on the basis ofdifferences in orders of magnitude in sonic speeds ofvarious fluid states (vapor/liquid/solid) when acousticwaves are propagated in a reservoir fluid going through

phase changes.

a ) 2~1/2 --.

Q

2( C~ f- -

2 ~ +

where C is the sonic speed in the fluid, Oz is an integerwhich defines the modes (Oz = I, first radial and Oz = 2.second radial. etc.). a,... is an Eigen value (IX". = 0 for

radial mode), I is the length of the resonator and a is dieradius of the resonator.

EXPERIMENTAL DETAILSAt die phase transition or onset due to the

magnitude of difference between sonic speeds indifferent phases (liquids, solids and vapor), the acousticresponse goes through a sharp, major change which isanalyzed with the mode tracking utility.

Hycal's AR system, consisting of a cylindricalresonator (0.25 inches in diameter) is made ofhastelIoywith one transducer at the top to generate acousticwaves through the fluid in the resonator, and the otherat the bottom to receive the signals that carryinformation about the fluids through the phasetransitions. One can detect the onset of liquid-solid orliquid-vapor transitions in fluids by analyzing theacoustic responses.

PROCEDURE

The stabilized live oil was charged to a pressure of8500 psia into the resonator cell which was maintainedat a temperature of 210°F (99°C). The resonatorpressure was then decreased by changing the volume ata very slow rate of 50 psia per minute at constanttemperature. The rate of depressurization decreasedwith time and reached approximately 5 psia per minutetowards the end of the experiment. The acoustic datawith volume, temperature and pressure were collectedand subsequently analyzed to detect dte onset of solidsprecipitation and bubble point. The system was thencleaned and prepared for another sample. The systemtemperature was changed to 220°F (105°C) and at thistemperature, the cell was charged to a pressure of 8500psia and the experiment was repeated as before.Measurements were completed at 230°F (110°C) and240°F (116°C). Similar measurements were made forlive oil widt 200/0 deaspbalted oil with 5000 ppm and10,000 ppm Inhibitor X and the onset of asphalteneprecipitation was detennined using AR at threetemperatures, 160°F (71°C), 210°F (99°C) and 240°F(116°C), respectively.

The assembly is housed in a well-insulatedcirculating air-bath with precise temperature conb"olfrom -40°C to 150°C. A digital pressure gauge is usedto measure pressures up to 10,000 psia and a platinumresistance thennometer is used to measure temperaturesprecisely. A linear velocity displacement transducerarrangement is used to accurately measure the volumeat any instant in time. The signal received at the receiveris processed through a low noise amplifier and thenthrough a fast analog to a digital converter (ADC).

Acoustic data acquired by the ADC, at a samplingrate of 100kHz, is synchronized by a trigger signalgenerated by a function generator. The acquisitioncomputer interfaced to the control computer displays theacoustic spectrum through a graphic interface and showsthe data. Pressure, temperature and volume data,gathered during acoustic data acquisition are alsodisplayed. The frequency spectrum presents variousexcited modes of the fluid contained in the resonator.Hycal's custom software has been used to track one ofthe modes along with temperature, pressure and volume.The results show the liquid-solid or liquid-vaportransitions.

RESULTS AND DISCUSSION

The normalized acoustic response for the live oilversus pressure results are shown in Figure I. The onsetof asphaltene precipitation is clearly shown at P s(pressure of 6855 psia). The acoustic response increaseddue to die increase in sonic speed when the nucleationof solid particles occuned in live oil (due to the liquid-solid transformation process). Further depressurization

The acoustic response in a fluid contained in acylindrical resonator can be represented by theresonance frequency and is related to sonic speed by the

following equation:

2

leads to a liquid-vapor transition (bubblepoint) atPb (pressure of 3221 psia).

where

Asphaltene onset pressures and bubblepointpressures for similar nms for die live oil at fourtemperatures (210, 220, 230 and 240°F) are presentedin Table 1. One can see the asphaltene precipitationonset has dropped from 6855 psiaat210°F to 6225 psiaat 240°F. Recent AR data on live oil widi 200/0deaspbalted oil confirmed die audlors' view that diesecombinations strengthen die colloidal suspension andsuppress die asphaltene precipitation pressures to someextent. Further attempts were made widt an Inhibitor Xat Hycal widi various concentrations in live oil plus a200/0 deasphalted oil mix to further suppress the solidsprecipitation onset pressure below die productionpressure (5000 psia was desired).

r.v.PRc,.TrTnPn4Ht

= fugacity of solid (aoo)= molar volume of solid (lit/mol)= pressure (atrn)= gas constant (cal/mol'K)= heat capacity of solid (cal/mol'K)= reference temperature (K)= triple point temperature (K)= triple point pressure (atm)= heat of fusion (cal/mol)

AR results at 5000 ppm and 10,000 ppm ofInhibitor X in live oil plus 20010 deasphalted oil arepresented in Table 1. The results confirmed that, at10,000 ppm levels, the solids precipitation onsetdropped from 4062 psia at 160°F (71°C) to3813 psiaat240°F (116°C), well below the production pressure of5000 psia. The bubblepoint pressure increased from1566 psia at 160°F (71°C) to 2586 psia at 240°F(116°C).

The solid beat capacity and the beat of fusion werealso tuned to matcb the oilier experimental dataparameters. Table 2 presents a comparison of ARexperimental data with the results of modeled data andpercent error. As shown, there was good agreement.Figure 4 presents a comparison of results.

CONCLUSIONS

An advanced acoustic resonance technique hasproven to be a useful tool to detect onset of asphalteneprecipitation in live black oils. It is an ideal probe forevaluating the effects of various new inhI"bitors on theasphaltene onset pressures before testing in actualreservoirs. This will make it more cost-effective and onecan make more efficient use of inhI"bitors. Modeling forfuture prediction of asphaltene precipitation can beachieved by incorporating high quality AR data.

Preliminary results are promising. Figure 2 presentsa comparison of the results presented in Table 1 andshows the effects of inhibitors on solids precipitationonset at various temperatures. Figure 3 presents theeffects of inhibitors on the bubblepoint pressures atvarious temperatures.

EOS MODELING

A single solid component asphaltene model statesthat solid precipitation occurs when the fugacity of thesolid component in the liquid phase is greater than thefugacity of the fluid of the component as a solid. Forthis model, the first data point (6855 psia and 210°F)was chosen as the reference point The solid fugacity forthis point was generated using the multiphase solidfugacity function of the Computer Modeling Group's(CMG) WINPROP. The solid fugacity at this point wasthen set equal to the fugacity of the asphaltenecomponent in the liquid phase predicted by the EOS.The fugacity of the solid phase at other experimentalconditions were then calculated from the followingmodel.

ACKNOWLEDGMENTS

The authors wish to thank Colin Thiessen,YunFeng Hu and William Gibb for their assistance inmaintaining the AR system assembly at Hycal andVivian Whiting for her assistance in the preparation ofthis manuscript

REFERENCES

Leontaritis, K.J. and Mansoori, G.A.: "AsphalteneDeposition: A Survey of Field Experiences andResearch Approaches", Journal of Petrol. Sci. and

Eng., 1988, 1,229.

3

Tuttle, RN.: "High Pour Point and AsphalteneCrude Oils and Condensates", JPT, 1983, 1192.

), Speight, J.G., Wernick, D.L., Gould, K.A.,Overfield, RE., Rao, B.M.L. and Savage, D.W.:"Molecular Weight and Association ofAsphaltenes: A Critical Review", Revue deL 'Institute Francais du Petro/e, 1985,40.

4. Fuhr, B.J., "Properties of Asphaltene from a WaxyCrude", Fuel, 1991,70, 1293.

5. Hammami, A., "Paraffm Deposition from CrudeOils: Comparison of Laboratory Results to FieldData", SPE paper 38776, SPE meeting, SanAntonio, TX, Oct. 5,1997.

6. Sivaraman, A. et al, "Acoustic Resonance: AnEmerging Technology to Identify Wax andAsphaltene Precipitation Onset in ReservoirFluids", Paper 97-96 presented at the 48th A TM ofthe Petroleum Society in Calgary, AB, Canada,June 8-11, 1997.

7. Sivaraman, A., "Advanced Acoustic Resonance forSolids Problems: Meeting Challenges in the New

Millennium", paper presented at the DeepStarAsphaltene Workshop held at Texaco EP1DFacility, Houston, TX, Oct. 22-23, 1998.

4

Table 1. Summary of Acoustic Resonance Results

Temperatureof (OC)

Pressure(psia) 160

(71)

210

(99)

220

(105)

230

(110)

240

(116)No. Sample

Live Oi) Solids onset 6855 6588 6419 6225

3221 3284 3276 3290Bubble point

2 Live oil + 200/0 deaspbalted oil +SOOO ppm of Inht"bitor X

7191 6OSG 5377Solids onset

Bubble point 1963 2385 2630

3 Live oil + 200/0 deasphalted oil +10,000 ppm of Inhibitor X

4062 3932 3813Solids onset

1566 2344 2586Bubble ooiot

Table 2. EOS Model Results

1 ",'];~',

O.8~

'tPI

a:u 0.6

:p

C/)

~

~

"0 0.4CI)

.~

m

E0

Z 0.2

Pb

0 L_~~~3000 5000 6000

Pressure (psis)

7000 8000I

4000

Figure 1. Acoustic Determination of Onset of AsphaltenePrecipitationfor a Live Oil at 210°F

9OOOr-- ~

Z-7000 I

ca.-co°0Do-e:](/)(/)e

a..

"6- -~c.~

'S-

---LiII8 c.+~ D8Ip ~

'%

5000 --u.1»+~ 0I+1~ x

---0--.

~250

3000 I

150

I , I

190 210

Temperature (OF)

, I

230

I ,

170

Figure 2. Acoustic Determination of Effects of Inhibitors onSolids Precipitation Onset at Different Temperatures

5000 I

4000~

-co.~Co-e::]C/)C/)e

a..

~Uv.~

3000~--LM c.+3)% ~ ~~

FLN8 c.+ 2O%D88p c.+ 11XXX1R1m x

1000 L-150

I , I

190 210

Temperature (OF)

--L-

230

. ,250170

Figure 3. Acoustic Determination of Effects of Inhibitors onBubble Point Pressures at Different Temperatures

8000

7000-co'inCo-

~~U)U)

~Q.

~~

~C)

Experirmr.- (M) SoIds 0rI88t

6000 C8QJI8I8d SoIkis Oneet

5000200

I I ,

240 250I , I , I

210 220 230

Temperature (OF)

Figure 4. Comparison of Experimental (AR) Solids PrecipitationOnset Data with EOS Modeled Data