scale prediction and inhibition for oil and gas production at high

Scale Prediction and Inhibition forOil and Gas Production at High

Temperature/High PressureChunfang Fan, Amy T. Kan, SPE, Ping Zhang, Haiping Lu, Sarah Work, Jie Yu, and Mason B. Tomson,

SPE, Rice University

Summary

With the advance of new exploration and production technolo-gies, oil and gas production has gone to deeper and tighter forma-tions than ever before. These developments have also broughtchallenges in scale prediction and inhibition, such as the preven-tion of scale formation at high temperatures (150–200�C), pres-sures (1,000–1,500 bar), and total dissolved solids (TDS)(>300,000 mg/L) commonly experienced at these depths. Thispaper will discuss (1) the challenges of scale prediction at hightemperatures, pressures, and TDS; (2) an efficient method to studythe nucleation kinetics of scale formation and inhibition at theseconditions; and (3) the kinetics of barite-crystal nucleation andprecipitation in the presence of various scale inhibitors and theeffectiveness of those inhibitors. In this study, nine scale inhibi-tors have been evaluated at 70–200�C to determine if they cansuccessfully prevent barite precipitation. The results show thatonly a few inhibitors can effectively inhibit barite formation at200�C. Although it is commonly believed that phosphonate scaleinhibitors may not work for high-temperature inhibition applica-tions, the results from this study suggest that barite-scale inhibi-tion by phosphonate inhibitors was not impaired at 200�C understrictly anoxic condition in NaCl brine. However, phosphonateinhibitors can precipitate with Ca2þ at high temperatures and,hence, can reduce efficiency. In addition, the relationships of scaleinhibition to types of inhibitors and temperature are explored inthis study. This paper addresses the limits of the current preditionof mineral solubility at high-temperature/high-pressure (HT/HP)conditions and sheds light on inhibitior selection for HT/HP appli-cation. The findings from this paper can be used as guidelines forapplications in an HT/HP oilfield environment.

Introduction

Despite the increasing usage of renewable energy, such as windpower and solar power, the world consumption of oil and gasincreases continually. In the meantime, conventional oil and gasreserves are rapidly being depleted, but tremendous deepwaterand unconventional oil and gas resources remain to be producedin the USA and worldwide. A major driving force for a number ofnew developments in the oil and gas industry has been the signifi-cant increase in demand for oil and gas along with the discoveryof deepwater and unconventional oil and gas resources that oncewere considered inaccessible and/or uneconomical. For example,new drilling and completion technologies have been developed tomake oil and gas production from low-permeability sandstone andshale possible. Meanwhile, there are remarkable developments indeep offshore technologies to allow more deepwater production.Exploration companies have been pushing drilling operations far-ther from shore because of technological improvements that allowthem to handle extreme depths and pressures. For example, theTiber oil field is approximately 7 miles below the Gulf of Mexico.Dyer and Graham (2002) pointed out that a number of HT/HP

fields in the North Sea have been in production, and new deep-water production offshore Brazil promises to be a major play foryears. Those HT/HP reservoirs are characterized by high pres-sures (12,000–15,000 psi), high temperatures (>175�C), andvery-high-salinity brines (TDS of approximately 300,000 ppm)(Dyer and Graham 2002). Dyer and Graham (2002) suggestedvarious inorganic mineral scales, such as insoluble barium sulfate,strontium sulfate, calcium sulfate, and calcium and magnesiumcarbonates, are expected in those HT/HP reservoirs. Additionally,halite scale could also be a problem because HT/HP reservoirbrines often have a very high salinity and the decreases of temper-ature and/or pressure, coupled with even small-percentage evapo-ration, during production could cause halite to precipitate.

The developments in deepwater oil and gas production havebrought more challenges than ever before in oilfield scale controland prediction. First, the solubility data of those mineral scales atHT/HP and high-ionic-strength conditions are very limited. Forexample, although Blount (1977) studied barite solubilities inwater up to 300�C and 1,400 bar (approximately 20,305 psi),most of his data are the solubilities of barite in pure water. Heonly studied barite solubilities in 0.2 and 4 molal NaCl solutionsfrom 100 to 250�C and from 14.5 to 7,252 psi. Second, the avail-able solubility data or models from different authors are ofteninconsistent with each other. Third, although more and moreresearch (Dyer and Graham 2002; Dyer and Graham 2003; Fanet al. 2009) on scale inhibition has been conducted at high temper-atures (>100�C), there is still a lack of knowledge about how toprevent scale formation at such HT conditions. Thermal stabilityof most commonly used inhibitors, such as phosphino polycarbox-ylic acid (PPCA), bis-hexamethylenetriamine-penta (methylenephosphonic) acid (BHPMP), diethylene triamine pentamethylenephosphonic acid (DTPMP), and nitrilotrimethylenephosphonicacid (NTMP), are not well studied. The interactions between thoseinhibitors and background electrolytes, such as Ca2þ, Mg2þ, orFe2þ, in very high-salinity brines and at high temperatures areespecially unclear. Also, the application of those scale inhibitors inan oilfield environment is one of the major concerns of engineersand management teams that need to make effective decisions forscale prevention or control at HT conditions. Such decisions areoften made by evaluating the field conditions, scaling tendency, in-hibitor performance, and cost. There are also a variety of methodsof applying scale inhibitors in the field, such as continuous-injec-tion and inhibitor-squeeze treatments. Inhibitor selection is alsodirectly related to how these inhibitors are applied in the field. Forexample, inhibitor-squeeze treatment requires inhibitors to have ahigh inhibitor retention on the rock formation for the treatment tohave a long lifetime. For continuous injection, such a requirementis generally not necessary, but inhibitor thermal stability becomesa major consideration. Overall, many aspects have to be consid-ered in order to make a good decision concerning scale preventionand control, expecially at HTs.

The objectives of this study are (1) to review the available sol-ubility data at HT/HP for common mineral scales, including bar-ite, anhydrite, and calcite, and to examine the accuracy of thesaturation index (SI) predicted by the scale-prediction software,ScaleSoftPitzerTM (SSP) at HT/HP; (2) to develop an efficientmethod to study nucleation kinetics of scale formation and

Copyright VC 2012 Society of Petroleum Engineers

This paper (SPE 130690) was accepted for presentation at the SPE InternationalConference on Oilfield Scale, Aberdeen, 26–27 May 2010, and revised for publication.Original manuscript received for review 18 March 2000. Revised manuscript received forreview 31 March 2011. Paper peer approved 4 April 2011.

June 2012 SPE Journal 379

Supplied by the NIOC Central Library

inhibition at HT/HP and high-TDS conditions; and (3) to evaluatethe performances of numerous scale inhibitors at various tempera-ture conditions.

Background Information

Thermodynamics of Solubility. For a mineral/water reaction,based on the classic Gibbs thermodynamic theory, a general solu-bility equation at a constant temperature and pressure can bedescribed by

Ksp ¼Y

i

avii ¼ exp �DG�

RT

� �; ð1Þ

where Ksp is the crystal solubility product, ai is the activity of theith species, �i is the stoichiometric coefficient of the ith species inthe solid phase, DG� is the change in standard-state Gibbs energyfor equilibrium at the temperature T (K), and R is the gasconstant.

Temperature Effect on Solubility. Using the relationshipbetween the change in Gibbs free energy (DG) for a reaction andthe enthalpy DH and the entropy DS at a specific temperature, Eq.1 at some temperature and Po (a reference pressure) can be rear-ranged as

lnðKspÞT;Po¼ �

DG�T;Po

RT¼ �

DH�T;Po

RTþ

DS�T;Po

R: ð2Þ

Pressure Effect on Solubility. The relation between the solubil-ity product at pressure P and at the reference pressure Po isdescribed as

lnðKspÞT;P ¼ lnðKspÞT;Po�

D �V�Po

RTðP� PoÞ

þ D�j�

2RTðP� PoÞ2

: ð3Þ

Theoretical prediction of equilibrium constants at HPs andHTs has been extensively studied by Helgeson and his colleagues(Helgeson 1967; Helgeson et al. 1981; Shock and Helgeson1988). In their studies, the temperature and pressure dependenceof the partial molar volume, the enthalpy DH, and the entropy DSare used with appropriate integration and differentiation to calcu-late the free energy of formation and the equilibrium constants atany temperatures and pressures. During the process, 10 to 88 ad-justable parameters are involved in the calculations, and the effectof ionic strength is still not included.

Experimental Techniques

Dynamic-Inhibition-Efficiency Test. The HT apparatus used forthis study consisted of four syringe pumps (Pharmacia Biotech P-500) and a flow loop with automatic data recording of differentialpressure (Fig. 1). The first two pumps were used for injecting cati-onic and anionic solutions (each at 120 mL/h). The third pumpwas used to inject inhibitors into the anionic-solution tubing. Thecombined flow rate of anionic solution and inhibitor solution is120 mL/h. At the end of the reaction loop, another pump (Pharma-cia Biotech P-500) was used to add microvolumes of 0.2-MEDTA solution (pH¼ 10) into the loop to stop mineral precipita-tion (5 mL/h). All compartments of the apparatus were connectedwith Hastelloy C or polyether ether ketone (PEEK) materials toavoid corrosion and inhibitor adsorption. Before each experiment,all solutions were sparged with Ar gas for approximately 10minutes, and then subjected to a vacuum for another 10 minutes toremove O2. The measured dissolved-oxygen concentration of thebrines after the gas sparging and vacuum exposure is near 0.1ppm or less. After that, the cationic and anionic solutions werepumped separately into two coils of 165-cm-long Hastelloy C tub-ing with 0.107-cm ID. Approximately 90 cm of the tubing wassubmerged in an oil bath for heating solutions. The heat-transferrate in the Hastelloy C tubing has been measured to be approxi-mately 25�C/s, which is equivalent to approximately 7 seconds forthe solution to be heated from 25 to 200�C. The heated solutionswere then combined by a tee into a 165.1-cm-long tube reactor(reactor volume¼ 0.817 mL). After this reactor, the fluid flowedthrough a 40-in.-long, 0.05-cm-ID PEEK tubing submerged par-tially in cooling water to cool the fluid to room temperature, andthen flowed though a backpressure regulator at 250 psia, whichwas used to prevent the fluid in the loop from vaporizing (water-vapor pressure at 200�C¼ 225 psi). A differential-pressure trans-ducer (Validyne DP 15-34) was connected to the front and backends of the flow loop to monitor the differential pressure in thereaction coil. Data acquisition was accomplished with a digitalmultimeter (Radio Shack) connected to a PC for data logging.Effluent samples were periodically analyzed by inductivelycoupled plasma optical-emission spectrometry (ICP-OES, PerkinElmer 4300DV) for Ba, S, and P concentrations as a double-checkfor barite nucleation. After each experiment, 0.2-M EDTA solu-tion (pH¼ 10) was pumped into the loop for 30 minutes to cleanthe tubing. Then, deionized water was pumped through foranother 30 minutes.

During inhibitor-screening tests, different concentrations of aninhibitor were tested. Because a small amount of inhibitors mayadsorb on the surface of the Hastelloy C tubing to create an inhibi-tor carryover effect from one concentration to another, coaxialtubing with fluorinated ethylene propylene (FEP) tubing as the

. . . . . . . . . . . . . . . . . .

. . . . . . . .

. . . . . . . . . .

Pump C

Pressure Transducer

BackpressureRegulator (250 psi)

Oil Bath

EffluentSampling

Inhibitor

Cations

Anions

SO42

Pump B

Pump A

Pump D

EDTA

Ba2+Cooling Coils

Fig. 1—Experimental apparatus.

380 June 2012 SPE Journal


inner tube was used. FEP tubing was chosen because it has ultra-low carry-over effect, high chemical resistance, and low perme-ability (Cole-Parmer Technical Library 2010). The ultralowcarry-over effect of FEP tubing has been verified experimentallyas well. No inhibitor residues were found in the effluents after aflush of EDTA solutions. This coaxial tubing was made by thread-ing a 1/16- (0.0625-) in. OD and 1/32-in.-ID FEP Teflon tubing intoa 1/8-in.-OD and 0.070-in.-ID Hastelloy C tubing (see Fig. 2).Hastelloy C tubing was used as the outside layer to make thecoaxial tubing capable of sustaining high pressure. Such coaxialtubing was tested for several weeks at 200�C to see whether anyleaking problems might occur. After intensive use and testing, nodistortion of the FEP tubing was found.

Static Inhibition-Efficiency Test. The inhibition-efficiency testsat low-temperature (70�C) conditions were conducted by staticbottle-test methods. The experimental protocol used in this studywas similar to that of He et al. (1994a, b). Cationic and anionic sol-utions were maintained at the desired temperature before mixing.Then, equal volumes of the cationic and anionic solutions weremixed in a jacketed glass reactor and kept at a constant tempera-ture by circulating water from a heating circulator (Julabo F25-MC). The mixed solution was stirred by a Teflon-coated magneticstirring bar. The time between the generation of a supersaturatedstate by mixing the two solutions and the first observed change inturbidity monitored by laser-light scattering using a green laserlight (k¼ 532 nm) is defined as the induction period (t0ind in the ab-sence of inhibitor, and tinh

ind in the presence of inhibitor).

Chemicals. BaCl2�2H2O (ACS reagent grade, Fisher Chemical)and Na2SO4 (ACS reagent grade, Fisher Chemical) were used tomake cationic and anionic solutions, respectively. NaCl (granular,Fisher Chemical) and CaCl2�2H2O (ACS reagent grade, FisherChemical) were used as background electrolytes. A buffer reagent[piperazine-1,4-bis(2-ethanesulfonic acid) sesquisodium salt(PIPES), Fisher BioReagent] and NaOH (Fisher Chemical) wereused to adjust the solution pH to 6.75. This material is known tohave negligible interactions with divalent ions. Various commer-cial-grade scale inhibitors, mainly BHPMP and two sulfonatedcarboxylate copolymers (SCCA and SCCB), were used in thisstudy, and a full list is shown in Table 1. 0.2-M EDTA solutionwas made by dissolving EDTA disodium salt (ACS reagent grade,Fisher Chemical), and its pH was adjusted by adding NaOH untilpH 10. Such a solution was used as the tubing-cleaning solution.

Results and Discussion

Predictions of Scale Formation at HT/HP. Mineral supersatu-ration is probably the most important factor related to kinetics ofscale nucleation and growth and can be expressed by the satura-tion index (SI). Using barite as an example, the SI is defined as:

SIbarite ¼ log½Ba2þ� � ½SO2�

4 � � cBa2þ � cSO2�4

Kbaritesp

( ); ð4Þ

where [Ba2þ] and [SO42–] and cBa2þ and cSO2�

4refer to concentration

(molality) and activity coefficients of barium and sulfate ions,respectively. Thermodynamically, if the SIbarite of an aqueous so-lution is greater than 0, the solution is supersaturated withrespected to barite and barite scale can form; if the SIbarite is lessthan 0, the solution is undersaturated and no barite scale can form;if the SIbarite is equal to 0, the solution is in equilibrium with barite.To solve Eq. 4, the concentration of cationic and anionic ions, theactivity coefficients of those ions, and the solubility product of themineral must be known. An accurate prediction of scale formationlargely relies on the accuracy of activity coefficients and solubilityproduct, which are not only dependent on temperature and pres-sure, but also on ionic strength and brine composition.

It is generally accepted that the Pitzer ion-interaction modelfor activity coefficients provides a good estimate of activity coef-ficients for high-TDS solutions (Pitzer 1995). In this study, all theSI values, including the ones of the mixed cationic and anionic so-lution for barite nucleation and inhibition at high temperatures,were calculated using SSP, in which the Pitzer equations wereused to calculate activity coefficients. A detailed description ofthe Pitzer equations can be found elsewhere (Pitzer 1973; Harvieet al. 1984; Pitzer 1995; Atkinson and Mecik 1997). Althoughother formulations can be used to calculate activity coefficients,

. . . . . . . .

0.125 in.

0.070? 0.070 in.

0.0625 in.

0.032

FEP

Hastelloy C

Fig. 2—Cross section of the coaxial tubing.

TABLE 1—SCALE INHIBITORS USED IN THIS STUDY

Abbreviation Chemical Name

BHPMP Bis-hexamethylene

triamine-penta(methylene phosphonic) acid

DTPMP Diethylene

triamine-penta(methylene phosphonic) acid

HDTMP Hexamethylene

diamine-tetra(methylene phosphonic) acid

MAC2000 Maleic acid copolymer

MAT1000 Maleic acid terpolymer

NTMP Nitrilotri(methylene phosphonic) acid

PPCA1900 Phosphinopolycarboxylic acid

PPCA3800 Phosphinopolycarboxylic acid

SCCA Sulfonated carboxylate copolymer

SCCB Sulfonated carboxylate copolymer



the Pitzer method is probably the most common method used byoilfield service and production companies.

As discussed in the background information, equilibrium con-stants of minerals are temperature and pressure dependent.Although theoretical predictions of equilibrium constants can gen-erally be obtained on the basis of the Helgeson theory, such calcu-lations require too many adjustable parameters. Given the lack ofthermodynamic data of HTs and HPs, in practice, equilibriumconstants of minerals are often calculated by empirical equations.For example, a well-known computer code, SOLMINEQ.88(Kharaka et al. 1988), which has been developed on the basis ofthe Helgeson theory to model mineral/water systems at high tem-peratures and pressures, contains such empirical Keq/temperaturefunctions—for example, the temperature dependence of the baritesolubility product (Langmuir and Melchior 1985):

lnK�sp;barite ¼ 136:035� 7680:41=TðKÞ�48:595log10TðKÞ : ð5Þ

In the SSP, a modified Eq. 5 is used to express both temperatureand pressure effects:

lnK�sp;barite ¼ 136:035� 7680:41

TðKÞ � 48:595log10TðKÞ

þð0:394� 0:0001119ðTðKÞ � 273:15ÞÞ � Patm

500

:

� � � � � � � � � � � � � � � � � � � ð6Þ

The accuracy of such models is dependent upon the accuracy andavailability of experimental data.

A summary of experimental data for common mineral-scalesolubility products published since 1950 is presented in Tables 2through 4 (barite, anhydrite, and calcite, respectively). Afterexamining these data, one can see that reliable solubility data ofcommon scales, such as barite, gypsum, anhydrite, and calcite,are rare at HTs, HPs, and high ionic strengths. These solubilitydata often come from only one or two research groups, and insome cases only one data point is available at certain conditions.Often, reported solubility data vary from one research group toanother. For example, Table 2 lists the available experimentaldata of barite solubility at HT/HP since 1960. Besides the dataobtained by the Brine Chemistry Consortium (BCC) at Rice Uni-versity in 2005, the data of Templeton (1960) and Blount (1977)are the only available experimental data over the range of condi-tions. A comparison of the solubility products between Templeton(1960) and Blount (1977) reveals variation, particularly at high-TDS conditions, as suggested by Fig. 3a. The calculated SI valuesof their solubility data by SSP are plotted in Fig. 3b, which showsthat the predictions of SI values by SSP are consistent with Tem-pleton’s data and Blount’s data, although a closer match wasfound between the predictions and Templeton’s data. The barite-solubility data from BCC (2005) and the corresponding SI valuescalculated by SSP are plotted in Fig. 4. This figure suggests thatat 1,000 psi with 1-M NaCl conditions, the predicted SI values areconsistent with the experimental data. Fig. 5a shows how baritesolubilities change with pressure at three different temperatures:96, 189, and 249�C. The plotted solubility data were adapted fromBlount (1977). As shown in Fig. 5a, the trends of barite solubilityvs. pressure are similar at all three temperatures: barite solubilityincreases with increasing pressure. The corresponding SI valuespredicted by SSP match well at 96 and 189�C, even at high pres-sures, but at 249�C the offset between the predicted value and ex-perimental data increases with increasing pressure (Fig. 5b), andthis will be the subject of future studies. Some of these deviationsappear to be systematic, and a systematic error of 60.2 in SI cor-responds to an error in concentration of 660%, which is muchgreater than typical field-data errors. Often, this will be the

. . . . . . . . . .

TABLE 2—EXPERIMENTAL DATA ON BARITE SOLUBILITY

PRODUCT AT HT/HP SINCE 1960

Authors Salt Matrix Temperature Pressure

Templeton (1960) NaCl (0–5 m) 25–95�C Atmosphere

Blount (1977) NaCl (0–4 m) 96–500�C 500–21,000 psi

BCC* (2005) NaCl (1 m) 24–190�C 1,000 psi

*Rice University, Brine Chemistry Consortium

TABLE 3—EXPERIMENTAL DATA ON ANHYDRITE SOLUBILITY PRODUCT AT HT/HP SINCE 1960

Authors Salt Matrix Temperature Pressure

Dickson et al. (1963); Blount and Dickson (1969) NaCl (0–6 m) 100–450�C 80–15,000 psi

Marshall et al. (1964); Marshall and Slusher (1968) NaCl (0–4 m) 125–200�C Vapor pressure of water

Templeton and Rodgers (1967) NaCl (0–5.76 m) 250–325�C Vapor pressure of water

TABLE 4—EXPERIMENTAL DATA ON CALCITE SOLUBILITY PRODUCT AT HT/HP SINCE 1950

Authors System Temperature Pressure

Ellis (1959, 1963) CaCO3–H2O–CO2–NaCl (0–1.0 mol/kg) 98–320�C 0.97–62.9 bar PCO2

Segnit et al. (1962) CaCO3–H2O–CO2 75–200�C 2–60 bar PCO2

Sharp and Kennedy (1965) CaCO3–H2O–CO2 200–600�C 42–1,433 bar (0–10% CO2)

Berendsen (1971) CaCO3–H2O–CO2 100–300�C 100–1,000 bar

Plummer and Busenberg (1982) CaCO3–H2O–CO2 0–90�C 1 bar

CaCO3–H2O–CO2–N2–NaCl (0–6.24 m)

CaCO3–H2O–CO2–N2–KCl (0–5.84 m)

Wolf et al. (1989) CaCO3–H2O–CO2–N2–CaCl2 (0–4.60 m) 10–60�C 1 bar; 0.0076–0.01 bar PCO2

CaCO3–H2O–CO2–N2–NaCl (0–6.33 m)–

CaSO4 (0.0–0.0597 m)

He and Morse (1993) CaCO3–H2O-Na-Mg-Cl-SO4-CO2 0–90�C 1 bar (30–98% CO2)

CaCO3–H2O–CO2–NaCl (1 m)–

BCC (2005) Mg (0.18 m)–SO4 (0.033 m)–Acetate 24–190�C 4–158 bar

(0.011 m)–Ca (0.06 m)



difference between the predictions that a brine might need noscale inhibitor and those that suggest use of a scale inhibitor.

Fig. 6a shows a pressure dependency of anhydrite-solubilitydata collected from Dickson et al. (1963) and Blount and Dickson(1969) at a constant NaCl concentration (1.9-m NaCl) and 150,200, and 249�C. At all three temperatures, anhydrite solubilitiesincrease with the increase in pressure. Fig. 6b is a plot of the cor-responding predicted SI values vs. pressure. Similar to Fig. 5b,Fig. 6b shows that at low temperature (150�C), the predicted SIvalues are consistent with the experimental data at all the testedpressure conditions, whereas at high temperature (200 and249�C), a large mismatch is found at approximately 17,500 psia.Very few reliable data are available, suggesting that more experi-mental work is needed at these HT/HP conditions.

The available experimental data of calcite solubility at varioustemperatures, pressures, TDS, and CO2 concentrations collectedsince 1950 are listed in Table 4. Among common scales, calcite isthe most-studied mineral, but the system of CaCO3-CO2-H2O isalso the most complex one, mainly because it involves more equi-librium constants, such as CO2 partitioning constants (KH ¼

PCO2=fCO2;aqg), ionization constants (e.g., K1¼fHCO�3 g�fHþg=

fCO2;aqg), stability constants (Kst¼fCaHCOþ3 g=fCa2þg�fHCO�3 g),and solubility products (Ksp¼fCa2þg�fCO2�

3 g). Each equilibriumconstant is a function of temperature and pressure. The SI valuescalculated by SSP, but based on the experimental data from Wolfet al. (1989) and Ellis (1959, 1963), are plotted in Fig. 7. As indi-cated by Figs. 7a and 7b, the predicted SI values are consistentwith the experimental data, even at high NaCl concentrations (> 4m, Fig. 7a) and high temperatures (200�C, Fig. 7b).

These comparisons of the predicted SI values as calculated bySSP and the available experimental data from literature indicatethat SSP can accurately calculate SI values of brines up to 200�C,12,000–15,000 psi, and approximately 300,000-ppm-TDS condi-tions. However, more experimental and modeling work is neededin order to get an accurate prediction of scale formation at higher-temperature (>200�C), -pressure (>15,000 psi), and -TDS (>300,000 ppm) conditions. For example, at HTs (>200�C) andHPs (>15,000 psi), both barite and anhydrite are more solublethan model predictions (Figs. 5b and 6b, respectively). Thismeans that model predictions at HTs and HPs may overestimate

Barite Solubility vs. TDS

0.00

0.10

0.20

0.30

0.40

0.50

0 2 4 6NaCl (m)

95ºC (Templeton)

100ºC (Blount)

(a) SI Prediction

-1

-0.5

0

0.5

1

0 1 2 3 4 5NaCl (m)

Bar

ite S

I

Templeton, 95ºCBlount, 100ºC

(b)

BaS

O4(

mm

)

Fig. 3—(a) Comparison of barite-solubility data from Templeton (1960) and Blount (1977) at various TDS conditions and (b) the cor-responding SI values predicted by SSP at the same conditions.

0.00

0.10

0.20

0.30

0.40

0 50 100 150 200

Temperature (ºC)

Ba

Solu

bilit

y (m

m) 1 m NaCl, BCC, 2005

1000 psi

(a)

-1.0

-0.5

0.0

0.5

1.0

0 50 100 150 200

Temperature (ºC)B

arite

SI

1 m NaCl, BCC, 2005

(b)

Fig. 4—(a) Barite solubilities [from BCC (2005)] vs. temperature and (b) the corresponding SI values predicted by SSP vs.temperature.

Barite Solubility vs. Pressure

0.0000.0050.0100.0150.0200.0250.0300.0350.040

0 5000 10000 15000Pressure (psia)

BaS

O4(

mm

)

96ºC 189ºC 249ºC

(a) SI Prediction

-1

-0.5

0

0.5

1

0 5000 10000 15000 20000Pressure (psia)

SI

96ºC

189ºC

249ºC

(b)

Fig. 5—(a) Barite solubilities (Blount 1977) vs. pressure and (b) SI values (predicted by SSP) vs. pressure.



scale risk, and in fact inhibitor treatment may not be needed in thefield. Therefore, precaution has to be taken for any scale controlat HTs and HPs.

Kinetics of Barite-Scale Formation and Inhibition. Barite-crystal nucleation at 175–200�C was studied in the aforemen-tioned HT apparatus in order to examine the reliability of thismethod and the feasibility of using the coaxial-tube reactor. Pre-

cipitation is monitored by measuring the change in barium con-centrations and differential pressures. Table 5 lists 11 brinecompositions used in the following experiments. A typical barite-nucleation result where Brine 1 was injected into the HT appara-tus is plotted in Fig. 8. As illustrated in Fig. 8, the concentrationof barium decreased immediately after injecting the solutions,while the differential pressure did not show an increase untilapproximately 22 minutes. Note that the spikes in elevated Ba

(a) (b)

-1

-0.5

0

0.5

1

0 2 4 6

NaCl Conc. (m)

Cal

cite

SI

Wolf

60ºC

-1.0

-0.5

0.0

0.5

1.0

0 200 400 600Temperature (F)

Cal

cite

SI

Ellis, 1 m NaCl, 12 atm CO2Wolf, 1 m NaCl, 0.01 atm CO2 Ellis, 0 m NaCl, 12 atm CO2

Fig. 7—The calcite SI values predicted by SSP vs. NaCl concentration (a) and vs. temperature (b). Note that experimental data arecollected from Wolf et al. (1989) and Ellis (1959, 1963).

Anhydrite Solubility vs. P

0.000.010.010.020.020.030.030.04

0 5,000 10,000 15,000 20,000 25,000Pressure (psia)

CaS

O4

(m)

150ºC 200ºC 249ºC

1.9 m NaCl

(a)SI Prediction

-1.00

-0.50

0.00

0.50

1.00

0 5,000 10,000 15,000 20,000 25,000Pressure (psia)

Anh

ydrit

e SI

150ºC 200ºC 249ºC

1.9 m NaCl

(b)

Fig. 6—(a) Anhydrite-solubility data vs. pressure at 150, 200, and 249ºC and (b) the corresponding SI values predicted by SSP atthe same conditions. Note that all the anhydrite-solubility data are from Dickson et al. (1963) and Blount and Dickson (1969).

TABLE 5—BRINES USED IN THIS STUDY

Brine

Cationic Brine Matrix Anionic Brine MatrixInh.* Conc.

(mg/l) SIBartieNaCl (M) Ca2þ (M) Ba2þ (mM) NaCl (M) Ca2þ (M) SO42– (mM)

1 1 0 2.26 1 0 2.26 0.5–1.0 1.23 (200�C)

2 1 0 0 – – – 1.0 –

3 1 0.05 0 – – – 1.0 –

4 1 0 4 – – – 1.0 –

5 1 0.2 4 – – – 1.0 –

6 1 0 8 1 0 0.4 0–1.0 1 (200�C)

7 1 0.1 8 1 0.1 0.6 0–1.0 1 (200�C)

8 1 0.2 8 1 0.2 0.8 0–1.0 1 (200�C)

9 1 0.025 1.64 1 0.025 1.64 0.5 1.33 (70�C)

10 1 0.025 2.26 1 0.025 2.26 0.5 1.61 (70�C)

11 1 0.025 0.64 1 0.025 0.64 0.5 0.14 (175�C)

0.08 (200�C)

*Specific inhibitors are described in the text and figure captions.



concentrations may be caused by mechanical problems with theapparatus. The Pharmacia syringe pump refilled for 10 secondsfor every 10 mL of flow. One can anticipate some fluctuation ofbarium and sulfate concentrations during the refilling process.However, the overall results indicate that barite precipitated rap-idly, but the pressure buildup is much delayed until the coaxial-tube reactor is substantially blocked. Similarly, it has beenobserved that calcite precipitation was better indicated by meas-

uring the pH change than by measuring the pressure buildup inthe tubing (Matty and Tomson 1988). At the end of the experi-ment, the inner FEP tubing was removed to determine the locationof the scale buildup. An image of the inner tube is shown in Fig. 9.White precipitate appeared at �60 cm from the influent end of thetubing. The X-ray diffraction (XRD) analysis of the solids isshown in Fig. 10; the solid is identified to be barite crystals. In thesame experiment, the effluents were collected and filtered by

Barite Nucleation (SI=1.23 at 200ºC)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 5 10 15 20 25 30 35

Time (min)D

iff. P

ress

ure

(Psi

)

0

20

40

60

80

100

120

140

160

Ba C

on. (

mg/

l)

Pressure Change Ba Con.

Fig. 8—The results of the barite-nucleation-kinetics study through monitoring differential-pressure change and barium-concentra-tion change in effluents. The experiment was performed at 200ºC, and the mixed brine contained 1M NaCl, 1.13 mM Ba21, and 1.13mM SO4

2–, which gives the SI value of barite as 1.23. Note that the concentration of barium decreased immediately after injectingthe solutions, while the differential pressure showed an increase after approximately 22 minutes. Both indicate that bariteprecipitated.

Influent Effluent

First appearance of barite scale

Fig. 9—Image of the inner tube that was used in the experiment charted in Fig. 8.

10 20 30 40 50 60 70

Inte

nsity

(cou

nts)

2 Theta (deg.)

Barite Standard

Sample From Tubing

Fig. 10—The plots of XRD spectrum of barium sulfate precipitates formed in the reaction tube at 200ºC.



membrane filtration. Small barite crystals were found on the mem-brane by scanning electron microscopy (SEM), which suggestssome precipitates did not attach on the tubing and were flushedout. The SEM images of the barium sulfate crystals are shown inFig. 11. From this figure, one can see the very uniform single crys-tals with a distinguished structure that matches barite-crystal mor-phology. The size of a single barite crystal is approximately 3.5lm long and 1.2 lm wide. If these barite particles were trapped inthe flowing tube, the crystals could grow bigger. Fig. 12 shows abarite particle stuck on an in-line filter installed at the end of theHT apparatus.

Two sets of barite-inhibition experiments were conducted byusing SCCB and BHPMP as inhibitors. Both sets of experimentswere performed at 200�C with barite SI¼ 1.23 (Brine 1). With 1-

mg/L SCCB, the differential pressures were generally maintainedat less than 0.1 psia for at least 150 minutes (Fig. 13). Some fluc-tuations in differential pressures, characterized by sudden rise anddrop of differential pressures, are observed between 0 and 150minutes’ reaction time. Such fluctuations in differential pressurescould also be caused by the syringe pumps, as explained previ-ously. Without SCCB, the tubing was blocked within 30 minutes,accompanied by a steady gradual increase in differential pressuresand a decrease in effluent barium concentrations (see Fig. 8). Inthe presence of 1-mg/L SCCB, the barium concentrations waskept constant (see Fig. 13) and the measured values are within 5%error margin compared with the barium concentration of the stocksolution (148 mg/L) used in this study. Taking systematic errorsinto the consideration, one can conclude that 1-mg/L SCCB

10 µm

Fig. 11—The SEM images of barium sulfate precipitates collected by filtration of the effluents. The experiment was performed at200ºC, and the mixed brine contained 1M NaCl, 1.13 mM Ba21, and 1.13 mM SO4

2–, which gives the SI value of barite as 1.23. Thecrystals are uniform single barite crystals and are approximately 3.5 lm long.

20 100

Fig. 12—An SEM image of barium sulfate precipitates formed on a screen of an in-line filter.

0 50 100 150 200 250

1802

1

00.20.40.60.8

1.21.41.61.8 160

140

120100

80

60

40

20

0

Time (min)

Diff

. Pre

ssur

e (P

si)

[Ba2+

] (m

g/l)

Barite Inhibition by Inhibitor SCCB (SI=1.23 at 200ºC)


1 mg/l SCCB 0.5 mg/l SCCB

Fig. 13—Plots of the effluent Ba21 concentrations vs. run time, and pressure vs. run time, after a supersaturated brine (bariteSI 5 1.23 at 200ºC) flowed through the HT apparatus. In these experiments, the initial Ba21 concentrations are 148 mg/L, and for thefirst 150 minutes, the initial concentration of SCCB is 1 mg/L. After that, the concentration of SCCB is dropped to 0.5 mg/l.



successfully prevented barite from precipitating. Once the con-centration of the inhibitor SCCB dropped to 0.5 mg/L, a largeamount of barite scale formed and blocked the tubing afterapproximately 50 minutes, indicated by both a gradual increase inpressures and a decrease in the barium concentrations (Fig. 13).Similar experiments were conducted at 200�C using BHPMP asan inhibitor. A typical result is shown in Fig. 14. With 1-mg/LBHPMP, the differential pressures were barely increased for atleast 120 minutes, which indicated that tubing was not severelyblocked (Fig. 14). However, the fact that barium concentrationsdropped by approximately 20% (see Fig. 14) suggests that baritescale did form in the tubing. After 120 minutes, the concentrationof BHPMP was reduced to 0.5 mg/L. A large amount of baritescale formed and blocked the tubing within 20 minutes, indicatedby both a continuous increase in pressure and a further decreasein barium concentration (Fig. 14).

The results from these two sets of experiments imply that bothBHPMP and SCCB can inhibit barite-scale formation at 200�C,while SCCB is more effective than BHPMP in terms of control-ling barite-scale formation at HTs under these conditions. A slowresponse of pressure change shown in Fig. 8, and the differencebetween the measurements of pressure changes and barium-concentration changes shown in Fig. 14, indicate that withoutsimultaneously measuring concentration change, results of the tra-ditional dynamic tube-blocking method can be misleading. Gener-ally, the severity of barite-scale buildup may be divided into thefollowing three stages: (1) nucleation and precipitation of baritescales; (2) attachment of barite to tubing surfaces; and (3) crystalgrowing to large particles. Simultaneously monitoring of bothpressure buildup and concentration changes will allow us to con-duct more-precise scale control.

As shown in Fig. 14, although BHPMP (1mg/L) did not com-pletely prevent barite-scale formation, the tubing was not blocked.If precipitates flushed out without attaching onto tubing walls, theflow would not be affected by the precipitates. Finding chemicalsthat can change the sizes, morphologies, and/or chemistry of pre-cipitates to improve their transportability could be a solution forscale control at HTs and HPs. For example, many studies (Xiaoet al. 2001; Tomson et al. 2003; Jones et al. 2006; Wang et al.2006; Barouda et al. 2007) have shown that crystal morphologyand size can be altered by inhibitors, but more work is needed inthis area.

Stability and Compatibility of BHPMP With Ca21

and Ba21

at 200ºC. Two solutions (Brines 2 and 3 in Table 5) containing1-mg/L BHPMP, 1-M NaCl, and 0- or 0.05-M Ca2þ ions, wereflowed through the HT apparatus at 200�C, at 1 and 0.5 mL/min.The BHPMP concentrations of the stock and the effluent solutionswere compared. In the absence of calcium, the concentration ofBHPMP in the effluents was the same as the BHPMP concentra-tion in the stock solution (1 mg/L), indicating that BHPMP wasstable at 200�C (Fig. 15a). Note that the stability-test duration isless than 1 minute in this study. Additional thermal-stability testsof chemicals under anoxic conditions will be reported in thefuture. In the presence of 0.05-M Ca2þ, BHPMP concentrationwas only 0.66 mg/L or 0.60 mg/L at 1- and 0.5-mL/min flowrates, respectively (Fig. 15a). This indicates that BHPMP was ei-ther adsorbed on the tubing wall or precipitated as a calcium salt.In another set of experiments, the compatibility of BHPMP withCa2þ and Ba2þ at 200�C was studied, in which the solutions(Brines 2, 4, and 5 in Table 5) contained approximately 1-mg/LBHPMP in the presence and absence of divalent cations (0.004-M

Barite Inhibition by BHPMP (SI=1.23 at 200ºC)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 20 40 60 80 100 120 140 160 180

Time (min)D

iff. P

ress

ure

(Psi

)

0

20

40

60

80

100

120

140

160

Ba

Con

. (m

g/l)


PMPHB l/gm 5.0PMPHB l/gm 1

Fig. 14—Plots of the effluent Ba21 concentrations vs. run time, and pressure vs. run time, after a supersaturated brine (bariteSI 5 1.23 at 200ºC) flowed through the HT apparatus. In these experiments, the initial Ba21 concentrations are 148 mg/L, and for thefirst 120 minutes, the concentration of BHPMP is 1 mg/L. After that, the concentration of BHPMP is dropped to 0.5 mg/L.

0

0.2

0.4

0.6

0.8

1

1.2

BH

PM

P (m

g/l)

Stock Solution1 ml/min 0.5 ml/min

With 0.05M Ca2+Without Ca2+

(a)

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60Time (min)

BH

PM

P C

on. (

mg/

l)

BHPMP onlyBHPMP & 0.004M BaBHPMP & 0.004M Ba & 0.2M Ca

(b)

Fig. 15—Results of BHPMP stability and compatibility with Ca21 and Ba21 at 200ºC. (a) The comparisons of BHPMP concentrationsin stock solution and the effluents at 1-mL/min (residence time 5 115 seconds) and 0.5-mL/min flow rate (residence time 5 230 sec-onds). (b) The concentration of BHPMP in the effluents vs. solution flowing time. Note that the flow rate is 1 mL/min.



Ba2þ ions and/or 0.2-M Ca2þ ions) and were flowed through theHT apparatus at 1 mL/min and at 200�C. The effluent solutionswere collected every 10 minutes for BHPMP measurement. Theresults are plotted in Fig. 15b and show that the BHPMP concen-trations in the effluents of the solutions with only BHPMP areconsistent with the BHPMP concentration in the stock solution. Inthe presence of 0.004-M Ba2þ, the BHPMP concentrations in theeffluents remain unchanged. However, with 0.2-M Ca2þ, theBHPMP concentrations in the effluents dropped from approxi-mately 1 mg/L to approximately 0.6 mg/L (Fig. 15b). This con-firms that BHPMP can react with calcium in solutions and reducethe overall inhibitor efficiency.

Effect of Ca21 on BHPMP Inhibition Efficiency at 200ºC. Theeffect of calcium ions on BHPMP inhibition efficiency has beenstudied by many researchers (Collins 1999; Boak et al. 1999;Xiao et al. 2001; Tomson et al. 2003; Boak et al. 2003; Sobie andLaing 2004; Fan et al. 2009). Generally, it is confirmed that cal-cium ions have a positive effect on BHPMP-inhibition efficiency,especially at low-temperature conditions (<100�C). However, atHTs (150 and 200�C), the experimental results from our previousstudy (Fan et al. 2009) show that calcium ions (0.05 M) have ei-ther no effect or a slight negative effect on barite-scale inhibitionby BHPMP at 0.5 mg/L. It is possible that there is not enoughBHPMP to inhibit barite formation owing to the precipitation ofCa-BHPMP at HT. In order to test this hypothesis, three sets ofexperiments with the solutions containing 0.0-, 0.1-, and 0.2-MCa2þ (Brines 6, 7, and 8 in Table 5), respectively, were conductedin the HT apparatus with 0- and 1-mg/L BHPMP. All barite-nucleation and -inhibition experiments were conducted at 200�Cwith SIbarite¼ 1. Figs. 16 through 18 are the results of barite-nucleation and -inhibition experiments with 0.0-, 0.1-, and 0.2-M

Ca2þ, respectively. The results suggest that at all three calciumconcentrations with 1-mg/L BHPMP, the tubing was not blocked(see Figs. 16b, 17b, and 18b), whereas without BHPMP the tubingwas blocked (see Figs. 16a, 17a, and 18a). In all three barite-nucleation experiments, scale-buildup time varied with calciumconcentration. The time for a substantial change in the differentialpressures is approximately 43 minutes at 0-M Ca, 87 minutes at0.1-M Ca, and 71 minutes at 0.2-M Ca. Barite-scale buildup ismuch slower in the presence of 0.1-M Ca than in solutions with-out Ca. However, the scale buildup at 0.2-M Ca is faster than at0.1-M Ca. Note that Brine 8 is supersaturated in respect to bothbarite and anhydrite. Also shown in Figs. 16 through 18 are theBa concentrations vs. time. The slope of the data reveals the ratesof precipitation. Table 6 is a list of the precipitation rates in thepresence and absence of BHPMP. Consistent with the scale-buildup times, the rate of precipitation is faster in the absence ofCa2þ than in the presence of Ca2þ. With 0.1-M and 0.2-M Ca2þ

in solutions, BHPMP (1 mg/L) appears to be able to completelystop barite precipitation (see Figs. 17b and 18b), whereas in theabsence of Ca2þ, barite scale was not completely inhibited by 1-mg/L BHPMP (see Fig. 16b). Note that Brine 7 contained moreSO4

2þ than Brine 6, while less barite precipitation was observed.

Evaluation of Scale Inhibitors. In this study, nine scale inhibi-tors have been evaluated with both a dynamic tube-blocking testand a static-bottle test. The dynamic tube-blocking tests were con-ducted at 175 and 200�C, whereas the static-bottle tests were per-formed at 70�C using the brine compositions of Brines 9 through11 in Table 5. In all tests, mixed brines contained 0.025-M Ca2þ and0.5-mg/L inhibitor. The experimental results are plotted in Fig. 19.As shown in Figs. 19a and 19b, most phosphonates, maleic acidcopolymers, and phosphinopolycarboxylic acids increase barite

Barite Nucleation Without Ca2+ (SI=1 at 200ºC)

0

1

2

3

4

0 10 20 30 40 50

Time (min)

Diff

. Pre

ssur

e (P

si)

0

200

400

600

800

[Ba2+

]. (m

g/l)


(a)Barite Inhibition Without Ca2+ (SI=1 at 200ºC)

0

0.5

1

1.5

2

2.5

0 50 100 150 200Time (min)

Diff

. Pre

ssur

e (P

si)

0100200300400500600700

[Ba2+

] (m

g/l)


1 mg/l BHPMP No Inh.

(b)

Fig. 16—The plots of the effluent Ba21 concentrations vs. run time, and differential pressure vs. run time, after a supersaturatedbrine flowed through the HT apparatus. (a) Barite SI 5 1, without Ca21 ions, at 200ºC. (b) Barite SI 5 1, without Ca21 ions, and withBHPMP at 200ºC.

(a) (b)Barite Nucleation With 0.1M Ca2+ (SI=1 at 200ºC)

00.20.40.60.8

11.2

0 20 40 60 80 100

Time (min)

Diff

. Pre

ssur

e (P

si)

200300400500600700

[Ba2+

] (m

g/l)


Barite Inhibition by BHPMP With 0.1M Ca2+

(SI=1 at 200ºC)

00.20.40.60.8

1

0 50 100 150 200

Time (min)

Diff

. Pre

ssur

e (P

si)

0

200

400

600

800

[Ba2+

] (m

g/l)


1 mg/l BHPMP

No Inh.

Fig. 17—The plots of the effluent Ba21 concentrations vs. run time, and differential pressure vs. run time, after a supersaturatedbrine flowed through the HT apparatus. (a) Barite SI 5 1, with 0.1-M Ca21 ions, at 200ºC. (b) Barite SI 5 1, with 0.1-M Ca21 ions andwith BHPMP at 200ºC. The comparison between these two figures suggests that calcium ions do improve BHPMP inhibitionefficiency.



induction time at 70�C and at SIbarite¼ 1.33 and 1.61. However,most inhibitors cannot inhibit barite precipitation at HTs and at SIas low as 0.08, except for SCCA and BHPMP. The poor inhibitionefficiency may be related to the inability to inhibit the fast baritenucleation at HTs (He et al. 1994b, 1995). BHPMP has been iden-tified as one of the best barite inhibitors (He et al. 1996). At HTs(175 and 200�C), BHPMP showed a reduced inhibition effect onbarite-scale formation. Surprisingly, one sulfonated carboxylatecopolymer with poor performance at low temperatures, SCCA,shows a strong inhibition effect in preventing barite-scale

TABLE 6—A LIST OF THE RATES OF CHANGE IN BARIUM

CONCENTRATION PER UNIT TIME*

D[Ba2þ]/Dt 0.0-M Ca2þ 0.1-M Ca2þ 0.2-M Ca2þ

No inhibitor (mg/l/min) �1.5 �0.25 �0.80

1 mg/l BHPMP (mg/l/min) �0.34 0.016 �0.04

*The rates of change in [Ba2þ] per unit time were obtained from Figs. 16 through

18.

Contro

l

BHPMP

DTPMP

HDTMP

MAC2000

MAT1000

PPCA1900

PPCA3800

SCCA

70°C, SI=1.33

175°C, SI=0.14

200°C, SI=0.08

1600

1400

1200

Inhi

bitio

n Ti

me

(s)

Inhi

bitio

n Ti

me

(s)

1000

1000

1500

2000

2500(b)

(a)

800

600

400

500

2000

0

70°C, SI=1.61

NTMP

Contro

l

BHPMP

DTPMP

HDTMP

MAC2000

MAT1000

PPCA1900

PPCA3800

SCCANTMP

Fig. 19—Comparison of induction times of barium sulfate precipitation in the presence of different scale inhibitors. (a) Inductiontime of barium sulfate precipitation at 70ºC and two different supersaturated brines of barite, SI 5 1.33 and 1.61. (b) Induction timeof barium sulfate precipitation at 175ºC and SI 5 0.14, and 200ºC and SI 5 0.08. Note that all the brines used in this study contained1-M NaCl, 0.025-M Ca21, and 0.5 mg/L of different scale inhibitors, and varied barium and sulfate concentrations that correspondto their barite SI values.

Barite Nucleation With 0.2M Ca2+ (SI=1 at 200ºC)

0

0.5

1

1.5

2

0 20 40 60 80

Time (min)

Diff

. Pre

ssur

e(P

si)

0

200

400

600

800

[Ba2+

] (m

g/l)


(a)Barite Inhibition by BHPMP With 0.2M Ca2+

(SI=1 at 200ºC)

00.050.1

0.150.2

0.250.3

0 40 80 120 160

Time (min)

Diff

. Pre

ssur

e (P

si)

0

200

400

600

800

[Ba2+

] (m

g/l)


1 mg/l BHPMP No Inh.

(b)

Fig. 18—The plots of the effluent Ba21 concentrations vs. run time, and differential pressure vs. run time, after a supersaturatedbrine flowed through the HT apparatus. (a) Barite SI 5 1, with 0.2-M Ca21 ions, at 200ºC. (b) Barite SI 5 1, with 0.2-M Ca21 ions andwith BHPMP at 200ºC. The comparison between these two figures suggests that calcium ions do improve BHPMP inhibitionefficiency.



formation at HTs (175 and 200�C). Its inhibition efficiency is evenbetter than that of BHPMP at HTs. This is consistent with previousobservations with SCCB. Both sulfonated inhibitors showed betterperformance than BHPMP at 200�C. A possible explanation is thatthose sulfonated polymeric inhibitors may form stable calciumcomplexes at HT to block barite-crystal growth, whereas BHPMPmay precipitate with calcium and render it unavailable for bariteinhibition. It may also suggest that sulfonated inhibitors are morestable than phosphonates at the HTs. However, sulfonated copoly-mers often show poor adsorption ability on rock-matrix surfaces.Hence, when inhibitor-squeeze treatment is used in the field, inhib-itor-adsorption characteristics on actual core materials have to betested first. For inhibitor continuous-injection treatment, inhibitor-adsorption characteristics may not be one of the most importantissues, while inhibitor stability is one of the major concerns, espe-cially at HTs (Cowan and Weintritt 1976). In an inhibitor continu-ous-injection treatment, inhibitors are injected continuouslythrough a capillary tubing. If inhibitors are not stable and/or formprecipitates at HTs, those precipitates may block the capillary tub-ing and cause a problem. Additionally, inhibitors may react withhigh-calcium brines to form pseudoscales. Therefore, when choos-ing a best inhibitor for oilfield-scale treatment, field conditions,scaling tendency, inhibitor performance, inhibitor-adsorption char-acteristics on actual core materials, inhibitor stability and thermalstability, methods for applying inhibitors, and the overall cost haveto be eavaluated.

Conclusions

A review of the available solubility data for barite, anhydrite, andcalcite has been made and the challenges of scale prediction atHTs, HPs, and high TDS have been discussed. The goodness ofthe fit between the available experimental solubility data and thecorresponding SI values predicted by SSP is excellent at mostconditions, except at a few very harsh conditions, such as 250�Cand 14,648 psia, at which only one data point is available. Theresults indicate that the solubility data of common mineral scalesat HTs, HPs, and high ionic strengths are limited and sometimesconflict with each other. This clearly indicates that more experi-mental work needs to be conducted at HT/HP conditions. Theresults also suggest that more caution is needed in scale preditionat harsh conditions for any field application. For example, in anultradeepwater production, well conditions such as temperatures,pressures, TDS, and alkalinity are often difficult to measure accu-rately. A small error in the measurement can cause a totally differ-ent scale prediction at these HT/HP conditions.

In this study, a modified HT apparatus based on the dynamictube-blocking method and lattice-ion monitoring has been built tostudy inhibition efficiency of various inhibitors at HTs. The ex-perimental results have shown that this apparatus affords a reli-able and efficient method for performing mineral-scale-inhibitorscreening under HT conditions. After conducting the barite-nucle-ation and inhibition experiments and evaluating nine scale inhibi-tors, it was found that (1) early detection of barite scale relies onmeasuring the lattice-ion concentration in addition to the pressurebuildup; (2) many inhibitors effectively inhibit barite scale atmoderate temperature (70�C) but are not effective at HTs(175–200�C); (3) barite-scale inhibition by phosphonate inhibitorswas not impaired at 200�C under strictly anoxic conditions and inNaCl brine, but phosphonate inhibitors can precipitate with Ca2þ

ions in the brine at HTs and hence reduce their inhibition effi-ciency; and (4) sulfonated carboxylate copolymers show a betterinhibition effect on barite-scale formation at HTs (175 and200�C). Note that the inhibitors were exposed to HTs for 12–230seconds in the testing conditions of this study, and the long-termthermal stability of inhibitors under anoxic conditions will bereported in the future. These results can be used as guidelines inselecting inhibitors for oilfield application in HT/HP conditions.

Nomenclature

ai ¼ activity of the ith speciesK1 ¼ ionization constant

KH ¼ CO2 partitioning constantKsp ¼ crystal solubility productKst ¼ stability constantPo ¼ reference pressureR ¼ gas constant

tinhind ¼ protection time, seconds

t0ind ¼ induction time, seconds

T ¼ temperaturecBa2þ ¼ activity coefficients of barium ionscSO2�

4¼ activity coefficients of sulfate ions

DG� ¼ change in standard-state Gibbs energyDG ¼ change in Gibbs free energyDH ¼ enthalpy

DH�T;Po¼ standard-state enthalpy at a reference pressure

D�j� ¼ change in standard partial molal compressibilityDS�T;Po

¼ standard-state entropy at a reference pressure

D �V� ¼ change in standard partial molal volume at infinite dilu-

tion for a given reaction�i ¼ stoichiometric coefficient of the ith species

Acknowledgments

This work was financially supported by a consortium of compa-nies including Baker-Petrolite, BP, Champion Technologies,Chevron, ConocoPhillips, Dow, Halliburton, Hess, Innovacion eIngenieria Sustentable, Kemira, Marathon Oil, MI SWACO(Schlumberger), Multi-Chem, Nalco, Occidental, Petrobras, SaudiAramco, Shell, Siemens, StatOil, Total, the National ScienceFoundation through the Center for Biological and EnvironmentalNanotechnology (EEC-0118007), the China-US Center for Envi-ronmental Remediation and Sustainable Development, and theAdvanced Energy Consortium.

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Chunfang Fan is a chemist at multi-chemVR , a Halliburton Serv-ice Company. He was previously a senior research scientist atthe Department of Civil and Environmental Engineering, RiceUniversity. He holds a PhD degree in geosciences from GeorgeWashington University.

Amy Kan is a senior research scientist in the Department ofCivil and Environmental Engineering at Rice University (DCEE-RU). She holds a PhD degree from Cornell University. Kan servesas an Associate Editor for the SPE Journal and as a member ofthe Executive Committee for the SPE International OilfieldScale Conference.

Ping Zhang is a research fellow with the DCEE-RU. He holds aPhD degree in civil and environmental engineering from RiceUniversity.



Haiping Lu is working with downstream chemical-industrialwater treatment for Baker Hughes. She was previously a post-doctoral associate of the Brine Chemistry Consortium at RiceUniversity. Lu holds a PhD degree in geosciences from GeorgeWashington University.

Sarah Work is a doctoral student at the DCEE-RU. Her researchis on carbon sequestration mineralization kinetics. She holds aBS degree in environmental science from New Mexico State

University and an MS degree in civil and environmental engi-neering from Rice.

Jie Yu is a research fellow with the DCEE-RU. She holds a PhDdegree in chemical engineering from Rice University.

Mason B. Tomson has been a professor of civil and environmen-tal engineering at Rice University since 1987. He holds a PhDdegree in physical chemistry from Oklahoma State University.



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