New Insights Into the Removal of Calcium Sulfate

7/25/2019 New Insights Into the Removal of Calcium Sulfate

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42 FALL 2011 SAUDI ARAMCO JOURNAL OF TECHNOLOGY

ABSTRACT

Calcium sulfate is one of the major scales that cause many

significant and serious operating problems in producing oil

and gas wells and in water injectors. Impermeable hard scale

deposits of calcium sulfate can severely impair the formation

permeability or lead to downhole equipment failure. Typically,

preventive treatments, such as the use of scale inhibitors, are

the most economical methods for calcium sulfate mitigation;

however, application of a cost-effective treatment is needed incase of emergency, when calcium sulfate precipitation occurs.

A comprehensive lab investigation was conducted to assess

the effectiveness of several remedial methods for calcium

sulfate removal. This article shows the amount of calcium

sulfate dissolved after its exposure to different reactive fluids

at 25 C and 50 C. In addition, it discusses the effect of

different factors on the efficiency of each remedial treatment.

These factors include pH, temperature, reactive fluid

concentration and presence of magnesium/iron ions.

Based on obtained results, several new findings were

identified. The presence of gypsum (CaSO4.2H2O) has anegative impact on the performance of mud acid treatments.

After the initial dissolution of gypsum in live mud acid,

dissolved calcium will precipitate as both calcium fluoride and

calcium sulfate in spent hydrochloric (HCl)/hydrofluoric (HF)

acid solutions. Gypsum has a higher solubility limit in live

HCl acid compared to its spent solutions, which results in the

reprecipitation of calcium sulfate in spent HCl. This solubility

of gypsum in acidic solutions could result in severe formation

damage. Live acids can initially dissolve any precipitated

calcium sulfate solids in wellbore areas, but the calcium sulfate

will reprecipitate in the formation rocks as the acid is spent.

Gypsum has a higher solubility limit in ethylene diamine

tetra-acetic acid (EDTA) solutions, compared to acidic

solutions. No reprecipitation of calcium sulfate occurred in

these solutions due to the fact that calcium ions exist as

complex ions and are not free to interact with other ions. The

dissolving power of EDTA was found to be a function of the

solution pH value. The dissolving power of EDTA for gypsum

was higher at high pH values. The presence of both mag-

nesium and iron (III) ions had a negative effect on gypsum

dissolution in the EDTA fluid. Compared to magnesium, iron

(III) ions resulted in a significant decrease in gypsum solubility

in EDTA. Dissolved magnesium ions in EDTA solutions can

reprecipitate as magnesium sulfate when gypsum is dissolved.

This reprecipitation is greater in lower pH EDTA solutions.

INTRODUCTION

Calcium sulfate has been cited in the literature as one of the

major scales that cause many significant and serious operating

problems in producing oil and gas wells and in water injectors.

Impermeable hard scale deposits of calcium sulfate can severelyimpair the formation permeability, thereby decreasing the well

injectivity or productivity1-4. In addition, calcium sulfate can

also negatively impact the well economics when it precipitates

in downhole equipment, such as electrical submersible pumps

(ESPs). This precipitation leads to pump failure due to over-

loading that causes serious damage to the pump components,

and as a result, costly workovers are required.

Calcium sulfate has two main stable crystal forms.

Gypsum, CaSO4.2H2O, is formed at low temperatures (i.e.,

T < 98 C), while anhydrite, CaSO4, is the predominant form

produced at high temperatures5. Gypsum, the most common

oil field calcium sulfate scale, is very difficult to remove. This

is mainly because it has relatively low solubility limits in

water, almost 2.36 kg in 1 m3 of water at 25 C. At higher

temperatures, calcium sulfate becomes more insoluble in water

as low as 1.69 kg in 1 m3 of water at 90 C. Besides temp-

erature, other factors affect gypsum solubility, such as the

solutions pH value and pressure. In general, calcium sulfate is

more soluble at low pH values and high pressures4, 6.

Many publications have reported on the precipitation of

calcium sulfate during different oil field operations, such as

water injection, acid stimulation and commingled hydro-

carbon/water production. The main cause of calcium sulfatescaling during these operations is the chemically incompatible

mixing of two fluids. For example, mixing injected seawater,

which is high in sulfate ions, with formation water, which

is high in calcium ions, will result in calcium sulfate pre-

cipitation once its solubility limit is exceeded. Similarly,

calcium sulfate precipitation can result from the incompatible

mixing of spent acid solutions with overflush fluids. It has

been reported7 that calcium sulfate precipitation occurred in

and around the ESPs of the Farida/Zelda reservoir wells

mainly after acid stimulation treatments. This was found to be

New Insights into the Removal of CalciumSulfate Scale

Authors: Dr. Mohammed H. Al-Khaldi, Ahmad M. Al-Juhani, Dr. Saleh H. Al-Mutairi and Dr. M. Nihat Gurmen


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SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2011 43

due to the incompatible mixing of spent hydrochloric

(HCl)/hydrofluoric (HF) acid and seawater overflush fluid.

Dissolved calcium ions united with sulfate ions from the

seawater to form calcium sulfate scale. Similarly, the conduct

of a chemical simulation8 revealed that detrimental acidizing

treatments in limestone/dolomite Dalan and Kangan

formations could result in calcium sulfate precipitation.

Simulation results indicated that in addition to precipitation

due to incompatible mixing, calcium sulfate may precipitate inspent acid solutions due to the rise in the solutions pH value.

Whereas, calcium sulfate initially is soluble in live acid

solutions, it reprecipitates as the solutions pH value

increases.

Typically, preventive treatments, such as the use of scale

inhibitors, are the most economical methods for calcium

sulfate mitigation2, 9; however, application of a cost-effective

treatment is needed when calcium sulfate precipitation occurs.

Chemical dissolvers offer alternatives to mechanical methods

for removing near wellbore calcium sulfate scaling. A

thorough literature survey reveals that chemical means rely on

the use of amino carboxylic acid salts. Ethylene diamine tetra-

acetic acid (EDTA) is one of the most effective dissolvers for

calcium sulfate, Fig. 110. In many reservoirs, EDTA has been

used to remove calcium sulfate formation damage11, 12. Severe

calcium sulfate damage experienced by a high temperature gas

condensate well in the Kalinovac field was removed12 using an

8 wt% tetrasodium EDTA treatment.

As can been seen, a number of studies have been performed

to investigate the solubility of calcium sulfate in HCl acid and

EDTA solutions; however, the studies have been conducted

within limited parameters. Therefore, the objectives of this

study are to: (1) Explore the solubility trend of calcium sulfate

in HCl acid and HCl/HF acids, (2) Investigate the dissolution

of calcium sulfate in EDTA solutions, and (3) Examine the

effects of pH, temperature and the presence of both iron and

magnesium on the solubility of calcium sulfate in HCl acid,

HCl/HF acid and EDTA.

THEORY

Equation 1 shows how calcium sulfate, gypsum, and scaling

can be expressed:

(1)

The potential for gypsum scaling measured thermodynamically

by the saturation index (SI). A positive SI value indicates that the

mixture solution is supersaturated and there is a potential for

scaling, while a zero or negative SI value shows that the mixture is

undersaturated and there is no scale tendency. The SI for gypsum

is the logarithm of the ion activity product of its two ion

components, calcium ions and sulfate ions, divided by itssolubility product, Eqn. 29:

(2)

where Ca2+ is the activity coefficient of the calcium ion,

SO42- is the activity coefficient of the sulfate ion, C i is the

concentration of the specie i (mole/L), and Ksp is the solubility

product of calcium sulfate salt, which is a function of

temperature and pressure. The ion activity coefficient in

electrolyte solutions can be calculated using solution ionic

strength and ion interaction coefficients. For example, the

calcium activity coefficient can be calculated using Eqns. 3

and 413:

(3)

(4)

where Im is the ionic strength, z is the valence (or oxidation)

number of the calcium ion, B is the ion interaction coefficient

and mj is the molality of interacting ion j (mol/kg). In the case

of the calcium ion, the interacting ion in the HCl acid solution

will be the chloride ion. In calculating a specie activitycoefficient, the ion interaction coefficients are zero for ions

with the same charge or unchanged ions. Many studies have

reported ion interaction coefficients for various ion pairs14.

HCl acid and HCl/HF acids dissociate to produce H+ ions.

In these acidic solutions, calcium sulfate is dissolved due to H+

attack on sulfate crystal sites, Eqns. 5 to 7:

(5)

(6)

(7)

The interaction of dissolved calcium ions with fluoride ions

can result in the precipitation of calcium fluoride, Eqn. 8:

(8)

Calcium fluoride precipitation is dependent on the

solutions pH value as will be discussed later. The EDTA is

a weak acid that dissociates stepwise as follows, Eqns. 9

to 12:

Fig. 1. Chemical and 3D structure of EDTA molecule10.


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(9)

(10)

(11)

(12)

The distribution of the acid species is dependent on boththe pH value and the dissociation constants, as shown in Fig.

215, 16. The dissociation constants of EDTA are: Ka1= 1.02

E-2, Ka2 = 2.14 E-3, Ka3 = 6.92 E-7, Ka4 = 5.5 E-1115, 16. The

first dissociation constant of EDTA, Ka1, is expressed in Eqn.

13 as:

(13)

The mechanism of calcium sulfate dissolution in the EDTA

solution is different from that in acidic solutions. Calcium

sulfate is mainly dissolved due to calcium chelation at pH > 4.

Equation 14 shows the reaction of tetrasodium EDTA with

calcium sulfate:


(14)

The calcium sodium EDTA complex is stable because of the

chelation bonds, Fig. 317. Similar to calcium, EDTA is also an

excellent chelating agent for both magnesium and iron, Eqns.

15 and 16:

(15)

(16)

The chelating agent affinity for an ion is defined by the

formation constant, KF, Eqns. 17 and 18:

(17)

(18)

The larger the formation constant is, the stronger is the

chelating agent-ion complex. Table 118 lists the formation

constants of EDTA with calcium, magnesium and iron (III)

cations. The presence of magnesium and iron will affect the

calcium sulfate dissolution in the EDTA solutions as will be

discussed later.

EXPERIMENTAL STUDIES

Materials

Calcium sulfate di-hydrate (CaSO4.2H2O) was obtained from

the Sigma-Aldrich company with a purity of more than 98%.

Magnesium chloride hexa-hydrate and iron chloride hexa-

hydrate were acquired from BDH laboratory supplies and

Fisher Scientific companies with a purity of 99% and 97%,

respectively. Ammonium bi-fluoride salts were supplied by a

local service company and were used as is. Solutions of mudacid (HCl/HF) at 9/1 wt%/wt% were prepared using

ammonium bi-fluoride and concentrated HCl acid solutions

at 37.5 wt%, Eqn. 19:

(19)

The tetrasodium EDTA salt was obtained from the DOW

Chemical company. The average pH value of tetrasodium

EDTA salt is 14. HCl acid at 20 wt% was used to adjust the

pH value of the EDTA solution to the desired value. All

IonFormation Constant,

Log KF18 at 25 C

Calcium, Ca2+ 10.70

Magnesium, Mg2+ 8.69

Iron (III), Fe3+ 25.7

Fig. 2. Distribution of EDTA species as a function of the solutions pH value at

25 C15, 16.

Fig. 3. Octahedral geometry of 1:1 metal EDTA ion complex17.

Table 1. The formation constants of EDTA with different ions


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reactive solutions of HCl acid, HCl/HF acid mixture and

EDTA were prepared using distilled water with a resistivity

greater than 18 .cm at room temperature.

Experimental Procedure

The solubility of gypsum in different reactive fluids was

investigated at 25 C and 50 C at atmospheric pressure.

Three sets of static solubility experiments were conducted

using 15 wt% HCl acid, 9 wt% HCl/HF acid, and 1 wt% and8 wt% EDTA solutions. These experiments were carried out

using 10 ml of reactive fluid mixed with 1 g of calcium sulfate

di-hydrate and equilibrated at the desired temperature using a

hot water bath. Solutions of HCl acid and HCl/HF acids with

gypsum solids were incubated for four hours. This soaking

time is enough to reach an equilibrium state of gypsum

dissolution in acidic solutions19. The mixtures of EDTA and

gypsum were soaked for nearly eight hours to reach the

reaction equilibrium state12.

Several test tubes were used to conduct each set of reactive

fluid/gypsum solubility experiments. At the end of the desired

soaking time, unreacted solids in each test tube containing a

reactive fluid/gypsum mixture were filtered using 1.2 m filter

paper. Then the pH value of certain filtered reactive fluid

samples of HCl acid and mud acid was adjusted using a few

drops of 3N NaOH to reach the desired value. This was done

to investigate the effect of the solutions pH value on the

gypsum solubility. The pH value of the EDTA solutions was

adjusted using 20 wt% HCl acid to the desired value before

they was mixed with gypsum. Each series of solubility

experiments was conducted in duplicate at 25 C and 50 C.

In another two series of experiments, magnesium chloride

and iron chloride were added to the solutions of HCl acid andEDTA at 1, 2 and 3 wt% before gypsum was added to the

final solution. These sets of experiments were performed to

investigate the effect of magnesium and iron (III) on gypsum

solubility in different reactive solutions.

The calcium, magnesium and iron concentrations in the

collected reactive fluid samples were measured using

inductivity coupled plasma. To measure the pH value of the

collected samples, an Orion model 250A meter and Cole-

Parmer Ag/AgCl single junction pH electrode were used.

Fluoride ion concentration was measured using a fluoride

selective pH electrode. Collected solid samples were analyzed

using X-ray powder diffraction (XRD).

RESULTS AND DISCUSSIONS

The solubility of gypsum (CaSO4.2H2O) in HCl/HF acids, HCl

acid and EDTA solutions was studied at atmospheric pressure.

The effects of the solution pH value, the temperature and the

presence of both magnesium and iron (III) on gypsum solubility

were also investigated. Table 2 gives a list of the static solubility

experiments conducted using different reactive fluids.

Figure 4 shows the solubility of gypsum in the HCl/HF acid

mixtureat 9/1 wt%/wt%, respectively, as a function of the

solutionspH valueat 25 C. Initially, calciumandsulfate

levels inthelive mudacid solutionreached nearly 3,300 mg/L

and 20,000 mg/L, respectively, due to gypsum dissolution

after a soakingtime of four hours at aliquid/solid ratioof

Fig. 4. Gypsum solubility in 9 wt% HCl/1 wt% HF acid as a function of the

solutions pH value at 25 C and atmospheric pressure.

Table 2. Gypsum static solubility experiments conducted using different reactive

fluids at atmospheric pressure and a liquid/solid ratio of 10 ml/1 g


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The reprecipitation of calcium ions as both calcium fluoride

and calcium sulfate when the pH of the mud acid solution

increased was due to the presence of free fluoride and sulfate

ions at relatively high pH values.

Figures 615, 16 and 715, 16 depict the distribution of

hydrofluoric and sulfuric acids, respectively. At low pH

values, both fluoride and sulfate ions are bound by hydrogen

ions, so they are not free to bind with calcium. Therefore,

when gypsum initially dissolves in a live mud acid solution,the dissolved calcium cannot combine with either fluoride or

sulfate ions because they are not free. Subsequently, when

hydrogen ions are consumed due to acid spending, both

fluoride and sulfate ions become free and combine with

calcium ions to precipitate as calcium fluoride and calcium

sulfate, Eqns. 1 and 8. This finding clearly shows the potential

detrimental effects of calcium sulfate scale in association with

mud acid treatments. Similarly, the reprecipitation of calcium

sulfate in spent acid solutions was observed during the HCl

acid/gypsum solubility experiments.

It was reported19 that gypsum solubility in HCl acid

solutions increased as the acid concentration increased up to

nearly 15 wt% and then decreased with further acid

concentration increases. The report did not investigate the pH

value effect on gypsum solubility in HCl acid. Figure 8 shows

the effect of the solutions pH value on gypsum solubility in

15 wt% HCl acid at 25 C. Compared to live HCl acid

solutions, gypsum had a lower solubility limit in spent HCl

acid. Calcium and sulfate levels in the live 15 wt% HCl acid

solution initially reached nearly 4,400 ppm and 14,000 ppm,

respectively, due to gypsum dissolution after a soaking time of

four hours at a liquid/solid ratio of 10 ml/1 g. Consequently,

raising the HCl acid solutions pH value from zero to nearly1 resulted in the reprecipitation of calcium sulfate as indicated

by a decrease in the concentration level of both calcium and

sulfate ions to nearly 1,800 ppm and 4,000 ppm, respectively.

XRD analysis of this precipitation indicated it was calcium

sulfate, Fig. 9. A similar gypsum solubility trend in concert

with the solutions pH value was also observed at high

temperatures.

Figure 10 shows the effect of the solutions pH value on

gypsum solubility in 15 wt% HCl acid at 50 C. From this

figure, three main conclusions can be drawn. First, from both

10 ml/1 g, Eqn. 7. Consequently, when the pH value of the

mud acid solutions containing these levels of calcium and

sulfate was raised to a higher value, white solid particles

precipitated, and the concentration of calcium, sulfate and

fluoride ions sharply decreased. For example, the level of

calcium ions decreased from 3,300 to 1,000; sulfate ions from

20,000 to 6,450 and fluoride ions from 6,450; mg/L to 3,000

mg/L when the solutions pH value was raised from zero to

nearly 1.5 using a few drops of 3N NaOH. This decrease

continued until the concentration of dissolved calcium ions

reached almost zero at a pH value of nearly 3.5. This behavior

indicated a precipitation process involving calcium, fluoride

and sulfate ions. XRD analysis of the collected white solid

material precipitated in the spent mud solutions indicated that

it contained both calcium fluoride and calcium sulfate, Fig. 5.

Fig. 5. XRD analysis of solid material precipitated in spent mud acid/gypsum

mixture solutions. The solid precipitate was a mixture of both calcium fluoride

and gypsum.

Fig. 6. Distribution of HF acid species as a function of the solutions pH value at

25 C15, 16.

Fig. 7. Distribution of H2SO4 species as a function of the solutions pH value at

25 C15, 16.

Fig. 8. Gypsum solubility in 15 wt% HCl acid as a function of the solutions pH

value at 25 C and atmospheric pressure.


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Figs. 8 and 10, it is obvious that the increase in reaction

temperature from 25 C to 50 C resulted in more gypsum

dissolution in the live HCl acid solution, such that the

dissolved calcium concentration increased from 4,400 to

nearly 8,000, respectively. Second, the effect of temperature

on gypsum dissolution in the HCl acid diminished when the

pH value of the acid solution exceeded 1. Finally, the increase

in the solutions pH value resulted in the reprecipitation of

calcium sulfate, also at high temperatures. Calcium sulfatereprecipitation also occurred in live HCl acid solutions when

they reached room temperature. The amount of the

reprecipitation was accounted for in the calculation of the

calcium sulfate solubility in the HCl acid solutions at 50 C.

Due to the fact that magnesium sulfate has a higher

solubility than that of calcium sulfate, the presence of

magnesium in reactive solutions should increase the gypsum

solubility in these solutions due to the so-called common ion

effect20. Figures 11 and 12 show that the presence of both

magnesium and iron (III) had no effect on the gypsum

solubility in the live HCl acid at 25 C and 50 C, up to

concentration levels of 4,000 ppm and 6,000 ppm,

respectively. These solutions also experienced a reprecipitation

of calcium sulfate when their temperature values reached

room temperature, Figs. 13 and 14.

It was reported21 that the solubility product of gypsum in

water is nearly constant at different temperatures up to 60 C

and at 25 C, it is 2.4E-5 M2. Figure 15 shows the solubility

product of gypsum in HCl acid, at different pH values, calculated

using Eqns. 2 to 4. Compared to water, it is obvious that the

solubility product of gypsum in live HCl acid is higher at

almost 3.79E-5; however, the solubility of gypsum in spent

HCl acid (pH > 2) approaches its solubility limit in water. Incontrast to the solubility behavior of gypsum in water, its

solubility limit in live HCl acid solutions increased with the

increase in temperature. For example, the gypsum solubility

product in HCl acid (pH = 0) increased from 3.79E-5 to

9.9E-5 M2 when the solution temperature was increased from

25 C to 50 C. This solubility trend of gypsum was not

observed in the EDTA solutions.

Fig. 10. Gypsum solubility in 15 wt% HCl acid as a function of the solutions pH

value at 50 C and atmospheric pressure.

Fig. 11. Gypsum solubility in live 15 wt% HCl acid as a function of dissolved

magnesium ion concentration at atmospheric pressure.

Fig. 12. Gypsum solubility in live 15 wt% HCl acid as a function of dissolved iron

(III) ion concentration at atmospheric pressure.

Fig. 13. Calcium sulfate reprecipitation in live 15 wt% HCl acid containing (a)

iron, and (b) magnesium ions, at different concentrations as a result of the

decrease in the solutions temperature value.

Fig. 9. XRD analysis of solid material reprecipitated in filtered HCl acid solutions

after reaction with gypsum. Reprecipitation occurred when the pH value was raised.


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Figure 16 shows the solubility of gypsum in an 8 wt%

EDTA solution as a function of the pH value. Both dissolved

calcium ions and sulfate ions, due to gypsum dissolution,increased as the solution pH increased. This is mainly due to

the fact that at higher pH values, the EDTA molecule exists as

a dissociated form, which means it has more chelation sites

(carboxylic groups), Fig. 2. From measurements of the calcium

and sulfate concentrations, it is clear that the temperature

change from 25 C to 50 C did not affect the gypsum solu-

bility in the EDTA solutions with a soaking time of 8 hours. In

addition, gypsum reprecipitation as a result of temperature

change did not occur in these solutions. Along with the

advantage of the EDTA solutions over HCl acid, gypsum has

higher solubility in these solutions compared to HCl acid; the

dissolved calcium in the EDTA solutions and the spent HCl

acid is nearly 8,000 ppm and 1,900 ppm, respectively.

The presence of both magnesium and iron affected the

gypsum solubility in the EDTA solutions, Figs. 17 and 18.

Compared to magnesium, iron (III) had a significant effect on

calcium sulfate dissolution in the EDTA solutions. This is mainly

due to the fact that EDTA has a higher affinity for iron than for

calcium, while it has a comparable affinity for both calcium

and magnesium, Table 2. In other words, in the presence of

iron (III), there will be fewer chelation sites for calcium

compared to those available in the presence of magnesium.

Fig. 14. XRD analysis of solid material reprecipitated due to temperature change

in live HCl/gypsum mixture solutions containing (A) 1 wt% MgCl2.6H2O and

(B) 1 wt% FeCl3.6H2O.

Fig. 15. Solubility product of gypsum in 15 wt% HCl acid as a function of the pH

value.

Fig. 17. Gypsum solubility in 8 wt% EDTA as a function of dissolved magnesium

ion concentration at 50 C and atmospheric pressure.

Fig. 16. Gypsum solubility in 8 wt% EDTA as a function of the solutions pH

value at atmospheric pressure.


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The presence of magnesium ions did not only decrease the

EDTAs ability to dissolve gypsum, but the ions also

reprecipitated as magnesium sulfate. The amount of this

reprecipitation of magnesium ions increased at low pH values,

Fig. 19. No reprecipitation occurred in the EDTA solutions

containing iron (III) ions, Fig. 20. This was clearly indicated,

Fig. 17, where the concentration of iron (III) in the EDTA

before gypsum dissolution (theoretical) is equal to gypsum afte

soaking for 8 hours.

CONCLUSIONS

The interaction between gypsum and different reactive fluids

(HCl/HF acid, HCl acid and EDTA) was studied in detail. The

effects of different parameters, such as solution pH value,temperature and presence of iron (III) and magnesium ions wer

investigated. The following conclusions can be drawn:

1. The presence of gypsum (CaSO4.2H2O) has a negative

impact on the performance of mud acid treatments. After

the initial dissolution of gypsum in the live mud acid,

dissolved calcium will precipitate as both calcium fluoride

and calcium sulfate in spent HCl/HF acid solutions.

2. Gypsum has a higher solubility limit in live HCl acid

compared to its spent solutions. This will result in

reprecipitation of calcium sulfate in spent HCl acid.

3. The reprecipitation of calcium sulfate in spent solutions ofHCl acid and HCl/HF acids could result in severe

formation damage. Live acids initially can partially

dissolve any precipitated calcium sulfate solids in the

wellbore area, but the dissolved calcium and sulfate ions

will reprecipitate in the formation rocks as the acid is spent.

4. Gypsum has a higher solubility limit in the EDTA

solutions, compared to acidic solutions. No reprecipitation

of calcium sulfate occurred in these solutions due to the

fact that calcium ions exist as complex ions and are not

free to interact with other ions.

5. The presence of both magnesium and iron (III) ions had a

negative effect on gypsum dissolution in the EDTA fluid.

Compared to magnesium, iron (III) ions resulted in a signi-

ficant decrease in gypsum solubility in the EDTA solutions.

6. Dissolved magnesium ions in the EDTA solutions could

reprecipitate as magnesium sulfate when the gypsum is

dissolved. This reprecipitation is higher in low pH EDTA

solutions.

ACKNOWLEDGMENTS

The authors would like to thank the management of SaudiAramco and Schlumberger for their support and permission to

publish this article. Special thanks go to the Chemistry and

Advanced Instruments Units of the R&D Center for their

analysis of different solutions and solids.

This article was presented at the SPE European Formation

Damage Conference,Noordwijk, theNetherlands, June7-10, 2011

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Fig. 18. Gypsum solubility in 8 wt% EDTA as a function of dissolved iron (III)

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Fig. 19. Magnesium reprecipitation in 8 wt% EDTA containing MgCl2.6H2O at

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Fig. 20. No ions reprecipitation in the solution of 8 wt% EDTA containing

FeCl3.6H2O at 1, 2, 3 wt% after gypsum dissolution. The solutions pH values

are (a) 4, (b) 8 and (c) 14.


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Dr. Saleh H. Al-Mutairi is a

Researcher with the Stimulation Group

in the Operations Services Division at

Saudi Aramcos Exploration and

Petroleum Engineering Center -

Advanced Research Center (EXPEC

ARC). His research interests include

well stimulation, formation damage, tubular corrosion

problems and rheology of fluids. Saleh has 11 years of

experience in well stimulation operations and research, andhe has published several pieces of work in this area.

He received his B.S. degree in Chemical Engineering and

an MBA degree, both from King Fahd University of

Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia.

Saleh also received a M.S. degree in Petroleum Engineering

from the University of Texas at Austin, Austin, TX. He

received his Ph.D. degree in Petroleum Engineering from

Texas A&M University, College Station, TX.

Dr. M. Nihat Gurmen is a Technical

Manager at Schlumberger for the

Arabian Market covering Saudi

Arabia, Kuwait and Bahrain. Based inal-Khobar, Saudi Arabia, he is a

company Subject Matter Expert for

stimulation products, fluids and

services. Nihat started his Schlumberger career in the Client

Support Laboratory in Sugar Land, TX, in 2004. In his

next assignment, Nihat transferred to Alice, a field location

in South Texas, as the District Technical Engineer. In his

current role in the Arabian Market, Nihat ensures that

correct technologies are applied in stimulation operations

and introduces new products and services as necessary.

Nihat received his B.S. degree in Chemical Engineering

from Bogazici University, Istanbul, Turkey, and earned a

Ph.D. degree from the University of South Florida, Tampa,

FL. He was a postdoctoral fellow at the University of

Michigan, Ann Arbor, MI, in the Fogler Research Group.

Nihat is an active member of the Society of Petroleum

Engineers (SPE) and coauthor of various journal, SPE and

patent publications.

BIOGRAPHIES

Dr. Mohammed H. Al-Khaldi joined

Saudi Aramco in 2001 as a Research

Engineer working in Saudi Aramcos

Exploration and Petroleum

Engineering Center Advanced

Research Center (EXPEC ARC).

During this time, he was responsible

for evaluating different stimulation treatments, conductingseveral research studies and investigating several

stimulation fluids. In addition, Mohammed was involved in

the design of acid fracturing treatments. As an award for

his efforts, he received the Vice Presidents Recognition

Award for significant contributions to the safe and

successful completion of the first 100 fracturing treatments.

Mohammeds research interests include well stimulation,

formation damage mitigation and conformance control.

He received his B.S. degree in Chemical Engineering

(with First Class Honors) from King Fahd University of

Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia.

Mohammed also received his M.S. and Ph.D. degrees in

Petroleum Engineering (with First Class Honors) fromAdelaide University, Adelaide, Australia.

He is an active member of the Society of Petroleum

Engineers (SPE). Mohammed has published more than 15

SPE papers and seven journal articles, and has two patents.

In 2011, he received the SPE Best Technical Paper Award,

winning first place in the 2nd GCC SPE paper contest.

Ahmad M. Al-Juhani is a Chemical

Engineering student at King Fahd

University of Petroleum and Minerals

(KFUPM), Dhahran, Saudi Arabia. He

is scheduled to graduate in August

2011.Ahmad did his training with the

Formation Damage and Stimulation Unit, Operation

Support Division, at the Exploration and Petroleum

Engineering Center - Advanced Research Center (EXPEC

ARC). During this period, he worked on several studies

involving pipe dope removal, calcium sulfate scale removal

and H2S scavenger evaluation.

Documents

New Insights Into the Removal of Calcium Sulfate