2010 Effect of pH and scale inhibitor concentration on phosphonate–carbonate interaction

Journal of Petroleum Science and Engineering 70 (2010) 10–27

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Journal of Petroleum Science and Engineering

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Effect of pH and scale inhibitor concentration on phosphonate–carbonate interaction

S. Baraka-Lokmane ⁎, K.S. SorbieDepartment of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, United Kingdom

⁎ Corresponding author. Tel.: +44 131 451 3189; fax:E-mail address: [email protected] (S. Ba

0920-4105/$ – see front matter © 2009 Published by Edoi:10.1016/j.petrol.2009.05.002

a b s t r a c t
a r t i c l e i n f o
Article history:Received 5 July 2007Accepted 14 May 2009

Keywords:carbonate rockscale inhibitorphosphonatecalcium dissolutioncorefloods

In this paper, we present results from five corefloods (RC1 to RC5) from the Jurassic Portlandian limestone(ф~19.80% and k=606 mD) using 5000 ppm, 10,000 ppm, 25,000 ppm and 27,000 ppm of partly neutralizedDiethylenetriamine pentamethylenephosphoric acid (DETPMP) at pH 4 and 2. The purpose of this study wasto study the effect of inhibitor concentration and pH on the inhibitor adsorption and on the evolution of theinhibitor and cation (calcium and magnesium) return concentrations. These corefloods were performedusing long cores (12 in.), which were treated with just 0.5 pore volume (PV) of inhibitor. Another purposewas to study the transport and inhibitor/carbonate rock interactions when less than 1 PV of inhibitor solutionis injected. This allows for consumption of the inhibitor during propagation and return, rather than saturatingthe core with many PV to full adsorptive capacity of the inhibitor/rock system. This study showed that thehigher the concentration of SI and lower the pH, the more calcium dissolution is observed (from the [Ca2+]effluents). In all treatments there is a decrease in the [Mg2+] effluent corresponding directly to the increasein calcium. The effluent cation results in the long corefloods which strongly support the view that bothmagnesium and calcium are binding quite strongly to the DETPMP scale inhibitor. These observations lead usto a number of conclusions on the factors that must be included in a full carbonate model. In particular, ourexperimental results, along with some simple modeling, greatly clarify the role of both calcium andmagnesium in the mechanism of the scale inhibitor retention in carbonate systems.

© 2009 Published by Elsevier B.V.

1. Introduction

A common problem, in oilfield reservoirs is scale deposition. Scaleis an assemblage of deposits that can develop in the formation poresnear thewellbore reducing formation porosity and permeability. It canblock flow by clogging perforations or forming a thick lining inproduction tubing (Fig. 1). The build-up of scale inside well bores andthe surrounding reservoir causes millions of dollars in damage everyyear (Mackay et al., 2003). Oilfield scales are inorganic crystallinedeposits that form as a result of the precipitation of solids from brinespresent in the reservoir and production flow system. This scaleformation is the result of changes in the ionic composition, pH,pressure and temperature of the brine. Common scales are calciumcarbonate (CaCO3) and barium sulphate (BaSO4). When the formationof sulphate or carbonate scale is a problem in producer wells, the mostcommon remedy is to treat the formation with scale inhibitorchemicals in a “squeeze” treatment (Fig. 2) (Crowe et al., 1994; Norriset al., 2001). Phosphonates are one of the most common types of non-polymeric scale inhibitors, which are used commercially in oilfieldoperations. The phosphonate scale inhibitor Diethylenetriaminepentamethylenephosphoric acid (DETPMP) is known to be amongstthe strongest adsorbing scale inhibitor onto carbonate.

+44 131 451 3539.raka-Lokmane).

lsevier B.V.

Adsorption is often the primary mechanism of SI retention duringsqueeze treatments of sandstone formations. The scale inhibitorretention mechanism can be more complex than simple adsorptionwithin carbonate reservoirs because the carbonate rock is a muchmore reactive substrate. In carbonate formations, scale inhibitors aredeliberately allowed to react with the formation and precipitate as theslightly soluble calcium salt. This can result in longer scale protectiontimes. Even fully neutralized inhibitors react with carbonates to formprecipitates (Crowe et al., 1994).

Reactions that govern the inhibitor squeeze and return are verycomplicated. Several factors, such as pH, [Ca2+], [Mg2+], temperature,rock mineralogy etc, affect the adsorption level and the shape of theadsorption isotherm (Jordan et al., 1994; Baraka-Lokmane and Sorbie,2004, 2006).

The Rice University Brine Chemistry Consortium has carried out alarge number of studies on the scale inhibitor retention in carbonate-rich formations during squeeze treatments (Kan et al., 1992, 2004a,b,Tomson et al., 2004; Kan et al., 2005). From these “batch” studies,some of their conclusions were:

(1) calcite is the primary solid responsible for phosphonate retentionand clay plays a secondary role in phosphonate retention;

(2) although formation mineralogy can be a factor, the primarycontrol of inhibitor retention and return is the pill acidity andconcentration; and

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Fig. 1. The build-up of scale inside a tube. Fig. 3. AFM image of the coreflood RC4 showing the morphology of the rock surface.

11S. Baraka-Lokmane, K.S. Sorbie / Journal of Petroleum Science and Engineering 70 (2010) 10–27

(3) salt forms at higher pill concentration. This is due to the calcitesurface poisoning that lowers the pH of phosphonate–calcitereaction.

The current study carries over some of this work into the realm ofdynamic corefloods and we compare our conclusions with those of theRice U. group. The purpose of this work is to study themechanisms andthe factors that affect the retention of the phosphonate scale inhibitorDETPMP in carbonate formations.We present coreflooding results usingDETPMP to study the effect of inhibitor concentration and pH oninhibitor adsorption and on the behaviour of the inhibitor and cation(calcium and magnesium) concentration. “Contained” corefloods wereperformed to study the transport and inhibitor/carbonate interactionsofthe phosphonate inhibitor when less than 1 PV is injected. This allowsthe analysis of the consumption of the inhibitor during propagation anddesorption, rather than saturating the core with many PV of SI solutionto reach full adsorptive capacity of the inhibitor/rock system.

2. Mineralogy of carbonate rock material

Carbonate rocks containmore than 50% of theworld's hydrocarbonreserves. The carbonate reservoirs have complex pore systems, mainly

Fig. 2. Squeeze treatment of producer well.

because they are particularly sensitive to post-depositional diagenesis,including dissolution, dolomitization and fracturing processes. Carbo-nate mineralogy is usually simple — principal minerals are calcite,dolomite, and minor quantities of clay (Choquette and Pray, 1970;Roehl and Choquette, 1985).

In order to carry out a systematic study in well-characterizedcarbonate cores, a permeable carbonate rockmaterial has been sourced.Samplematerial was acquired from theAlbion StoneQuarries Ltd, Isle ofPortland. This rock originates from the Portland Basebed stone and is anopen texturedoolitic limestone fromthe Jurassic Portlandian Formation.The b3.5m thick Basebed is the lower part of the Freestone series,whichcomprises the Portland Roach, PortlandWhitbed and Portland BasebedFormations and is underlain by a cherty series.

In this study, five methods were used to characterize the rockmaterial: X-ray diffraction (XRD), petrographic thin section, atomic forcemicroscopy (AFM), scanning electron microscopy (SEM) and energydispersive X-ray (EDX). XRD analysis was carried out both onwhole rockand on the fine particle fractions. The results of the bulk rock XRDanalysis show that the rock material is composed mainly of pure calcite(98 to 99.9%). Quartz constitutes between 2 and ~0.1% of the rock.Analysis of the fines showed that no clay minerals are present. Figs. 3and 4 characterize the morphology of the rock surface showing thecalcite crystals and the shell fragments. The petrography analysis showsthat the rock material is a pure porous marine oolitic limestone

Fig. 4. AFM image of the coreflood RC4 showing the calcite crystals and the shellfragments.

Fig. 6. SEM photo showing cement containing micrite and sparite crystals, around theooid grains (M: micrite, C: calcite, O: ooid).

12 S. Baraka-Lokmane, K.S. Sorbie / Journal of Petroleum Science and Engineering 70 (2010) 10–27

composed of fine-grained remains ofmicroorganismswith calcite shells,ooids (Fig. 5) with micrite and sparite calcite cement (Fig. 6). The rockpresents a fine, darkmatrix ofmicrite (microcrystalline calcite), which isalso present as cement in the pore space. It forms a regular coatingaround grains and appears as a dark line in thin sections. The thinsections show the presence of 3 major types of porosity: a highintergranular porosity (Fig. 5), a moldic, secondary porosity, created bydissolution of the carbonate shell fragments later in the diagenetichistory of the limestone (Fig. 6) and a high microporosity which occursbetween the micritic matrix of the sample (Fig. 5). The petrophysicalmeasurements of the rockmaterial show that the limestone has porosity(by helium porosimeter), Φ~19.80%, grain density, ρ=2.61 g/cm3 andpermeability, k~606 mD.

3. The squeeze strategy

In squeeze treatments, the objective is to retain the maximumamount of inhibitor in the formation either by

(1) adsorption on the rock substrate by a physico chemical processor

(2) precipitation (or phase separation) in a controlled manner, atsome distance from the near wellbore area. This is generallyachieved by adjusting the injected solution chemistry ([Ca2+]concentration, pH, temperature etc.).

Following the treatment, the inhibitor slow release from the rockwill be governed by the inhibitor adsorption or retention process,which is specific to the particular inhibitor, the rock matrix and thebrine chemistry of interest (Crowe et al., 1994; Graham and Mackay,2000; Jordan and Sjursaether, 2004).

In the field, the procedure for applying such a chemical treatmentto an oil or gas producing well normally involves the following fivestages (Fig. 2):

(1) Preflush: a pre-flush package which generally consists of a verydilute solution mostly in seawater of the inhibitor eventuallymixed with a surfactant, is injected into the well. This bothcools the near wellbore formation and “prepares” the rocksurface.

(2) Inhibitor injection: the main inhibitor slug, also dissolved inseawater, is squeezed into the near wellbore area.

(3) Overflush: the main inhibitor slug is displaced to the requireddistance from the wellbore by injecting seawater into the well.

Fig. 5. General texture of the oolitic limestone, showing the elongated and roundedooids, which are formed from micrite resulting the rough surfaces of the ooid grains.Note that blue epoxy resin is used to fill pore space (C: calcite, M: micrite, O: ooid). (Forinterpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)

(4) Shut-in: the well will then be shut in for a specified period(usually for 6 to 24 h) in order to allow the inhibitor to interactwith the rock substrate.

(5) Back production: the well is then put onto back production andthe inhibitor is then expected to return to the wellbore at aconcentration above the threshold level, minimal inhibitorconcentration (MIC), for a long period of time (squeeze lifetime).

The success of the treatment, or the squeeze lifetime, can bemonitored either by analyzing the inhibitor concentration or byassaying the scaling ions in the return brine. As soon as the scaleinhibitor return concentration falls below the minimum inhibitorconcentration (MIC) that prevents the scale formation, the squeeze isconsidered to be over and retreatment is recommended.

Successive squeeze treatments can be optimized either from anexamination of the actual treated field return data or from amodellingof the lab coreflood inhibitor return data performed under simulatedfield conditions. This is achieved by use of the industry standard,Squeeze VI software, developed by Heriot-Watt University, to enableto derive an adsorption isotherm from the scale inhibitor effluentresults either from the field or from the lab coreflood data. The shapeof the inhibitor adsorption isotherm, Г, describes the relationshipbetween the level of inhibitor adsorption at the rock surface (in mg/g)and the injected scale inhibitor solution. Once the scale inhibitor/rockadsorption isotherm has been derived for the mineralogy, tempera-ture and pressure conditions present in the specific reservoir, it maythen be used to optimize the design of the squeeze treatment for thereservoir formation.

4. Dynamic corefloods experiments

Coreflood experiments in the laboratory are designed to simulatethe reservoir squeeze treatment. These tests are a vital tool inunderstanding the mechanisms controlling scale inhibitor/rock inter-action since: they allow complete temperature and pressure control,they enable to determine the complete mineralogical petrophysicalcharacteristics of the rock, andwith them it is possible to performpost-scale inhibitor treatment petrography to quantify the magnitude ofretained scale inhibitor and its mineralogical structure and location.

The main steps in the corefloods broadly follow the field inhibitortreatment stages and are as follows:

(1) seawater saturation, permeability measurement and corecharacterization;

(2) injection of the main scale inhibitor slug (containing a lithiumtracer) followed by a shut-in of 24 h; and

Fig. 7. Coreflood equipment for corefloods RC1 to RC5.


(3) seawater postflush or back production until the inhibitorconcentration drops below 0.5 ppm (estimated MIC level).

(4) Permeability measurement to check if the treatment has causedany damage in the reservoir properties and a detailed petro-graphy analysis is carried out.

The coreflooding procedures adopted in this study are designed tostudy the inhibitor retention and return characteristics in a single phaseflow. The corefloodswere conducted using a bench top assembly with aresin-coated outcrop carbonate core (Figs. 7 and 8). The linepressure forthese floods was approximately equal to 20 psi. The corefloods wereconducted at room temperature. The full experimental procedure forcorefloods RC1 to RC5 is outlined in Appendix A.

These floods were performed in long cores (12q length and 1qdiameter). The purpose of these corefloods is to study the transport ofthe scale inhibitor (SI) solution and the SI/carbonate interactions, byinjecting less than 1 PV of SI into these cores. In fact, 0.5 PV of5000 ppm (RC1 and RC4), 10,000 ppm (RC2), 25,000 ppm (RC3) and27,000 ppm (RC5) seawater solutions of partly neutralized DETPMP atpH 4 (RC1, RC2 and RC5) and at pH 2 (RC3 and RC4) were injected intothe respective cores but all other fluids (postflush seawater, andlithium tracer solution) were at pH 6. After SI injection through theinlet end of the core, the flow was reversed and the postflush wasinjected through the former outlet of the core with the SI beingproduced from the former inlet (Fig. 9). This allows the evaluation ofthe SI slug during propagation and return, rather than saturating thecore with many PV to reach full adsorptive capacity of the SI/rock

Fig. 8. Schematic diagram of the experimental set up for the res

system. This was felt to be a more appropriate type of flood to use fordeveloping reactive transport models for SI/carbonate systems.

The composition of the synthetic seawater used for the corefloodtests is presented in Table 1. The scale inhibitor (SI) used for thecorefloods RC1, RC2, RC3, RC4 and RC5was a generic Diethylenetriaminepentamethylenephosphoric acid (DETPMP). The partly neutralizedDETPMP was dosed with lithium tracer, in a ratio of 1:100. The scaleinhibitor solutions were filtered through a 0.45 µm membrane filterprior to use.

The effluent samples were analyzed for the cation concentrations,[Ca2+], [Mg2+], [Fe2+/3+], and also for [Li+], by inductively coupledplasma (ICP). The DETPMP concentration was also determined by ICPbased upon the phosphorus content. The desired dilutions wereprepared using a Microprocessor-controlled dilutor (Gilson Dilutor401). For the calibration, matrix matched standards were used tosuppress the interferences of the brine solution.

5. Permeabilities measurements

Pressure drops (ΔP) between inlet and outlet of the cores weremonitored throughout all of the experiments. Variable flow rate vs. ΔPmeasurements were performed to determine the pre- and post-floodpermeabilities. Table 2 shows the calculated values of permeability forthe pre- and post-treatment stages.

In corefloods RC1, RC2, RC3, RC4 and RC5, before the scale inhibitortreatment, the permeabilities were equal to 596mD, 606 mD, 602mD,609mD and 600mD respectively. After the SI treatment an increase of20% (k=718 mD), 50% (k=909 mD), 65% (k=992 mD), 58%(k=960 mD) and 63% (k=975 mD) were observed respectively forthe floods RC1, RC2, RC3, RC4 and RC5. Table 2 shows that we observean increase in permeability as the concentration of the SI increasesand the pH of the SI decreases.

6. Inhibitor analysis

0.5 PV of 5000 ppm, 10,000 ppm, 25,000 ppm and 27,000 ppm ofpartly neutralized DETPMP at pH 4 (RC1, RC2 and RC5) and at pH 2(RC3 and RC4) were injected into the core but all other fluids(postflush seawater and lithium tracer solution) were at pH 6. After SIinjection from the inlet end of the core, the flowwas reversed and thepostflush was injected at the former outlet of the core with the SIbeing produced from the former inlet (Fig. 9). These experiments aredesigned in order to check what is the most important factor thatimproves the SI adsorption, is it the high concentration of the SI or thelow pH value of the SI.

in-coated coreflooding experiments, corefloods RC1 to RC5.

Fig. 9. Experimental procedure for the 12 inch long carbonate core (RC1).


6.1. Injection and initial postflush

Fig. 10 shows the back production with seawater at pH 6, i.e.reversed flow after the 0.5 PV SI slug injection in cores RC1 and RC2,followed by a shut-in period. The SI effluent profiles show animmediate spike in SI concentration, and then a gradual decrease init. In these floods, the scale inhibitor adsorption can be visualized fromthe retardation between scale inhibitor and the lithium tracer effluentconcentrations. In floods RC1 and RC2, the lithium effluent also showsa similar initial spike mirroring the SI one (they are mixed), followedby a drop to a concentration ratio (C/Co)~0.17 and 0.38 (respectively)and then a sudden rise as the injected lithium tracer from theseawater post flush breaks through in the reverse flow as shown inFig. 10. It is clear from the RC1 and RC2 tracer results in Fig. 10 thatthere is quite a high level of mixing (dispersion) in the core since thepostflush (injected at the outlet end) “catches” the leading edge of thetracer injected from the inlet with the SI slug. Likewise, it is evidentthat there is some level of SI adsorption since there is a long low [SI]tail concentration with [SI]N1 ppm for ~300 PV.

Table 1The composition of brine solution used for the tests.

Component Concentration(g/l)

Ion Concentration(ppm)

NaCl 24.08 Sodium (Na+) 10,890CaCl2.2H2O 2.34 Calcium (Ca2+) 428MgCl2.6H2O 11.44 Magnesium (Mg2+) 1368KCl 0.877 Potassium (K+) 460NaSO4 4.38 Sulphate (SO4

2−) 2960Chloride (Cl−) 19,773

6.2. Seawater postflush

Figs. 11 and 12 present the early (~100 PV) and late (~1800 PV)postflush SI return profiles, for corefloods RC1 to RC5. As expected, theinitial SI return concentrations are low and they also drop morerapidly for these floods since only 0.5 PV of SI solution have beeninjected. In the early post flush, the floods RC3 and RC5 presents thehighest SI returns since the corefloods have been performed with thehighest DETPMP concentration i.e. 25,000 ppm active DETPMP at pH 2and 27,000 ppm active DETPMP at pH 4, respectively. Floods RC1, RC2,RC3, RC4 and RC5, treatedwith 5000 ppm at pH 4,10,000 ppm at pH 4,25,000 ppm at pH 2, 5000 ppm at pH 2, and 27,000 ppm activeDETPMP at pH 4, respectively show lower [SI] returns. Thus after 10 PVof postflush for flood RC3 SI return concentration is ~780 ppm,~697 ppm for flood RC5, ~287 ppm for flood RC2, ~163 ppm for floodRC4 and ~69 ppm for flood RC1.

Fig. 12 shows that the [SI] return concentration reaches a value of1 ppm after ~300 PV for floods RC1, RC2 and RC4, after 355 PV for floodRC3 and after 596 PV for flood RC5. The 0.70 ppm return concentrationis reached after 400PV for floods RC1 to RC4 and after 691PV for flood

Table 2Brine permeability values before and after the scale inhibitor treatment.

Coreflood SIconcentration(ppm)

pH of theinjectedSI solution

Permeability(k) (mD)

Permeability(k) (mD)

Increase inpermeability(%)Pre-treatment Post-treatment

RC1 5000 4 596 718 20RC2 10,000 4 606 909 50RC3 25,000 2 602 992 65RC4 5000 2 609 960 58RC5 27,000 4 600 975 63

Fig. 10. Normalised (C/Co) inhibitor and lithium concentration profiles obtained during initial adsorption and postflush stages, floods RC1 and RC2.

Fig. 11. Inhibitor return profiles, early postflush (−0.40–N100 PV), Dequest 2066 (DETPMP) Aqueous for flood RC1 to RC5.


RC5. Finally the 0.5 ppm return concentration (MIC) is reached after~700PV for all five corefloods.

7. Cation analysis — calcium and magnesiumeffluent concentrations

Fig. 13 presents the [Ca2+], [Mg2+], [Li+] and [SI] concentrationprofiles during the main treatment and the early postflush stages forcoreflood RC5. In this figure, very distinct spikes in SI, lithium and

Fig. 12. Inhibitor return profiles, late postflush (100–N1800 PV), Deques

calcium concentrations are observed, which correspond at the sametime with a dip in the magnesium concentration. This is an importantobservation and has been explained by the binding of the magnesiumand the calcium to the DETPMP (Baraka-Lokmane and Sorbie, 2006).During the injection of the SI, we observe a loss in magnesiumconcentration, because the magnesium is adsorbed together with theSI. After the shut-in,we observe a spike in themagnesium concentrationbecause it is released by the rock together with the SI during thepostflush.

t 2066 (DETPMP) Aqueous [SI]=9–N0.40 ppm, floods RC1 to RC5.

Fig. 13. Normalised (C/Co) inhibitor, lithium and recorded calcium, magnesium concentration profiles obtained during initial adsorption and postflush stages, flood RC5.


During the postflush, two effects influence the effluent calciumconcentration, some calcium remains bound to the DETPMP but, inaddition carbonate rock dissolution also occurs and this phenomenonis predominant. This explains the increase in calcium concentration inthe effluents.

A direct comparison of the evolution of the [Ca2+] and [Mg2+]effluent concentrations for the five corefloods is shown in Figs. 14

Fig. 14. Recorded calcium concentration profiles obtained du

Fig. 15. Recorded magnesium concentration profiles obtained d

and 15 respectively. Clearly, the levels of both Ca and Mg concentra-tions correlate with that of the [SI], with a larger effect being seen bothat high SI concentration and at a lower injection pH. This effect will bediscussed and also modelled in more detail below.

Calculations of the amount of dissolution of calcium carbonatefrom the rock matrix were carried out. Table 3 shows SI adsorption,calcium carbonate dissolution and gain in magnesium during the

ring adsorption and postflush stages, floods RC1 to RC5.

uring adsorption and postflush stages, floods RC1 to RC5.

Table 3Scale inhibitor adsorption, calcium carbonate (dissolution) and magnesium (gain)during the main treatment and postflush stages for floods RC1 to RC5.

Coreflood Inhibitoradsorption(mg/g)

CaCO3 dissolution(mg)

Mg gain(mg)

MTa PFb MT PF

RC1 (SI, pH=4Li solution, pH=6)

0.073 0.93 (0.003) 211.24 (0.70) 1.53 14.14


0.075 1.27 (0.004) 248.59 (0.82) 2.07 182.34


0.087 1.49 (0.005) 481.23 (1.59) 2.31 198.65


0.080 1.15 (0.004) 420.04 (1.38) 0.82 117.33


0.084 1.17 (0.004) 475.19 (1.57) 1.16 168.71

The values in brackets are in mg/g.a MT = main treatment.b PF = postflush.

Table 5Percentages of SI retained by the rock.

Coreflood % SI retained % SI retained

Modelling Mass balance (experiment)

RC1 10.6 15.9RC2 6.8 4.8RC3 2.4 3.6RC4 13.1 14.7RC5 4.1 2.4


main treatment and the postflush stages, calculated using massbalance.

Fig. 14 shows that for the different corefloods we observe first aspike in calcium concentration: 693 ppm (RC1), 754 ppm (RC2),1213 ppm (RC3), 1106 ppm (RC4) and 1261 ppm (RC5). Then thecalcium levels drop to 412 ppm (RC1), 407 ppm (RC2), 457 ppm (RC3),449 ppm (RC4) and 441 ppm (RC5) and remain finally more or lessconstant until the end of the coreflood.

Fig. 14 and Table 3 show that coreflood RC4 (5000 ppm SI at pH 2)presents a higher total calcium concentration in the effluents (ca.421.19 mg or 1.38 mg/g vs. ca. 212.17 mg or 0.70 mg/g) than corefloodRC1 (5000 ppm SI at pH 4). The lower pH of the injected SI solution incoreflood RC4 is thus responsible for the high calcium dissolution.

Fig. 14 and Table 3 show also that coreflood RC5 (27,000 ppm SI atpH 4) presents comparable levels of calcium elution (ca. 476.36 mg or1.57 mg/g vs. ca. 482.72 mg or 1.59 mg/g) than flood RC3 (25,000 ppmSI at pH 2). These results show that the slightly higher SIconcentration for coreflood RC5 (27,000 ppm DETPMP) has compen-sated the lower pH value (pH=2) of coreflood RC3. Coreflood RC3(25,000 ppm SI at pH 2) presents a higher calcium elution (ca.482.72 mg or 1.59 mg/g vs. ca. 421.19 mg or 1.38 mg/g) than corefloodRC4 (5000 ppm SI at pH 2) thus pointing to the effect of increased SIinjected concentration on the calcium release from the rock. This isalso valid for the corefloods performed at pH 4, ca. 476.36 mg or1.57 mg/g for coreflood RC5 (27,000 ppm SI), ca. 249.86 mg or0.82 mg/g for coreflood RC2 (10,000 ppm SI at pH 4) and ca. 212.17 mgor 0.70 mg/g for coreflood RC1 5000 ppm SI).

Table 4 shows that all five corefloods show a very small SIadsorption level, Γ~0.073mg/g (RC1), ~0.075mg/g (RC2), ~0.087mg/g (RC3), ~0.080 mg/g (RC4) and ~0.084 mg/g (RC5) respectively.These values are roughly estimations as the cores have been treatedwith only 0.5 PV of SI solution. Table 5 shows that we obtain apercentage of scale inhibitor return varying between 84.51% and

Table 4Values of scale inhibitor adsorption calculated using mass balance.

Coreflood SI injected(mg)

Total SI retained(mg)

% SI return % SI retained SI adsorption(mg/g)

RC1 73.65 15.21 84.80 15.21 0.073RC2 145.78 7.02 95.19 4.81 0.075RC3 364.28 13.26 96.36 3.64 0.087RC4 80.11 11.78 85.30 14.70 0.080RC5 527.42 12.63 97.60 2.40 0.084

97.60%, thus the percentage of scale inhibitor retained in the rockvaries between 2.40% and 15.21%.

8. pH values

The effluent pH profiles for all five corefloods RC1 to RC5 are shownover the entire main treatment and postflush periods in Fig. 16. SIinjection is performed at pH=4 for corefloods RC1, RC2 and RC5 andat pH=2 for corefloods RC3 and RC4. In all these floods, a sharp rise inpH just after the shut-in can be observed and then the pH valuesstabilize at ~pH 7 until the end of the post flush. Fig. 17 shows also twoprofiles of the effluent pH values for coreflood RC4: a first profilepresenting pH values in the same order as for floods RC1 to RC5(varying between 6.7 and 8.4), and a second profile with much lowerpH values varying between 2.50 and 4.30. The lower pH values werethose measured immediately after elution from the core. The higherpH values were measured when the collected effluent samples hadbeen protected with test tubes lids and left standing for some timeafter elution. For these latter samples, CO2 has escaped from theeffluents, thus explaining the observed high pH values.

Fig. 17 shows that the high pH values have been modelled withMultiScale software (Table 6 and Fig. 18) (mineral scale predictionsoftware by Petrotech, Haugesund, Norway), and the low pH valueshave been modelled with PHREEQC modeling (Tables 7 and 8,simulations 1 to 5) (geochemical modelling program developed byAppelo and Gerba in 1993 for the U.S. Geological Survey).

9. Modelling of pH prediction, SI returns and Ca/Mg/SI interaction

In this study, we have carried out three types of modelling:

(1) the prediction of pH evolution has been performed first withthe help of two softwares: MultiScale and PHREEQC;

(2) secondly we investigated whether we can reproduce the SIreturn concentration profiles using a simple adsorptionisotherm approach.; and

(3) finally, the behaviour of the Ca and Mg concentration in the coreeluents has been explained by a simple equilibrium SI–Mg2+

binding model.

9.1. pH prediction

The pH of the effluents for our corefloods have been predicted withthe help of two models: PHREEQC and MultiScale.

9.1.1. MultiScale modellingMultiScale is a computer programdesigned to calculate equilibrium

in systems containing brines of different composition, gas, oil, andsolids. As the input, the user defines temperature, pressure, and theconcentration of each compound; MultiScale then uses the appro-priate chemical and thermodynamic equations to calculate the finalphases, which are present, and the equilibrium composition of eachphase. MultiScale can be used to predict the scaling tendency of NaCl,BaSO4, SrSO4, CaSO4, FeS, CaCO3 and FeCO3, as well as the final pH ofthe mixing of two solutions.

Fig. 16. Measured pH values for floods RC1 to RC5.

Fig. 17. Measured pH values for flood RC4.


Fig. 18 shows Effluents pH values modelled with the help ofMultiScale software. The input pH values are equal to 6 and 4. Thesoftware MultiScale does not accept pH values lower than 4, in thecase of carbonate system. Fig. 18 shows that the pH values are around7. Table 6 shows that the measured pH values are in the same order tothose predicted with the software MultiScale.

9.1.2. PHREEQC modellingThe second software used to predict the effluent pH values from the

different coreflooding experiments is the PHREEQCmodelling, which isa powerful geochemical modelling program developed by Appelo andGerba in 1993 for theU.S. Geological Survey. PHREEQC iswritten in the Cprogramming language and it is designed to perform a wide variety oflow-temperature aqueous geochemical calculations. The program isbased on equilibrium chemistry of aqueous solutions interacting withminerals, gases, solid solutions, exchangers, and adsorbing surfaces.PHREEQC is used for simulating a variety of reactions and processes innatural waters or laboratory experiments.

Table 7 shows the input data set. In this program, seawater is definedby using themajor-ion data, it is equilibrated with calcite, to investigatethe chemical evolution. In Table 7, PCO2

is the partial pressure of CO2, i.e.

Table 6Measured and modelled pH values using MultiScale software.

Input pH Measured pH values Modelled pH values (MultiScale)

6 7.2600 7.06334 6.8650 7.0628

the gas phase pressure of CO2 in equilibriumwith the carbon dioxide insolution. The Saturation Index, SI is a numeric value indicating whetherwater is scale-formingor corrosive. It takes accountof factors such aspH,total alkalinity, calcium hardness and water temperature. The idealSaturation Index range is between−3 and +3.

Mole transfer is relative to the moles in the phase assemblage;positive numbers indicate an increase in the volume of the phasepresent, that is, precipitation; negative numbers indicate a decrease inthe volume of the phase present, or dissolution. Saturation Index: “–”indicates that Saturation Index calculation not possible because one oftheconstituent elementswasnot in solution.Mole transfer: “–” indicatesthat no mole transfer of this mineral was allowed in the simulation.

Selected results from the output data are presented in Table 8. Inthe simulation the input pH values vary between 6 and 2, thetemperature is equal to 20 °C and values of CO2(g) vary between −4and 4. Table 7 shows that the resulting pH values decreases as inputCO2(g) increases. In the different simulations, the seawater has the logPCO2

specified by the input except for the simulations 5, 10 and 15where the seawater has the log PCO2

equal to about 2.5. In thesimulations 5, 10 and 15 the input CO2(g) value is equal to 4.

The different simulations show that the seawater is supersaturatedwithdolomite for input CO2(g) equal to−4,−2 and0 (simulations 1 to 3,6 to8and11 to13). This indicates that thermodynamically, dolomitizationshould occur, thus dolomite should precipitate. The seawater is under-saturated with dolomite for input CO2(g) equal to 2 and 4 (simulations 4,5, 9, 10, 14 and 15). This indicates that dolomite should dissolve.

Table 8 shows that no mole transfer of dolomite is allowed in thedifferent simulations. In the simulation 1, the moles of CO2 in the

Fig. 18. Effluents pH values modelled with the help of MultiScale software.


phase assemblage decreased by about 21 mmol, i.e. about 21 mmoldissolved into solution. Likewise, about 25 mmol of calcite dissolved.This figure is shown in all the simulations except for simulations 11and 12, where the input pH is equal to 2 and the values of CO2(g) areequal to−4 and −2. In the simulations 11 and 12, about 8 mmol and6 mmol escaped from the seawater solution.

These different simulations presented in Table 8 have shown thatwith the help of PHREEQC modelling we can reproduce the effluentspH values obtained in our coreflooding experiments.

9.2. Isotherm derivation

The SQUEEZE V software was used to derive adsorption isotherms,Γ(C) for the different corefloods, from the SI effluent concentrationsfrom floods RC1 to RC5. The isotherm describes the adsorption of thescale inhibitor on to the rock substrate. Once the SI adsorptionisotherm in the carbonate rock has been derived, it may then be usedin turn tomodel the coreflood. This “completes the cycle” since, wherethe model calculations match the experimental coreflood concentra-tions, then the isotherm is essentially validated. However, there is anissue in these “contained” floods since the full length of the core is notexposed to the SI solution and also the same end of the12q core is usedfor SI injection and back production. Thus, by assuming a shorter“effective length” of the core, the best fits for flood RC1 (performedwith 5000 ppm SI at pH 4) and RC3 (performed with 25,000 ppm SI atpH 2) are represented in Figs. 19 and 20 with their correspondingFreundlich isotherms (i.e. k and n values) (squeeze V program, Heriot-Watt University, Scotland).

Table 7PHREEQC modeling input data set.

TITLE Example 3, part A.–Calcite equilibrium at log PCO2=−4.0 and 20C.

SOLUTION 1 Seawaterunits ppmpH 2.00temp 20.0Ca 428.0Mg 1368.0Na 10,890.0K 460.0S(6) 2690.0

EQUILIBRIUM_PHASESCO2(g) −4.0Calcite 0.0

END

The Freundlich sorption isotherm model is the following

C = k:Cn ð1Þ

where k and n are constants, Г is the mass per dry unit weight of solid(mg/g) of the SI adsorbed, and C is the concentration in ppm of the SIin the brine solution.

Figs. 19 and 20 show the experimental and modelled SI returnscurves over the full SI concentration range for floods RC1 and RC3(points = experimental [SI]; solid lines = modelling results). Thesefigures show a satisfactory but not perfect match between experi-mental and the modelling results. Flood RC1 shows a better fitbetween modeling results and experimental results than flood RC3.This is probably because the calcium and the magnesium ions arestrongly involved in the SI/carbonate rock interaction, specially for thecoreflood performed with high SI concentration (25,000 ppm) andlower pH value (pH=2) and in this case a single adsorption isothermmay not provide a complete description of the process.

From the modelling results in Figs. 19 and 20, we can predict theamount of SI (for example, as a % of the injected amount) retained in thecore at the end of the flood. These predictions are compared with theexperimentalmass balancemeasurements inTable 5 and it can benotedthat both results agree extremely well. However, we cannot be satisfiedwith this “pseudo-isotherm” approach although if it fits the experi-mental data very well since we are aware that the SI/calcite interactionin the core is more complex than a simple adsorption process.

In the literature, we can find three views on the precise mechanismsthroughwhichSI species are retainedwithin the reservoir formation e.g.adsorption, precipitation, surface condensation, coupled adsorption/precipitation etc.

These three views are:

(1) the Heriot-Watt view, where retention is described in generalterms through an adsorption isotherm, Г(C), or alternatively by atwoparameter precipitation/dissolutionmodel,Π(C) (solubility)and r2 (dissolution rate) (Sorbie et al., 1991a,b, 1992, 1993a,b,1994, Sorbie and Gdanski, 2005);

(2) the Halliburton model, which approaches adsorption from amineralogical perspective and relates the total rock adsorptionto the adsorption on each mineral component of the rock(Gdanski and Funkhouser, 2001, 2002, 2005); and

(3) the Rice University view, in which for example, phosphonateretention is related to the solubility of a sparingly solublecalcium phosphonate precipitate and refers to high, medium

Table 8PHREEQC modeling results.

Simulation Input data pH LogPCO2

Saturation Index Mole transfer(mmol)

Calcite Dolomite CO2 Calcite Dolomite

1 Temperature=20 °C 8.143 −4.00 0.00 0.81 −20.75 −24.84 –

CO2 (g)=−4.0pH=6

2 Temperature=20 °C 7.113 −2.00 0.00 0.75 −2.312 −1.987 –

CO2 (g)=−2.0pH=6

3 Temperature=20 °C 5.977 0.00 0.00 0.45 −0.501 −0.146 –

CO2 (g)=0.0pH=6

4 Temperature=20 °C 4.751 2.00 0.00 −0.13 −3531 −0.885 –

CO2 (g)=2.0pH=6

5 Temperature=20 °C 4.493 2.46 0.00 −0.28 −10,000 −130.2 –

CO2 (g)=4.0pH=6

6 Temperature=20 °C 8.141 −4.00 0.00 0.81 −0.1254 −0.329 –

CO2 (g)=−4.0pH=4

7 Temperature=20 °C 7.112 −2.00 0.00 0.75 −2.225 −2.063 –

CO2 (g)=−2.0pH=4

8 Temperature=20 °C 5.977 0.00 0.00 0.45 −50.04 −14.65 –

CO2 (g)=0.0pH=4

9 Temperature=20 °C 4.750 2.00 0.00 −0.13 −3531.0 −88.60 –

CO2 (g)=2.0pH=4

10 Temperature=20 °C 4.493 2.46 0.00 −0.28 −10,000.0 −130.3 –

CO2 (g)=4.0pH=4

11 Temperature=20 °C 8.026 −4.00 0.00 0.58 7.853 −8.204 –

CO2 (g)=−4.0pH=2

12 Temperature=20 °C 7.012 −2.00 0.00 0.55 6.083 −9.615 –

CO2 (g)=−2.0pH=2

13 Temperature=20 °C 5.926 0.00 0.00 0.35 −40.63 −21.18 –

CO2 (g)=0.0pH=2

14 Temperature=20 °C 4.736 2.00 0.00 −0.16 −3515.0 −0.946 –

CO2 (g)=2.0pH=2

15 Temperature=20 °C 4.482 2.47 0.00 −0.30 −10,000.0 −136.5 –

CO2 (g)=4.0pH=2


and low solubility phases of this precipitate (Kan et al., 1992,2004a,b; Tomson et al., 2004; Kan et al., 2005).

These three views described above are probably all “right” undercertain circumstances. Sorbie and Gdanski (2005) have published adetailed comparison of the Heriot-Watt and Halliburton models showingthat the two models are very similar in broad principle and differ only insome details (e.g. the form of the desorption rate law). For example, bothmodels couldmatch somevery detailed experimental SI core effluent datawith equal accuracy by changing only some model parameters.

In the case of a carbonate system, although the “pseudo-isotherm”

approach fits the data very well, we are aware that the SI/calciteinteraction in the core is more complex than a simple adsorptionprocess. Future work will focus on improving the model for describingthis process. The modeling presented in section 9.3 takes some initialsteps towards a full model for the SI/carbonate interaction taking intoaccount the involvement of both calcium and magnesium ions.

9.3. A simple model of the Ca and Mg behaviour

The role of calcium andmagnesium ion, which are known to interactdirectly with the DETPMP, has been studied in this work. DETPMP

adsorption is significantly improved by the presence of Ca2+ ions andeffectively poisoned by the presence of excess Mg2+ ions. Sorbie andLaing (2004) have shown that calcium and magnesium may bind toDETPMP,whichwasdenotedbyA for simplicity. Thesolutionequilibriumbinding reactionswhich occur in a Ca/Mg/DETPMP system are as follow:

Ca + A XKCa

Ca � A ð2aÞ

Mg + A XKMg

Mg � A ð2bÞ

The stability constants KCa and KMg, are given by:

KCa =XCa � AXCa � XA

ð3aÞ

KMg =XMg � AXMg � XA

ð3bÞ

For DETPMP, these quantities are known to be KMg=6.3×1010 andKCa=5.0×1010. Suppose we start with initial quantities of Ca, Mg, SI

Fig. 19. RC1 — linear coreflood model, best fit. Freundlich isotherms: k=6000, n=0.25.

Fig. 20. RC3 — linear coreflood model, best fit. Freundlich isotherms: k=4500, n=0.45.

Table 9Values of percentage of SI bound to Ca and Mg using Eqs. (4a and b) and (5) for floodsRC1 to RC5.

Coreflood (XCaA/XMgA) % SI–Cabound

% SI–Mgbound

(XCaA/XMgA) % SI–Cabound

% SI–MgboundEq. (5) Eq. (4a and b)

RC1 0.1402 12 88 0.1496 13 87RC2 0.1436 13 87 0.1641 14 86RC3 0.1431 13 87 0.1797 15 85RC4 0.1432 13 87 0.1457 13 87RC5 0.1428 12 88 0.1796 15 85


(as A) in solution, denoted XCa0, XMg0and XA0

. It is quite straightforwardto rearrange the system of equations to obtain the following twoimplicit equations for XCa and XMg:

XCa =XCaO

1 + KCaXAO

1 + KCaXCa + KMgXMg

h i ð4aÞ

XMg =XMgO

1 + KMgXAO

1 + KCaXCa + KMgXMg

h i ð4bÞ

Eqs. (3a) and (3b) can be solved numerically in a spreadsheet but theactual quantity of interest is the equilibrium value of (XCaA/XMgA), i.e. theratio of SI bound to Ca to SI bound toMg. For very large values of stabilityKCa,KMg, as is the case here, an approximate equation for this ratio is givenby:

XCaA

XMgA

!e KCa � XCaOKMg � XMgO

!ð5Þ

Table 9 summarizes results for the calculated ratio using Eqs. (4aand b) and (5) from the five experimental corefloods RC1 to RC5. Thenumerically predicted value of (XCaA/XMgA) varies between 0.1402 and0.1797, implying that 12 to 15% of DETPMP is bound to Ca and 85 to88% of DETPMP is bound to Mg. This is quite consistent with theresults we observe in our experiments where the retained SI causes adecrease below the stock of magnesium during the adsorption stage ofthe treatment followed by a post shut-in value of magnesium

Table 10XRD, XRF and XPS results.

Sample XRD analysis XPS analysis

Calcite(CaCO3)(%)

QuartzSiO2

(%)

Albite Na(AlSi3O8)(%)

MicroclineK(AlSi3O8)(%)

Phosphorus P(%)

RC1 96.0 4.0 0.160RC2 99.0 1.0 0.170RC3 97.8 1.8 0.4 0.350RC4 98.8 1.2 0.210RC5 74.0 2.0 117.0 7.0 0.300


concentration above the stock as the SI (bound mainly to Mg) is backproduced. Further work is in progress to model more appropriatelythe corefloods (RC1 to RC5) taking into account the involvement ofcalcium and magnesium in the SI/carbonate rock interaction.

Fig. 23. Image of calcite cement in core RC5. Note the undamaged calcite crystals andcalcite crystals with microwormholes.

10. Post-coreflood petrographic analysis

The rock material used in floods RC1 to RC5 was analysed after theSI treatment, to check for any evidence of dissolution of the rockmatrix and/or precipitation within the rock matrix. A post-corefloodpetrography analysis has been performed on cores RC1 to RC5. These

Fig. 21. Image of calcite cement in core RC3. Note the undamaged calcite crystals.

Fig. 22. Image of calcite cement in core RC4. Note the undamaged calcite crystals.

analyses include the study of the thin sections as well as the XRD, XPS,AFM, SEM and EDX measurements.

XRD analysis carried out both on the whole rock and on the fineparticle fractions has shown that the rock material is almost pure

Fig. 24. Larger magnification of Fig. 23.

Fig. 25. SEM photo showing masses of SI salts on the ooids (coreflood RC1).

Fig. 26. SEM photo showing masses of SI salts on the ooids (coreflood RC2).

Fig. 27. Image of RC3 after the scale inhibitor treatment. Note the SI salts.

Fig. 29. SEM Photo of coreflood RC4, showing SI salts occupying the void space.


calcite. Core RC1 is about 96% calcite and 4% quartz. Core RC2 is about99% calcite and 1% quartz. Core RC3 is about 97.8% calcite, 1.8% quartzand 0.4% plagioclase. Core RC4 is about 98.8% calcite and 1.2% quartz.Core RC5 is about 74% calcite, 2% quartz, 17% plagioclase and 7%microcline (Table 10).

The analysis of the thin sections as well as the SEM analysisperformed on corefloods RC3 and RC4 have shown that the treatment of

Fig. 28. Energy dispersive X-ray spectrum

these floods with 0.5PV of respectively 25,000 ppm and 5000 ppm ofDETPMP at pH 2 has not induced the formation of worm holes or microworm holes (Figs. 21 and 22). The SEM analysis carried out on core RC5performed with 27,000 ppm DETPMP at pH 4 has shown that the highconcentration of the SI solution has induced the formation of microworm holes formed on the calcite crystals (Figs. 23 and 24). This isprobably due to the high concentration of the SI solution (27,000 ppm).Indeed, the coreflood RC5 showed also the highest level of calciumdissolution (Ca=481.55mg or 1.58mg/g) (see section 7). SEM analysiscarried out on cores RC1 to RC5 showed the presence of SI saltsoccupying the void spaces (Figs. 25 and 28) and also formed on the ooidgrain surfaces (Fig. 26). Large amount of SI salts have also been observedon the RC3 core, 25,000 ppm SI solution at pH 2 (Fig. 27).

The EDX analysis has revealed clearly the presence of phosphorustogether with calcium, chloride and sodium (Fig. 28). The structure ofthe vacuolar structure of the phosphorus calcium salt shows that the salthas been precipitated, confirming that the mechanism of the DETPMPretention in carbonate rocks is of a precipitation type (Figs. 29–31).

The Atomic Force Microscopy (AFM) analysis has shown the shapeof the calcium phosphonate crystals (Figs. 32 and 33); indeed, theAFM technique allows a higher resolution than the SEM technique and3-D images of the rock surfaces.

The X-ray photoelectron spectrometer (XPS) analyses have beencarried out in order to estimate the percentages of phosphorus (from the

(EDX) of the SI salt showed on Fig. 31.

Fig. 30. Larger magnification of Fig. 29. Fig. 32. AFM image showing the structure of the calcite phosphate crystal in coreflood RC4.


scale inhibitor) retained in the cores. XPS analysis (Table 10) carried outafter the scale inhibitor treatment showthat theamountof phosphorus inthe rock matrix varied between 0.16% (RC1) and 0.35% (RC3). Thus floodRC3 (25,000 ppm DETPMP at pH 2) shows more retained phosphorus(0.35%) than floods RC5 (0.30%), RC4 (0.21%), RC2 (0.17%) and RC1(0.16%). These results are in agreementwith the calculated values of the SIadsorption levels reported in Table 4 (see section 7). Table 4 shows thatthe coreflood RC3 exhibits also the highest level of SI adsorption(Γ~0.087 mg/g). These results confirm that we have more SI retentionwith high SI concentration and low pH values, and the low pH parameterseems to be more important than the concentration of the SI solution.

11. Summary and conclusions

Detailed results are presented from a series of DETPMP scaleinhibitor (SI) carbonate corefloods using Jurassic Portlandian chalk(Φ~19.8% and k~600 mD). Five of these were “contained” long (12q)corefloods injecting 0.5PV of SI as follows:

flood RC1 — 5000 ppm DETPMP at pH 4;flood RC2 — 10,000 ppm DETPMP at pH 4;flood RC3 — 25,000 ppm DETPMP at pH 2;flood RC4 — 5000 ppm DETPMP at pH 2; andflood RC5 — 27,000 ppm DETPMP at pH 4.

Postflush using pH 6 seawater brine was injected through theoriginal core outlet end of the above 5 corefloods and SI was backproduced from the original inlet.

Fig. 31. Larger magnification of Fig. 30 of the SI salt containing phosphorus, calcium,chloride, and sodium.

The main conclusions of this study are as follows:

(1) the results from the “contained” long corefloods RC1 to RC5showed that:(a) the higher the concentration of SI and lower the pH, the

more calcium dissolution is observed (from the [Ca2+]effluents);

(b) in all treatments there is a decrease in the [Mg2+] effluentconcentration corresponding directly to the increase in the[Ca] concentration;

(c) the post shut-in pH in all of these floods shows a peakbetween pH 7 and 8.5 which gradually decreases to pH~7;

(d) the core permeability (k~600 mD) increases in all casesindicating that the stimulation effect through calcitedissolution is greater than the permeability reducing effectinduced by the deposition of (Ca and Mg) phosphonatesalts; and

(e) the lowering of the injected SI solution pH (i.e. pH=2) ismore important than an increase in the SI concentration(i.e. 27,000 ppm DETPMP) for improving the SI adsorptionin the core.

(2) The prediction of the pH evolution has been performed withthe help of two softwares: MultiScale and PHREEQC. The futurework will consist of using a batch modelling in order tosimulate more appropriately our laboratory experiments.

(3) The coreflood effluent cation analysis strongly support the viewthat both magnesium and calcium are binding quite strongly tothe SI (i.e. Ca–A and Mg–A where A = DETPMP). The calciumeffluent concentration always shows an increase during SIinjection since the effect of dissolution (which increases theeffluent [Ca2+] concentration) dominates that of Ca–A reten-tion. This is further supported by our simple model of Ca–A and

Fig. 33. Larger magnification of Fig. 32.


Mg–A bindingwhich allows us to estimate the relative amountsof each of these species, as the ratio, (XCaA/XMgA). Thenumerically predicted value of (XCaA/XMgA) varies between0.1402 and 0.1797, implying that 12 to 15% of the DETPMP hasCa bound to it and 85 to 88% has Mg bound to it. This is inexcellent agreement with our experimental results(from theeffluent data) and explains why the Mg effluent concentrationshows a clear dip during the SI placement and a peak during theseawater postflush period.

(4) The modelling, using the derivation of an effective pseudo-isotherm (with a Freundlich isotherm), is able to produce goodmatches (notpredictions) of theSI return concentration curves forall five core floods RC1 to RC5. We are able to predict the final %SIretained in the core by modelling and this can be compared withthe experimental mass balance results. In fact, excellent quanti-tative agreement is seen. However, we are not satisfied by thisapproach since a simple adsorption model does not, in our view,represent all what is happening in these floodsWe are aware thattheSI/calcite interaction in the core ismore complex than a simpleadsorption process. Future work will focus on improving themodel for describing the SI/carbonate interaction taking intoaccount the involvement of both calcium and magnesium ions.

(5) The petrography analysis has shown that corefloods RC3 and RC4(at pH 2) did not induce the formation of worm holes or microworm holes, but coreflood RC5 performed with 27,000 ppmDETPMP at pH 4 has shown that the high concentration of the SIsolution has induced the formation of micro worm holes on thecalcite crystals. The analysis of the structure of the phosphonate-calciumsalt shows that this salt has beenprecipitated, confirmingthat the mechanism of the DETPMP SI retention in carbonaterocks is of a precipitation type. The Atomic Force Microscopy(AFM) analysis has revealed the shape of the calcium phospho-nate crystals. Thin sections, SEM, EDX, XPS and AFM analysis ofthe cores after the different corefloods have shown the presenceof SI salts formed within the void space and on the ooid grains.The EDX analysis of the SI salts has revealed the presence ofphosphorus. The XPS analysis has shown that corefloods RC3(25,000 ppmDETPMPat pH 2) showsmore retained phosphorus(0.35%) than floods RC5 (0.30%), RC4 (0.21%), RC2 (0.17%) andRC1 (0.16%). These results show that we have more SI retentionwith both a high SI concentration and a low pH value, the low pHparameter seeming to be more important on SI adsorption thanthe concentration of the SI solution.

Acknowledgements

The sponsors of the Heriot-Watt University, Flow Assurance andScale Team (FAST) JIP are thanked for their support, input andpermission to publish this work: Baker Petrolite, BG Group, BP,Champion Technologies, Chemtura, Chevron, Clariant, ConocoPhillips,Halliburton, Hydro Oil and Energy, M I Production Chemicals, Nalco,Petrobras, REP, Rhodia, Shell, Solutia, Statoil and Total. We thank DrNorbert Kohler for his constructive review of the manuscript.

Appendix A. Detailed experimental procedures for “contained”corefloods RC1 to RC5

Experimental procedure for coreflood RC1

1. Brine saturation: saturation of the core with brine solution (pH=6)at room temperature and at flow rates of 30, 60, 90, and 120 ml/h,1 h each. The core was then shut in over 24 h.

2. Pre-treatment brine permeability measurement: brine permeabilitywas measured at room temperature at flow rates of 30, 60, 90, 120,and 240 ml/h in forward direction of flow, 10 min each (brine with

pH=6). measurement of permeability k1 between P1 and P2 and k2between P2 and P3 (see Figs. 2.3 and 2.5).

3. Lithium solution injection: 50 ppm lithium in brine solution(pH=6) was injected at a flow rate of 30 ml/h, 30 samples of2 ml were collected (sample interval every 4 min).

4. Brine solution injection: brine solution (pH=6) was injected at aflow rate of 30 ml/h, 30 samples of 2 ml were collected (sampleinterval every 4 min).

5. Scale inhibitor injection: injection of 0.5 PV (15.18 ml) 5000 ppmactive SI-labelled with 50 ppm Li tracer — in brine solution(pH=4) at a flow rate of 30 ml/h then shut in. The core was thenshut in over 24 h.

6. Postflush is carried out with lithium solution injection: 50 ppmlithium in brine solution (pH=6) from the outlet (P3) (seeFigs. 2.3 and 2.5) at a flow rate of 30 ml/h. Sample schedule = N80samples of 2 ml collected (sample interval every 4 min); 60samples of 10 ml collected (sample interval every 20 min);approximately 500 samples of 20 ml collected (sample intervalevery 40 min), until the active inhibitor concentration fell belowapproximately 0.5 ppm active inhibitor.

7. Post-treatment brine permeability measurement: brine permeabilitywas measured at room temperature at flow rates of 30, 60, 90, 120,and 240 ml/h in forward direction of flow, 10 min each (brine withpH=6). Measurement of permeability k1 between P1 and P2 and k2between P2 and P3 (see Fig. 2.5).

8. Measurement of dead volume: after completing all the stepsdescribed above, the core assembly was dismantled and the deadvolume in the system was measured.


1. Brine saturation: saturation of the core with brine solution(pH=6) at room temperature and at flow rates of 30, 60, and90 ml/h, 1 h each. The core was then shut in over 24 h.

2. Pre-treatment brine permeability measurement: brine permeabilitywas measured at room temperature at flow rates of 30, 60, 90, and120 ml/h in forward direction of flow, 30 min each.(brine withpH=6).

3. Lithium solution injection: injection of 0.25 PV (7.59 ml) 50 ppmLithium in brine solution (pH=6) from the inlet at a flow rate of30 ml/h, 30 samples of 2 ml were collected (sample interval every4 min).

4. Brine solution injection: injection of 0.25 PV (7.59 ml) brinesolution (pH=6) from the inlet at a flow rate of 30 ml/h, 30samples of 2 ml were collected (sample interval every 4 min).

5. Lithium solution injection: continuous injection of 50 ppm Lithiumin brine solution (pH=6) from the outlet at a flow rate of 30ml/h,30 samples of 2 ml were collected (sample interval every 4 min).

6. Brine solution injection: continuous injection of brine solution(pH=6) from the outlet at a flow rate of 30 ml/h, 30 samples of2 ml were collected (sample interval every 4 min).

7. Scale inhibitor injection: injection of 0.5 PV (15.18 ml) 10,000 ppmactive SI-labelled with 50 ppm Li tracer — in brine solution(pH=4) from the inlet at a flow rate of 30 ml/h then shut in. Thecore was then shut in over 24 h.

8. Postflush is carried out with lithium solution injection: 50 ppmlithium in brine solution (pH=6) was injected from the outlet ata flow rate of 30 ml/h. Sample schedule = N80 samples of 2 mlcollected (sample interval every 4 min); 60 samples of 10 mlcollected (sample interval every 20 min); approximately 500samples of 20 ml collected (sample interval every 40 min), untilthe active inhibitor concentration fell below approximately0.5 ppm active inhibitor.

9. Post-treatment brine permeability measurement: brine permeabil-ity was measured at room temperature at flow rates of 30, 60, 90,


120, and 240 ml/h in forward direction of flow, 10 min each (brinewith pH=6).

10. Measurement of dead volume: after completing all the stepsdescribed above, the core assembly was dismantled and thedead volume in the system was measured.


1. Brine saturation: saturation of the core with brine solution (pH=6)at room temperature and at flow rates of 30, 60, and 90 ml/h, 1 heach. The core was then shut in over 24 h.

2. Pre-treatment brine permeability measurement: brine permeabilitywas measured at room temperature at flow rates of 30, 60, 90, and120 ml/h in forward direction of flow, 30 min each (brine withpH=6).

3. Lithium solution injection: 50 ppm Lithium in brine solution(pH=6) was injected at a flow rate of 30 ml/h, 60 samples of2 ml were collected (sample interval every 4 min).

4. Scale inhibitor injection: injection of 0.5 PV (15.18 ml) 25,000 ppmactive SI (pH=2) from the inlet at a flow rate of 30 ml/h then shutin. The core was then shut in over 24 h.

5. Post flush is carried out with iodide solution injection: 200 ppmSodium Iodide in brine solution (pH=6) was injected from theoutlet at a flow rate of 30 ml/h. Sample schedule =N80 samples of2 ml collected (sample interval every 4 min); 60 samples of 10 mlcollected (sample interval every 20 min); approximately 500samples of 20ml collected (sample interval every 40min), until theactive inhibitor concentration fell below approximately 0.5 ppmactive inhibitor.




2. Pre-treatment brine permeability measurement: brine permeabilitywas measured at room temperature at flow rates of 30, 60, 90, and120 ml/h in forward direction of flow, 30 min each (brine withPH=6).



5. Scale inhibitor injection: injection of 0.5 PV (15.18 ml) 5000 ppmactive SI-labelled with 50 ppm Li tracer — in brine solution(pH=2) at a flow rate of 30 ml/h then shut in. The core was thenshut in over 24 h.

6. Post flush is carried out with brine solution injection: brine solution(pH=6) was injected from the outlet at a flow rate of 30 ml/h.Sample schedule = N80 samples of 2 ml collected (sample intervalevery 4 min); 60 samples of 10 ml collected (sample interval every20 min); approximately 500 samples of 20 ml collected (sampleinterval every 40 min), until the active inhibitor concentration fellbelow approximately 0.5 ppm active inhibitor.


Note: analyze Li, Ca, Mg and Fe in steps 3 and 4; analyze Li, “P”, Ca,Mg and Fe in steps 5 and 6.



2. Pre-treatment brine permeability measurement: brine permeabilitywas measured at room temperature at flow rates of 30, 60, 90, and120 ml/h in forward direction of flow, 30 min each (brine withPH=6).



5. Scale inhibitor injection: injection of 0.5 PV (15.18 ml) 25,000 ppmactive SI-labelled with 50 ppm Li tracer — in brine solution(pH=6) at a flow rate of 30 ml/h then shut in. The core was thenshut in over 24 h.

6. Post flush is carried out with brine solution injection: brine solution(pH=6) was injected from the outlet at a flow rate of 30 ml/h.Sample schedule = N80 samples of 2 ml collected (sample intervalevery 4 min); 60 samples of 10 ml collected (sample interval every20 min); approximately 500 samples of 20 ml collected (sampleinterval every 40 min), until the active inhibitor concentration fellbelow approximately 0.5 ppm active inhibitor.


Note: analyze Li, Ca, Mg and Fe in steps 3 and 4; analyze Li, “P”, Ca,Mg and Fe in steps 5 and 6.

References

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2010 Effect of pH and scale inhibitor concentration on phosphonate–carbonate interaction

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