Isothermal Titration Calorimetry Study of Brine-Oil-Rock Interactions

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Isothermal Titration Calorimetry Study of Brine-Oil-Rock Interactions Jacquelin E. Cobos, Peter Westh, and Erik Gydesen Søgaard Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00512 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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Isothermal Titration Calorimetry Study of Brine-Oil-Rock Interactions Jacquelin E. Cobos,† Peter. Westh,‡ and Erik G. Søgaard∗,† †Department of Chemistry and Bioscience, Aalborg University, Esbjerg, Denmark ‡Department of Science and Environment, Roskilde University, DK-4000 Roskilde, Denmark E-mail: [email protected]

Abstract Isothermal Titration Calorimetry (ITC) is a technique that allows to accurately determine the thermodynamic parameters that characterize a binding interaction between two molecular systems. However, this technique have not had a wide application in petroleum science. This study is an attempt to determine the adhesion of different fluids onto a rock surface through ITC experiments. Two artificial brines with different ionic composition were titrated into chalk powder and then crude oil was added to those systems in order to mimic the processes that take place in an oil reservoir. In addition, the wettability alteration process associated with smart water flooding was investigated from a thermodynamic point of view. The results from the ITC experiments suggest that the interaction between smart water and chalk+brine+oil systems is both exothermic and endothermic. The exothermic heat response indicates chemisorption of sulfate (SO42− ) onto the mineral lattice. Whereas, the endothermic response proved the substitution of carboxylate complexes from the chalk surface by magnesium (M g 2+ ). The ITC results also show that the performance of diluted seawater seems to be higher than smart water with increased sulfate concentration. This is due to dynamic processes like brine dilution resulting in an increased osmotic pressure.

Introduction

carbonates. Different authors have attributed the oil recovery increase to a wettability alteration process in which the wetting conditions of the rock surface changes from oil-wet towards a mixed-wet state. This process could be linked to multiple-ion exchange (MIE) which is related to the adsorption of SO42− and co-adsorption of Ca2+ and/or M g 2+ onto the rock surface and the desorption of carboxylic acids and bases from the oil attached to the rock surface. 3 4 5 6 Zhang et al. 3 proposed that the adsorption of SO42− onto chalk lowers the positive rock surface charge which leads Ca2+ to approach the rock-brine interface and react with the adsorbed carboxylic groups. In this process, SO42− serves as a catalyst, which helps the adsorption of Ca2+ /M g 2+ to the rock surface. 5 7 Strand

Carbonate reservoirs hold more than 50% of the known petroleum reserves worldwide. Less than 30% of the oil in place from those reservoirs has been recovered due to the presence of fractures that compromises the oil displacement, low permeability, heterogeneous rock properties and wettability of the formation. It has been previously demonstrated that oil recovery can be improved by tuning the ionic content and salinity of the injection fluid for waterflooding, which affects the oil-brine-rock interactions. 1 2 Recently, a considerable effort has been focused on understanding the recovery mechanisms associated with smart water flooding in

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et al. 5 determined that the co-adsorption of SO42− and Ca2+ onto the chalk surface improved the oil recovery when the temperature was increased. However, SO42− and Ca2+ coadsorption decreased at temperatures higher than 100◦ C. Strand et al. 5 confirmed through Zeta potential measurements that SO42− , Ca2+ and M g 2+ are active ions and their reactivity changes with temperature. It was also determined that Ca2+ and M g 2+ have the same affinity for the rock surface at room temperature; however, M g 2+ becomes more reactive at high temperatures (90-100◦ C) and assist in the wettability alteration process by displacing both Ca2+ and calcium-carboxylate complexes (-COOCa). By help of imbition tests Fathi et al. 4 showed that the presence of sodium (N a+ ) and chloride (Cl− ) in seawater decreases the imbibition rate. It means that the ultimate oil recovery also decreases because those ions are only active in the double layer (not part of the inner Stern layer of the chalk/brine interface). This consequently may affect the access of the potential determining ions (SO42− , Ca2+ and M g 2+ ) toward the calcite surface. The ionic strength in general and more detailed ionic composition of the brine play a role in the wettability modification process. Youself et al. 8 obtained additional oil recovery by diluting seawater. The authors indicated that the low salinity water (LSW) changes the rock wettability toward a more water wet state and also improves the connectivity in the rock pore system. Alotaibi et al., 9 Zhang and Zarma 6 and Harrasi et al. 10 also proved that LSW improved oil recovery in carbonate reservoirs. The EOR effects of LSW in limestone were attributed to the dissolution of anhydrite (mineral contained in the rock material), which in turn increases the concentration of SO42− and Ca2+ in low salinity water. 11 The aim of the present research was to elucidate the rock-brine-oil interactions by using Isothermal Titration Calorimetry (ITC) that has been proved as a direct appraisal of the thermodynamics behind the binding interaction between molecular systems. ITC measures directly the heat absorbed or released during a binding interaction. It is a highly sensitive

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calorimetric technique that has mainly been used in the areas of physico- and bio-chemical research areas. The popularity of this technique is due to its speed, robustness and small sample consumption. 12 Although there are different techniques for studying binding reactions, ITC is the only method that measures directly the enthalpy change (∆H) of a binding interaction. Consequently, its degree of accuracy is higher than other alternatives, which determine the enthalpy from theoretical calculations (e.g. van’ Hoff method). In this study, Isothermal Titration Calorimetry (ITC) is used to described and quantified complex binding interactions between oil, brine and chalk in an accurate and efficient manner. The interaction energies obtained by ITC are the result of different endothermic and exothermic events that take place at the rock surface and between the brine-oil and brine-rock interfaces. Therefore, those energies could be used to increase the predictive capabilities of surface complexation models (SCM), which gives a description of the adsorption phenomena in the reservoir.

Materials and Methods Rock Material Dan chalk material obtained in AggersundDenmark by Dankalk A/S, was used in the experiments. The Dan chalk sample was dried at 100 ◦ C, ground with a ball mill and then sieved to mesh between 50 to 100 µm. X-ray Fluorescence Technique (XRF) was used to determine elemental composition of the chalk material. The results obtained through Lab-X 3000 XRF equipment are presented in Table 1. The mineral composition of the chalk sample was analyzed with Aeris, X-ray Power Diffraction instrument (XRD), from PANalytical. The qualitative results, shown in Figure 1, indicate that calcite is the major mineral. Brines Artificial formation water similar to Valhall and Ekofisk fields, synthetic seawater (SW) and


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Figure 1: X-ray Power Diffraction (XRD) analysis of Dan chalk material Table 1: Outcrop Dan Chalk X-ray Fluorescence (XRF) Analysis

density meter, which has an accuracy of 0.001 g/cm3 and reproducibility of 0.0005 g/cm3 .

Compound mass % CaO 91.90 M gO 0.93 SiO2 5.25 Al2 O3 0.76 P 2 O5 0.36 K2 O 0.13 SrO 0.42 F e 2 O3 0.23 Total 100

Table 2: Ionic composition, density and ionic strength of Valhall brine (VB), Ekofisk brine (EFB), diluted seawater (100D*SW) and modified seawater (SW*0NaCl*4S) Ions (mmol/L) Ca2+ M g 2+ SO42− N a+ Cl− HCO3− K+ Ba2+ Sr2+ Density (g/cm3 ) Ionic strength (mmol/L)

modified seawater were prepared in the laboratory by mixing reagent grade salts with deionized water (DI). 13 The brines ionic composition, density and ionic strength is shown in Table 2. The terminologies VB, EFB, and SW denote Valhall brine, Ekofisk brine and seawater, respectively. The prefix D indicates times of dilution of seawater by adding deionized water. Therefore, the terminology 100D*SW represents 100 times diluted seawater. The modified seawater is represented by SW*0NaCl*4S, which means that the seawater has been partly depleted in NaCl and has four times more SO42− than normal concentration. The density of the brines was measured at ambient temperature (23 ◦ C) with DMA 35 Anton Paar portable

VB

EFB

29.25 7.87 0.70 995.96 1064.56 8.92 4.70 0.00 0.00

99.92 21.89 0.00 1155.56 1423.30 3.95 7.36 1.842 8.51

0.13 0.45 0.24 4.50 5.83 0.02 0.1 0.00 0.00

12.99 44.52 96.03 194.08 125.07 2.02 2.02 0.00 0.00

1.041

1.06

0.997

1.01

6.57

472.69

1112.72 1559.43

100D*SW Modified SW

Crude Oil A crude oil from a Danish North Sea chalk reservoir was employed in the ITC experiments. 13 The acid number (AN) and basic number (BN) were measured by a Metrohm autotitrator unit, Tiamo 2.4. The oil density and viscosity were determined by DMA 35 Anton Paar density meter and a PVS rheometer from


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Brookfield, respectively. The crude oil properties are shown in Table 3.

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titrant were injected into the cell for 10 seconds. The number of injections was 7 with an interval of time of 420 seconds between them. Initially, VB or EFB brine were titrated into 1 g of chalk powder. Then oil was added to the resulting rock+brine system. Finally, 100 times diluted seawater (100D*SW) or modified seawater (SW*0NaCl*4S) was added to the rock+brine+oil system. AlGeer et al. 17 showed the existence of a threshold temperature (70 ◦ C - 75 ◦ C) at which potential determining ions modified the wettability of carbonate rocks toward a more waterwet state. Considering this threshold temperature and the fact that pore fluids are generally in equilibrium with rock minerals at temperatures higher than 70 ◦ C, 18 all the ITC experiments were performed at 75 ◦ C. At this temperature, oil-brine-rock binding interactions, which are responsible for the observed wettability alteration in previous studies, 3 19 were determined. A known chemical reaction was used as a reference in order to validate the ITC equipment. As suggested by Baranauskien et al., 20 1 mM of AgN O3 was titrated into the calorimeter cell containing 0.1 mM of NaCl at 25 ◦ C. The obtained thermogram was fitted with a nonlinear regression procedure by using a one site binding model (Independent). The obtained enthalpy change (∆H) for the reaction was -66.28 KJ/mol and the enthalpy value reported in the literature for this reaction is -65.72 KJ/mol. It is also important to mention that the microcalorimeter has a permanently mounted reference, which is optimized for an ampoule and sample with a total heat capacity of 8.43 J/K. The data obtained in each titration was analyzed with N anoAnalyzeT M software from TA Instruments.

Table 3: Crude oil properties AN BN Viscosity Density g/cm3 (mg KOH)/g (mg KOH)/g mPa.s 0.52 1.60 11.94 0.862

ITC Experiments The isothermal titration calorimetry unit (ITC) measures the heat released or absorbed in a binding interaction between a liquid titrated into the cell containing a liquid or a solid substance. In the present case, different brine solutions and/or oil were adsorbed to chalk powder or a more complex matrix made of chalk and brine or combinations of chalk+brine+oil. The obtained heat flow signal, indicates the power applied to the control heater in cal/sec or J/sec to keep the temperature inside of the calorimeter cell constant. For example, when an exothermic reaction takes place, the power in the control heater decreases to restore the temperature. Depending on the data collection preference, this event can be plotted exo up or exo down. Exo down indicates that the thermogram is following the decrease in the control heater power and exo up shows the heat released in the calorimeter cell. In the experiments, the default data preference was exo up. Therefore, positive peaks in the thermograms show an exothermic event and negative peaks indicate an endothermic event. The integration of the heat power over time gives the heat (q) in the system that participates in the wetting of the chalk powder. This heat is equal to the enthalpy change (∆H) at isobaric and isothermal conditions. 14 15 16 The titration measurements were performed with TAM III, multi-channel, microcalorimetric system from TA Instruments. The microcalorimeter of the TAM III unit has a precision of ± 100 nW and the thermostat has an accuracy of ± 0.1◦ C. To carry out the titrations, a syringe was filled with one of the selected fluids (brine or oil) and the working cell with Dan chalk powder. Typically, 9.948 µl of

Results and Discussion Addition of Formation Water to Chalk In the first set of experiments, two synthetic brines (VB and EFB) were added separately to Dan chalk powder in order to simulate an oil field reservoir with formation water before the


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Table 4: Heat and enthalpy change (∆H) values for Valhall brine (VB) and chalk interaction

oil invasion. When the aqueous fluid contacts the chalk lattice, surface complexes are formed at the water-mineral interface. 21 22 23 The raw signal registered in the calorimeter when Valhall brine was titrated into the chalk is presented in Figure 2. As can be seen, the obtained heat flow signal for the brine-rock interaction is positive. According to data collection, those peaks show an exothermic event.

Injection 1 2 3 4 5 6 7

Heat [VB] ∆H (µJ) (ionic strength/L) (KJ/ionic strength) 4.52E+05 1.11 -40.80 1.39E+05 2.23 -6.26 9.14E+04 3.34 -2.75 4.27E+04 4.45 -0.97 3.73E+04 5.56 -0.67 2.79E+04 6.68 -0.42 1.57E+04 7.79 -0.20

affects the pair creation processes. Therefore, the relative reduction of ionic strength when the brines interact with the chalk is smaller for EFB than for VB. This means that it is much harder for the unpaired Ca2+ ions to reach the positive charged rock surface. In this sense, the first peak indicates that Ca2+ ions in the bulk solution take energy from the heat reservoir to overcome the repulsion from the chalk to get close to the mineral surface. PHREEQC, aqueous modeling software provided by the U.S. Geological Survey (USGS), was used to simulate the distribution of species in EFB and VB. Those results indicate that the concentration of calcium decreased when the brines are in equilibrium with calcite. About 8 % of calcium was absorbed in the case of VB and only 1% in the case of EFB. A reduction of calcium in the brines was also confirmed by Inductively Coupled Plasma (ICP). It means that the chalk surface becomes more positively charged and a counter ion distribution takes place in the brine to a greater extent. It is also observed in Figure 3 that only small exothermic peaks are obtained after the first titration with EFB. This suggests that EFB wetted the dry chalk surface less intensive than VB. Other experimental data is in accordance with this observation since a water-wetness percentage of 25% was obtained through flotation experiments for chalk aged with VB and 17% for chalk aged with EFB. 13 This water-wetness difference probably occurs because EFB does not contain SO42− and has a high concentration of Ca2+ and M g 2+ . The heat from the reservoir developed by the injection of EFB into the chalk powder and the

Figure 2: Heat flow vs time for the VB-chalk system

The integration of the area between peaks and the baseline that can be seen in Figure 2 gives the heat developed by each injection of VB (Qinjection ). It can be used to calculate ∆H by Eq (1). In this equation, [C] is the molar concentration of the ligand titrated into the calorimeter cell, and Vinjection represents the volume of each injection (9.948 µl). Notice that [VB] in Table 4 is the ionic strength of VB and represents the concentration of all ionic compounds in the brine in mmol/L. It is observed in Table 4 that the maximum enthalpies occurs at the earliest points in the titration and decreases in intensity as the chalk powder is saturated with VB. Qinj = ∆H × [C] × Vinjection (1) The interaction between the Ekofisk brine and the chalk powder can be seen in Figure 3. The heat flow signal for this interaction shows an endothermic peak in the first titration and only small exothermic peaks in the following titrations. EFB has a high concentration of Cl− but a low concentration of HCO3− and SO42− , which


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Table 5: Heat and enthalpy change (∆H) values for Ekofisk brine (EFB) and chalk interaction Injection 1 2 3 4 5 6 7

Heat [EFW] ∆H (µJ) (ionic strength /L) (KJ/ionic strength) -4.59E+05 1.56 29.61 -1.73E+04 3.1 0.56 6.97E+03 4.65 -0.15 9.15E+03 6.21 -0.147 4.22E+03 7.76 -0.05 1.32E+04 9.31 -0.14 1.08E+04 10.86 -0.10

also kept at 75 ◦ C. Figure 4 shows the heat flow signal for the injection of crude oil into the chalk powder that contains VB, which is identified by chalk+VB+oil. As can be seen in the thermogram, the addition of oil to the system picks up energy from the surroundings (endothermic reaction). This is because the cohesion of the brine is higher that the cohesion of oil. Therefore, when a part of the oil penetrates the brine film to reach the rock surface and becomes physisorbed, energy from the heat reservoir is needed, especially for the first oil injection. A part of the oil will form a layer on top of the brine and create a new interface with a second diffuse layer (See Figure 5). The heights of the endothermic peaks are decreasing in the following injections. In other words, it is higher at the beginning of the process than at the end because in the last case oil was added to the oilbrine-chalk interfaces and not only to a brinechalk interface. Hopkins et al. 24 showed that the adsorption of the acidic components of the oil onto the chalk surface occurs immediately upon contact due to the strong electrostatic attraction between the negatively charged carboxylic compounds in the crude oil and the positively charged sites on the rock surface. 25 It was done by collecting effluent samples of the produced oil and measuring its Acid Number (AN) as a function of pore volume injected. The heat absorbed by the titration of oil into the chalk+VB system is presented in Table 6. The enthalpy values for the oil and chalk+VB interaction were computed by using a modification of Eq (1). The amount of oil as a function of injection number was used to calculate the

Figure 3: Heat flow vs time for the EFB+chalk system

corresponding (∆H), calculated with Eq (1), can be found in Table 5. It can be noticed that the highest ∆H value corresponds to the first injection and only small enthalpy changes occurs in the next titrations. As it was explained previously, EFB requires energy from the system to reach the rock surface at the beginning. From the results presented in Table 4 and Table 5, it can be observed that the enthalpy change (∆H) mole per ionic strength for EFB, without taking into account the first injection, is much smaller than the enthalpy for VB. It can be seen in Table (2) that VB has a high concentration of HCO3− and SO42− and a lower concentration of Cl− . This ionic composition enhanced the formation of calciumhydrogencarbonate and calcium-sulphate pairs in the brine. Therefore, those ions reached the chalk surface much easier than the EFB unpaired ions. It can also be understood that when VB was brought onto the chalk surface, more surface complexes were formed at the surface sites (e.g.,> CaH2 O+ and > CO3− ) due to the adsorption of the active species of this brine (i.e., SO42− ). Addition of Crude Oil to the Brine+Chalk System Crude oil was titrated into the previous brinechalk systems in order to determine the oilchalk interactions when a water film is present. The temperature for this set of experiments was


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amount of oil that penetrates the chalk+EFB interface to become physisorbed to the surface. According to Hunter, 27 the thickness of the double layers is inversely proportional to the ionic strength of brine. Therefore, the two diffuse layers of the brine-chalk and brine-oil interfaces shrinks at higher ionic strength. In this sense, EFB created a little thinner water film in comparison with VB due to a high concentration of divalent ions in the brine. A schematic representation of brine-oil and brine-rock interfaces is given in Figure 5. This figure shows only schematically the conditions in chalk-brine-oil systems. In reality, a lot of movements takes place at 75 ◦ C. It can be expected that the interfaces very often collided with each other. Thus, some exchange of oil components from the two separated interfaces will occur. Similarly, reconstruction of the chalk surface after each disturbing injection to the three phase system is expected to take place. This will also result in a heat exchange with the reservoir. In general, it is important to highlight that in both systems, chalk+VB and chalk+EFB, oil absorbs energy from the surroundings in its forced attachment to the chalkbrine system.

Figure 4: Heat flow vs time for the chalk+oil+VB system

enthalpy change (∆H), which is expressed in terms of the oil mass added to the system. Table 6: Heat and enthalpy change (∆H) values for oil interaction with Valhall brine (VB) and chalk system Injection 1 2 3 4 5 6 7

Heat (µJ) -1.31E+05 -5.56E+04 -4.18E+04 .-3,04E+04 -3.21E+04 -3.20E+04 -2.77E+04

Oil ∆H (g oil) (J/g oil) 8.57E-03 15.24 1.71E-02 3.24 2.57E-02 1.62 3.43E-02 0.89 4.9E-02 0.75 5.14E-02 0.62 6.00E-02 0.46

In the second part of the experiment, the same crude oil was added to the chalk+EFB system. As shown in Figure 6, the peaks for this interaction are higher than the ones for the chalk+VB+oil system. The possible explanation for this energy difference could be related to the chemical composition of the formation brine that is in contact with the mineral surface before the oil invasion. As can be seen in Table 2, EFB has a higher ionic strength (1559.43 mmol/L) than VB (1112.72 mmol/L). Kovscek et al. 26 indicated that the adsorption of the oil components depends upon the stability of the water film. Since the ionic strength of EFB is higher, the concentration of counterions in the double layer is also higher than in VB. Therefore, more energy was spent for a similar

Figure 5: Representation of brine-chalk and brineoil interfaces. (a) Stern layer, (b) brine-chalk diffuse layer, (c) brine-oil diffuse layer, (d) Stern layer [Reconstructed from Hunter 27 ]

The heat and enthalpy change values for the interaction between oil and chalk+EFB system are shown in Table 7. It is observed that for each oil injection, ∆H for the chalk+EFB system is higher than for the chalk+VB system.


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seen that interaction between the smart fluids and chalk+brine+oil systems is mainly exothermic. Zhang et al. 29 proposed that the adsorption of SO42− from the imbibing seawater onto the chalk mineral surface is responsible for the observed wettability alteration. Strand et al, 5 also observed that SO42− and Ca2+ increase the water-wetness of the rock. The authors studied the adsorption of SO42− onto the chalk surface through chromatographic separation of SO42− and a non-adsorbing tracer thiocyanate (SCN ) at water-wet sites in the rock. Sakuma et al. 18 used electronic structure calculations based on density functional theory (DFT) to model the ionic substitution that occurs at the calcite surface. The results from this study showed that the incorporation of SO42− into the calcite surface, covered by a mono-layer of water molecules, is exothermic (-11 kJ/mol). Consequently, the exothermic peaks that can be seen in Figure 7 could be the result of SO42− chemisorption, which in turn lowers the positive charge of the surface and M g 2+ displaces the carboxylate complexes from the chalk lattice. This in turn, changes the contact angle of the chalk surface and therefore the wettability of the rock in both systems is altered. Sakuma et al. 18 determined that the contact angle for a 10% surface substitution varies dramatically for mixed-wet and oil-wet surfaces. For instance, the contact angle for an oil-wet surface changes from 180◦ to 109◦ and the contact angle for a mixed wet surface (90◦ ) is 48◦ after substitution with smart water. The heat and enthalpy values that can be found in Table 8 point out that the symbiotic interaction between the modified seawater and the chalk surface is higher for system 1 (chalk+VB+oil) than for system 2 (chalk+EFB+oil). This could indicate that the degree of SO42− chemisorption and substitution of Ca2+ by M g 2+ at the chalk surface is greater for system 1 than system 2. Experimental data from Sohal et al. 13 indicates that the interaction between the modified seawater and system 1 was greater than its interaction with system 2 at the same pressure and temperature conditions. Field-scale waterflooding projects showed that the performance of the seawater

Figure 6: Heat flow vs time for the Chalk+EFB+oil system Table 7: Heat and enthalpy change (∆H) values for oil interaction with Ekofisk brine (EFB) and chalk Injection 1 2 3 4 5 6 7

Heat (µJ) -1.56E+05 -9.47E+04 -6.03E+04 -3.75E+04 -2.96E+04 -2.20E+04 -1.81E+04

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Oil ∆H (g oil) (J/g oil) 8.57E-03 18.16 1.71E-02 5.52 2.57E-02 2.34 3.43E-02 1.09 4.29E-02 0.69 5.14E-02 0.43 6.00E-02 0.30

Addition of modified seawater to the Chalk+Brine+Oil System The effect of ”smart water” or modified seawater was studied in the following set of experiments. The fluid titrated into the chalk+brine+oil systems was depleted in N aCl and spiked with 4 times SO42− as it is suggested in the literature. 4 13 Fathi et al. 4 pointed out that the active ions, Ca2+ , M g 2+ and SO42− , can easily penetrate the electrical double layer (EDL) to interact in the Stern Layer when the injection fluid is depleted in N a+ and Cl− . Puntervold et al. 28 indicated that spiking the injection fluid with SO42− increases the oil recovery, however, it is not advisable at high temperatures (100 ◦ C) due to anhydrite (CaSO4 ) precipitation. The thermograms obtained by the titration of modified seawater into the chalk+brine+oil systems are displayed in Figure 5. It can be


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bility alteration process in which the wetting properties of the rock are changed from a oilwet or mixed-wet state to an water-wet state. Zhang and Sharma 6 obtained a water-wetness enhancement of the carbonate rock by reducing the salinity of the water. The authors reported an oil increase of about 25.04% from reservoir carbonate core plugs. Yousef et al. also studied the effect of LSW in Middle Eastern carbonate reservoir core plugs by different diluted versions of seawater. The experiments showed that the oil recovery increases with a dilution up to 20 times. 8 In this study, the effect of the low salinity waterflooding was analyzed by ITC experiments in which 100 times diluted seawater was titrated into the chalk+VB+oil and chalk+EFB+oil systems. The heat flow signal for both systems is presented in Figure 8. Contrary to the previous experiments, the interaction between 100 times diluted seawater and the rock+brine+oil systems are both partly endothermic and exothermic. Sakuma et al. 18 indicated that the incorporation of M g 2+ into the calcite surface is an endothermic process (5kJ/mol) while the adsorption of SO42− is exothermic (-8kJ/mol). Bearing this in mind, the exothermic peaks could represent the adsorption of SO42− onto the chalk surface and the endothermic peaks could indicate the displacement of Ca2+ and Ca2+ carboxylate complexes from the surface by its interaction with M g 2+ . The exothermic peaks in both systems are also a consequence of osmotic pressure. When 100*DSW was titrated into the chalk+brine+oil systems, the water from the low concentrated brine will move into the formation water (EFW or VB) and create an osmotic pressure, normally called, disjunction pressure. Large amounts of energy are released to the surroundings in this process. The EDL of counter-ions is also expanded because of a less suppressive force exerted by the ions in the bulk solution. It means that not so many counter ions are present very close to the chalk surface, making it easier for the physisorbed oil components to escape from the surface. Note that restructuring processes are also taking

Figure 7: Heat flow vs time for the titration of SW*0NaCl*4S into chalk+VB+oil (system 1) and chalk+EFB+oil (system 2)

injection in the Ekofisk oil field was better than in the Valhall oil field. The difference was attributed to the lower temperature of the Valhall field (90 ◦ C) compared to the Ekofisk field (130 ◦ C). 30 This is also a result of the amount of oil physisorbed to the rock surface in the EFB system, which is higher than in the VB system. Table 8: Heat and enthalpy change values for titration of SW*0NaCl*4S into system 1 and system 2 Inj. 1 2 3 4 5 6 7

System 1 Heat ∆H (J) (KJ/mol) 0.017 -3.58 0.015 -1.64 0.016 -1.13 0.015 - 0.78 0.015 -0.66 0.006 -0.20 0.025 -0.76

System 2 Heat ∆H (µJ) (KJ/mol) -0,037 7.83 0,011 -1.17 0,007 -0.48 0,005 -0.27 0,004 -0.19 0,005 -0.18 0,006 -0.17

Addition of diluted seawater to the Chalk+Brine+Oil System Low salinity waterflooding (LSW) has significantly contributed to the hydrocarbon recovery for several decades in both sandstone and carbonate reservoirs. Several studies in the literature indicate that the main mechanism involved in the improved recovery is a wetta-


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place at the rock surface and even if these processes will be exothermic in nature, they need to borrow some energy from the reservoir at the beginning.

The overall enthalpy change for the titration of modified seawater and dilute seawater into the chalk+brine+oil systems is presented in Figure 7. SW*0NaCl*4S:1 and 100D*SW:1 represents the interaction of the chalk+VB+oil system with modified seawater (SW*0NaCl*4S) and diluted seawater (100D*SW), respectively. On the other hand, SW*0NaCl*4S:2 and 100D*SW:2 indicate the interactions for the chalk+EFB+oil system. By comparison, the interaction enthalpy for 100D*SW is greater than SW*0NaCl*4S for both systems. For instance, ∆H for the second injection of SW*0NaCl*4S into the system 1 is -1.64 KJ/mol while the injection of 100D*SW gives a ∆H of around -75.5 KJ/mol. Those results indicate that the performance of diluted seawater is better than modified seawater for both systems. Hiort et al. 22 built a geochemical thermodynamic model based on spontaneous imbibition experimental data from Zhang and co-workers. 3 The authors proposed that low-salinity brine causes rock dissolution in order to maintain the chemical equilibrium in the system. Consequently, the oil compounds absorbed onto the dissolved mineral surface are released. Yousef et al. 8 also reported anhydrite dissolution in cores flooded with low salinity brine. Nuclear magnetic resonance (NMR) measurements, performed by the Yousef and co-authors, showed anhydrite dissolution in cores flooded with low salinity brine. Those measurements indicated enhanced pore system connectivity between macro and micro-pores. Hirasaki 31 elaborated a model based on the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory to determine the effect of surface forces (repulsive EDL, van der Waals and hydration forces) on the wettability of rockbrine-oil systems. In this model, it is assumed that the rock surface does not dissolve or restructure. This means that the surface is not chemically reactive and remains structurally intact at any time. Chen et al., 32 on the other hand, demonstrated that restructuring of surfaces and microscopic changes in the rock morphology reduces the effective adhesion energy, which thereby increases the water-wettability.

Figure 8: Heat flow vs time for the titration of 100 times diluted seawater into chalk+VB+oil (system 1) and chalk+EFB+oil (system 2)

The integration of the peaks that can be observed in Figure 8 with respect of the baseline gives the heat per injection of 100 times diluted seawater (100D*SW) into system 1 (chalk+VB+oil) and system 2 (chalk+EFB+oil). The enthalpy change (∆H) for both systems was calculated by using the heat values and the ionic concentration of 100D*SW. It can be noticed in Table 9 that ∆H for system 2 is lower than for system 1 as it occurs when modified seawater was brought into similar chalk+brine+oil systems. Table 9: Heat and enthalpy change values for the titration of 100D*SW into system 1 and system 2 Inj. 1 2 3 4 5 6 7

System 1 Heat ∆H (J) (KJ/mol) 0.027 -414.9 0.010 -75.5 0.008 -42.7 -0.031 119.8 0.014 -41.5 0.006 -15.9 -0.024 51.4

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System 2 Heat ∆H (µJ) (KJ/mol) 0.008 -127.9 -0.008 58.3 -0.007 37.9 0.010 -37.2 -0.005 16.1 -0.007 17.9 -0.008 16.5


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Energy & Fuels

Figure 9: Enthalpy change for modified seawater (SW*0NaCl*4S) and diluted seawater (100D*SW) with system 1 (chalk+VB+oil) and system 2 (chalk+EFB+oil)

The authors observed through Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM), topographical changes in the smooth calcite surfaces after dilution. Those changes were also shown by Imaging and Surface Forces Apparatus (SFA) measurements. Chen and co-workers concluded that the adsorption or dissociation between the mineral surface and crude oil is followed by a rapid surface dissolution, which occurs at short times (seconds to hours). After this process, surface restructuring (roughening) takes place and it affects both measured zeta potentials and calculated adhesion energy.

VB. As it was expected, the adsorption of the carboxylic molecules at the chalk surface depends on the thickness of the water film. A brine with a higher ionic strength produce a thinner layer but more ionic dense interface that interacts more strongly with the charged parts of the oil. This phenomenon was observed in the ITC results when the same crude oil was brought into the calorimeter cell. The adsorption of active components of the oil is higher for the chalk+EFB than for the chalk+VB system. The thermograms obtained by the titration of the modified seawater, 100D*SW and SW*0NaCl*4S, clearly indicate the interaction between the potential determining ions (PDI) and the rock-brine-oil systems. Effectively, the mechanism associated with the wettability alteration in carbonates at temperatures above 70◦ C is the substitution of Ca2+ with M g 2+ from the rock surface (endothermic process) and the chemisorption of SO42− onto the chalk lattice (exothermic process). According to the ITC results, the performance of diluted seawater seems to be higher than modified seawater at the same pressure and temperature conditions for both chalk+VB+oil and chalk+EFB+oil. The possible explanation for this difference is that dilution effect of this brine is not only related with the physical ion exchange at the rock surface

Conclusions The isothermal titration calorimetry experiments (ITC) can be used to determine the binding interaction between the chalk, and different fluids. It requires small amounts of materials to be performed and gives a better understanding of the crude oil, brine and rock equilibrium. The results from the ITC experiments indicated that the ionic composition of the reservoir brine plays an important role in the initial wettability of the rock. For instance, the chemisorption of Ekofisk brine onto the chalk surface is lower than Valhall brine. This is consistent with the published data, which indicates that EFB makes the rock less water-wet than


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but also with dynamic processes like brine dilution resulting in an increased osmotic pressure.

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Isothermal Titration Calorimetry Study of Brine-Oil-Rock Interactions

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