Performance of Smart Water in Clay-Rich Sandstones: Experimental

Sep 27, 2018 - In addition, monovalent cations are found to have stronger impact on changing wettability toward a water-wet state than are divalent ca...
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Performance of Smart Water in Clay-Rich Sandstones: Experimental and Theoretical Analysis Armin Bazyari,† Bahram S. Soulgani,†,* Mohammad Jamialahmadi,† Abolfazl Dehghan Monfared,‡ and Abbas Zeinijahromi§ †

Department of Petroleum Engineering, Ahwaz Faculty of Petroleum Engineering, Petroleum University of Technology, Ahwaz, Iran Petroleum Engineering Department, Faculty of Petroleum, Gas and Petrochemical Engineering, Persian Gulf University, Bushehr 75169-13817, Iran § Australian School of Petroleum, The University of Adelaide, Adelaide, South Australia 5005, Australia Downloaded via UNIV OF SUNDERLAND on September 27, 2018 at 22:32:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Smart water (SW) has been recognized as an effective yet environmentally friendly technique for enhanced oil recovery in both carbonate and sandstone reservoirs. However, owing to complexities of oil properties, rock compositions, and ion characteristics, the performance of smart water is not well-understood. This paper attempts to derive insights on how smart water performs in clay-rich sandstones. A comprehensive mechanistic study is carried out on synthetic sandpacks that contain different clay types (kaolinite and montmorillonite) and clay concentrations (3 and 8 wt %), under injection of three SWs (0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2). Extensive experiments and modeling are utilized to investigate wettability alteration at microscopic and macroscopic scales, including swelling index test, zeta potential measurement, coreflooding test, contact angle measurement, particle analysis of effluent, differential pressure analysis across the sandpacks, and disjoining pressure isotherm analysis. The theoretical results of disjoining pressure isotherm analysis show that wettability alteration is more accurately indicated by the maximum peak of the disjoining pressure curve than by the area below the positive section of that curve. This is confirmed by contact angle measurements and recovery factors (RFs). In addition, monovalent cations are found to have stronger impact on changing wettability toward a water-wet state than are divalent cations. We also find that there might exist a minimum salinity below which the expansion of the double layer reaches its maximum. Decreasing the salinity below this minimum value is found not to affect the sample’s wettability. Coreflooding tests show that total RF in the montmorillonite sandpacks is higher than in those made up of kaolinite. In general, a direct relationship is found between clay concentration and RFs. Furthermore, it is found that fines migration and wettability alteration are the dominant mechanism in kaolinite sandpacks, while clay swelling, wettability alteration, and a salt-in effect have been reported to be more significant in montmorillonite sandpacks.

1. INTRODUCTION

Clay swelling results from the chemical interaction between montmorillonite and brine.11 The two main types of clay swelling are osmotic and crystalline. Osmotic swelling depends on the CEC of clays. When brine contacts clay, if the total charge of cation in a given interlayer is greater than that in the brine, water molecules will migrate to the space between clay layers. This swells the interlayer of the clay. Osmotic swelling is directly proportional to the CEC of the clay. In contrast, crystalline swelling is associated with the hydration of cations in the clay interlayers. The former mechanism can happens at interlayer distances up to 10−22 Å, but the latter one occurs only at interlayer separation greater than 22 Å. Unlike montmorillonite, kaolinite has no space between sheets, and therefore the brine only is able to reduce the surface charge of kaolinite particles (chemical interaction), followed by fines migration as a physical interaction.12−15 Theories for how using SW can enhance oil recovery in sandstone include fines migration,16 pH increases,17 multicomponent ion exchange,18 salt-in and salt-out effect,19 and expansion of double layer (EDL).20 In sandstone reservoirs,

Sandstone reservoirs usually contain clay minerals, especially kaolinite and montmorillonite. When clay minerals contact an aqueous solution that is injected during drilling and production, formation damage often results.1−3 The past decade has seen the introduction of smart water (SW) flooding (SWF) for enhanced oil recovery from carbonate and sandstone reservoirs. However, when utilizing SWF in clay-rich sandstone, the decreased ionic strength of injection brine reduces the electrical attraction between pore surfaces and fines, which in turn causes fines detachment and permeability reduction as well as improving sweep efficiency. Moreover, clay swelling and permeability impairment have also been encountered where montmorillonite contacts SW.4−6 Proposed remedies to prevent this include using nanofluid and increasing divalent cations.7,8 The cation exchange capacity (CEC) of clay particles has the greatest influence on both rock mechanics and pore structural properties.9 Montmorillonite has very high cation exchange capacity (70−100 mequiv/100 g), but kaolinite’s is very low (2−6 mequiv/100 g).10 Ion exchange in clays and other minerals is also dependent on the crystalline structure of the mineral and on the chemical composition of any brine in contact with the mineral surfaces. © XXXX American Chemical Society

Received: May 11, 2018 Revised: September 4, 2018

A

DOI: 10.1021/acs.energyfuels.8b01663 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

much smaller than colloidal particles in electrolyte solution. Total forces as a function of distance h can be calculated as32

the high positive charge in connate water attracts the negative charge in minerals (quartz and clays) and the acidic group of oil. Accordingly, removing oil from the rock surface can be expedited by weakening connate water’s positive charge. Using SW that contains a low concentration of monovalent cations expands the double layer and presumably extracts more oil.17 Oil recovery from cores containing clay minerals has been shown to be facilitated by clay swelling, emulsification, and detachment of fines, and their movement through the porous media.21−23 Low-salinity flood tests on a sandstone core yielded more fines in the effluent and greater oil recovery than did high-salinity flood tests, suggesting that fine migration aided oil recovery.16 How SWF is affected by the presence of connate water, crude polar components, clay minerals, and other parameters was also discussed.24−26 On the other hand, SWF could result in clay mineral movement, flocculation, and redeposition, which would tend to cause pore plugging and formation damage.23 Reduction of ionic strength in the presence of high CEC clay minerals, especially montmorillonite, led to clay swelling, which in turn reduced the accessibility of pore spaces to fluid flow.24,27 Fine migration occurs due to changes in hydrodynamic and electrical forces. In the near wellbore region, high-pressure drawdown could increase the flow velocity beyond the critical velocity and thereby release fines.28 In reservoirs far away from the wellbore, flow velocity hardly exceeds 1−5 ft/day. Hence, fluid flow into porous media is laminar, and the hydrodynamic force can be ignored.29 Therefore, the main type of force in reservoirs is electrical and is the sum of electrostatic double layer repulsion, London−van der Waals attraction, Born (repulsion), and acid−base interaction (attraction or repulsion). The Born and acid−base interaction forces can be disregarded.30 Hence, the most important component of interaction forces are electrostatic double layer and London−van der Waals that are determined by DLVO theory.31−33 The ionic strength of brine dictates the extent of interaction forces between fines and pore wall surfaces.23 Therefore, to design the best SW requires determining the optimum salinity, based on a pore-scale understanding of rock, oil, and brine. One consideration is that fines detachment fosters enhance oil recovery (EOR) by increasing the microscopic displacement efficiency, because blocking pore throats and diverting fluid flow through nonsweep pores brings about new path channels and subsequently, higher oil recovery.34−36 This work presents experimental and theoretical mechanistic studies to investigate the role of various parameters as well as the underlying mechanisms in using SWF to improve oil recovery from clay-rich sandstones that contain kaolinite or montmorillonite. The concentrations used in SW are 0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2. Then, the SWs are injected in sandpacks that were enriched with a clay content of 0.3 wt % and 8 wt %. The results of the experiments were analyzed using the swelling index test, contact angle measurement, recovery factor, effluent particle size evaluation, differential pressure across the sandpack, and disjoining pressure calculation

Ft(h) = Fa(h) + Fr(h)

(1) 32

2.1. Disjoining Pressure. Hirasaki developed a novel sensor using the DLVO theory to investigate water film thickness and disjoining pressure, based on intermolecular interaction. The intermolecular forces include van der Waals attractive forces, electrostatic double layer repulsive forces, and structural forces:32,40 Π t = Πel = Π vw + Πs

(2)

where Πt is the disjoining pressure at the rock/water/oil interfaces, Πel is the electrostatic double layer forces, Πvw is van der Waals forces, and Πs is structural force. 2.1.1. Electrostatic Double Layer Force. Electrostatic double layer force occurs when two similarly charged surfaces approach each other in electrolyte solution. Two approaching charged bodies in a vacuum interact based on Coulomb’s law. But, on the basis of ions of electrolyte brines, the electrostatic double layer forces can be repulsive, attractive, or a combination. This force were estimated by zeta potentials ξ1 and ξ2 for the desired surface, but they can instead be approximated by41 ji 2ψr1ψr 2 cosh(κh) − ψr12 − ψr 22 zyz Πel(h) = nbkBT jjj zz z j (sinh(κh))2 { k

(3)

where ψ1 and ψ2 are the reduced potential, ψri = (eζi/kBT), k is the reciprocal Debye−Huckel double layer length, nb is the ion density in the bulk brine, and kB is the Boltzmann constant. The reciprocal Debye−Huckel double-layer length was obtained by ij 2N e 2I yz κ = jjj A zzz j εr ε0KBT z k {

1/2

(4)

where εr is the relative permittivity (dielectric constant) of the brine used in the coreflooding experiments, which is assumed to be 78.4; ε0 is the permittivity of free space, 8.854 × 10−12; kB is the Boltzmann constant; T is the temperature; NA is Avogadro’s number; e is the electron charge, 1.6 × 10−19 C; and the ionic strength I is I=

1 2

n

∑ cizi2

(5)

i=1

where n is the number of ionic species, ci is the molar concentration of ith ion, and zi is the charge of ith ion.42 2.1.2. London−van der Waals Force. The London−van der Waals force between the two media made of the same material is generally attractive. Accordingly, this force is identified as the strength of attachment between two media. For two parallel plates, the London−van der Waals attractive force is expressed as32

∏vw(h)=−

2. DLVO THEORY The DLVO theory37,38 (after Derjaguin, Landau, Verwey, and Overbeek) was originally employed to determine stability of colloidal particles in aqueous solution by measuring attraction Fa and repulsion Fr forces. However, this theory was also intended to characterize colloidal adsorption and other applications.39 The DLVO theory assumes that ions are

(

h

) )

A 15.96 λ + 2 lw

(

h

12πh3 1 + 5.32 λ

lw

2

(6)

where A is the Hamaker constant in an oil/water/solid system, λlw is the London wavelength, and h is the distance between the two plates. Israelachvili43 formulated an approximation to calculate Hamaker constant for materials 1 and 2 separated by material 3: A = Av=0 + Av>0 B

(7) DOI: 10.1021/acs.energyfuels.8b01663 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels A= +

3 ijj ε1 − ε3 yzzijj ε2 − ε3 yzz zz zzjj kT jj 4 jk ε1 + ε3 z{jk ε2 + ε3 z{ (n12 − n32)(n22 − n32) 3Pve

3. MATERIALS AND METHODS 3.1. Fluids. Brines were prepared by dissolving NaCl and CaCl2 in deionized water. Table 1 presents properties of connate water and the

8√ 2 (n12 + n32)1/2 (n22 + n32)1/2 [(n12 + n32)1/2 (n22 + n32)1/2 ]

(8)

Table 1. Injection Brines Characterizations

where Av=0 is the zero-frequency energy and includes the Keesom and Debye dipolar contribution, Av>0 is the dispersion energy and includes the London energy contribution, ϵ1, ϵ2, and ϵ3 are the static dielectric constants of the three media, n1 is the refractive index in the visible part of the spectrum extrapolated to zero frequency, ve is the electronic absorption or ionization frequency (which is assumed to be the same for each material and is similar for water, quartz, and alkanes but is smaller for aromatic compounds), T is the absolute temperature (°K), k is Boltzmann’s constant (1.38 × 10−23 J K−1), and P is Planck’s constant (6.63 × 10−34 J·s). The Hamaker constant for oil/silica in water is approximately 1 × 10−20 J.32,43 Melrose44 used Hamaker constants ranging from 0.3 to 0.9 × 10−20 J. 2.1.2.1. Hamaker Constant Calculation for Clay Particles. The Hamaker constants for various clay particles were calculated as follows:44 For montmorillonite, A=

9651nkBTγ 2exp( −2κcdc)

(

dc−3 + (dc + Δ)−3 − 2 dc +

Δ 2

, −3

)

connate water

κcdc = 1

salt

g/L

g/L

g/L

g/L

NaCl CaCl2 TDS pH ionic strength (mol/L)

164 30 194 6.92 3.6173

3 0 3 7.25 0.0513

0.5 0 0.5 7.12 0.0085

0 3 3 7.04 0.081

density (g/cm3)

viscosity (cp)

acid no. (mg KOH/g)

base no. (mg HCl/g)

aspheltene content (%)

0.872

36

2.53

1.62

2.1

3.2. Sandpack. The sand grains and the kaolinite and montmorillonite particles were provided by KianKaveh Pharmaceutical Chemical Complex. Compositional analysis of montmorillonite and kaolinite, which was performed by X-ray diffraction (XRD), is shown in Tables 3 and 4, respectively. A methylene blue test was

Table 3. Characterization of Montmorillonite

(10)

where n is ion concentration, kB is Boltzmann’s constant, and T is absolute temperature. γ2 is defined as ÄÅ Z É ÅÅ e δ/2 − 1 ÑÑÑ2 2 Å ÑÑ γ = ÅÅÅ Z Ñ ÅÅÇ e δ/2 + 1 ÑÑÑÖ (11) Zδ is the reduced Stern potential given by Van Olphen45 as ÄÅ É yzi d − δ yÑÑÑÑ ÅÅÅij σ 0 zzÑÑ − 1zzzzjjj Zδ = lnÅÅÅjjjj zÑÑ ÅÅ β(2 cos Z − 2)1/2 ÅÅÇk δ (12) {k δ {ÑÑÑÖ

specifications (%mass)

value

montmorillonite feldspar quartz anorthite muscovite calcite CEC (meq/100 g) average particle size (μm)

87.5 (mass %) 5.2 (mass %) 4.7 (mass %) 1.2 (mass %) 1 (mass %) 0.4 (mass %) 75.3 3

Table 4. Characterization of Kaolinite

where σ0 is the constant surface charge density and can be calculated from the CEC and the specific surface area, δ is the thickness of the Stern layer (∼3 Å) σ0, and 1/2

(13)

where ϵ is the dielectric constant of the fluid. The above equations imply that increasing the CEC of a clay particle would decrease its Hamaker constants. 2.1.3. Structural Forces. In contrast to electrostatic double layer forces and London−van der Waals forces, structural forces act only at distances of less than 5 nm.32 The structural interaction was calculated as Πs(h) = A s e−(h / hs)

SW#3

Table 2. Crude Oil Properties

For montmorillonite the particle thickness, Δ, is on the order of d. For kaolinite, illite, and plagioclase,

i ϵnkT yz zz β = jjj k 2π {

SW#2

three SWs. The crude oil was obtained from the Gachsaran oil field in southwest Iran. To remove the solid particles from the crude oil, the samples were centrifuged at 4500 rpm and then put through 3 μm filter paper. The density was 0.872 g/cm3, and the viscosity was 36 cP. Table 2 represents the oil characteristics at 25 °C.

(9)

A = 1306nkTγ 2dc 3

SW#1

specifications

value

kaolinite chloride sulfate heavy metals (such as Pb) loss on drying (600 C) CEC (meq/100 g) Average particle size (μm)

>99 (mass %) 0.025 (mass %) 0.1 (mass %) 0.005 (mass %) 0.15 (mass %) 4.3 3

carried out to measure CECs of kaolinite and montmorillonite based on ASTM D7503-10.47,48 All impurities were removed from the sand grains, using a two-stage procedure. In the first stage, the sand grains were washed by toluene to liquidate organic impurities (4 h), acetone was used to remove toluene (4 h), the sand grains were washed with deionized water to remove the acetone (8 h), and the sand grains were dried at 40 °C under atmospheric pressure (48 h) (to avoid changing their physical properties). In the second stage, the sand grains were rinsed in 15 wt % HCl to remove inorganic impurities (4 h), the sand grains were washed with deionized water, the initial and final brines were confirmed to have the same pH (which verified that the

(14)

where As is the coefficient and hs is the characteristic decay length for the exponential model. This study used a coefficient of 1.5 × 1010 Pa and a decay length of 0.05 nm.32 C

DOI: 10.1021/acs.energyfuels.8b01663 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels acid had been completely removed), and the final brine was dried at 40 °C under atmospheric pressure (48 h). To prepare the sandpack, which would simulate a sandstone reservoir with clay, sand grains were sieved so as to select those that were 180−250 μm. Then, the desired amounts of kaolinite or montmorillonite were added to sand grains, and these mixtures were stirred until homogeneous. A 12 cm-long sleeve was then prepared. Before the sleeve was packed, a mesh (mesh number of 200) was securely glued to one side, to hinder sand production. A prepared mixture was placed into the sleeve in increments of 10 g and packed slightly by a uniaxial compression apparatus after each increment. Finally, another mesh of the same size as the first was pasted on the other side of the sleeve. Figure 1 depicts a schematic of a sandpack.

applied for disc preparation, using 3 and 8 wt % kaolinite and montmorillonite. To ensure that all particles were coated with oil, 20 pore volumes of oil was injected at 0.1 cc/min into the packed clay particles. In the next step, 15 g of oily mixture was extracted from the sleeve and placed in a high-pressure uniaxial cylinder at 5000 psi for 5 min. The disc was then retrieved from the cylinder by a hydraulic jack and polished to obtain a smooth and homogeneous surface. Prior to contact angle measurement, the discs were immersed in the oil sample at 60 °C and atmospheric pressure for 21 days to provide an oil-wet surface. To evaluate wettability, a disc was placed into the experimental cell that was filled with desired SWs. Afterward, oil was injected into the contact angle apparatus’ cell using the Isco pump at a rate of 0.05 cc/min through the 1/16 inch needle to introduce an oil droplet on the disc surface, at which point an image was captured for analysis of wettability. The schematic of the contact angle measurement apparatus is shown in Figure 2.

Figure 2. Contact angle apparatus schematic. 3.5. Core Flooding Setup. The core flooding setup comprised a core holder, three transfer vessels, a 40-bar pressure transducer, a realtime data acquisition system, an Isco syringe pump (Mmodel 65D), and a computer. Figure 3 has a schematic of the core flooding setup. To have laminar fluid flow in the sandpacks, 0.1 cc/min fluid velocity (0.933 ft/day) was utilized25. Table 6 gives the properties of the sandpacks.

4. RESULTS AND DISCUSSIONS 4.1. Swelling Index Test (SIT). As a preliminary step, the swelling index of montmorillonite was evaluated based on the French standard NF XP 84-703.52 In the porous media, due to montmorillonite’s very high CEC, contact with SW makes it swell and narrows its flow paths, which increases capillary forces. Figure 4 shows that the swelling indices of montmorillonite in the NaCl SWs were also very high. In contrast, the CEC of kaolinite is very low. Hence, the swelling values of kaolinite in contact with the SWs could be ignored. Figure 4 illustrates the SIT outcomes in the presences of connate water, 0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2. It is worth mentioning that the amount of clay swelling also affect zeta potential measurements that will be discussed in the next sections. 4.2. Montmorillonite−SWs−Oil System. Disjoining pressure isotherm analysis (DPIA) is used to predict how attraction and repulsion forces affect wettability alteration on a given surface. DPIA was applied in this study to the clay/brine and oil/brine interfaces. DPIA usually incorporates structural, London−van der Waals, and electrostatic double layer forces, of which the last two are long-range. However, this study

Figure 1. Schematic of Sandpack. The mesh number was chosen to allow clay particles with average size of about 3 μm to pass easily through, but to block sand grains. 3.3. Zeta potential measurement. In this work, zeta potential between oil/brine and clay/brine was measured within 1 mV of the actual value, by a Zetasizer Nano ZS (Malvern Instruments, UK), which uses electrophoretic light scattering. Brine/clay and brine/oil suspensions were prepared following the procedure proposed by Zhang.49 Table 5 presents characteristics of the various combinations of brine and either clay or oil, taken at 25 °C. 3.4. Contact Angle. Contact angle measurement has been used to evaluate the wettability alteration effect of different agents and/or brines for EOR.50,51 We used drop shape analysis to calculate the contact angle of 3 and 8 wt % montmorillonite and kaolinite discs. The contact angle measurements were performed at 25 °C and atmospheric pressure for three different SWs: 0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2. The captive drop apparatus comprised a high-resolution digital camera, Isco pump, cell, 50 cc transfer vessel, and 1/16 inch needle. A similar procedure illustrated in section 3.2 was

Table 5. Zeta Potential Measurements kaolinite/brine

montmorillonite/brine

oil/brine

salts

concn (%wt)

zeta potential (mV)

pH

zeta potential (mV)

pH

zeta potential (mV)

pH

NaCl

0.05 0.3 0.3

−38.11 −30.03 −10.14

6.24 6.05 6.12

−44.4 −36.18 −13.2

6.83 6.41 6.05

−31 −23 −14.19

5.43 5.37 4.98

CaCl2

D

DOI: 10.1021/acs.energyfuels.8b01663 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 3. Schematic of the coreflooding apparatus: (1-) Isco syringe pump; (2) valve; (3) transfervessel; (4) core holder; (5) sandpack; (6) hydraulic pump; (7) overburden gauge; (8) differential pressure; (9) data acquisition; (10) computer; (11) fraction collector.

While, some research has been carried out on the effects of zeta potential and ionic strength on disjoining pressure isotherm, what is not yet clear is the impact of the Hamaker constant on it.53−55 Using Reerink and Overbeek’s44 method and the CECs of the two clays, Hamaker constants were calculated as 3.1 × 10−20 J and 2.2 × 10−20 J for kaolinite and montmorillonite, respectively. Results of zeta potential measurement for oil/brine and clay/brine were listed in Table 5. For both cases, in the presence of monovalent cations, zeta potential measurements were shifted to higher negative values. As can be inferred from the results, zeta potential was greater in the presence of kaolinite compared to its value in the presence montmorillonite. For example, zeta potential measurements in the 0.05 wt % NaCl brine at the interface of montmorillonite/ 0.05 wt % NaCl brine and kaolinite/0.05 wt % NaCl brine were −44.4 mV and −38.11 mV, respectively. In fact, montmorillonite swelled in contact with SW, and its surface charge density had decreased. Considering the zeta potential concept, the ability of montmorillonite particles to adsorb opposite charges was reduced. In the other words, the decrease in counterion adsorption was a result of the decrease in zeta potential values. The zeta potential of clay/brine was more negative in comparison with the case of oil/brine because the negative charge in the surface of clay particles was higher than charges of negative polar components of the oil samples. As shown in Figure 5, the disjoining pressure isotherms of 0.05 wt % NaCl and 0.3 wt % NaCl were positive in the presence of montmorillonite while 0.3 wt % CaCl2 gave negative disjoining pressures isotherm. On the basis of Table 5, the zeta potential of 0.3 wt % CaCl2 had a lower negative value than that of the brines of 0.3 and 0.05 wt % NaCl. According to eq 4, the

Table 6. Key Parameters of Sandpacks samples 8 8 3 3

wt wt wt wt

% % % %

kaolinite montmorillonite kaolinite montmorillonite

length diameter porosity permeability (cm) (cm) (%) (md) 12 12 12 12

2.53 2.53 2.53 2.53

31.5 33 26.4 27

79 62 354 348

SWi (%) 19.5 36.8 25.48 30.25

Figure 4. SIT in contact with 0.05 wt % NaCl, 0.3 wt % NaCl, and 0.3 wt % CaCl2.

disregarded structural forces because they are short-range (under 5 nm) and therefore dominated by the other two forces. DPIA requires calculating the Hamaker constant, zeta potential of the oil/brine, and clay/brine interfaces, and ionic strength of the electrolyte. E

DOI: 10.1021/acs.energyfuels.8b01663 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

NaCl due to the EDL mechanism could be ignored. This finding for the first time, theoretically proved the hypothesis mentioned by some researchers that there is a critical salinity below which EDL mechanisms are ineffective56−58· Another core finding from Figure 5 was related to the influence of the divalent cation on the reciprocal Debye−Huckel length which was more effective than the influence of monovalent cation; confirmed by negative disjoining pressure through injection of 0.3 wt % CaCl2 into both kaolinite and montmorillonite sandpacks. Hence, in these cases the EDL mechanism had not contributed to the EOR process. 4.3. Kaolinite−SWs −Oil System. Figure 6 demonstrates disjoining pressure isotherm curves for sandpacks containing

Figure 5. Disjoining pressure isotherm of montmorillonite/brines.

Debye−Hückel length was a function of ionic strength. Higher ionic strength of 0.3 wt % CaCl2 in comparison with brines of 0.3 and 0.05 wt % NaCl resulted in a decrease of the Debye− Hückel length. Thus, by injecting 0.3 wt % CaCl2, a negative disjoining pressure resulted which was in good agreement with the assumption of compacting double layer. According to results and calculations, it was firmly concluded that unlike 0.05 wt % NaCl and 0.3 wt % NaCl brines, the 0.3 wt % CaCl2 brine was supposed to be ineffective in EOR through the EDL mechanism. As a result, divalent cations played a more important role than monovalent cations in disjoining pressure isotherms. There were two ideas to describe relations between DPIA and wettability alteration; the first one was the area below disjoining pressure isotherm which represents available energy for removing oil droplets from rock surfaces and the second one was the maximum disjoining pressure. To clarify the ambiguity of what aspects of these two arguments were of the most importance and reliability for identifying wettability alteration, we compared them in this research. The maximum disjoining pressure isotherm was 169.54 KPa for 0.05 wt % NaCl brine and 159.54 KPa for 0.03 wt % NaCl brine. On the basis of Table 7, the ultimate oil recovery of a sandpack containing

Figure 6. Disjoining pressure isotherm analysis of kaolinite/brines.

kaolinite in contact with three different SWs including 0.3 wt % NaCl, 0.05 wt % NaCl and 0.3 wt % CaCl2. As mentioned priory, Hamaker constant for determining attractive London Van-der Waals forces was for kaolinite particles which was higher than that of montmorillonite with 2.2 × 10−20J. Comparing Kaolinite with Montmorillonite, its larger Hamaker constant along with its lower zeta potential in clay/brine interfaces caused greater attraction forces which indicated that double layer compaction had occurred. In addition, the disjoining pressure isotherm peaks for kaolinite were lower than those of montmorillonite. The disjoining pressure isotherm value in the presence of kaolinite/0.3 wt % NaCl was 14.22 KPa while its corresponding value for montmorillonite was 159.54 KPa. Hence, it could have been inferred that the contribution of EDL mechanisms to EOR from the kaolinite clay-rich sandpack was not as much as the contribution of montmorillonite sandpack. This finding was also justified by RF results (see Figure 7). The sharp differences between disjoining pressure isotherm curves caused by reducing salinity from 0.3 wt % to 0.05 wt % NaCl in the kaolinite case, led to further oil recovery; while for montmorillonite with the same salinity values, little differences were observed for the disjoining pressure isotherm and almost the same RF. To find the role of EDL as the main EOR mechanism for kaolinite, effluent particles size analysis was performed, and differential pressure across the sandpacks was analyzed. As could be deduced from Figure 9b, no increase in differential pressure was observed during injecting 0.3 wt % NaCl into sandpacks with 3 wt % kaolinite. Moreover, particle size analysis of the effluent which was carried out every 10 min,

Table 7. RF (% OOIP) of Sandpacks by Injection of 0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2. The Error of Calculation is 0.4% of OOIP

3 3 8 8

wt wt wt wt

% % % %

montmorillonite kaolinite montmorillonite kaolinite

0.3 wt % NaCl

0.05 wt % NaCl

0.3 wt % CaCl2

47.8 57.2 35.2 54

1.2 0.8 4.2 2.2

1.6 3.2 1 2.6

8 wt % montmorillonite was 51.95% of original oil in place (OOIP) during injection of 0.3 wt % NaCl at secondary mode; while only 0.8% additional OOIP was achieved in tertiary mode. However, the area below the positive disjoining pressure isotherm for 0.05 wt % NaCl brine was approximately three times greater than that of 0.3 wt % NaCl brine. Therefore, it was concluded that the peak of positive disjoining pressure has significant impact on EOR compared to the area below the positive portion of disjoining pressure isotherm. Nonetheless, it was important to mention that the amount of additional oil recovery by lowering salinity from 0.3 wt % NaCl to 0.05 wt % F

DOI: 10.1021/acs.energyfuels.8b01663 Energy Fuels XXXX, XXX, XXX−XXX

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of particles in the effluent were detected that confirmed the fines migration mechanism. Furthermore, increase in differ ential pressure and detection of particles in effluent were observed for the case of injecting 0.05 wt % NaCl in the sandpacks containing 3 and 8 wt % kaolinite. However, no increase in differential pressure and presence of particles in effluent were observed during injecting 0.3 wt % CaCl2 into both 3 and 8 wt % kaolinite sandpacks. Released fines were able to block some pore throats. Therefore, fluid flowed in other accessible pores which led to increasing sweep efficiency; and consequently, higher oil production. In fact, fines migration may intensify the formation damage by reducing permeability and increase oil recovery by diverting the flow path in EOR. The results of dominant mechanisms are presented in Table 8. Table 8. Dominant Mechanisms of EOR during Injection 0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2 in the 3 and 8 wt % Montmorillonite and Kaolinite Sandpacks

Figure 7. Contact angle measurement for 3 and 8 wt % montmorillonite and kaolinite with connate water, 0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2.

0.3 wt % NaCl 3 wt % montmorillonite EDL-clay swelling 3 wt % kaolinite EDL

did not show any particles. Nevertheless, by injecting into 8 wt % kaolinite-sandpacks at the same salinity, both increase in differential pressure across the sandpack (Figure 8b) and existence

8 wt % montmorillonite EDL-clay swelling 8 wt % kaolinite EDL-fines migration

0.05 wt % NaCl 0.3 wt % CaCl2 EDL-clay swelling

salt-in effect

EDL-Fines Migration EDL-clay swelling

salt-in effect

EDL-fines migration

salt-in effect

salt-in effect

4.4. Effect of Clay Concentration and Different SWs on EOR. The disjoining pressure equation determines the wettability alteration by the EDL mechanism for EOR by using different variables including clay/brine/oil zeta potential, ionic strength of brine, and Hamaker constant. Furthermore, some physical reservoir parameters are not considered in the disjoining pressure isotherm formulation, namely porosity, permeability, fractures, concentration of minerals, and original rock wettability. In other words, disjoining pressure examines the wettability alteration and EOR qualitatively.32,56 According to Table 7, sandpack with 3 wt % montmorillonite was flooded in order to determine the RF values. Corefloods were conducted with sequential injection of 0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2 which resulted in 54%, 2.2%, and 2.6% of OOIP of oil recovery, respectively. The experiments were repeated for concentration of 8 wt % and RF values became 57.2%, 0.8%, and 3.2% of OOIP, respectively. Flood experiments were also performed on sandpacks containing kaolinite. RF values obtained during the injection of 0. 3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2 into 3 wt % kaolinite were 35.2%, 4.2%, and 1% of OOIP, respectively. While, the EOR from sandpacks containing 8 wt % kaolinite led to 47.8% of OOIP, 1.2% of OOIP, and 1.6% of OOIP; as the porous media was flooded with the above-mentioned sequence of SWs. Experimental results demonstrated that the RF ratio of 0.05 wt % NaCl and 0.3 wt % NaCl in the 3 and 8 wt % montmorillonite sandpacks were 0.037 and 0.014, respectively. The corresponding values in the case of 3 and 8 wt % kaolinite were 0.12 and 0.025, respectively. Experimental results revealed that as the concentration of clay decreased, the EOR efficiency of second stage of SW injection increased. Moreover, comparing total amount of produced oil revealed that the ultimate value of RF increased from 40.4% to 50.6% of OOIP by increase in kaolinite concentration from 3 wt % to 8wt %. Similarly, when montmorillonite concentration was changed

Figure 8. Drainage and imbibition process in 8 wt % clay concentration montmorillonite (a) and kaolinite (b).

Figure 9. Drainage and imbibition process in the presence of 3 wt % montmorillonite (a) and kaolinite (b). G

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conducted. As can be seen at Figure 8 differential pressure across sandpacks increased and the presence of kaolinite particles in the effluents was also detected. Therefore, fines migration mechanism and the wettability alteration contributed to the EOR process. Moreover, particle size analysis of the effluent in the case of a sandpack containing 8 wt % kaolinite showed the existence of particles. Additionally, 4% of total OOIP was produced by injection of 0.05 wt % NaCl. Wettability altered from slightly oil wet toward mixed wet when 0.3 wt % CaCl2 was injected into containing 3 and 8 wt % montmorillonite sandpacks, and contact angles changed to 81° and 69°, respectively. Similarly, for sandpacks containing 3 and 8 wt % kaolinite contact angle measurements were 90° and 91°. It was clear that changes of wettability were more intense on montmorillonite rather than on kaolinite. There are a number of factors including brine salinity, cation type, pH and physical properties of particles such as surface area and CEC that affect the Hamaker constant, zeta potential and reciprocal Debye−Huckel double layer length. As demonstrated in Figure 10, there were three layers with different

from 3 wt % to 8 wt %, the RF increased from 58.6% to 61.2% of OOIP. It could be evident that there is a direct relation between the ultimate oil recovery and clay concentration. This finding was in good agreement with the results reported by Tang and Marrow.16,35 4.5. Contact Angle. The experiments were designed to find out the effects of ion types and salinity level on wettability alteration of the clay-coated sandpacks. Contact angle measurements of crude oil with connate water and three different types of SWs (i.e., 0.03 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2) on 3 and 8 wt % kaolinite and montmorillonite discs were carried out at ambient temperature and pressure. The results are depicted by Figure 7. The wettability of rock depends on the stability of water film on the rock surface. The stability of the water film is under the influence of interactions between rock/brine/oil interfaces. In sandstones, the two main interactions are electrostatic interactions between oil/brine charges and rock/brine charges in addition to the hydrogen bond between the polar component of oil and the rock surface.57,59 Contact angle is a function of the disjoining pressure isotherm, which consists of different forces including the Van-der-Waal attractive force, electrostatic double layer force, and the structural force.32 Brine salinity and cation types are important factors in electrostatic interactions. Injection of 0.3 and 0.05 wt % NaCl produced repulsive interactions in two oil/brine and rock/brine interfaces which resulted in more stable water film. It was approved by disjoining pressure isotherm analysis (Figures 5 and 6). Furthermore, contact angle measurements were in good agreement with electrostatic interactions of rock/brine/oil charges which showed the transition from the oil-wet state toward the water-wet state. For the case of 0.3 wt % CaCl2 injection, the disjoining pressure was calculated to be negative which displayed no improvement in oil recovery by the EDL mechanism. Creating hydrogen bonds due to interactions of 0.3 wt % CaCl2 and polar functional groups in crude oil named as the salt-in effect mechanism caused the removal of oil from the rock surface.56 The connate water contact angles were approximately 120° in both concentrations of kaolinite and montmorillonite. By using 0.3 wt % NaCl, wettability shifted from the slightly oil-wet toward the water-wet state. Changes in wettability were greater in the presence of montmorillonite rather than kaolinite. Results of contact angles were in good agreement with the disjoining pressure isotherm analysis. Meanwhile, isotherm disjoining pressure analyses revealed that the peak of curves and the area below the positive section had greater values for the case of montmorillonite. The contact angles of 3 and 8 wt % montmorillonite in contact with 0.05 wt % NaCl brine were 70° and 66°, respectively. In addition, the corresponding values for 3 and 8 wt % kaolinite were 82° and 80°, respectively. These results clearly demonstrated that wettability alteration in the montmorillonite cases had occurred sharper in comparison with kaolinite cases. Similarly, this behavior was observed for 0.3 wt % NaCl brine. Therefore, more volumes of oil were produced by injection of 0.3 and 0.5 wt % NaCl in sandpacks containing montmorillonite: presented in Table 7. Although, there was a slight difference (2 deg) between the contact angle of 0.3 wt % NaCl and 0.05 wt % NaCl in 3 wt % kaolinite, the share of RF in 0.05 wt % NaCl to total RF was considerable (10%). This value for sandpacks containing 3 and 8 wt % montmorillonite were 3.20% and 1.06%, respectively. To support contact angle measurement results, the particle size analysis of effluents was

Figure 10. Diffuse electrical double layer46

potentials around the charged particles. According to charges of particles, the counterions scatter to neutralize the particle’s surface by forming an ion layer which was different from the bulk electrolyte solution. The highest electrical potential occurred at the stern layer. The stern layer is the first layer surrounding the charged particle which forms the molecular capacitor with the fixed charge. In the second layer (diffuse layer), the electrical potential decreases with the distance from charged particles, and finally in the bulk solution the electrical potential becomes zero. Because clay particles have negative charges, the approach of Ca2+ toward the particles surfaces caused a reduction in negative charges of the clay particles. More ionic strength of Ca2+ led to forming a highly positive charged stern layer which resulted in decreasing the electrical potential of the diffuse layer. The high density of Ca2+ around the clay particles caused compaction of the Debye−Hückel length. In other words, the zeta potential as a representative of the electrical charge of the diffuse layer decreased. Due to a lower ionic strength of Na+, the density of Na+ around the charged particles decreased. Hence, Na+ ions were not able to neutralize the negative charges of clay particles as much as Ca2+. Consequently, a thicker diffuse layer and more negative zeta potential resulted. H

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the solubility of organic materials in the brine until below the critical ionic strength. The salting-in effect mechanism works based on this assumption that a lower concentration of divalent cations especially Ca2+ in SW breaks some bonds of carboxylic materials of crude oil from the clay surfaces through the destruction of bridges of high concentration of Ca2+ of connate water from one side and increasing organic materials solubility by forming hydrogen bonds to another side. Accordingly, organic materials were released from the clay surfaces, and consequently the wettability altered toward the water-wet state. Because of the higher CEC of montmorillonite compared to that of kaolinite, the injection of 0.3 wt % CaCl2 led to more frequent disruption of carboxylic groups from the montmorillonite particles. It is worth noting that as the clay concentration became higher, the initial adsorption of polar components of crude oil onto clay surfaces increased and it was expected that injection of 0.3 wt % CaCl2 could have resulted in higher RF. As shown from Table 7, by increasing the concentration of montmorillonite from 3 wt % to 8 wt %, the amount of recovered oil was increased from 2.6% to 3.2% of OOIP. Moreover, in the 3 and 8 wt % kaolinite bearing sandpacks RF values were 1% and 1.6% of OOIP, respectively, because of the lower CEC of kaolinite, which was in good correspondence with the mentioned salt-in effect. Although, analysis of the effluents of injection of 0.3 wt % NaCl brine in the 8 and 3 wt % kaolinite sandpacks did not show any particles, differential pressure analysis revealed a slight growth in the 8 wt % kaolinite sandpack (Figure 8). Furthermore, Figure 11 depicts the initial state of particles in pores. In fact, due to very high salinity of connate water and very strong attraction electrical forces, particles adhere to pore surfaces. Reducing water salinity made some of kaolinite particles detached from pore walls and tended to move through porous media. Increasing DP in 8 wt % kaolinite-sandpacks confirmed that attached particles were released, however effluent analysis did not detect any particles which meant they were not produced. These results clearly revealed detachment and redeposition mechanism in this sandpack. This mechanism caused some released particles blocked pore throats. Therefore, sweep efficiency would be increased. It is worth noticing that DP behavior (see Figure 9) and effluent analysis in the 3 wt % kaolinite-sandpack did not confirm this phenomenon. Comparison these two sandpacks showed that as kaolinite concentration increased, the possibility of detachment and redeposition of particles increased. Besides, DPIA for this salinity revealed only low positive values inferring that wettability alteration due to EDL mechanisms had been low. Actually, 47.8% of OOIP in 8 wt % kaolinite-sandpacks was related to detachment and redeposition of particles as well as EDL; while in the 3 wt % kaolinite sandpacks, 35.2% of OOIP was only merely because of the EDL mechanism. For the montmorillonite particles in contact with 0.3 wt % NaCl, very High positive values of disjoining pressure isotherms were achieved. Therefore, wettability alteration was definitely one of the main mechanisms behind EOR in 3 and 8 wt % montmorillonite-sandpacks. Effluents analysis did not detect any particles, however, DP behavior showed a sharp increase as depicted by Figures 8 and 9. Regarding SIT it could be deduced that clay swelling was another mechanism in those cases. In fact, by swelling the montmorillonite particles during injection of 0.3 wt % NaCl, oil-phase was forced to move through porous media; and consequently, more amount of oil was produced.

Overall, decreasing the salinity led to more negative zeta potentials at both rock/brine and oil/brine interfaces. An increasing divalent cation in the brine caused the zeta potential of oil/brine and clay/brine interfaces to become slightly negative in comparison with monovalent cations. Therefore, divalent cations had a minor effect on the wettability alteration compared to monovalent cations. Furthermore, lower CEC of kaolinite resulted in a greater Hamaker constant which in turn led to stronger attractive London−van der Waals forces. On the basis of Table 7, RF values were justified by contact measurements. As could be seen, RF records of montmorillonite-sandpacks in contact with 0.3 wt % CaCl2 were higher than those of kaolinite sandpacks. Outputs of contact angle tests were normally in good agreement with disjoining pressure analysis. Figure 13 shows schematically the connection between contact angle and disjoining pressure isotherm. Positive disjoining pressure represents the water wet state. It is worth mentioning that the disjoining pressure isotherm analysis was not able to take the concentration of clay into consideration. 4.6. Dominant Mechanisms behind EOR. During injection of 0.3 wt % CaCl2, due to negative values of disjoining pressure isotherms in all distances, the EDL mechanism could not contribute to improving oil recovery in both types of sandpacks containing kaolinite and montmorillonite. Swelling index test revealed that the amount of swelling of montmorillonite particles in contact with 0.3 wt % NaCl and 0.05 wt % NaCl was greater than that of 0.3 wt % CaCl2. Considering the injection of SW sequences, it was not possible to have additional oil recovery due to the swelling of montmorillonite particles. Because 0.3 wt % CaCl2 brine was flooded after 0.3 wt % NaCl and 0.05 wt % NaCl brine. Furthermore, results of the swelling index test for kaolinite particles in contact with three desired SW samples did not indicate any growth in particle size. Hence, it could be concluded that the swelling of montmorillonite and kaolinite particles were not the reason for improving oil recovery by injection of 0.3 wt % CaCl2. Another piece of evidence that confirmed ineffectiveness of clay swelling as a cause of improvement in oil recovery was that no increase in differential pressure occurred across the sandpacks by injection of 0.3 wt % CaCl2, while the expected increase was observed for 0.3 and 0.05 wt % NaCl (Figures 8 and 9). Moreover, due to lower CEC of kaolinite, clay swelling was very negligible in contact with different SWs; hence, the clay swelling mechanism could be ignored. Consequently its role was disregarded on EOR of desired clay-rich sandpacks. Besides, in the kaolinite-sandpacks, based on the results illustrated by both Figure 8 and 9, there was no increase in differential pressure across the sandpack. As a result, fines migration mechanism had no contribution to EOR. It is well accepted that clay minerals can act as cation exchanger in a replacing order as follows:17 Li+ < Na + < K+ < Mg 2 + < Ca 2 + < H+

(15)

The equation below shows the removal of the cation from the clay surface:60 Ca 2 + − clay + H 2O V clay − H+ + Ca 2 + + OH− (16)

Regarding previous discussions, the mechanism behind this incremental oil recovery was supposed to be salt-in effect. In this mechanism divalent cations of connate water especially Ca2+ act as a bridge between organic materials of crude oil and clay surfaces. In addition, salinity reduction is able to increase I

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Figure 11. In the equilibrium conditions, connate water coated pore walls. Due to high electrical attraction forces, fine particles adsorb on rock surfaces. This phenomena resulted in negative disjoining pressure isotherm.

Figure 12. Demonstrate EOR mechanism during injection of SW in the kaolinite sandpacks. Reduction salinity causes desorption of clay particles from pore surfaces. Released particles are able to increase sweep efficiency mechanism by blocking some pores which resulted in EOR.

that injection of 0.05 wt % NaCl could have altered wettability more than 0.3 wt % NaCl toward water-wet state. Surprisingly, in 8 wt % kaolinite-sandpacks only 1.2% of excessive OOIP was produced which meant wettability alteration had happened weakly. In the other words, one of the big advantages of this

On one hand, Kaolinite particles had high positive disjoining pressure isotherm values in contact with 0.05 wt % NaCl; Figure 6. On the other hand, disjoining pressure isotherm had slightly positive values in the previous step in which particles were contacting 0.3 wt % NaCl brine. Hence, it was expected J

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Figure 13. Communication between contact angle and disjoining pressure. As shown in panels a, b, and c, by increasing contact angle, the peak of disjoining pressure is increased.

montmorillonite, similar to the case of injection of 0.3 wt % NaCl. The summery of mechanisms are given in Table 8.

study was regarding this fact that DPIA is not able to interpret the results of EOR in the clay-rich sandstones with high certainty and additional tests were required. Besides, very slight increase in DP and lack of particles in effluent proved that production of 1.2% of OOIP was related to detachment and redeposition of particles. For 3 wt % kaolinite particles noticeable DP increase clearly indicated the fines migration mechanism. According to Figure 12, lowering salinity from 3 wt % NaCl to 0.05 wt % NaCl caused particles to begin moving through the porous media due to weakness of the electrical forces. In this case, particles were found through effluent analysis which meant they were able to move easily inside the pores. In this sandpack despite 8 wt % kaolinite-sandpacks, results of very high positive disjoining pressure isotherms were in good agreement with both DP behavior and effluent analysis. Injection of 0.05 wt % NaCl in the 3 wt % kaolinitesandpacks resulted in 4.2% of OOIP which was noticeable in comparison with the total RF (40.4% of OOIP). Actually, as illustrated by Figure 12, more moving particles made some of them pass through the pore throats and produced oil stick to themselves, while many of them blocked the pore throats; and hence, increased the sweep efficiency sharply. On the basis of Figure 4, although the amounts of swelling of montmorillonite particles in contact with 0.3 and 0.05 wt % NaCl were very high, the difference between SIT values for the given variety of salinity were negligible. In addition, the maximum values of disjoining pressure isotherms for these two salinities were approximately the same (Figure 5). Furthermore, in the previous discussion we proved that the main characteristics of DPIA to evaluate the performance of SWs on RF and contact angle was the peak of curves, and the area below the positive disjoining pressure isotherms was not a good indicator for prediction. Regarding the above discussions, we argued that lowering the salinity from 0.3 wt % NaCl to 0.05 wt % NaCl could not produce a greater volume of oil. In the other words, the maximum EDL had happened in 0.03 wt % NaCl. Therefore, the share of 0.05 wt % NaCl on RF was negligible. This hypothesis was in good agreement with RF outcomes. Hence, the amount of recovered oil in 3 wt % montmorillonite during injection of 0.3 wt % NaCl brine was 54% of OOIP, and the excess oil produced by changing the injected SW to 0.05 wt % NaCl was only 1.95% of OOIP. Likewise, RF in 8 wt % montmorillonite-sandpack for 0.3 wt % NaCl and 0.05 wt % NaCl brines were 57.4% of OOIP and 0.8% of OOIP, respectively. As a result, it was found that EDL and clay swelling were the main mechanisms behind additional oil recovery during the injection of 0.05 wt % NaCl in the 3 and 8 wt %

5. CONCLUSIONS Predicting the behavior and efficiency of SWF in clay-rich sandstone reservoirs requires a study of pore-scale and macroscale analysis. In this paper, different tests were conducted by using three different SWs (0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2), two different clay types (montmorillonite and kaolinite), and two clay concentrations (3 and 8 wt %). DPIA was implied as a microscopic scale evaluation method. In addition, contact angle measurements and core-flooding tests were carried out as macro-scale experiments. Furthermore, to better understand the mechanisms of SWF in each sandpack, SIT, particle size analysis of effluent, and differential pressures across sandpacks were utilized. On the basis of the extensive experiments in this study, the following conclusions can be drawn: • Disjoining pressure isotherm analysis cannot evaluate RF quantitatively. Using this pore-scale method was only a good indicator for the qualitative evaluation of RF. • In the presence of montmorillonite, there was a threshold salinity below which the EDL mechanism cannot be considered in EOR. Therefore, it was crucial to find this limit of salinity based on studied rock compositions. • The detachment of oil from the clay surfaces was a function of the disjoining pressure peak and the corresponding area below the positive disjoining pressure was not a good indicator for prediction of RF. • Cation type had significant impact on disjoining pressure isotherms. Monovalent cations (0.05 and 0.3 wt % NaCl) generated positive disjoining pressure isotherm values; however, divalent cations (0.3 wt % CaCl2) resulted in negative disjoining pressure isotherm values. • In the presence of kaolinite, decreasing salinity resulted in detachment of particles which improve sweep efficiency by the fines migration mechanism. • Injection of 0.3 wt % CaCl2 improved the oil recovery by a salting-in effect mechanism. In addition, because this mechanism depends on CEC, the impact of the saltingin effect mechanism in the sandpacks containing montmorillonite particles was greater than that of kaolinitesandpacks. • RF outcomes revealed that as clay concentration increased, RF was improved. Moreover, RF in the presence of montmorillonite was higher compared to that in the presence of kaolinite, provided by DPIA as a K

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pore-scale method and contact angle measurement as a macroscale method.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bahram S. Soulgani: 0000-0003-4038-7572 Abolfazl Dehghan Monfared: 0000-0003-1026-8839 Abbas Zeinijahromi: 0000-0002-3088-6952 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.energyfuels.8b01663 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.8b01663 Energy Fuels XXXX, XXX, XXX−XXX