The interfacial properties of clay-coated quartz at reservoir conditions

Fuel 262 (2020) 116461

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Full Length Article

The interfacial properties of clay-coated quartz at reservoir conditions a,⁎

b

b

c

Bin Pan , Changping Gong , Xiaopu Wang , Yajun Li , Stefan Iglauer

T

d

a

Department of Chemical and Petroleum Engineering, University of Calgary, Calgary T2N 1N4, Canada National Engineering Equipment Testing and Detection Technology, China University of Petroleum (East China), Qingdao 266555, China School of Petroleum Engineering, China University of Petroleum (East China), No. 66, Changjiang West Road, Qingdao 266580, China d School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Australia b c

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 and CH4 wettabilities Mineral-CO2 Mineral-CH4 and mineral-brine surface energies Kaolinite-coated quartz Montmorillonite-coated quartz

Shale interfacial properties are important for CO2 geo-sequestration and CH4 recovery in shale formation. In this work we use a newly-developed and well-defined shale model (i.e. a clay-coated quartz) to systematically evaluate the influence of kaolinite and montmorillonite, as primary constituents of shales, on CO2 shale-wettability, and shale-CO2, shale-CH4, and shale-water surface energies. Specifically, we measure CO2 wettability of clay-coated quartz and the related CO2-brine and CH4–brine interfacial tensions at various pressures and temperatures. We calculate the surface free energies of the clay-coated quartz versus CO2 and CH4 using Neumann’s equation. The results demonstrate that clay coating leads to a less hydrophilic surface at a low temperature (i.e. 300 K), while it renders the surface more hydrophilic at a high temperature (i.e. 353 K); however, clay coating has only a small influence on the quartz-CO2 and quartz-CH4 surface energies. In addition, higher CO2 pressures always result in less water-wet surfaces for clean, kaolinite-coated and montmorillonite-coated quartz samples at the temperatures tested (i.e. 300 K and 353 K). Moreover, higher CO2 and CH4 pressures lead to smaller mineralCO2 and mineral-CH4 surface energies, respectively. An increase in temperature shows a complicated effect, i.e. it increases the surface energies of mineral-CO2 while it reduces those of mineral-CH4 (slightly) and clay-coated quartz-brine systems. For similar pressure and temperature values, the surface energies of mineral-CO2 system are always smaller than those of the corresponding mineral-CH4 systems. The results can aid the predictions of CO2 storage capacity, leakage risk assessments, and CH4 recovery.

1. Introduction In carbon oxide (CO2) geo-storage (CGS) and in conventional gas reservoirs, shale formations can act as a favorable caprock due to their extremely low permeability and high capillary pressures [1,2]. It has also been proposed that shale formations can be potential CO2 sinks, especially when combined with enhanced methane (CH4) production [3–5]. For CO2 storage and/or enhanced CH4 production in shale formations, CO2 and CH4 wettabilities of shale are key parameters which determine CO2 storage capacity, leakage risk and CH4 distributions, reserves and recovery [3,6–12]. Note that clay minerals are usually filled in the pores or adsorbed onto the surfaces of the shale [13–16], while, it is still not clear about how clay separately influences shale wettability due to the complexity of shale structure and composition [17,18]. In Pan et al. [17], a well-defined shale model (i.e. clay-coated quartz) was developed to investigate how clay influenced CH4-shale wettability at reservoir conditions (i.e. high pressure and high

temperature). The results indicated that at a high pressure CH4 atmosphere, clay coating de-wets shale at a low temperature, while it intensifies shale wettability at a high temperature. However, how clay influences CO2-shale wettability at reservoir conditions has not been studied so far, despite its importance for CGS. Furthermore, brine-CO2, brine-CH4, mineral-CO2, mineral-CH4 and mineral-brine surface energies (γLG , γSG and γSL ) are vital factors which influence CO2 storage and CH4 production [19–22]. While γLG can be measured and is well understood [21,23–26], γSG and γSL are not accessible experimentally [27], and can only be derived via semi-empirical methods [28–30]. For example, in terms of Neumann’s equation of state, Ameri et al. [28] calculated γSG for sandstone-CO2; Kaveh et al. [29] calculated γSG for silty shale-CO2; and Arif et al. [30] calculated γSG for quartz/mica/coal-CO2. However, γSG for shale-CH4 has not been reported so far, despite its importance for CH4 production. Various protocols have been developed to coat pure substrate surfaces (e.g. quartz [17] and polymer [31]) with clay, in order to mimic

⁎

Corresponding author. E-mail addresses: [email protected] (B. Pan), [email protected] (C. Gong), [email protected] (X. Wang), [email protected] (Y. Li), [email protected] (S. Iglauer). https://doi.org/10.1016/j.fuel.2019.116461 Received 31 August 2019; Received in revised form 17 October 2019; Accepted 18 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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the property of real rock. Considering the primary composion in shale is quartz, we continue to use the clay-coated quartz as a well-defined shale model in this work. This model is a favorable substitution for real shale rocks in terms of shale wettability study, due to its relatively simple chemical composition [17]. In this work, we use the proposed model to systematically study shale wettability and their surface free energies. Specifically, we measure the CO2 wettability of clay-coated quartz, and γLG for brine-CO2 and brine-CH4, respectively, at reservoir conditions (i.e. elevated pressures and temperatures). We calculate the corresponding γSG for mineral-CO2, mineral-CH4 and γSL for mineralbrine. 2. Methodology 2.1. Sample preparation and characterization Alpha-quartz single crystals (10 mm × 5 mm × 2 mm) were supplied by Dade Quartz Co. Ltd, China. For the contact angle experiments, a careful cleaning procedure is required to remove any organic contaminants from the quartz substrates, which otherwise has a dramatic impact on the results [32,33]. Thus, the samples were flushed with acetone and immersed in fresh piranha solution (1 vol 30 wt% H2O2 + 3 volumes 98 wt% H2SO4) for 30 mins; both H2O2 and H2SO4 were provided by Sinopharm Chemical Reagent Co., Ltd. Subsequently, the soaked quartz crystals were covered with clean aluminum foil and heated for 8 hrs at 353 K. To coat the clean quartz surfaces, a 2 wt% kaolinite (or montmorillonite) in aqueous 1.5 wt% NaCl suspension was prepared by adding kaolinite (or montmorillonite) powders into the aqueous 1.5 wt% NaCl solution. The purity of NaCl is ≥99 mol%. Kaolinite and montmorillonite were purchased from Sigma-Aldrich. The chemical formulas for kaolinite and montmorillonite are Al2O7Si2·2H2O and MgNaAl5(Si4O10)3(OH)6, respectively. The average particle sizes for these two clay minerals were 1050 nm and 900 nm, respectively (measured with a Zetasizer Nano instrument, Malvern, UK), while their total organic contents (TOC) were 335 mg/kg and 3093 mg/kg, respectively [17]. The prepared suspensions were stirred for 2 hrs at 323 K and then the clean quartz samples were immersed and aged in the suspensions for 6 hrs (at 353 K). Finally, the aged substrates were covered with clean aluminium foils and heated for another 6 hrs (at 353 K). The root mean square (RMS) roughness of the clean, kaolinite-coated and montmorillonite-coated quartz were 483.7 pm, 629.8 nm and 863 nm respectively (measured via JPK NanoWizard 4 and Bruker Multi mode 8 Atomic Force Microscope); the respective coverages of kaolinite and montmorillonite were 37.5% and 39.0% (The SEM images were processed and analysed using MATLAB. Specifically, the raw SEM images were converted into binary images (black (clay coating area) and white (non-coating area)); subsequently the black and white areas were quantified by counting the pixels; from this ratio the clay coverage was obtained.) [17]. The SEM images of clean quartz, kaolinite-coated and montmorillonite-coated quartz can be referred to our previous report [17].

Fig. 1. Illustration of the contact angle and the interfacial tension measurements.

droplet moved along the surface, θa and θr were simultaneously measured through the leading and trailing edges of the droplet. All the measurements were repeated more than 10 times and the average values were reported. The standard deviation of the contact angle measurements was 3°-4°. 2.3. Brine-CO2 and brine-CH4 interfacial tension measurements The brine-CO2 and brine-CH4 interfacial tensions (γLG ) were measured with a Krüss DSA 100 instrument using the pendant drop technique [35]. The measurement chamber was flooded with (low pressure) CO2 or CH4 for 10 mins firstly and then adjusted to pre-set pressures (0.1 MPa, 5 MPa, 10 MPa, 15 MPa, 20 MPa, and 25 MPa) and temperatures (300 K, 323 K, and 353 K). Subsequently, a 6–7 uL pendant drop (5 wt% NaCl brine) was generated at the end of a cylindrical steel tube with an outer diameter of 1.2 mm and an inner diameter of 0.6 mm. The density of 5 wt% NaCl brine was measured as 1053 kg/m3 at ambient condition (i.e. 0.1 MPa and 25 °C) and it was assumed to be constant, independent with pressures and temperatures in this work. The densities of CH4 and CO2 at various pressures and temperatures were collected from literatures [36,37]. γLG was then calculated using the ASDA method [38]. All measurements were repeated more than 10 times and the average values were reported in this work. The standard deviations of γLG were within 3 mN/m. 2.4. Mineral-CO2, mineral-CH4 and mineral-brine surface energy calculations To obtain the mineral-fluid surface energy, we first calculated the equilibrium contact angles (θe ) from the measured θa and θr , Eq. (1) [39],

θe = arccos ⎛ ⎝ ⎜

2.2. Contact angle measurements

where ℵa =

Advancing (θa ) and receding (θr ) brine contact angles of the mineral-CO2-brine systems were measured with a Krüss DSA 100 instrument using the tilted plate method [34], Fig. 1. The substrates were placed into the measurement chamber at pre-set temperatures (300 K or 353 K); the chamber was then flooded with (low pressure) CO2 for 10 mins. Afterwards, the CO2 pressure inside the measurement chamber was increased to prescribed values (0.1 MPa, 5 MPa, 10 MPa, 15 MPa, and 20 MPa). Once the pressure inside the chamber stabilized, a 6–7 μL droplet (5 wt% NaCl brine) was dispensed onto the 17° tilted substrate surface. Note that a 17° tilting angle guaranteed that the dispended droplet moved very slowly over the tilted substrate. Just before the

ℵa cosθa + ℵr cosθr ⎞ ℵa + ℵr ⎠ ⎟

(

sin3 θa 2 − 3cosθa + cos3 θa

)

1 3

and ℵr =

(1)

(

sin3 θr 2 − 3cosθr + cos3 θr

)

1 3

.

Then we used Neumann’s equation of state to calculate γSG and γSL [40]. Note that Neumann equation proposes that γSL is a function of γSG and γLG and that the solid–liquid adhesion free energy per unit area is equivalent to the geometric average value of the liquid cohesive work and the solid cohesion work [40]. Thus, Eqs. (2) and (3) were derived (a detailed derivation can be found in Ameri et al. [28],

γSL = γSG + γLG − 2 γSG γLG [1 − β (γSG − γLG )2]

(2)

) 2]

(3)

γSG = γSL + γLG − 2 γSW γLG [1 − β (γSL − γLG 2

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Furthermore, at 300 K, when CO2 pressure increased from 5 MPa to 20 MPa, θa for kaolinite-coated quartz and montmorillonite-coated quartz increased from 26° to 65° and 25° to 60°, respectively. This trend is qualitatively similar to the data reported by Fauziah et al. [43]. In their work, the dynamic contact angle was measured on porous clay substrates at 305 K; the reported θa for kaolinite and montmorillonite increased from 40° to 58° and from 70° to 105°, respectively for a pressure increase from 5 MPa to 20 MPa. Although some parameters (temperature, surface roughness, brine composition) were slightly different, the main difference lies in the porous character of the samples Fauziah et al. [43] tested, as the existence of pores could increase contact angle significantly [44]. In addition, molecular dynamics simulation reported that CO2 adsorption capacity onto kaolinite increased dramatically for the phase change from gaseous to supercritical state [45], which means that stronger CO2 affinity to kaolinite at high pressure, consistent with our results. Physically, a higher water contact angle at higher pressures is caused by stronger intermolecular forces between the non-aqueous phase and the solid surface [46–48] and the associated reduction of surface energies between solid and non-aqueous phase [30]. In addition, at the same pressure and temperature conditions, all three mineral surfaces showed a stronger affinity to CO2 than CH4. For a specific substrate and at a constant temperature, the effect of pressure on CO2 wettability was clearly more significant than for CH4 wettability. This is due to the larger CO2 density (when compared to the CH4 density) at the same thermo-physical condition and the larger density increase for CO2 than for CH4 for the same pressure interval [47,48]. Note that the detailed contact angle data for CH4 was reported in our previous work [17].

Equation (3) is combined with the Young’s equation (4),

cosθe =

γSG − γSL γLG

(4)

So that Eq. (5) is obtained,

cosθe = 1 − 2

γSL γLG

[1 − β (γSL − γLG )2]

(5)

Note that γSL and β can be determined via least-squares regression (e.g. in MATLAB); θe is a function of pressure, temperature and solid type; γLG is a function of pressure, temperature and gas type; and γSL and β are functions of temperature and solid type but independent of pressure [30]. Finally, γSG can be obtained from Eq. (3). Note thatγSG is a function of pressure, temperature, gas type and solid type. 3. Results and discussion 3.1. Effect of pressure on contact angle High CO2 pressures de-wetted all mineral surfaces (i.e. θa and θr increased with pressure for clean, kaolinite-coated and montmorillonite-coated quartz as shown in Fig. 2 and Table 1). For example, for pure quartz, at 353 K, when the pressure increased from 0.1 MPa to 20 MPa, θa increased from 0° to 55°, consistent with literature data [41,42], where a θa increase from 0° to 56° for 40 nm RMS roughness quartz-water-CO2 [41] and from 0° to 50° for 560 nm RMS roughness quartz-water-CO2 [42] were reported, respectively, when the pressure increased from 0.1 MPa to 20 MPa at 343 K. 70

300 K, Clean quartz 353 K, Clean quartz 300 K, Kaolinite-coated quartz 353 K, Kaolinite-coated quartz 300 K, Montmorillonite-coated quartz 353 K, Montmorillonite-coated quartz

60

[°]

50

3.2. Effect of temperature on contact angle The effect of temperature on wettability is complex [6] and depends on the intrinsic wettability of the substrate [49]. Herein, the contact angle for clean quartz increased with an increasing temperature, e.g., at 20 MPa, when the temperature increased from 300 K to 353 K, θa increased from 46° to 55°, consistent with literature data [41,42]. AlYaseri et al. [42] reported a θa increase from 30° to 50° for a temperature increase from 296 K to 343 K at the same pressure (i.e. 20 MPa); Sarmadivaleh et al. [41], reported a θa increase from 35° to 53°, when the temperature increased from 296 K to 343 K, again at the same pressure. However, for kaolinite-coated and montmorillonitecoated quartz, the contact angle decreased with an increasing temperature. The trend was consistent with a literature report that CO2 sorption on hydrated montmorillonite decreased with an increasing temperature at a high pressure [50], while inconsistent with previous hypothesis that the contact angle would increase with an increasing temperature for inherently hydrophilic surfaces [49]. However, this hypothesis was reached in terms of contact angle measurements on pure dolomite surface, which has a different surface chemistry when compared to clay-coated quartz. Note that water hydration shells may exist in the internal clay structures. Theoretically, in the presence of CO2, Solvated Na+ in the interlayer tends to migrate onto the clay surface, to screen the basal oxygen bonding with water molecules, thus de-wetting clay surface [51]. This explained a larger water contact angle for clay-coated quartz than for clean quartz at 300 K, see section 3.3 below. However, a higher temperature leads to stronger hydrated clay swelling [52], thus a decreasing Na+ surface density resulted, which contributes to the reduced water contact angle for clay-coated quartz. Thus, higher temperature led to stronger water-wet property.

40

30

20

10

0 0

5

10 Pressure [MPa]

15

20

(a)

60

300 K, Clean quartz 353 K, Clean quartz 300 K, Kaolinite-coated quartz 353 K, Kaolinite-coated quartz 300 K, Montmorillonite-coated quartz 353 K, Montmorillonite-coated quartz

50

r

[°]

40

30

20

10

0 0

5

10 Pressure [MPa]

15

20

3.3. Effect of clay coating on quartz wettability

(b)

Fig. 2. Effect of pressure and temperature on a) θa and b) θr (mineral-CO2-5 wt % NaCl brine).

As is known, surface roughness influences θ, thus, we converted the apparent θa and θr measured on the rough surface (Fig. 2) to θs 3

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Table 1 Experimental data used in Figs. 2 and 3. Temperature [K] 300

Temperature [K] 353

Substrates

Pressure [MPa]

θa [°]

θr [°]

θs, a [°]

θs, r [°]

θa [°]

θr [°]

θs, a [°]

θs, r [°]

Quartz

0.1 5 10 15 20 0.1 5 10 15 20 0.1 5 10 15 20

0 15 30 35 46 0 26 42 45 65 0 25 36 40 60

0 8 20 25 40 0 11 34 36 55 0 11 23 34 53

NA

NA

NA

20 22 39 40 57 23 25 32 40 56

0 22 26 37 43 0 7 12 20 30 0 0 7 17 24

NA

20 32 46 48 67 23 33 42 45 63

0 31 44 47 55 0 23 29 31 42 0 15 20 25 40

20 30 35 36 46 23 27 30 33 45

20 21 23 28 35 23 23 24 28 33

Kaolinite

Montmorillonite

70

(including θs, a and θs, r ) on an idealized perfectly smooth surface, Fig. 3 via Eq. (6) [53],

300 K, Clean quartz 353 K, Clean quartz 300 K, Kaolinite-coated quartz 353 K, Kaolinite-coated quartz 300 K, Montmorillonite-coated quartz 353 K, Montmorillonite-coated quartz

60

50

−

−

θA is the measured apparent contact angle (i.e.θa and θr ). r is the ratio of apparent surface area to the projected surface area. It was assumed that clay particles are adsorbed as a monolayer on a perfectly flat quartz surface. The base radius of the sessile brine droplet was considered as a constant 2 mm for a simplicity. Kaolinite and montmorillonite coverages on the clean quartz were 37.5% and 39%, − (compare section 2). Thus, r for kaolinite and montmorillonite were 1.0625 and 1.085 respectively. At low temperatures, clay coating de-wetted quartz (versus CO2); while at high temperatures, clay coating intensified water-wetting of quartz, consistent with analogue CH4 systems [17]. The underlying mechanism has been indicated in section 3.2. Furthermore, when the pressure was larger than 5 MPa, θs, a for montmorillonite-coated quartz were always smaller than that for kaolinite-coated quartz at the same pressure and temperature conditions, Fig. 3a. This difference is caused by the different surface charge on the two different clay mineral surfaces. As there are more negative charges on the montmorillonite surface than that on kaolinite surface [54], thus montmorillonite has a higher surface polarity than kaolinite; and consequently, montmorillonite has a stronger intrinsic hydrophilicity than kaolinite.

,

[°]

40

30

20

10

0 0

5

10 Pressure [MPa]

15

20

(a)

60

300 K, Clean quartz 353 K, Clean quartz 300 K, Kaolinite-coated quartz

50

353 K, Kaolinite-coated quartz 300 K, Montmorillonite-coated quartz 353 K, Montmorillonite-coated quartz

40

,r [°]

(6)

cosθA = r cosθs

3.4. Effect of pressure and temperature on γLG We measured γLG for 5 wt% NaCl brine-CO2 and 5 wt% NaCl brineCH4 systems, respectively, at various temperatures (i.e. 300 K, 323 K and 353 K) and pressures (i.e. 0.1 MPa, 5 MPa, 10 MPa, 15 MPa, 20 MPa and 25 MPa), Fig. 4, as these are relevant for CO2 storage in gas reservoirs and natural gas production. At 0.1 MPa, the measured γLG for all the gas-brine systems were very similar, ranging from 64.4 mN/m to 71.8 mN/m. This is caused by the similar density differences in CO2-5 wt% NaCl brine and CH4-5 wt% NaCl brine at 0.1 MPa, prevailing at various temperatures [20,55]. As the pressure increased from 0.1 MPa to 10 MPa, γLG for CO2 deceased sharply while further pressure increase from 10 MPa to 25 MPa, γLG only slightly decreased. This trend was consistent with a literature report that γLG for CO2-brine decreased steeply with an increasing pressure when the pressure was lower than a threshold (around 10 MPa), then decreased mildly with a further pressure increase [26]. Furthermore, γLG for CO2-brine almost increased with temperature for all the pressures except 0.1 MPa, consistent with literature results [56]. In

30

20

10

0 0

5

10 Pressure [MPa] (b)

15

20

Fig. 3. Effect of pressure and temperature on a) θs, a and b) θs, r on an idealized perfectly smooth surface.

4

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pressure increases. Consequently, the stronger interaction between gas and solid results, and thus a lower γSG is caused at a higher pressure [30,57]. The γSG (CO2) dependence on temperature is because the cohesive energy density of CO2 decreases with temperature [58], while the solid cohesive energy density is almost independent of temperature [59], thus the difference in cohesive energy densities between CO2 and solid increases with an increasing temperature. Consequently, the corresponding interaction is weak and γSG (CO2) increases with temperature [30]. In contrast, the insignificant effect of temperature on γSG (CH4) may be due to the cohesive energy density of CH4 less sensitive to temperature. In addition, γSG ( CH4) was much larger than the corresponding γSG (CO2) at the same thermodynamic and mineral surface conditions. This difference (i.e. ΔγSG) became more significant at high pressures as shown in Fig. 6. However, at the same temperature, when pressures were higher than 5 MPa, the corresponding ΔγSG for abovementioned three solid substrates were always larger than 33 mN/m. The difference between γSG (CH4) and γSG (CO2) may be attributed to their different mass density and cohesive energy density at the same thermodynamic conditions.

Fig. 4. Effect of pressure and temperature on γLG for gas-brine (5 wt% NaCl).

contrast, γLG for CH4-brine decreased with both an increasing pressure and an increasing temperature. 3.6. Effect of temperature on γSL 3.5. Effect of pressure, temperature and clay coating on γSG

For CH4-brine-mineral and CO2-brine-mineral systems, higher temperatures enhanced γSL for quartz, while reduced γSL for kaolinite-coated quartz and montmorillonite-coated quartz, Fig. 7. Arif et al. [30] also reported that as the temperature increased, γSL increased for quartz, while decreased for mica and coal. The increase in γSL for quartz at higher temperatures can be attributed to a larger number of water molecule desorption from quartz surface [60], while the decrease in γSL for clay-coating at a higher temperature may be due to the stronger interaction between water and clay, which is induced by larger surface energies of clay at higher temperatures [61].

As reported in Tables 2 and 3 and depicted in Fig. 5, γSG (CO2) decreased with pressure and increased with temperature, consistent with literature data for (hydrophilic) quartz, sandstone, mica, and various coals [28,30]. However, γSG (CH4) decreased with pressure slightly, while, it was not very sensitive to temperature (only a small decrease). In addition, clay coating only had a minor influence on γSG at high pressure, while, had a more apparent influence on γSG at 0.1 MPa and 300 K. Fundamentally, gas (CO2 and CH4 here) cohesive energy densities increase and approach to the substrate cohesive energy density as the Table 2 γSG and γSL for CO2-brine (5 wt% NaCl)-solid. Temperature [K]

Pressure [MPa]

Solid

θe [°]

cosθe

γLG [mN/m]

γSG mN/m]

γSL [mN/m]

β 10-4]

300 K

0.1 5 10 15 20 0.1 5 10 15 20 0.1 5 10 15 20 0.1 5 10 15 20 0.1 5 10 15 20 0.1 5 10 15 20

Clean quartz

0 12 25 30 43 0 19 38 41 60 0 18 30 37 56 0 27 32 42 49 0 15 21 26 32 0 8 14 21 32

1.000 0.979 0.906 0.865 0.731 1.000 0.947 0.788 0.760 0.502 1.000 0.950 0.870 0.798 0.552 1.000 0.894 0.846 0.743 0.657 1.000 0.965 0.934 0.902 0.848 1.000 0.991 0.972 0.933 0.847

71.80 33.50 25.00 24.70 24.66 71.80 33.50 25.00 24.70 24.66 71.80 33.50 25.00 24.70 24.66 64.50 49.40 34.00 27.20 26.40 64.50 49.40 34.00 27.20 26.40 64.50 49.40 34.00 27.20 26.40

73.02 30.06 21.60 21.30 21.27 74.31 27.16 18.74 18.46 18.42 73.69 28.17 19.74 19.45 19.41 65.71 45.27 27.45 20.49 19.70 65.07 47.62 31.12 24.23 23.43 65.66 48.28 31.76 24.82 24.02

0.1721

2.24

0.6323

2.25

0.4321

2.22

0.7582

2.54

0.1352

2.59

0.0914

2.94

353 K

Kaolinite quartz

Montmorillonite quartz

Clean quartz

Kaolinite quartz

Montmorillonite quartz

5

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Table 3 γSG and γLS for CH4-brine (1.5 wt% NaCl)-solid*. Temperature K]

Pressure MPa]

Solid

θe [°]

cosθe

γLG [mN/m]

γSG [mN/m]

γSL [mN/m]

β [10–4]

300 K

0.1 5 10 15 20 0.1 5 10 15 20 0.1 5 10 15 20 0.1 5 10 15 20 0.1 5 10 15 20 0.1 5 10 15 20

Clean quartz

0 14 19 29 30 0 21 23 33 42 0 18 30 37 56 0 14 19 29 30 0 12 12 16 17 0 8 7 11 20

1.000 0.972 0.944 0.874 0.865 1.000 0.933 0.919 0.834 0.743 1.000 0.950 0.870 0.798 0.552 1.000 0.972 0.944 0.874 0.865 1.000 0.977 0.979 0.962 0.957 1.000 0.991 0.993 0.982 0.937

71.40 64.90 62.30 57.70 55.90 71.40 64.90 62.30 57.70 55.90 71.40 64.90 62.30 57.70 55.90 67.50 62.30 60.00 56.30 53.50 67.50 62.30 60.00 56.30 53.50 67.50 62.30 60.00 56.30 53.50

73.59 64.66 61.59 56.24 54.18 77.27 64.63 61.12 55.10 52.81 85.05 64.51 61.17 55.42 53.22 70.27 61.56 57.89 52.21 48.11 67.53 61.37 58.70 54.46 51.31 68.36 61.89 59.10 54.68 52.54

0.8028

2.19

4.3673

2.32

9.5731

2.82

1.9497

2.41

0.1761

2.16

0.2436

2.38

323 K

Kaolinite quartz

Montmorillonite quartz

Clean quartz

Kaolinite quartz

Montmorillonite quartz

* θe was calculated based on the dynamic contact angle data measured for1.5 wt% NaCl brine in Pan et al. [17]. 45 40 35 ΔγSG [mN/m]

30 25

Clean quartz

20 15

Kaolinite-coated quartz

10

Montmorillonite-coated quartz

5 0 0

5

10

15

20

Pressure [MPa]

Fig. 6. Effect of pressure on the difference (ΔγSG) between CH4-mineral and CO2-mineral interfacial tensions at 300 K.

Fig. 5. Effect of pressure and temperature on γSG for CO2 and CH4.

4. Conclusions In the present study, we used a well-defined shale model (i.e. claycoated quartz) and measured CO2 wettability at reservoir conditions. We calculated mineral-CO2, mineral-CH4, and mineral-brine surface energies using the Neumann equation. The following conclusions were reached:

Fig. 7. Effect of temperature on γSL at various temperatures.

1) Higher CO2 pressures led to larger brine contact angles (i.e. less 6

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hydrophilic mineral surfaces) for all mineral surfaces. Furthermore, higher CO2 pressures resulted in a lower mineral-CO2 surface free energy. Thus, as the formation depth increases, it can become less favorable to store CO2 [62]; especially when the formation rock becomes hydrophobic, potential leakage can probably take place. 2) Temperature had a complex influence on the interfacial properties of the minerals. For CO2 on clay-coated quartz, higher temperatures caused smaller brine contact angles and larger mineral-CO2 surface energies. Clay coating de-wetted the quartz surface at low temperature; however, clay coating intensified quartz hydrophilicity at higher temperature.

[16]

[17]

[18]

[19]

[20]

These results can enlighten the analysis on the CO2 storage capacity predictions, leakage risk assessments, and CH4 reserves estimates and associated production efficiency enhancement.

[21]

Acknowledgements

[22]

The authors thank the valuable discussions with Dr. Hossein Hejazi from University of Calgary. The authors wish to acknowledge financial assistance from the Natural Science Foundation of China (51874330; 41602143; 51509260), Shandong Provincial Natural Science Foundation (ZR2019QEE037), Chinese Scholarship Council, and the University of Calgary Global Research Initiative in Unconventional Hydrocarbon Resources-Beijing Site, Kerui-MITACS Accelerate Research Fund Application Ref. IT09328.

[23]

[24]

[25]

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