Coal Wettability After CO2 Injection - Energy & Fuels (ACS Publications)

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Coal Wettability after CO2 Injection Ahmed Zarzor Al-Yaseri, Hamid Roshan, Xiaomeng Xu, Yihuai Zhang, Mohammad Sarmadivaleh, Maxim Lebedev, Ahmed Barifcani, and Stefan Iglauer Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01189 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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Coal Wettability after CO2 Injection

A. Z. Al-Yaseri1*, H.Roshan2, X. Xu3, Yihuai Zhang1, M. Sarmadivaleh1, M. Lebedev4, A. Barifcani1and S. Iglauer1

1

Curtin University, Department of Petroleum Engineering, Kensington, Australia

2

School of Petroleum Engineering, University of New South Wales, Sydney, Australia

3

China University of Mining and Technology, Beijing, China

4

Curtin University, Department of Exploration Geophysics, Kensington, Australia

*email address of corresponding author: [email protected]

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Abstract Increasing energy demand and associated global warming are unarguably the two major challenges that the world currently faces. One of the ideas to reduce the carbon footprint while increasing the efficiency of the energy extraction is the CO2 sequestration in coal seams. This can additionally enhance the coal bed methane production. However, this process depends on many factors, among which coal wettability is of particular importance especially because of its pressure and temperature dependency. In order to evaluate this process, coal wettability was tested by measuring the contact angle of CO2/water as a function of pressure, temperature and salinity (DI water and brine (5 wt% NaCl + 1 wt% KCl) i.e. wt% is the weight percentage of salt to water. The results show that the CO2/water contact angle increases significantly with increasing pressure, temperature and salinity indicating more effective CO2 wetness of coal. This in turn can reduce the CO2 residual trapping capacities and increase methane recovery. Furthermore, we demonstrated that CO2 density correlates well with coal wettability.

Keywords: Wettability, Coal, Contact angle, CO2 storage, CBM, ECBM

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1. Introduction Recently CO2 emission has been recognized as a major cause of climate change. Emissions are

continuously

increasing

due

to

increased

energy

demand

as

global

industrialisation proceeds. Such increase in energy demand has given rise to the fossil fuel consumption and hence generation of CO2 [1]. CO2 geo-sequestration (CGS) has therefore attracted a considerable attention as a potential solution for increased CO2 emission [2, 3, 4, 5, 6, 7, 8]. It is however important to understand the nature of interaction between the fluid and the formation in order to safely store CO2 underground [9]. CGS is an efficient method to reduce these emissions, while enhancing oil and gas recovery in unconventional petroleum reservoirs [1, 10, 11] i.e. unmineable coal seams for CO2 trapping and enhanced coal bed methane (ECBM)

production [12,13,14,15,16]. As an

example, Reznik et al. [17] conducted an extensive laboratory study and showed that only 30–50% of the original methane in coal bed methane (CBM) can be recovered by natural desorption whereas the CO2 adsorption with pressure of 5.5 MPa can completely replace the methane and leads to 100% desorption of in situ methane. Therefore, CO2 sequestration in coal is a potential mechanism to maximize the commercial production from these reserves [18]; however, ECMB recovery by CO2 sequestration involves complex processes that are not yet fully understood [18]. Wettability is the ability of a fluid to maintain contact with a solid surface that can be measured through contact angle of the fluid with the solid phase [19]. Furthermore, wettability is directly related to the recovery efficiency and injection pressure design of CO2 sequestration projects [13, 16]. In addition, there are many other phenomena affecting the efficiency of ECBM. For instance, CO2 occupies relatively small micro-pores of the coal matrix after injection while brine stays in the larger pores [13]. CO2 wettability of coal is also

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higher than methane [20] but the CO2 leads to coal swelling, and fracturing of minerals [21], which can create the residual permeability. In addition, CO2 storage in coalbeds for ECBM is significantly different from the conventional reservoirs (CO2 can be dissolved in water or oil), because coalbeds act as both reservoir and source rock [22]. As mentioned, the CO2/coal/brine wettability is a particular factor in ECBM, especially due to high amount of water produced from most of the CBM wells [23,24] with salt concentration varying from 200 mg/L to 170,000 mg/L [25]. Such wettability is mainly a function of pressure, temperature, coal rank, and brine salinity [13,26,27]. It has been widely reported in the literature that the CO2 wettability in water/coal [13,26,27] and water/minerals [28,29,30,31,32,33,34] systems increases with increasing pressure. Kaveh et al. [27] showed that at a certain pressure a sudden increase in CO2 density is experienced where the excess sorption (both surface and capillary adsorption) is in fact reducing and surprisingly the contact angle linearly increases. Moreover, the sorption behaviour of CH4 and CO2 in coal was investigated by Hu et al. [35] based on molecular dynamic simulation and their results showed that coals adsorb more CO2 than CH4. The effect of temperature and brine salinity at different pressures however have not yet been entirely investigated for CO2/water/coal system; which is especially important where temperatures and brine salinities can vary significantly in subsurface formations[36]. Especially it has been shown that temperature and salinity have significant effects on CO2quartz [30,31,32,37], shale [9, 34] and Mica [33] wettabilities. In addition, the depth of the coalbeds varies from outcrop to more than 4000 m with average pressure gradient of 0.01027 MPa/m [38]. However, it is preferred to store the CO2 at temperatures and pressures above its critical point (304 K, 7.38 MPa) where CO2 absorption into coal is relatively high [39, 40].

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Therefore we measured the advancing and receding water contact angles of CO2–brine on a coal substrate at different pressures (0.1, 5, 10, 15, 20 MPa), temperatures (308, 323, and 343 K), and salinities (DI water and brine (5 wt% NaCl + 1 wt% KCl)). The results were then analysed with a newly proposed physical model [32, 34] derived based on the sharp-kink approximation [41, 42].

2. Experimental methodology The coal sample used in this study was extracted from a proposed storage site in Pingdingshan Ten colliery, Henan Province of China. There are three groups of coal seams at this site, a) Ding, Ji and Wu Groups: the gas pressure of Ding group varies from 0.6 MPa to 2.16 MPa corresponding to the depth of 400 m to 800 m, b) Ji group with pressure ranging from 0.5 MPa to 2.55 MPa corresponding to the depth of 430 m to 800 m and c) Wu group having pressure between 0.67 MPa to 2.45 MPa. The sample used in this study is collected from the Ding group with the average depth of 750 m, 1.6 to 2.3 m thickness and 14 m3/t gas content with the maximum gas pressure of 1.14 MPa. The block was then processed into coal powder, cylinder core and sub-blocks to conduct different measurements. Coal powder was used for proximate analysis (following China National Standards [43]) and to determine the coal rank [44]. Details of the tested coal sample are summarized in Table 1 with fixed carbon content up to 53.22% and volatile matter content as 36.43% (the tested coal is a typical sub-bituminous coal rank). Moreover, the zeta potential of the tested coal sample suspended in brine (5 wt% NaCl + 1 wt% KCl) was measured with a zeta potentiometer (Brookhaven ZetaPALS) at ambient conditions (298K and 0.1MPa), Table 2 i.e. zeta potential measurements were carried out with sample powder

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at a constant pH. All zeta potential were measured three times to take into the account the possible scatter of the data.

Table 1, Physical properties of the tested coal Ρ (g/cm3)

Mad (%)

Vdaf(%)

Aad(%)

Cf (%)

1.35

6.53

36.43

3.82

53.22

ρ, bulk density (with pores); Mad, moisture content (%Vol, air dry base); Vdaf, volatile matter (%, dry ash free); Aad, ash yield (%, air dry base); Cf, fixed carbon content.

Table 2, Zeta potentials (ZP) measured for the coal sample Sample No.

T

ZP

(K)

(mV)

1

298.15

-6.7

2

298.15

-9.15

3

298.05

-5.55

average

298.12

-7.13

standard deviation

0.0408

1.838

The intact coal substrate sample used for the contact angle measurement has a surface roughness of 1.1µm (RMS roughness measured with an atomic force microscopy instrument, model DSE 95-200). The surface roughness was used to correct the measured contact angles for roughness induced changes (REF) thus reproducibility of the results (Figure 1) [45]. It has been reported that a higher surface roughness resulted in lower advancing and receding contact angles [32,46]. 6 ACS Paragon Plus Environment

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Before

110 µm

(a)

(b)

After

(c)

63µm

(d)

Figure 1. Atomic force microscopy topography images of the coal surface used in the experiments (before and after testing), (a) and (c) deflection signal, different heights are colour coated before and after measurement; (b) and (d) 3D topography of the substrate. RMS roughness is 1.1 µm before and 1.3 µm after measurement.

In addition, the scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM) and x-ray micro-computed tomography (µCT) was used to select a region of the sample with lowest amount of micro-fractures in order to ensure that the droplet is not affected by unnecessary seepage into the sample (Figure 2).

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a

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coal

fracture

10 μm

EHT=15KV

b

Aperture size=30 μm

fracture fracture

coal

1mm Figure 2. a) SEM image of the coal sample, b) Raw Two-dimensional µCT image of the coal sample at 3.4 µCT.

The sample was cleaned by removing any surface contaminations before each test by exposing it to air plasma for 5min [47, 48, 49, 50].The substrate was then placed in a high pressure high temperature cell. The cell was pressurized with CO2 using a high precision syringe pump (ISCO 500D; pressure accuracy of 0.1% FS) until a pre-set value was obtained (Figure 3). When the desired pressure was attained, a droplet (average volume ∼ 7µL) of de-

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gassed brine (5 wt% NaCl + 1 wt% KCl) or deionized (DI) water (vacuumed for more than 10 hours) was dispensed onto the tilted substrate (tilting angle 12o) and advancing and receding contact angles were measured simultaneously [32] using a high definition video camera (Basler scA 640–70 fm, pixel size = 7.4 µm; frame rate = 71 fps; Fujinon CCTV lens: HF35HA-1B; 1:1.6/ 35 mm).This procedure was repeated for different pressures (0.1, 5, 10, 15, and 20MPa) and temperatures (308, 323, and 343K). It should be noted that brine was saturated with CO2 using a mixing reactor [51] although the earlier studies showed that the contact angle is not influenced by such equilibration [7].

Θ at 308 K and 5 MPa

Θ at 323 K and 10 MPa

0.75mm

0.75mm

Figure 3. Schematic diagram of the high temperature/high pressure contact angle measurement apparatus used for contact angle measurements.

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Results and discussion From zeta potential measurements on coal sample, an average surface potential of -7.13 mV at average pH of 8.3 was obtained [52,53]. Interestingly the coal surface had a slightly negative charge. It is known that surface charges can influence the contact angle and other rock properties [54,55,56] however; Roshan et al. [34] showed that such influence on contact angle can be only significant if the negative potential of the sample surface is below -10 mV. We therefore use the equation proposed by Al-Yaseri et al. [32] for neutral substrates where the liquid contact angle is related to density of gas phase in the system [57,58,59].

cosθ =

∆ρ

γ lg

I −1

(1)

The expansion of equation 1 gives:

cosθ = −

I

γ lg

ρg + (

I

γ lg

ρlf − 1)

(2)

Where θ is the contact angle of the liquid, γ lg is the interfacial tension between fluid phases I = −∫

∞ zmin

V ( z ) dz , is the van der Waals potential integral [42, 57, 58], and ∆ρ = ρlf − ρ g ( ρ g

is the gas density and ρlf is the film liquid density which is a function of the liquid and gas densities for a specific substrate [47]. The advancing and receding contact angles are presented in Figures 4 and 5, respectively where both advancing and receding contact angle significantly increased with increasing pressure for brine + CO2 and DI water + CO2 systems i.e. this is well consistent with literature data [13,26,27]. For instance, the advancing contact angle of brine at 343.15 K increased from 75 ° to 152 ° when the pressure increased from 0.1 to 20 MPa (Figure 4).

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Figure 4. Advancing and receding water contact angles for CO2/Brine as a function of pressure and temperature, RMS= 1.1µm.

Furthermore, temperature had a considerable effect on the contact angle, the contact angle increased for all pressures with an increase in temperature. This is consistent with most literature data on quartz, mica and carbonate substrates [31,32,60,61]; however, there are other studies showing that the contact angle can decrease with temperature [33,37,62].

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In order to analyse the results, equation 1 was therefore used. The measured advancingreceding water contact angles (Figures 4 and 5) were combined to obtain the Young’s contact angle e.g. equation 2 [63,64].

Figure 5. Advancing and receding water contact angles for CO2/DI water as a function of pressure and temperature, RMS= 1.1µm.

 rA cosθ A + rR cosθR   rA + rR  

θl = arccos 

(2)

With

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1/3

1/3

    sin3 θ A sin3 θR rA =  r =  and R   3 3  2 − 3cosθ A + cos θ A   2 − 3cosθR + cos θR 

The obtained contact angles were then plotted vs the CO2 density for both Brine + CO2 and DI water + CO2 systems (Figure 6). As seen from this figure, the contact angle of brine + CO2 and DI water+CO2 systems follow a well-defined linear correlation for each specific temperature (296, 323 and 343 K) e.g. three lines approach unity. The slope and intersect of each linear relationship along with its coefficient of determination (R2) are reported in Table 3.

Table 3. The slope, intersect and R2 of each fitted line (Eq. 1) to the experimental data of contact angle vs CO2 density at different temperatures. -

Brine+CO2

-

DI water+CO2

-

Temperature (K)

slope

intersect

R2

slope

intersect

R2

296

-0.001

0.391

0.97

-0.001

0.814

0.98

323

-0.0012

0.235

0.98

-0.0014

0.721

0.92

343

-0.0015

0.127

0.91

-0.0019

0.641

0.97

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Figure 6. Water contact angles vs CO2 density fitted with a linear equation (Eq.1) for CO2/Brine and CO2/DI water systems. The slope and intersect of each linear relationship is reported in Table 1.

It is seen from Figure 6 that the higher the density of CO2 (because of the increase in pressure), the higher the contact angle obtained. Looking at equation 1, it is seen that an increase in pressure significantly reduces ∆ρ and to some extent γ lg however the ∆ρ pressure dependency is more pronounced [65]. The ratio of

∆ρ

γ lg

therefore reduces with

pressure thus increasing contact angles e.g ρ lf variation with pressure is insignificant 14 ACS Paragon Plus Environment

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whereas

ρg

considerably

increases

with

pressure

increase

thus

reducing

∆ ρ ( = ρ lf − ρ g ) . On the other hand, the contact angle increases with temperature for brine+CO2 and DI water+CO2 systems. It is also observed from Eq. (1 or 2) that ∆ρ , γ lg and I are temperature dependent. It has been documented that the increase in temperature increases both the gasliquid interfacial tension and density difference between fluid phases especially at relatively higher pressure (≥ 5 MPa). However, the sensitivity of density difference to temperature is much more pronounced than the gas-liquid interfacial tension [31,65]. Therefore the ratio of

∆ρ

γ lg

will increase with increase in temperature which in turn reduces the contact angle

consistently. However, the increase in contact angle with temperature was seen from the results. Al-Yaseri et al [66] investigated the effect of temperature on van der Waals forces and showed that the increase in contact angle with temperature is related to decrease in van der Waals forces. In addition, the contact angles are consistently higher for brine than DI water for all pressures and temperatures (Figures 4 and 5) consistent with literature data for quartz and mica [32,33, 37,60.67]. Salinity can affect the density difference, interfacial tension between fluids and van der Waals potentials. The effect of salinity on interfacial tension is more significant than the effect of salinity on the density difference while such effect on van der Waals potentials is not yet fully understood. If assumed that the van der Waals potentials are unaffected by salinity, the increase in salinity increases the interfacial tension between CO2 and brine (> ) causing the contact angle to increase (equation 1).

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I

γ lg

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3. Conclusion We systematically measured advancing and receding water contact angles of the system brine (5 wt% NaCl + 1 wt% KCl as well as Deionised water), coal and CO2 at different pressure (0.1, 5, 10, 15, and 20 MPa) and temperature (296, 323 and 343 K) conditions. The results show that the contact angle of brine is higher than DI water and the contact angle of both increased with increasing pressure or temperature. Consequently, coal becomes increasingly CO2-wet with increasing pressure (CO2 density) which increases the seepage rate of CO2 into the coal. Moreover, CO2 wets the coal surface at high pressure and temperature more significantly leading to more efficient recovery of methane e.g. mainly due to wider distribution of CO2 in micro-pores and higher CO2 adsorption. It has been also shown that CO2 density can be used to estimate the water-CO2 contact angle on coal.

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