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Water Sorption and Distribution Characteristics in Clay and Shale: Effect of Surface Force Jing Li, Xiangfang Li, Keliu Wu, Xiangzeng Wang, Juntai Shi, Liu Yang, Hong Zhang, Zheng Sun, Rui Wang, and Dong Feng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00927 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 10, 2016
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Energy & Fuels
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Water Sorption and Distribution Characteristics in Clay and Shale: Effect of Surface Force
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Jing Li a, Xiangfang Li a, Keliu Wu a,b*, Xiangzeng Wang c, Juntai Shi a, Liu Yang d,
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Hong Zhang a, Zheng Sun a, Rui Wang a, Dong Feng a
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a
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102249, P.R. China.
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b
Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N1N4, Canada
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c
Shaanxi Yanchang Petroleum (Group) Corp. Ltd., Xi’an 710075, P.R. China.
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d
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Beijing 102249, P.R. China
MOE Key Laboratory of Petroleum Engineering, China University of Petroleum (Beijing), Beijing
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum ,
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* Corresponding author: Keliu Wu
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Tel: +1 4039663673; E-mail address:
[email protected] 12
Abstract
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Characteristics of sorption and distribution of water in nanoporous shale are topics of great
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interest to evaluate unconventional reservoirs. Also, a study of surface force of water/solid
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interaction at nanoscale is significant for understanding the storage of initial water and the fate of
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residual treatment liquid in shale systems. In this work, thickness and stability of water film were
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investigated by vapor sorption experiments on clay and shale samples. Meanwhile, an approach
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based on surface forces (disjoining pressure), which resulted in the instability of adsorbed film
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transition into condensed bulk liquid, was developed to describe molecule/pore-wall interactions.
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Our experimental results directly demonstrated the occurrence of capillary condensation in
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hydrophilic clay minerals, however, water would not entirely fill in shale nanopores even under high
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moist conditions. This remarkable finding mainly due to the inaccessibility of water molecules to
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micropores of hydrophobic organic matter. In addition, the water distribution characteristics are also
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significantly influenced by pore scale. Under a moist condition with certain relative humidity (e.g.
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RH=0.98), the water distributed in hydrophilic inorganic pores with different sizes was mainly
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classified as: (i) capillary water in small pores (e.g. < 6~7 nm), and (ii) water film in large pores
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(e.g. > 6~7 nm). In contrast, the surface repulsion prevents water condensing and likely results in a
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monolayer water film sorption in hydrophobic organic pores (e.g. θ=100°). Therefore, in actual shale
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system with initial moisture content, the inorganic microporosity totally blocked by water might be
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unable for gas transport or storage, while the hydrophobic organic pores mainly provide effective
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space for gas accumulation.
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Keywords: Shale, Clay, Water sorption, Water distribution, Surface force. ACS Paragon Plus Environment 1
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1 Introduction Shale gas, as a typical unconventional energy resource, has become an increasingly important [1]
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part of the world’s gas resource
. The production of recoverable reserves from shale formation
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mainly depends on gas storage ability and flow capacity
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within shale gas reservoir under actual condition may be a significant factor that influences the
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evaluation of gas deliverability. Present studies showed that the methane adsorption capacity of
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moist shale samples would significantly reduce when compared with dry samples
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the presence of pre-adsorbed water would obviously decrease the apparent permeability (or diffusion
9
coefficient) and increase the stress sensitivity of shale matrix
[2-5]
. However, the initial water saturation
[6~7]
. Meanwhile,
[8~10]
. Therefore, understanding how
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water stores and distributes in shale system is essential to reserve estimations and production
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predictions.
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Gas shales are heterogeneous rocks with complex mineralogical compositions
[11]
. Meanwhile,
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a significant difference of wettability exists between organic matters and inorganic minerals, which
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directly results in a much more complicated water distribution characteristic inside porous shale.
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Generally, the organic pores (e.g. kerogen pores) forming during hydrocarbon accumulation and
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generation processes are hydrophobic and almost without water
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Hu et al. (2014) [13] showed that water molecules could gather as clusters near functional groups in
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kerogen pores, and experimental results by Prinz et al. (2005) [14] indicated that water could adsorb
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on hydrophilic sites in organic pores of low-rank coals. Contrast with organic pores, water storage in
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inorganic pores seems to be much clearer. Especially for the clay minerals, they are usually
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hydrophilic and present a strong affinity to water. Korb et al. (2014)
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samples collected form oil shale formation and found that the initial water mainly stored in inorganic
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pores. Ruppert et al. (2013) [16] investigated the accessibility of pores to methane and water by
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ultra-small-angle neutron scattering study (USANS) and indicated that inorganic pores less than 30
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nm display a strong affinity to water. Based on the spontaneous imbibition experiments, liquid water
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presented a strong capacity to imbibe into shale samples due to the effect of inorganic matter [17~19].
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Meanwhile, the additional driving forces, such as electrostatic interactions (a part of disjoining
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pressure) [20], chemical osmosis [17] and adsorption effect [18] might significantly contribute to the
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strong intake of water or brine by shale during imbibition process. Based on water or vapor sorption
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experiments, Yua. et al. (2012)
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nanopores of shale inorganic minerals (such as quartz and chlorite) by field-emission scanning
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electron microscope (FESEM) images, and clay-rich shale presented more moisture uptake than
[21]
[12]
. However, simulation results by
[15]
analyzed the original
directly showed the water condensation behavior within
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organic-rich shale [22-23]. Generally, the spatial distribution of water in shale gas reservoir has been categorized as two
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forms, namely (i) hydration water attached to clay surface area, also referred as clay-bound water; (ii)
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capillary water trapped in pore network system, also referred as capillary-bound water [24~25]. Due to
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the electric charge in clay surface, water film could strongly adsorb on the external surface of
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particles or internal surface inside pores though hydrogen bond and electrostatic force. Based on
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tight rock analysis (TRA) technology, the clay bound water content (CBW) in gas shales could reach
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about 2.63~7.19% of the total sample volume
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pores should be paid more attention. However, to the best of our knowledge, the present studies only
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illustrated a qualitative description of water occurrence in shale system, but the quantitative
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characteristics of water distribution inside both inorganic and organic microporosity with different
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surface chemistry (e.g. wettability) are still poorly understood.
[26]
. Thus the water distribution inside these inorganic
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In this work, firstly, the surface interactions which dominate the stability, thickness and
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wettability of thin liquid film on substrate were introduced, and a mathematical model for
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determining sorption characters of water inside different pore sizes was established (in section 2).
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Subsequently, water sorption and distribution in pure clay and shale samples were investigated by
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our experimental approach (in section 3), and the comparison between calculated results and
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experimental data was conducted to verify our proposed model (in section 4). Finally, sorption and
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distribution characteristics of water within clay and shale porous media have been further discussed
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as complete wetting case and partial wetting case, respectively. Our present work tries to pave a path
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for characterizing water distribution in both hydrophilic inorganic and hydrophobic organic porosity,
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and further demonstrates a better insight on the impact of water on gas storage and transport in shale
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systems.
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2 Theory
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2.1 Surface Forces
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For thin wetting films, the additional forces caused by interactions between film and solid
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surface are becoming significant [27]. This action of surface forces is always the primary reason
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dominating the stability, thickness and wettability of adsorbed film on substrate. Considering the
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additional forces, the pressure in the thin film could be different from that in bulk liquid, and this
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difference can be described as disjoining pressure Π(h). Based on Derjaguin et al. (1987)’s theory [28],
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the total disjoining pressure is characterized by the sum of molecular component Πm, electrical
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component Πe, and structural component Πs. More details for each term can be found in other ACS Paragon Plus Environment 3
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published works [27-29], and the relationship between film thickness h and disjoining pressure Π(h) is
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given by:
Π ( h ) = Π m ( h) + Π e ( h ) + Π s ( h) h − εε (ζ − ζ ) A Π m ( h ) = H3 ; Π e (h ) = 0 1 2 2 ; Π s ( h) = ke λ h 8π h 2
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(1)
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Where, AH is the Hamaker constant, J. ε0 is the permittivity of vacuum, F/m; εr is the relative
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permittivity of film, dimensionless; ζ1 and ζ2 are the electric potentials of two interfaces between
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film, respectively, mV; k is the constant of structural force, N/m2; λ is the characteristic thickness of
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hydration layer, nm.
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In this work, the values of these parameters refer to other published works. For molecular
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component, the value of Hamaker constant AH for describing water sorption on inorganic adsorbents
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[30-32]
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from 0.1×10-20 to 1×10−20 J. For electrical component, the value of potentials difference △ζ=ζ1-ζ2
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mainly depends on the electric charge of solid surface and ion inside electrolyte liquid
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an empirical value of △ζ=0~80 mV is used to described nano-scale water film adsorption on
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substrate [34-36]. For structural component, the characteristic thickness of hydration layer λ for water
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approximates to 1~2 nm [34-36], however the value of coefficient k, which is related to the wettability
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of solid surface, is much more complicated, and it will be discussed later.
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2.2 Contact angle
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(e.g. sand stone, clay, silt and siliceous shale) and organic adsorbents [33] (e.g. graphite) ranges
[20, 33]
. Here,
Considering the surface force interactions on spreading and wetting dynamics, the contact angle θ of liquid drop on flat surface by Frumkin-Derjaguin approach is given as [36]: h − A H εε 0 (ζ 1 − ζ 2 ) λ + kλe ] cos θ = 1 + (1 / γ l g ) ∫ Π (h)dh = 1 + (1 / γ l g ) ⋅ [ 2 + 2h0 8π h0 h0 2
∞
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(2)
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Where, γlg is liquid-gas surface tension, the value of 72.5 mN/m is used in this study; h0 is the
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equilibrium thickness of wetting film. For the contact angles measured by macroscopic liquid drops,
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the equilibrium condition can be simplified as Πm(h0)+Πe(h0)+Πs(h0)=0
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case, this equilibrium thickness approach to infinity in condition of Π(h0)=0, and the contact angle
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calculated by Eq.(2) is 0°. However, in the partial wetting case, there are several roots of equation
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Π(h0)=0 and the smallest one is chosen as the equilibrium thickness due to thicker films are
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metastable or unstable [37].
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[36]
. In the complete wetting
Generally, the behavior of wetting film on substrate surface can be categorized as three states [28-29]
: stable, metastable, and unstable. The stable case, representing continuous positive disjoining ACS Paragon Plus Environment 4
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pressure, refers as complete wetting of solid surface, while the unstable state, representing
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continuous negative disjoining pressure, indicates ideal complete non-wetting (however this ideal
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surface completely non-wetted by water cannot be found in nature
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characterized by both of positive and negative forces, refers as partial wetting.
[38]
). The metastable state,
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Based on the research of Churaev et al. (1995a, b and c) [34-36], the changed structure of water
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molecule near to the solid-liquid interface mainly depends on the wettability of substrate surface. As
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shown in Fig.1, two different orientations of polar molecules on hydrophilic and hydrophobic
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surface are illustrated. The dipole moment of polar molecules are oriented preferably vertical to
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hydrophilic surface which is caused by an attractive structural force (k>0), whereas the parallel
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orientation to hydrophobic surface is result of a repulsive structural force (k
