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Supercritical Methane Sorption on OrganicRich Shales over a Wide Temperature Range Feng Yang, Congjiao Xie, Shang Xu, Zhengfu Ning, and Bernhard M. Krooss Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02628 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017
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Energy & Fuels
Supercritical Methane Sorption on Organic-Rich Shales over a Wide Temperature Range
Feng Yang,a,b,* Congjiao Xie,a Shang Xu,a Zhengfu Ning,b Bernhard M. Krooss c
a
Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of
Geosciences, Wuhan 430074, PR China b
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing),
#18, Fuxue Rd, Changping, Beijing 102249, PR China c
Energy and Mineral Resources Group (EMR), Institute of Geology and Geochemistry of Petroleum and
Coal, Lochnerstr. 4-20, RWTH Aachen University, 52056 Aachen, Germany * Corresponding Author:
[email protected] 1
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ABSTRACT: Methane sorption on organic-rich shales over wide temperature and pressure ranges (30
2
to 120 °C, up to 25 MPa) is analyzed by the pore filling/potential theory. The supercritical
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Dubinin-Astakhov (SDA) sorption model using a density term instead of the pseudo-saturation vapor
4
term is extended to methane sorption isotherms of shales with high accuracy. A modified adsorption
5
potential approach is suggested to analyze the temperature dependence of supercritical methane sorption
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on shales. The temperature-invariant characteristic curves are obtained using the modified adsorption
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potential approach. A characteristic curve equation derived from the SDA model is provided to predict
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sorption isotherms at other temperatures using one isotherm. The physical meaning of the characteristic
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curve has been discussed, and it comprehensively reflects the available pore space for methane sorption
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and the affinity between methane molecules and organic matter. According to methane characteristic
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curves of shales and clay minerals, shales in the gas window show higher affinity than shales in the oil
12
window and clay minerals, though the clay minerals may provide comparable adsorbed volume. The
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adsorption characteristic energy shows a parabolic-like shape with a minimum approximately around Req
14
=1.1%, which are related with the evaluation of porosity of shales. This study advanced the fundamental
15
understanding about the dynamic process of methane sorption on shales.
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KEYWORDS: shale gas; sorption isotherm; adsorption potential; characteristic curves
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Energy & Fuels
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Introduction
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Fine-grained sedimentary rocks, such as mudstones and shales, contain abundant nanometer- to
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micrometer-sized pores. These narrow pores create intense fluid-rock interaction that may lead to complicated
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fluid storage and transport process [1]. An important component of hydrocarbon storage in organic-rich shales is
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gas sorption on organic matter and clay minerals. Sorption is a general term, since sorption data measured on the
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organic-rich materials may include a combination of adsorption onto the pore surface and absorption within the
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organic matter [2]. Sorbed gas may significantly contribute to the original gas in place (OGIP) of unconventional
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reservoirs. Remarkably, 50–80% of the total amount of natural gas in some of the largest shale gas plays in North
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America is postulated to be trapped as adsorbed phase in the pores of the rocks [3].
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Concerns about the accurate evaluation of gas content and diffusion kinetics have led to many experimental
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studies about gas sorption on shales. Significant progress has been achieved in the controls on sorption capacity
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of shales [4–12]. However, data on high-temperature high-pressure sorption isotherms of shales are still scare. In
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particular, the burial depth of the Paleozoic shales in the Upper Yangtze region of China is mostly in a range of
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2000–4000 m [13–16], which indicates that the temperature and pressure of shale reservoirs are in the range of
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60–120 °C and 20–40 MPa, assuming the hydrostatic pressure and normal geothermal gradients (0.01 MPa/m,
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0.03 °C/m). Experimental techniques employed in obtaining sorption data have to be optimized and at the same
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time the measuring conditions have to be extended to in-situ conditions of deep shales while many published
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sorption data are limited to moderate pressures (< 15 MPa) and temperatures (< 60 °C) [4–7].
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The accurate modeling gas sorption behavior is important for OGIP estimation and production simulation.
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Methane sorption isotherms of shales, commonly IUPAC type I in shape, are often modeled by the Langmuir and
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less frequently the Dubinin-Astakhov (D-A) models [6–9, 17–21]. Recently, sophisticated theoretical models and
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molecular simulation techniques, which consider both the adsorbate-adsorbent interaction and pore structure
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characteristics of adsorbents, are also introduced to model gas sorption on shales [22–27]. Several investigators
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found that the D-A equation based on potential theory provides a better fit than Langmuir model on sorption data
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of coals [19–21]. The D-A model, originally developed for subcritical gas in microporous adsorbent, has been
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extend to supercritical gas sorption using the definition of pseudo-saturation vapor pressure (Ps, Table 1, [28–
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30]). Under the shale reservoir conditions, most gases (including CH4 and CO2) are supercritical and the
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properties of the adsorbed fluid (e.g. density and volume) are not well defined due to the absence of a clear
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phase-transition. Though a series of approaches have been proposed to characterize the Ps and ρads (adsorbed
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phase density, Table 2, [31–32]), these schemes do not always work well in supercritical gas sorption at wide
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pressure and temperature ranges, especially when the pressure is larger than the pseudo-saturation vapor pressure
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(P > Ps) [21]. The adsorption potential becomes negative at elevated pressures (> ~10 MPa) (Figure 1), which
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goes against Dubinin’s postulates in potential theory. It was recently proposed that the D-A equation can be
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applied to supercritical conditions by replacing the Ps term by the ρads and pressure by gas density [33–35]. The
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modified supercritical Dubinin-Astakhov (SDA) model has achieved good results in describing high-pressure
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nitrogen and methane sorption on coals [33–35]. Furthermore, since gas sorption is temperature dependent, a
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sorption model should be able to reliably predict the temperature dependence of supercritical gas sorption. The
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characteristic curve in Dubinin-Polanyi potential theory offers a method to investigate the temperature
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dependence of sorption. Though the SDA model was applied to describe supercritical methane sorption on shales,
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but the characteristic curve (another important issue associated with the potential theory) of methane on shales
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hasn’t been studied [20–23]. The classic characteristic curve method only works at low to moderate pressure
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range (P < Ps, Figure 1), and cannot be extended to high pressure sorption. The modified characteristic curve
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described below can be applied to a much wider pressure and temperature ranges, and provides new insights into
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the dynamic process about methane sorption on shale systems.
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In this work, we examined the Dubinin-Polanyi potential theory on high pressure methane sorption on
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shales. New approach was proposed to extend the potential theory to supercritical gas sorption over wide
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temperature and pressure ranges. A set of high pressure sorption data for Paleozoic shales from Sichuan Basin
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has been measured at temperatures from 30 to 120 °C and pressure up to 25 MPa. Experimental data were
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parameterized by SDA-based excess sorption equations, and the validity of temperature dependence of
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supercritical methane sorption model has been tested. A modified adsorption potential method was proposed to
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calculate the characteristic curves for supercritical gas sorption, and then a rigorous function from the SDA
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equation was also developed to describe the modified characteristic curve, other than using an empirical
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polynomial equation. Furthermore, the physical meaning of characteristic curve has been elucidated by
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comparing characteristic curves of different kinds of shales and clay minerals. To the best of our knowledge, this
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is the first attempt to explain the interaction between adsorbent (shale) and adsorbate (methane) from the
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perspective of characteristic curve.
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Experimental section 2.1
Samples
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Four Paleozoic shales were collected from the field standard stratigraphic section in Changning area in
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Southern Sichuan Basin (Table 3). The Sichuan Basin, a kind of cratonic basin in the western Yangtze platform,
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has undergone multi-stage tectonic evolution [13]. The Upper Ordovician-Lower Silurian shales widely develop
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in the Upper Yangtze region of southern China. The Upper Ordovician Wufeng Formation (O3w) formed in the
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deep-water continental shelf with water retention depositional environment, and was conformably overlain by
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Lower Silurian Longmaxi Formation (S1l). The anoxic environment in Upper Ordovician caused by water
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retention is favorable for the preservation of organic matter. In the Silurian, the anoxic environment was
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damaged because of a gradual sea level decline [14,15]. The transformation from anaerobic environment into
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normal oxygen deposition environment affects adversely the preservation of organic matter. Consequently, the
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TOC (total organic carbon) content of the studied stratigraphic section decreases from Upper Ordovician Wufeng
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Formation to Late Silurian Longmaxi Formation. Fresh shale samples were collected from outcrop profiles in
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Changning structure. The Changning stratigraphic section is frequently used as comparative study about marine
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shales in China [16]. The first shale gas well in China (Changxin1) was completed in this area in 2008. The
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Paleozoic shales in Sichuan Basin are currently considered as the key targets for shale gas exploration in China.
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Several national shale gas fields operated by PetroChina and Sinopec have been developed in this area. Total
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shale gas production from Weiyuan-Changning area was more than 2300 million m3 in 2016.
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Shale samples were first ground into powder with average particle size of 100 mesh and prepared for XRD
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analysis, TOC, and adsorption measurements. Results from organic geochemistry and XRD measurements are
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presented in Table 3. TOC content of the samples, based on LECO CS230 carbon/sulfur analyzer results, ranges
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from 0.89% to 4.83%. The equivalent vitrinite reflectance (Req), derived from the bitumen reflectance, is around
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2.8%, which indicates that the samples are at the highly over-mature stage (Req > 2.0%).
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2.2
Pore structure characterization by nitrogen physisorption
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Low-pressure nitrogen adsorption measurements were conducted on Micromeritics Gemini VII 2390t to
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investigate the pore structure characteristics of shale samples. Powder samples were first degassed at 105°C in
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vacuum about 12 h to remove adsorbed moisture. Then degassed samples weighting 0.5–1.0 g were exposed to
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nitrogen at 77 K along a series of precisely controlled gas pressures. Nitrogen adsorption-desorption isotherms
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were obtained over the relative equilibrium pressures (P/P0) range of 0–0.995. The Brunauer-Emmett-Teller
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(BET) and t-plot methods were applied to estimate the total specific surface area and micropore surface area,
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respectively. Pore size distribution (PSD) was interpreted by using Barrett-Joyner-Halenda (BJH) approach.
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Energy & Fuels
2.3
High-temperature high-pressure methane sorption experiments
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High-temperature high-pressure (HTHP) sorption measurements were performed on a manometric setup.
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The setup was consisted by a low-temperature zone (< 40 °C) and a high-temperature zone (Figure 2). In order to
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protect the temperature-sensitive parts from high temperatures, the low-temperature zone was installed for the
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reference cell (the valves and pressure transducer) while the sample cell was placed in the high-temperature zone.
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A high-precision Keller pressure transducer with 30 MPa range (PAA-33X type, 0.01% precision of full scale
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capacity guaranteed in 10–40°C) is used to monitor the pressure. The sample cell is sealed by metal face fitting
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(VCR®, Swagelok) with nickel gaskets having a 0.5 µm filter, which is used to prevent sample particles from
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entering the valves. Temperature readings for both heating zones are taken from a Pt-100 resistance temperature
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detector (RTD) with an estimated accuracy of 0.1 K. Volumes of the reference (VRC = 6.125 ± 0.005 cm3) and the
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sample cell (VSC = 51.26 ± 0.02 cm3) were determined by multiple helium expansions at a specified reference
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temperature (TRC = TSC = 39 °C). Methane sorption measurements are performed on dry shale samples (~100
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mesh), and the measuring procedure has been documented in previous studies [8,9,36]. Sufficient time was
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allowed for gas to reach “technically sorption equilibrium” in the RC and SC. Certainly, substantially long
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equilibration times require a leak-tightness of the setup. An acceptable leakage rate is below 5 mbar/h, which is
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determined prior to experiment using helium at a representative pressure.
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The large void volume is one of the major sources of uncertainties in manometric method for excess
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sorption isotherm measurements [37]. Prior to each experiment, SC was packed with sample as much as possible.
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In this study, the typical used amount of the dry samples is 50–80g, which corresponds to Vvoid/VRC values of
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3.4–4.2 measured using helium expansions. Furthermore, blank methane expansion tests were performed on steel
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cylinders of different sizes placed in the SC at the target temperature. The blank methane sorption isotherms of
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stainless steel cylinders with different “void volumes” were interpolated to acquire the blank expansion value of
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shale samples at an equivalent void volume. From the “raw” expansion data measured on a shale sample, the 7
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blank expansion values were subtracted to obtain the final corrected excess sorption isotherm. These blank
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sorption tests were carried out to identify experimental artifacts, and have been used in previously international
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inter-laboratory comparison study to assess the reproducibility of sorption isotherms on shales [38]. The error in
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the excess sorption values determined from the experimental data can be estimated using the Gauss error
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propagation theory. In this study, the calculated uncertainty of measured excess sorption increases with pressure
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and is estimated to be 0.019 mmol/g at the high-end experimental pressure values.
133 134 135
The excess sorbed mass (mexc), also denoted “Gibbs surface excess”, is calculated through the following mass balance: = − (, )
(Eq. 1)
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Here Vvoid is the pore space that is not occupied by the powered sample in SC and determined by helium
137
expansion; mtotal is the total mass of adsorbate (methane) transferred into the SC; ρgas(T, P) is free gas density
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calculated using the GERG equation of state (EOS) provided by Kunz et al. [39].
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The mass balance calculations in the manometric method of sorption measurement rely on an EOS to
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calculate the gas density (or gas compressibility factor) at certain pressures and temperatures. There are
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numerous different EOSs available, however the most commonly used EOSs are the cubic equations of
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Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK), and the multi-parameter Setzmann and Wagner (Se-W,
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[40]) and Span and Wagner (Sp-W, [41]) EOSs. Currently, Se-W and Sp-W are the most accurate EOSs for CH4
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and CO2 respectively, and have been used for instance in the National Institute of Standards and Technology
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(NIST) Chemistry WebBook. More recently, Se-W and Sp-W EOSs are incorporated in the multi-component
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EOS GERG 2004 and GERG 2008 by Kunz et al. [39]. The GERG EOS is used as International Organization for
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Standardization standard (ISO 20765–2: 2015) for natural gases. Figure 3 demonstrates calculated methane
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density at 30 and 100 °C by PR, SRK, Se-W and GERG EOSs and density difference between those and Se-W
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EOS. Clearly, the methane density calculated by the cubic PR and SRK EOSs deviates significantly from that
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based on the more accurate Se-W and GERG EOSs.
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Excess adsorbed mass was converted to excess adsorbed amount of substance (nexc, expressed in mmol/g or
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mol/kg) for consistency with earlier published data. The relationship between surface excess amount and the
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absolute amount adsorbed is: = − = 1 −
154
(Eq. 2)
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Where nexc (mmol/g) is the Gibbs excess sorption; nabs (mmol/g) is the absolute adsorbed gas amount at pressure
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P (MPa); ρgas and ρads is the bulk phase (free gas) and (average) adsorbed phase density, respectively; Vads is the
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volume of the adsorbed phase, and can be calculated as = / .
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2.4
Isotherm Models
2.4.1 Adsorbed phase density
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Excess sorption can be converted into absolute sorption by determining either the adsorption volume or the
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adsorbed phase density. However, both the ρads and Vads are not accessible to direct measurement. It’s common to
162
use approximate density values of the adsorbed phase or its volume. The transformation methods from the excess
163
sorption to absolute sorption using constant adsorbed phase density assumptions are summarized in Table 2. The
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van der Waals (VDW) approximation assumed that the adsorbed phase consists of molecules having the VDW
165
constant, namely, the adsorbed phase density of methane is 0.373 g/cm3 [28]. This value is less than the liquid
166
density approximation (0.421 g/cm3 for methane) [31]. Ozawa et al. [32] regarded the supercritical fluid as
167
superheated liquid, and proposed a modified liquid method which corrected the thermal expansion of the
168
adsorbate. The adsorbed phase density can also be estimated under certain assumptions from excess sorption
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isotherm which has a maximum in excess sorption (graphical method). In this work, we examined these
170
approximations above to describe our experimental data, and all the experimental isotherms recorded at different
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temperatures were fitted in the same regression.
172
2.4.2 Supercritical Dubinin-Astakhov model
173
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The classic Dubinin-Astakhov (D-A) equation, based on the mechanism of pore filling, is given by = " #$% &−
174
' (
)
(Eq. 3)
175
where n0 is the maximum (absolute) sorption capacity; A is the adsorption potential, and can be calculated as
176
A = RT-( /); R is the universal gas constant; T is the absolute temperature; Ps is the saturation vapor
177
pressure for the adsorbate; t is a structure heterogeneity parameter; E is the adsorption characteristic energy.
178
When t = 2, the D-A equation refers to D-R equation.
179
The D-A equation is usually used to describe the adsorption of subcritical vapors. Methane is in
180
supercritical state in shale gas reservoirs. Above critical temperature, gas cannot condense, and thus does not
181
exhibit a saturated vapor pressure. Several investigators proposed the concept of pseudo-saturation vapor
182
pressure [28–30]. However, these adaptations cannot well accommodate supercritical gas sorption at wide
183
pressure and temperature ranges, especially when the experimental pressure is larger than the pseudo-saturation
184
vapor pressure (P > Ps) [21]. Previous work on the meaning of the Dubinin’s isotherm has indicated that the term
185
Ps is not necessarily the saturation pressure, but is related to the energy required to compress the gas to the
186
sorbed phase density at the sorbent surface [42, 43]. If so, a form of Dubinin’s isotherm can be applied to a much
187
wider pressure and temperature ranges with gas density rather than pressure because density is more meaningful
188
at supercritical conditions [33–35]. The modified supercritical Dubinin-Astakhov (SDA) equation [33–35],
189
which substitutes the pressure term in D-A equation with a density term, can be applied to supercritical sorption
190
and is expressed as
191 192
= " #$% &− .- . / 0/1/ )
(Eq. 4)
The modified supercritical Dubinin-Astakhov (SDA) equation is applied to the excess sorption data as
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(Eq. 5)
194
#$% &− .- . / 0/1/ )
234 = " 1 −
193
The above function can be written in the form of adsorption volume: 234 =
195
56 7
1 −
#$% &− .- .
/ 0/1/ )
(Eq. 6)
196
where W0 is the limiting adsorption volume and represents the total available adsorption space. W0 is a
197
property of adsorbent and assumed to be constant in order to be consistent with pore filling theory. Previous
198
investigation on supercritical fluids sorption on porous carbons has shown that adsorbed phase density is
199
likely temperature-dependent [44,45]. Eq. 6 contains four parameters, W0, E, t and ρads. They can be
200
obtained by simultaneously performing multi-variables nonlinear regression over the sorption data recorded
201
at different temperatures. During the process of fitting SDR equation against experimental data, W0, E, t are
202
regarded as characteristic parameters of adsorbent and ρads is temperature-dependent. The optimal fit is done
203
with Microsoft excel using a build-in program, solver, to minimize the square sum of residual (SSR):
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SSR = ∑> ?@:
205 206
23;
3
(% ) − 4 (% )
~1.1 %) is larger than shales in the oil window
376
(Req < ~1.1 %) with the same TOC content (Figure 11). Organic matter-hosted pores develop during the thermal
377
maturation of organic matter. Under the same TOC content, shales in gas window always have more pores and
378
larger limiting adsorption volume than shales in oil window.
379
Clay minerals also contribute a lot to the limiting adsorption volume (Figure 11). There are abundant pores
380
in clay minerals, which provide adsorbed volume and sorption capacity for methane. The later stage of modified
381
characteristic curves (or the volume adsorbed) of sample CN_32 is comparable with CN_22, which is probably
382
related with its high TCM. The limiting adsorption volume of clay minerals decrease in the order:
383
montmorillonite > illite > chlorite > kaolinite, which is slightly different from the sequence of sorption capacity:
384
montmorillonite > kaolinite > illite > chlorite. This is caused by the difference in adsorbed phase density. The
385
fitted adsorbed phase density in kaolinite is about 404 kg/m3, while it is around 200 kg/m3 in illite. The limiting
386
adsorption volume of montmorillonite is 18.6 cm3/kg, which is larger than all the shale samples. The limiting
387
adsorption volume of illite is about 11.1 cm3/kg, which is comparable to matured shales with TOC content being
388
6%. The considerable limiting adsorption volume in clay minerals supports the argument that clay minerals
389
contribute a lot to methane sorption capacity of low-TOC and clay-rich dry shales [7,8].
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4.5
Effect of thermal maturity on characteristic curves
391
The limiting adsorption volume of clay minerals is comparable to organic-rich shale. However, it does not
392
necessarily mean that the clay minerals play the same role in sorption process. It’s interesting to note that at the
393
initial stage (low pressure stage in gas sorption) of the characteristic curves, the adsorbed volume of clay
394
minerals is always less than shales under the same adsorption potential condition (Figure 12). Figure 12
395
illustrates the characteristic curves of representative shales at different thermal maturities. At the beginning of
396
methane sorption (low pressure stage), the adsorption potential of methane on shales and clay minerals decrease
397
in this order: shales in the gas window > shales in the oil window > clay minerals when the adsorbed volume is
398
equal (Figure 12b). This indicates that the heat for methane sorption on shales in the gas window is larger than
399
that on shales in the oil window and clay minerals. This also reflects the higher affinity of methane molecules for
400
sorption on organic matter than on clay minerals. Nonpolar gases like methane are preferentially attracted by
401
hydrophobic organic matter rather than by hydrophilic inorganic matter [57]. Furthermore, organic matter of
402
shales in the gas window is relatively rich in aromatic function groups, while the organic matter of shales in the
403
oil window is relatively rich in hydrophilic oxygen-containing functional groups and aliphatic function groups
404
[58]. The characteristic curves show that organic matter in shales in the gas window has stronger affinity for
405
methane than organic matter in shales in the oil window.
406
The adsorption characteristic energy E is a parameter to characterize the apparent heat of sorption system.
407
Figure 13 illustrates the effect of thermal maturity on adsorption characteristic energy of shales. Adsorption
408
characteristic energy shows a parabolic-like shape with a minimum approximately at the end of oil window (Req
409
≈1.1%), which is consistent with the evolution of porosity in New Albany shales with maturation. Mastalerz et
410
al. found that the porosity are high in immature shales (Req < ~ 0.5%), and decline with increasing maturity to
411
minima in the late mature samples (Req ≈ 1.1%, the end of oil window), and later increase towards the
412
postmature shale (Req > ~1.1%, gas window) [59]. The decline in porosity during the late mature stage is caused 20
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413
by the pore filling by oil or solid bitumen, which reduce the sorption sites and the sorption system release less
414
heat. Many pores in organic matter are generated by secondary cracking of oil and bitumen in postmature shales
415
(Req > ~1.1%). The interaction between methane and postmature shales enhances and the sorption system release
416
larger heat.
417
The parabolic-like trend between the characteristic energy and thermal maturity of shales is analogous to
418
but different from the previously reported parabolic shape in methane sorption on dry coals [60]. For dry coals,
419
sorption capacity shows a parabolic dependence on coal rank (thermal maturity). For dry shales, no obvious
420
relationship exists between sorption capacity and maturity. However, the characteristic energy instead of sorption
421
capacity shows a parabolic trend with thermal maturity in shales. This can be attributed to the difference in the
422
mineral compositions of coals and shales. In dry shales, the relationship between sorption capacity and thermal
423
maturity is covered by clay minerals because of low TOC content. Thus, total sorption capacity is a function of
424
not only TOC but also TCM in shales. Clay minerals contribute considerable adsorption volume (Figure 11), but
425
the sorption heat on clay minerals is less than organic matter (Figure 13). Adsorption characteristic energy is the
426
heat released by the sorption system-mainly the heat when methane sorption on high energetic sorption sites
427
(organic matter). The organic matter content of coals is high (commonly larger than 60%). Because methane
428
molecules preferentially adsorb on high energetic sorption sites, it can be inferred that probably both sorption
429
capacity and sorption heat of dry coals show a parabolic correlation with rank. Methane sorption characteristic
430
on coals and shales can be unified using parabolic trend from the point of sorption thermodynamic. Detailed
431
investigations about the thermodynamic property of methane sorption on shales will be discussed in future.
432
5
Conclusions
433
High-temperature high-pressure sorption data for methane on shales from Sichuan Basin have been
434
obtained at 30–120°C and pressures up to 25 MPa using a specially designed two-temperature-zone manometric
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435
setup. The sorption isotherms over wide temperature ranges have been successfully described by the supercritical
436
Dubinin-Astakhov (SDA) sorption model. Methane characteristic curves were obtained using a new expression
437
of adsorption potential, and it is found that if the thermal expansion of adsorbed phase is considered, the
438
characteristic curves are temperature-invariant. A characteristic curve equation is provided and able to predict
439
methane sorption at other temperatures based on the easily tested sorption isotherm at room temperature. The
440
modified characteristic curves comprehensively characterize the available pore space for sorption and the affinity
441
of methane molecules. The later stage of the modified characteristic curves (limited adsorption volume) is
442
mainly controlled by the available pore space provided by organic matter and clay minerals. The initial stage of
443
the characteristic curves reflects the affinity of methane molecules for sorption on organic matter. According to
444
the characteristic curves, shales in the gas window show higher affinity than shale in the oil window and clay
445
minerals, though the clay minerals may provide comparable adsorption volume. The adsorption characteristic
446
energy shows a parabolic-like shape with a minimum approximately around Req =1.1%, which are related with
447
the evaluation of porosity of shales.
448
Acknowledgments
449 450 451
China (Grant No. 51604249, 41690134, 41572109), and State Key Laboratory of Petroleum Resources and
452
References
453 454 455 456 457 458 459 460 461 462 463
(1)
The authors would like to acknowledge the financial support of the National Natural Science Foundation of Prospecting Independent Research Subject (Grant No. PRP/open-1606).
Amann-Hildenbrand, A.; Ghanizadeh, A.; Krooss, B.M. Transport properties of unconventional gas systems. Mar. Petrol. Geol. 2012, 31, 90–99.
(2) Montgomery, S.L.; Jarvie, D.M.; Bowker, K.A.; Pollastro, R.M. Mississippian Barnett Shale, Fort Worth basin, north-central Texas: Gas-shale play with multi-trillion cubic foot potential. AAPG Bull. 2005, 89, 155–175. (3) Curtis, J. B. Fractured shale-gas systems. AAPG Bull. 2002, 86, 1921−1938. (4) Chalmers, G.R.L.; Bustin, R.M. Lower Cretaceous gas shales in northeastern British Columbia, part 1: geological controls on methane sorption capacity. Bull. Can. Petrol. Geol. 2008, 56,1–21. (5) Ross, D.J.K.; Bustin, R.M. The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs. Mar. Petrol. Geol. 2009, 26, 916–927. (6) Zhang, T.W.; Ellis, G.S.; Ruppel, S.C.; Milliken, K.; Yang, R.S. Effect of organic-matter type and thermal 22
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Page 23 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508
Energy & Fuels
maturity on methane adsorption in shale-gas systems. Org. Geochem. 2012, 47, 120–131. (7) Ji, L.M.; Zhang, T.W.; Milliken, K.; Qu, J.L.; Zhang, X.L. Experimental investigation of main controls to methane adsorption in clay-rich rocks. Appl. Geochem. 2012, 27, 2533–2545. (8) Gasparik, M.; Ghanizadeh, A.; Bertier, P.; Gensterblum, Y.; Bouw, S.; Krooss, B.M. High-pressure methane sorption isotherms of black shales from the Netherlands. Energy Fuels 2012, 26, 4995–5004. (9) Gasparik, M.; Bertier, P.; Gensterblum, Y.; Ghanizadeh, A.; Krooss, B.M.; Littke, R. Geological controls on the methane storage capacity in organic-rich shales. Int. J. Coal Geol. 2014, 123, 34–51. (10) Vandewijngaerde, W.; Piessens, K.; Dusar, M.; Bertier, P.; Krooss, B.M.; Littke, R.; et al. Investigations on the shale oil and gas potential of Westphalian mudstone successions in the Campine Basin, NE Belgium (well KB174): Palaeoenvironmental and palaeogeographical controls. Geol. Belg. 2016, 19, 1–11. (11) Yang, F.; Ning, Z.F.; Zhang, R.; Zhao, H.W.; Krooss, B.M. Investigations on the methane sorption capacity of marine shales from Sichuan Basin, China. Int. J. Coal Geol. 2015,146, 104–117. (12) Yang, F.; Ning, Z.F.; Wang, Q.; Zhang, R.; Krooss, B.M. Pore structure characteristics of lower Silurian shales in the southern Sichuan Basin, China: Insights to pore development and gas storage mechanism. Int. J. Coal Geol. 2016, 156, 12–24. (13) Zhou, Q.; Xiao, X.M.; Tian, H.; Pan, L. Modeling free gas content of the Lower Paleozoic shales in the Weiyuan area of the Sichuan Basin, China. Mar. Petrol. Geol. 2014, 56, 87–96. (14) Li, Y.F.; Zhang, T.W.; Ellis, G.S.; Zhao, D. Depositional environment and organic matter accumulation of Upper Ordovician-Lower Silurian marine shale in the Upper Yangtze Platform, South China. Palaeogeogr. Palaeocl. 2017, 466, 252–264. (15) Chen, C.; Mu, C.L.; Zhou, K.K.; Liang, W.; Ge, X.Y.; Wang, X.P.; et al. The geochemical characteristics and factors controlling the organic matter accumulation of the Late Ordovician-Early Silurian black shale in the Upper Yangtze Basin, South China. Mar. Petrol. Geol. 2016, 76, 159–175. (16) Dai, J.X.; Zou, C.N.; Liao, S.M.; Dong, D.Z.; Ni, Y.Y.; Huang, J.L.; et al. Geochemistry of the extremely high thermal maturity Longmaxi shale gas, southern Sichuan Basin. Org. Geochem. 2014, 74, 3–12. (17) Clarkson, C.R.; Haghshenas, B. Modeling of supercritical fluid adsorption on organic-rich shales and coal. SPE 164532 presented at the Unconventional Resource-USA in The Woodlands, Texas, USA, 10–12 April, 2013. (18) Rexer, T.F.T.; Benham, M.J.; Aplin, A.C.; Thomas, K.M. Methane adsorption on shale under simulated geological temperature and pressure conditions. Energy Fuels 2013, 27, 3099–3109. (19) Tian, H.; Li, T.F.; Zhang, T.W.; Xiao, X.M. Characterization of methane adsorption on overmature Lower Silurian-Upper Ordovician shales in Sichuan Basin, southwest China: Experimental results and geological implications. Int. J. Coal Geol. 2016, 156, 36–49. (20) Chen, L.; Jiang, Z.X.; Liu, K.Y.; Ji, W.M.; Wang, P.F.; Gao, F.L.; et al. Application of Langmuir and Dubinin-Radushkevich models to estimate methane sorption capacity on two shale samples from the Upper Triassic Chang 7 Member in the southeastern Ordos Basin, China. Energ. Explor. Exploit. 2017, 35, 122– 144. (21) Clarkson, C.R.; Bustin, R.M.; Levy, J.H. Application of the mono/multilayer and adsorption potential theories to coal methane adsorption isotherms at elevated temperature and pressure. Carbon 1997, 35, 1689–1705. (22) Chareonsuppanimit, P.; Mohammad, S.A.; Robinson Jr., R.L.; Gasem, K.A.M. High-pressure adsorption of gases on shales: Measurements and modeling. Int. J. Coal Geol. 2012, 95, 34–46. (23) Mohammad, S.A.; Arumugam,A.; Robinson Jr., R.L.; Gasem, K.A.M. High-pressure adsorption of pure gases on coals and activated carbon: Measurements and modeling. Energy Fuels 2012, 26, 536–548. (24) Mosher, K.; He, J.J.; Liu, Y.Y.; Rupp, E.; Wilcox, J. Molecular simulation of methane adsorption in micro23
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Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553
and mesoporous carbons with applications to coal and gas shale systems. Int. J. Coal Geol. 2013, 109–110, 36–44. (25) Zhao, T.Y.; Li, X.F.; Zhao, H.W.; Li, M.F. Molecular simulation of adsorption and thermodynamic properties on type II kerogen: Influence of maturity and moisture content. Fuel 2017, 190, 198–207. (26) Tang, X.; Ripepi, N.; Stadie, N.P.; Yu, L.J.; Hall, M.R. A dual-site Langmuir equation for accurate estimation of high pressure deep shale gas resources. Fuel 2016, 185, 10–17. (27) Tang, X.; Ripepi, N.; Luxbacher, K.; Pitcher, E. Adsorption models for methane in shales: review, comparison and application. Energy Fuels 2017, 31, 10787–10801. (28) Dubinin, M.M. The potential theory of adsorption of gases and vapors for adsorbents with energetically nonuniform surfaces. Chem. Rev. 1960, 60, 235–241. (29) Kapoor,A.; Ritter, J.A.; Yang, R.T. On the Dubinin-Radushkevich equation for adsorption in microporous solids in the Henry’s Law region. Langmuir 1989,5,1118–1121. (30) Do, D.D. Adsorption analysis: Equilibria and kinetics, Imperial College Press: London, 1998. (31) Agarwal, R.K.; Schwarz, J.A. Analysis of high pressure adsorption of gases on activated carbon by potential theory. Carbon 1988, 26, 873–887. (32) Ozawa, S.; Kusumi, S.; Ogino, Y. Physical adsorption of gases at high pressure. IV. An improvement of the Dubinin-Astakhov adsorption equation. J. Colloid Interface Sci. 1976,56, 83–91. (33) Sakurovs, R.; Day, S.; Weir, S.; Duffy, G. Application of a modified Dubinin-Radushkevich equation to adsorption of gases by coals under supercritical conditions. Energy Fuels 2007, 21, 992–997. (34) Day, S.; Duffy, G.; Sakurovs, R.; Weir, S. Effect of coal properties on CO2 sorption capacity under supercritical conditions. Int. J. Greenhouse Gas Contr. 2008, 2, 342–352. (35) Jeon,P.R.; Choi, J.; Yun, T.S.; Lee, C.H. Sorption equilibrium and kinetics of CO2 on clay minerals from subcritical to supercritical conditions: CO2 sequestration at nanoscale interfaces. Chem. Eng. J. 2014, 255, 705–715. (36) Krooss, B.M.; Van Bergen, F.; Gensterblum, Y.; Siemons, N.; Pagnier, H.J.; David, P. High-pressure methane and carbon dioxide adsorption on dry and moisture-equilibrated Pennsylvanian coals. Int. J. Coal Geol. 2002, 51, 69–92. (37) Mohammad, S.A.; Fitzgerald, J.E.; Robinson Jr., R.L.; Gasem, K.A.M. Experimental uncertainties in volumetric methods for measuring equilibrium adsorption. Energy Fuels 2009, 23, 2810–2820. (38) Gasparik, M.; Rexer, T.F.T.; Aplin,A.C.; Billemont, P.; De Weireld, G.; Gensterblum, Y.; et al. First international inter-laboratory comparison of high-pressure CH4,CO2 and C2H6 sorption isotherms on carbonaceous shales. Int. J. Coal Geol. 2014, 132, 131–146. (39) Kunz, O.; Klimeck, R.; Wagner, W.; Jaeschke, M. The GERG-2004 wide-range equation of state for natural gases and other mixtures; VDI Verlag: Dusseldorf, 2007. (40) Setzmann, U.; Wagner, W. A new equation of state and tables of thermodynamic properties for methane covering the range fromthemelting line to 625 K at pressures up to 1000 MPa. J. Phys. Chem. Ref. Data 1991, 20, 1061–1151. (41) Span, R.; Wagner, W. A new equation of state for carbon dioxide covering the fluid region from the triple-point temperature to 1100 K at pressures up to 800 MPa. J. Phys. Chem. Ref. Data 1996, 25, 1509– 1596. (42) Cerofolini, G.F. The Dubinin-Radushkevich (DR) equation: history of a problem and perspectives for a theory, in: D.H. Everett (Ed.), Colloid Science, The Royal Society of Chemistry, 1983, 59–83. (43) Cerofolini, G.F. Multilayer adsorption on heterogeneous surfaces, J. Low Temp. Phys. 1972, 6,473–486. (44) Zhou, L.; Zhou, Y.; Li, M.; Chen, P.; Wang, Y. Experimental and modeling study of the adsorption of supercritical methane on a high surface area activated carbon. Langmuir 2000,16,5955–5959. 24
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554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589
Energy & Fuels
(45) Do, D.D.; Do, H.D. Adsorption of supercritical fluids in non-porous and porous carbons: analysis of adsorbed phase volume and density. Carbon 2003,41, 1777–1791. (46) Gregg, S.J.; Sing, K.S.W. Adsorption, surface area and porosity, 2nd ed.; Academic Press: New York, 1982. (47) Myers,A.L.; Monson,P.A. Adsorption in porous materials at high pressure: Theory and experiment. Langmuir 2002, 18, 10261–10273. (48) Zhou, Y.P.; Zhou, L. Fundamentals of high pressure adsorption. Langmuir 2009, 25, 13461–13466. (49) Charoensuppanimit, P.; Mohammad, S.A.; Robinson Jr., R.L.; Gasem, K.A.M. Modeling the temperature dependence of supercritical gas adsorption on activated carbons, coals and shales. Int. J. Coal Geol. 2015, 138,113–126. (50) Sakurovs, R.; Day, S.; Weir, S.; Duffy, G. Temperature dependence of sorption of gases by coals and charcoals. Int. J. Coal Geol. 2008, 73, 250–258. (51) Dong, X.H.; Liu, H.Q.; Guo, W.; Hou, J.R.; Chen, Z.X.; Wu, K.L. Study of the confined behavior of hydrocarbons in organic nanopores by the potential theory. Fluid Phase Equilibr. 2016, 429, 214–226. (52) Ambrose, R.J.; Hartman, R.C.; Diaz-Campos, M.; Akkutlu, I.Y.; Sondergeld, C.H. Shale gas-in-place calcualtions part I: New pore-scale considerations. SPE J. 2012, 17, 219–229. (53) Huan, X.; Zhang, X.B.; Wei, H.W. Research on parameters of adsorption potential via methane adsorption of different types of coal. J. China Coal Soc. 2015, 40, 1859–1864. (54) Wu, S.; Tang, D.Z.; Li, S.; Chen, H.; Wu, H.Y. Coalbed methane adsorption behavior and its energy variation features under supercritical pressure and temperature conditions. J. Petrol. Sci. Eng. 2016, 146, 726–734. (55) Loucks, R.G.; Reed, R.M.; Ruppel, S.C.; Jarvie, D.M. Morphology, genesis, and distribution of nanometer-scale pores in siliceous mudstones of the Mississippian Barnett Shale. J. Sediment. Res. 2009, 79, 848–861. (56) Milliken, K.L.; Rudnicki,M.; Awwiller, D.N.; Zhang, T.W. Organicmatter-hosted pore system, Marcellus Formation (Devonian), Pennsylvania. AAPG Bull. 2013, 97, 177–200. (57) Yang,F.;
Xie,C.J.;
Ning,Z.F.;
Krooss,
B.M.
High-Pressure
methane
sorption
on
dry
and
moisture-equilibrated shales. Energy Fuels 2017,31,482–492. (58) Lis, G.P.; Mastalerz, M.; Schimmelmann, A.; Lewan, M.D.; Stankiewicz, B.A. FTIR absorption indices for thermal maturity in comparison with vitrinite reflectance Ro in type-II kerogens from Devonian black shales. Org. Geochem. 2005, 36, 1533–1552. (59) Mastalerz, M.; Schimmelmann,A.; Drobniak,A.; Chen, Y.Y. Porosity of Devonian and Mississippian New Albany shale across a maturation gradient: Insights from organic petrology, gas adsorption, and mercury intrusion. AAPG Bull. 2013, 97, 1621–1643. (60) Bush, A.; Gensterblum, Y. CBM and CO2-ECBM related sorption processes in coal: A review. Int. J. Coal Geol. 2011, 87, 49–71.
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590
591 592 593 594 595 596
597 598 599 600
601 602 603 604 605
Figures
Figure 1. The classic adsorption potential ( H = IJKL(MN /M) ) of methane calculated using pseudo-saturation vapor pressure based on Dubinin’s [28] and reduced Kirchhoff equations [29]. It should be noted that the classic adsorption potential becomes negative at elevated pressures (> ~10 MPa), which goes against Dubinin’s postulates in potential theory.
Figure 2. Schematic diagram of the high-temperature high-pressure (HTHP) sorption apparatus (modified after [9]).
Figure 3. Comparison of (a) methane density at 30 and 100 °C calculated using the PR, SRK, Se-W and GERG EOSs and (b) density difference between those and Se-W EOS.
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606 607 608 609
610 611 612 613
Figure 4. (a) Low-pressure N2 adsorption-desorption isotherms for the shale samples; (b) Pore size distributions of the samples by using BJH method on the adsorption branch of the isotherms.
Figure 5. Methane excess sorption isotherms measured on dry shale samples at different temperatures. The error bars represent the estimated uncertainties in the amount of gas adsorbed according to the Gauss error propagation theory.
614 615 616 617
Figure 6. The extrapolated plot of methane excess sorption against gas density for sample CN_23. It should be noted that there is some uncertainty in the extrapolation beyond experimental pressure (gas density).
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618 619 620
Figure 7. The modified characteristic curves for methane on sample CN_22 based on different adsorbed
621 622 623 624 625
The density of adsorbed phase was calculated using (a) VDW constant approximation [28]; (b) modified
626 627 628 629 630
gas density methods. The modified adsorption potential is calculated using H = IJKL:OPQN /ORPN < (Eq. 8). liquid density approximation [32]; (c) liquid density at boiling temperature and ambient pressure [31]. Characteristic curve data using the VDW constant and liquid density approximations for adsorbed phase density display larger dispersity than the modified liquid density method.
Figure 8. Modified characteristic curves for methane on shale samples. The modified adsorption potential is calculated using H = IJKL:OPQN /ORPN < (Eq. 8). The lines represent the fitted characteristic curves using Eq. (10), and the fitted correlation coefficient R2 are also labeled.
631 632 633
Figure 9. Measured and predicted methane excess sorption isotherms at 120 °C. Excess sorption isotherms at 120 °C are predicted by characteristic curve equations obtained from isotherms at 30–100 °C.
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634
635 636 637 638 639 640 641
642 643 644 645 646 647
Figure 10. Effect of organic richness (TOC) and clay minerals (TCM) on modified methane characteristic curves of shales with thermal maturity level in the gas window. Semi log coordinate is used for comparison. The later stage (high pressure stage) of the modified characteristic curves is mainly controlled by the pore space provided by organic matter and clay minerals. The available pore volume for gas sorption on shales with high TOC and TCM are always large.
Figure 11. Comparison of the limiting adsorption volume as a function of TOC for shales at different thermal maturity levels (this study and Posidonia shales [9] and Barnett shales [6]) and clay minerals [7]). The limiting adsorption volume of shales in the gas window is larger than shales in the oil window with the same TOC content. The clay minerals also contribute to the limiting adsorption volume.
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648 649 650 651 652 653 654 655 656 657 658
659 660 661 662 663
Figure 12. Effect of thermal maturity (Req) on modified methane characteristic curves of shales. The lines represent the fitted characteristic curves using Eq. (10). Generally, though the later stage of characteristic curves of the samples studied are controlled by TOC and clay mineral (Figure 10), the adsorbed volume at the initial stage of characteristic curves (gas sorption at low pressure) decrease in this order: shales in the gas window > shales in the oil window > clay minerals when the adsorption potential is equal, which indicates that shales at the gas window show higher affinity than shales at the oil window and clay minerals. It should be noted that the initial stage of characteristic curves of our four samples are almost the same because they are at the same thermal maturity level. Sample CN_22 represented this set of highly over-mature samples and was plotted.
Figure 13. Effect of thermal maturity (Req) on adsorption characteristic energy of shales (this study and Posidonia shales [9] and Barnett shales [6] and clay minerals [7]). Thermal maturity of shales are approximately divided as: Immature: (Req < ~ 0.5%); Oil window: ~0.5% < Req < ~1.1%; Gas window: Req > ~1.1%.
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Tables Table 1: Various methods for the estimation of pseudo-saturation vapor pressure Ps. Method
= (/4 )= 4
Dubinin’s equation Reduced Kirchhoff equation
= 4 #$% &
STEU SV
.
WX YV
]
Dubinin [28]
a
@ZSTEU /SV
ln = + _
Antoine equation a
Ref
Expression
/ 1 −
SV
)
S
Kapoor et al.[29]
b
Do [30]
c
S
Pc and Tc are the critical pressure and critical temperature of the adsorbate.
b
Tnbp is the temperature at the normal boiling point.
c
C and D are Antoine parameters.
Table 2: Various methods to estimate the adsorbed phase density. Method
Expression
Ref
= 8a4 /(04 )
VDW constant approximation
=
Liquid density approximation
a
Agarwal and Schwarz [31]
= #$%b−c × ( − )d
Modified Liquid density
Dubinin [28]
b c
Ozawa, et al. [32]
Graphical method
-
-
Optimization method
-
-
a b c
Pc and Tc are the critical pressure and the critical temperature of the adsorbate, and M is molar mass.
is the liquid density at boiling temperature and ambient pressure.
is the boiling temperature at ambient pressure and c is thermal expansion coefficient of the adsorbate.
Table 3: Geochemical characteristics, mineralogy composition, and pore structure of the shale samples. Sample
Age
TOC
Req
(wt %)
(%)
Quartz + feldspar (wt %)
Total
Carbonates (wt %)
clays (wt %)
SBET b
Vt c
Dd
Smicroe
Vmicrof
(m2/g)
(cm3/kg)
(nm)
(m2/g)
(cm3/kg)
CN_11
O3 w
4.83
2.8
74.0
0.0
26.0
28.75
43.0
6.0
11.8
4.7
CN_22
S1l
2.87
2.8
55.4
25.4
17.2
14.96
18.5
4.9
4.8
1.9
CN_23
S1l
2.92
2.8
57.4
27.4
36.2
14.19
19.7
5.6
6.0
2.5
CN_32 a
S1l
0.89
2.8
25.3
27.7
45.5
14.95
26.7
7.1
3.6
1.5
a
Pore structure parameters of CN_32 were provided based on the results on a copy sample from the sampling sites; b. Total
specific surface areas were determined by the BET method; c. Total pore volumes were evaluated from the adsorbed amount of N2 at the highest relative pressure point; d. average pore diameter; e. Micropore surface area interpreted by the t-plot method; f. t-plot micropore volume.
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Table 4. Fitting parameters of SDA model for methane sorption on shales at different temperatures. Shales
TOC
Req
Thermal
W0
E
t
α
8869.6
1.56
0.0004
0.0089
6273.6
1.14
0.0028
0.0071
7747.8
1.32
0.0012
Gas window
0.0073
6559.8
1.37
0.0017
0.5
Oil window
0.0109
6687.5
1.41
0.0000
0.5
Oil window
0.0095
6572.2
1.44
0.0005
3
(wt %)
(%)
Maturity
(cm /g)
(J/mol)
CN_11
4.83
2.8
Gas window
0.0081
Sichuan
CN_23
2.92
2.8
Gas window
shale
CN_22
2.87
2.8
Gas window
CN_32
0.89
2.8
WIC_143
14.1
WIC_149
11.7
Posidonia shale a
Barnett shale b
a
Sample
HAR_038
9.3
0.9
Oil window
0.0089
4930.7
1.15
0.0011
HAD_103
6.7
1.5
Gas window
0.0091
7159.4
1.38
0.0010
HAD_115
7.7
1.5
Gas window
0.0095
7243.1
1.50
0.0009
HAD_119
7.7
1.5
Gas window
0.0142
5689.9
1.23
0.0016
HAD_123
10.5
1.5
Gas window
0.0117
7116.2
1.47
0.0010
Lee C-5-1
7.9
0.58
Oil window
0.0073
7806.4
1.95
0.0000
Tarrant A-3
7.05
0.81
Oil window
0.0071
7476.7
1.75
0.0000
Blakely #1
6.6
2.01
Gas window
0.0106
7414.3
1.42
0.0016
Smectite
0.0
-
-
0.0186
8881.5
1.99
0.0003
Clay
Illite
0.0
-
-
0.0111
4490.0
1.27
0.0038
mineral c
Kaolinite
0.0
-
-
0.0078
5639.6
1.43
0.0002
Chlorite
0.0
-
-
0.0079
5832.1
1.66
0.0024
Methane sorption data of the Posidonia shales were from Gasparik et al. [9]; b. Methane sorption data of Barnett
shales were from Zhang et al. [6]; c. Methane sorption data of the clay minerals were from Ji et al. [7].
32
ACS Paragon Plus Environment