Water vapor sorption on Marcellus shale: measurement, modelling and thermodynamic analysis

Supplemental Materials

Xu Tanga*, Nino Ripepia,b, Katherine A. Valentinec,d, Cigdem Kelesb, Timothy Longc,d, Aleksandra Gonciaruke

(a. Virginia Center for Coal and Energy Research, b. Department of Mining and Minerals Engineering, c. Department of Chemistry, d. Macromolecules Innovation Institute, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060, United States; e. Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, UK)

1. Sample characterization

Two methods are used to characterize material properties of the Marcellus shale sample: proximate analysis, image analysis, pore structure analysis (N2@77K), and XRD spectrum analysis.

1.1 Proximate analysis

The proximate analysis is conducted using Q500 Thermogravimetric Analyzer (TA Instruments, New Castle, DE) in the Department of Mining and Mineral Engineering at Virginia Tech. Figure S-1 shows the weight change of Marcellus shale as a function of time under different temperatures. The moisture, volatiles, fixed carbon and ash of the Marcellus shale makes up 0.9%, 9%, 3.9% and 86.1% of the shale sample, respectively.

Figure S-1 Proximate analysis of Marcellus shale sample

1.2. Imaging analysis

The environmental SEM (FEI Quanta 600 FEG) in ICTAS Nanoscale Characterization and Fabrication Laboratory at Virginia Tech is used here to characterize the organic matter and pores of the Marcellus shale. Scanning Electron Microscope (SEM) is an automated image analysis system which uses back scattered electrons (BSE) and energy dispersive X-ray signals from a scanning electron microscope to create digital images across the sample surface. Each pixel in these images contains information, derived from BSE and energy dispersive spectrometry (EDS), on the chemistry of the mineral, matrix, or organic component that make up an individual small region under the electron beam. The unit is coupled to a computer, which controls the movement of the sample stage and the electron beam. The compute also collects and analyzes the data from the various measurements. In particular, data on the chemistry of each pixel site was converted by the computer to qualitative data on mineralogy due to the association of chemical elements present. Figure S-2(a,b,c) shows the SEM image of the Marcellus shale. The organic matter is distributed as agglomerations of different shapes as shown in Figure S-2(a,b). Figure S-2(c,d) show that the organic matter is rich in pores from nanoscale to microscale with different shapes.

Figure S-2 SEM image for the Marcellus samples; dark areas in (a) and (b) represent the organic matter using a backscattered electron mode; (c) and (d) show the pore size and shape of the selected area of the samples. 1.3 Pore structure analysis

Gas sorption was conducted using a Micromeritics ASAP 2020 analyzer (Norcross GA, USA) using ultrapure N2 adsorbates (BOC gases, Nottingham, UK). All samples were degassed for 15 hours at 110

. Approximately 1-2.3 gram coal samples were used for each N℃ 2 sorption isotherms including using a filler rod. Sample tube bulbs were immersed in liquid N2 at approximately 77K. Figure S-3 shows the nitrogen sorption isotherm on Marcellus shale under 77 K.

Figure S-3 Nitrogen ad/desorption isotherm in Marcellus shale under 77K.

The BET specific surface area using N2 was calculated by the linear BET relationship under the relative pressure ranges from Rouquerol’s approach, which includes (1) both the resulting parameter C BET is positive and the intercept on the ordinate of the BET-plot is positive, and (2) the term Vad(po-p) should continuously increase with p/po (ISO 9277: 2010). The calculated BET specific surface is 32.32 m2/g. The pore size distribution is obtained using classic BJH model based on the adsorption branch of the nitrogen isotherms as shown in Figure S-4.

Figure S-4 BJH pore size distribution of Marcellus shale: triangle represents incremental pore volume, bold dot represent accumulative pore volume

1.4 XRD spectrum analysis

Bulk mineralogical composition of the Marcellus shale sample was analyzed by means of powder X-ray diffraction (XRD). XRD measurements were performed on a Bruker D8 Advance diffractometer using CoKa radiation generated at 40 kV and 35 mA. Randomly orientated powder samples were illuminated through a fixed divergence slit of 0.6 mm. A scintillation detector recorded the diffracted beam at a counting time of 1.5 s between an angle interval of 2o–80o 2ϴ. Powder shale samples (100-150 μm) was used to do the analysis. The results are shown in Figure S-5. Three minerals are observed in this shale sample: pyrite, quartz and calcite.

Figure S-5 X-ray diffraction powder patterns of the mudrock set measured between an angle interval 2ϴ of 2o–82o.

2. Water vapor ad/desorption tests

2.1 Introduction of test instruments

The TA Instruments Q5000 thermo-gravimetric sorption analyzer (TA Instruments, New Castle, DE) is used for measuring water vapor adsorption/desorption isotherms under controlled temperature and relative humidity (RH) conditions. Relative humidity, also called relative pressure (P/Po), is the ratio of the partial pressure of water vapor (P) to the saturation vapor pressure (Po) of water at a given temperature. In this instrument, both the sample and reference chambers are exposed to the same conditions of temperature and humidity. The net effect on the sample weight change is therefore only from moisture adsorption or desorption. Figure S-3 shows the Illustration of the instrument.

Figure S-6 Illustration of the TA instruments Q5000 Sorption analyzer (Provided by TA instruments and used with permission)

This instrument is based on the gravimetric method with a high performance thermo-balancer with a signal resolution of 0.01µg/0.05ug, a sensitivity of 0.1ug/0.5µg, and a weighing accuracy of ±0.1 % with ±0.01% precision. The humidity chamber is an insulated tri-level aluminum chamber containing de-ionized water, which is controlled and maintained by a pair of mass flow controllers. By adjusting the amount of “dry” and “wet” gases flowing through the controller, the software is capable of maintaining a desired relative humidity level from 0 to 98% RH with ±1% accuracy. Identical sensors are also connected to the sample and reference chamber to provide a continuous indication of humidity. The constant temperature (±0.1 ) of the test chamber is controlled by four thermoelectric devices in℃ conjunction with a thermistor in a closed-loop system using Peltier control elements, which ranges from 0.1 to 150 at ambient pressure. ℃ A sample is exposed to a series of humidity step changes at constant temperature. Hemispherical quartz pans are utilized as the sample and the reference pan.

2.2 Test procedures

Shale samples from the Marcellus shale formations were obtained from one shale gas well in the central Appalachian region, West Virginia, United States. The shale specimen was ground and sieved using 0.38–0.83 mm metal sifters and placed in a drying oven at 105 °C for 24 h to dry. After drying, the prepared sample was stored in a desiccator prior to adsorption measurements. Water vapor ad/desorption measurements were conducted using the TA Instruments Q5000 Thermo-gravimetric Sorption Analyzer in the group of Dr. Timothy Long, Department of Chemistry, at Virginia Tech.

3-5mg of the dried shale was utilized in each test for water vapor ad/desorption at three separate temperatures (303.15, 313.15, and 328.15 K). The relative vapor pressure was stepped from 0 – 0.95 and back to 0 with a 0.05 increment, while maintaining an isotherm at one of the experimental temperatures. Each relative vapor pressure step was continued until equilibration of the sample weight occurred (<0.01% change over 10 min). A pre-drying step was utilized at the start of each run in which the temperature was held at 60 °C with 0 relative vapor pressure until the sample weight equilibrated. Weight gain/loss of the pre-dried sample during this hysteresis test was attributed to water adsorption and desorption.

2.3 Test results

Table S-1 shows the raw experimental data for water vapor ad/desorption isotherms under 303.15, 313.15 and 328.15 K. It is worth noting that the maximum relative pressure only goes to 0.8 under 328.15K because of instrument limitation.

Table S-1 Raw experimental data for water vapor ad/desorption isotherms

P/P0Adsorption uptake

(mmol/g)P/P0

Adsorption uptake (mmol/g)

P/P0Adsorption uptake

(mmol/g)0.05 0.0512 0.05 0.0460 0.05 0.03670.1 0.0972 0.1 0.0876 0.1 0.0725

0.15 0.1340 0.15 0.1222 0.15 0.10220.2 0.1662 0.2 0.1534 0.2 0.1298

0.25 0.1970 0.25 0.1834 0.25 0.15470.3 0.2278 0.3 0.2135 0.3 0.1783

0.35 0.2603 0.35 0.2449 0.35 0.20320.4 0.2952 0.4 0.2779 0.4 0.2296

0.45 0.3369 0.45 0.3159 0.45 0.25960.5 0.3862 0.5 0.3607 0.5 0.2950

0.55 0.4413 0.55 0.4113 0.55 0.33890.6 0.4949 0.6 0.4616 0.6 0.3869

0.65 0.5440 0.65 0.5070 0.65 0.43090.7 0.5918 0.7 0.5484 0.7 0.4679

0.75 0.6395 0.75 0.5894 0.75 0.49950.8 0.6907 0.8 0.6307 0.8 0.5275

0.85 0.7507 0.85 0.6769 0.75 0.52730.9 0.8342 0.9 0.7357 0.7 0.5194

0.95 1.0156 0.95 0.8583 0.65 0.50450.95 1.0395 0.95 0.8744 0.6 0.46170.9 0.9478 0.9 0.8269 0.55 0.3933

0.85 0.8850 0.85 0.7873 0.5 0.33540.8 0.8379 0.8 0.7543 0.45 0.2978

0.75 0.7977 0.75 0.7244 0.4 0.26450.7 0.7615 0.7 0.6956 0.35 0.2322

0.65 0.7273 0.65 0.6632 0.3 0.20610.6 0.6779 0.6 0.5987 0.25 0.1804

0.55 0.5840 0.55 0.4959 0.2 0.15380.5 0.4840 0.5 0.4120 0.15 0.1256

0.45 0.4087 0.45 0.3516 0.1 0.09520.4 0.3559 0.4 0.3088 0.05 0.0567

0.35 0.3138 0.35 0.27310.3 0.2756 0.3 0.2361

0.25 0.2392 0.25 0.20260.2 0.2029 0.2 0.1628

0.15 0.1702 0.15 0.14020.1 0.1346 0.1 0.1083

0.05 0.0878 0.05 0.0674

303.15 K 313.15 K 328.15 KP0=4.247 kPa P0=7.3849 kPa P0= 15.7621 kPa

Reference:

TA Instruments-Q5000SA Brochure ： http://www.tainstruments.com/wp-content/uploads/2012-Sorption-Brochure.pdf