9980
2008, 112, 9980–9982 Published on Web 06/14/2008
High Pressure Pair Distribution Function Studies of Green River Oil Shale Karena W. Chapman,*,† Peter J. Chupas,† Randall E. Winans,† and Ronald J. Pugmire‡ X-ray Science DiVision, AdVanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, and Departments of Chemical Engineering and Chemistry, UniVersity of Utah, Salt Lake City, Utah ReceiVed: May 2, 2008; ReVised Manuscript ReceiVed: June 1, 2008
The compression behavior of a silicate-rich oil shale from the Green River formation in the pressure range 0.0-2.4 GPa was studied using in situ high pressure X-ray pair distribution function (PDF) measurements for the sample contained within a Paris-Edinburgh cell. The real-space local structural information in the PDF, G(r), was used to evaluate the compressibility of the oil shale. Specifically, the pressure-induced reduction in the medium- to long-range atom distances (∼6-20 Å) yielded an average sample compressibility corresponding to a bulk modulus of ca. 61-67 GPa. A structural model consisting of a three phase mixture of the principal crystalline oil shale components (quartz, albite and Illite) provided a good fit to the ambient pressure PDF data (R ∼ 30.7%). Indeed the features in the PDF beyond ∼6 Å, were similarly well fit by a single phase model of the highest symmetry, highly crystalline quartz component. The factors influencing the observed compression behavior are discussed. Introduction
Experimental Methods
The development of strategies enabling the economic and environmentally acceptable recovery of fossil fuel resources contained within the considerable global oil shale deposits may represent an important step toward our long-term energy security.1,2 In addition to the general need to characterize this resource, it is important to understanding the impact of pressure on the oil shale as an analogue for shockwaves associated with in situ explosive fracturing of the rock expected as part of potential recovery strategies.2 Here we use pair distribution function (PDF) analysis of high energy X-ray scattering data to probe the compression behavior in an oil shale from the Green River formation at pressures up to 2.4 GPa.3,4 The Green River formation, in the western United States, is one of the richest and most extensive oil shale resources worldwide, containing ∼20 wt% organic matter, predominantly as amorphous kerogen solid, integrated in a matrix of silicateand carbonate-based minerals.5,6 The PDF method7,8 is well suited to study composite materials of this type as it provides local structural information independent of long-range order and therefore probes both crystalline mineral and amorphous organic components of the oil shale. The real-space local structural information in the PDF can be fit using a least-squares approach to evaluate and/or optimize the structure and/or composition of various model systems. Moreover, as the PDF is related to the probability of finding an atom at a given radial distance from another atom, peaks observed in the PDF correspond directly to interatomic distances within the sample. By following the pressure-dependence of the peaks within the PDF (i.e., the interatomic distances), it has been possible to probe the compressibility of the oil shale.
A slab of oil shale from the Green River formation was collected from an exposed surface outcrop of the kerogen-rich Mahogany Zone (R7) in the Piceance Creek Basin, Colorado (N 40°04′02.6′′, W 108°14′41.5′′). Visual microscopic analysis suggested that the sample contained predominately silicate minerals possibly due to depletion of the carbonate mineral components typical of Green River oil shale through weathering of the outcrop.5 A cylindrical oil shale core of diameter 3.5 mm, freshly cut perpendicular to the layer plane, was placed in a boron-epoxy gasket with a 3.5 mm diameter sample cavity. Brass plugs were used to fill the ends of the sample cavity which were not in the incident X-ray beam. The gasket/sample was sandwiched between a pair of opposed conical anvils with tungsten carbide inserts, within a VX5 model ParisEdinburgh (PE) pressure cell.3,9 A controlled force was applied to the anvils using a hydraulic fluid pump, generating near-hydrostatic pressures at the sample. Compared to the diamond anvil cell apparatus more commonly used for high pressure X-ray scattering experiments, the PE cell compresses large sample volumes allowing macro-scale samples to be probed while retaining the sample microstructure, for example, the layered structure of the oil shale. The PE cell was mounted on the instrument at beamline 11-ID-B at the Advanced Photon Source, Argonne National Laboratory with the incident (and scattered) beam directed through the gasket Via the gap between the anvils. High-energy X-rays (90.48 keV, λ ) 0.1370 Å) were used in combination with a MAR-345 image plate area detector to record diffraction images to moderately high values of momentum transfer (Qmax ∼ 22 Å-1).10 The 1 × 1 mm X-ray beam probed an approximate sample volume of 1 × 1 × 3.5 mm at ambient pressure. At high pressure, where the reduced gap between the anvils limited the incident and scattered beams, data were accumulated for longer to maintain the signal-to-noise ratio. Data were collected at applied forces corresponding to 0, 100, 200, 300, and 400 bar fluid pressure. The resulting sample pressures were estimated
* To whom correspondence should be addressed. E-mail: chapmank@ aps.anl.gov. † Argonne National Laboratory. ‡ University of Utah.
10.1021/jp803900s CCC: $40.75
2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 27, 2008 9981
Figure 1. r-smoothed PDFs, G(r), for a Green River oil shale upon compression to 2.4 GPa.
to be 0.0, 0.2, 1.0, 1.7, and 2.4 GPa based on the pressureinduced compression measured for separate NaCl sample based on the known third-order Birch-Murnaghan equation of state for NaCl (K0 ) 23.5897 GPa and K0′ ) 4.8206).11 Additional ex situ scattering data were collected for the recovered sample and for a separate uncompressed sample core to Qmax ∼ 27 Å-1. Raw image data were processed using Fit-2D.12,13 The sample-to-detector distance was refined using a CeO2 NIST standard. The PDFs were extracted within PDFgetX2, 14 subtracting the contributions from the sample environment and background to the measured diffraction intensity. Corrections for multiple scattering, X-ray polarization, sample absorption, and Compton scattering were then applied to obtain the structure function S(Q). Direct Fourier transform of the reduced structure function F(Q) ) Q[S(Q) - 1] up to Qmax gave G(r), the PDF. The resolution of the measured PDF is ultimately limited by the finite Q-range (Q ) 4π sin θ/λ) used in the Fourier transform, which is limited in turn by the maximum scattering angle and the wavelength. Here, the intensity data were measured to high values of Qmax to enable an accurate Fourier transform and to minimize aberrations in the PDF such as “termination ripples” which propagate through G(r) as high frequency noise. Additionally, the in situ high pressure PDF data were smoothed by r averaging over the period of the termination ripples (2π/Q) to allow pressure-dependent trends in the PDF to be more readily distinguished. The PDFs of the individual mineral components and composites were calculated within PDFFIT15 using the known structures of quartz (SiO2),16 “low” albite (NaAlSi3O8),17 and Illite (KAl2(AlSi3O10)(OH2)).18 Results and Discussion The PDFs of the oil shale contain distinct peaks to large distance (∼20 Å) as is consistent with the long-range, periodic order of the crystalline mineral components (Figure 1). No pronounced structural transitions were evident upon compression to 2.4 GPa with the general features in the PDF shifting monotonically to lower r, with reduced interatomic distances and sample volume. The most marked changes occurred upon initial compression (0 to 0.2 GPa) with approximately linear changes evident thereafter. Some features of the PDFs, the aggregate of multiple overlapping correlations of similar length, changed shape slightly (i.e., broadened or sharpened) as correlations from different components and crystallographic axes compressed at different rates. Some changes in PDF peak intensities were evident in the recovered sample relative to an uncompressed sample (see Supporting Information), which may
Figure 2. (a) Experimental and calculated PDFs, G(r), for an uncompressed oil shale from the Green River formation with (b) the contributions from quartz, albite and Illite to the calculated PDF.
be associated with variation of the mineral composition between the (most likely) different sample volumes probed or changes in static structural disorder. The pressure-induced sample compression was estimated from the r-scaling factor (i.e., compression rate) required to overlay the general features (6-20 Å) of the high pressure PDF data (0.2-2.4 GPa). The estimated average linear compression rate (βr ) -dr/(r dP)) of ca. 0.005-0.0055 GPa-1 corresponds to a volumetric compressibility (βV ) -dV/(V dP) ) 3βr) of ca. 0.015-0.0165 GPa-1 and a pressure-invariant bulk modulus (K ) 1/βV) of ca. 61-67 GPa for this oil shale. Although different compressibilities would be expected for the different components and along different crystallographic axes within this nonhomogeneous system, the features in the moderate-resolution PDFs, being the aggregate of multiple correlations, inherently reflects an averaged behavior which actually facilitates the estimation of the compressibility using this approach. The low-r region was excluded from this analysis as it has been well established that the behavior of local atomic distances in such framework minerals can differ significantly from that of the long-range average structure with the compression mechanism likely to involve correlated tilting of relatively rigid polyhedral units within the structure.19,20 Accordingly, the observed compression behavior is biased toward the crystalline mineral components of the system which contribute sharp peaks to the PDF in the medium to long length scales. The high resolution PDF (Qmax ) 27 Å-1) measured ex situ for the uncompressed oil shale is shown in Figure 2a. At low r, the PDF is dominated by well defined peaks which are characteristic of tetrahedral network silicates (and aluminosili-
9982 J. Phys. Chem. C, Vol. 112, No. 27, 2008 cates): Within the tetrahedra, dSi-O ∼ 1.6 Å; along the tetrahedral edges, dO · · · O ∼ 2.6 Å; and between connected tetrahedral centers, dSi · · · Si ∼ 3.1 Å. The absence of characteristic peaks at 1.2 (dC-O), 2.1 (dMg-O), and 2.4 Å (dCa-O) found in dolomite21 and calcite22 (the major carbonate minerals in the Green River formation) is consistent with the carbonate-depleted state of the present sample. Well defined peaks associated with the amorphous organic component would be limited to the low-r region, although it is difficult to assign these correlations in the experimental data as they are likely to have low intensity relative to those involving the more strongly scattering Si/Al atoms. The experimental PDF was compared with PDFs calculated for models of the major silicate minerals in Green River oil shale: quartz, albite and Illite (see Figure 2b).5 The features in the PDF at larger distance (r > 6 Å) were remarkably well reproduced by a single phase model of quartz (Rr)6-20Å ) 30%), the highest symmetry component of the oil shale (hexagonal; P3221; a ) 4.9 Å, c ) 5.4 Å). The lower symmetry albite (triclinic; C-1) and Illite (monoclinic; C2/c) minerals had greater distributions of atomic distances at these length scales, with correlations overlapping such that distinct features were not readily distinguished at high-r. However, the sharp features at low-r, in particular the intensity of the peak at 1.6 Å (dSi-O), could only be reasonably fit by including the additional (albite and Illite) silicate minerals, fixing the atomic displacement parameters, peak broadening and dampening parameters based on the values fit using the quartz model (for r ) 6-20 Å). The PDF calculated for a composite model, the sum the individual PDFs calculated for the three phases weighted by the published silicate abundance of 1.21 (quartz): 1.26 (albite): 1 (Illite),5 was in good agreement with the experimental PDF data (Rr)1-20Å ) 30.7%). This model assumes that any contributions to the measured PDF from interparticle (i.e., surface) correlations are small relative to the intraparticle correlations being modeled, that is, with typical particle/domain sizes greater than ∼20 Å. As the medium- to long- range features of the PDF used to evaluate the compression behavior can be largely attributed to the quartz component, to a first approximation, the measured compression rate may be expected to closely approach that of quartz. However, the value of the bulk modulus implied by the measured compression is significantly larger (i.e., a reduced compression) than that known for bulk quartz (Kquartz ∼ 37.12(9) GPa).23 Indeed, the value of the bulk modulus is also larger than a weighted average for the three major crystalline mineral components in the oil shale of ca. 50 GPa (Kalbite ) 54.2(7) GPa;24 KIllite ) 61.4(4) GPa).25 Although nanoscale materials have been known to have increased bulk moduli relative to the bulk material,26 here the mineral grain size is unlikely to be sufficiently fine for this effect to be substantial. Instead, the reduced compression rate of the measured component is likely to be associated with the multicomponent, microstructured nature of the oil shale. For example, a softer component in the composite may compress preferentially, and act as a static shock absorber, such that a smaller force is effectively transmitted to the harder mineral components. Such components could include the amorphous organic material, which has similarities to soft materials like rubbers,27,28 and/or the necessarily more compressible (hydrocarbon-filled) “pores” within the mineral matrix of the oil shale.29 We have demonstrated that the local structural information in the form of a PDF can be used measure the compressibility ab initio in the absence of a structural model. Extensions of this approach can be readily envisioned to measure compressibilities in glasses, nanoscale materials and other noncrystalline systems.
Letters Acknowledgment. Work performed at Argonne and use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The VX5 PE cell was purchased as part of the SNAP project supported by Grant DE-FG02-03ER46085. We acknowledge financial support from Chevron. We thank Darren R. Locke for the microscopic analysis and useful discussions and Marcus Wigand and Sean Norris for collecting the sample. Supporting Information Available: Details of the high pressure experiment and PDF data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Rogner, H. H. Annu. ReV. Energy EnV. 1997, 22, 217–262. (2) Andrews, A. Oil Shale: History, IncentiVes, and Policy; Congress Research Service, The Library of Congress: Washington, DC, 2006, RL33359. (3) Mezouar, M.; Le Bihan, T.; Libotte, H.; Le Godec, Y.; Hausermann, D. J. Synchrotron Radiat. 1999, 6, 1115–1119. (4) Chapman, K. W.; Chupas, P. J.; Kurtz, C.; Locke, D. R.; Parise, J. B.; Hriljac, J. A. J. Appl. Crystallogr. 2007, 40, 196–198. (5) Brons, G.; Siskin, M.; Botto, R. I.; Guven, N. Energy Fuels 1989, 3, 85–88. (6) Siskin, M.; Katritzky, A. R. Science (Washington, DC) 1991, 254, 231–237. (7) Egami, T.; Billinge, S. J. L., Underneath the Bragg Peaks: Structure Analysis of Complex Materials; Oxford/Pergamon Press: New York, 2004. (8) Nield, V.; Keen, D. A., Diffuse Neutron Scattering from Crystalline Materials; Oxford/Clarendon Press: Oxford, 2001. (9) Klotz, S.; Hamel, G.; Frelat, J. High Pressure Res. 2004, 24, 219– 223. (10) Chupas, P. J.; Qiu, X.; Hanson, J. C.; Lee, P. L.; Grey, C. P.; Billinge, S. J. L. J. Appl. Crystallogr. 2003, 36, 1342–1347. (11) Decker, D. L. J. Appl. Phys. 1971, 42, 3239–3244. (12) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Ha¨usermann, D. High Pressure Res. 1996, 14, 235–248. (13) Hammersley, A. P. ESRF Internal Report, 1997, ESRF97HA02T. (14) Qiu, X.; Thompson, J. W.; Billinge, S. J. L. J. Appl. Crystallogr. 2004, 37, 678. (15) Proffen, T.; Billinge, S. J. L. J. Appl. Crystallogr. 1999, 32, 572– 575. (16) Prewitt, C. T.; Sueno, S.; Papike, J. J. Am. Mineral. 1976, 61, 1213– 1225. (17) Young, R. A.; Mackie, P. E.; Vondreele, R. B. J. Appl. Crystallogr. 1977, 10, 262–269. (18) Guggenheim, S.; Chang, Y. H.; Vangroos, A. F. K. Am. Mineral. 1987, 72, 537–550. (19) Dove, T. M. Am. Mineral. 1997, 82, 213–244. (20) Chapman, K. W.; Chupas, P. J.; Kepert, C. J. J. Am. Chem. Soc. 2005, 127, 15630–15636. (21) Steinfink, H.; Sans, F. J. Am. Mineral. 1959, 44, 679–682. (22) Graf, D. L. Am. Mineral. 1961, 46, 1283–1316. (23) Levien, L.; Prewitt, C. T.; Weidner, D. J. Am. Mineral. 1980, 65, 920–930. (24) Downs, R. T.; Hazen, R. M.; Finger, L. W. Am. Mineral. 1994, 79, 1042–1052. (25) Faust, J.; Knittle, E. J. Geophys. Res.-Sol. Ea. 1994, 99, 19785– 19792. (26) Tolbert, S. H.; Alivisatos, A. P. Science (Washington, DC) 1994, 265, 373–376. (27) Saxby, J. D. Fuel 1981, 60, 994–996. (28) Kerogens are known to exhibit glass-rubber transitions (see Parks, T. J.; Lynch, L. J.; Webster, D. S.; Barrett, D. Energy Fuels 1988, 2, 185– 190. ). Although such transitions have been found to occur at temperatures above that of the present study, the processes used to isolate the kerogen from the mineral matrix in those studies also eliminates the smaller organic molecules. Consequently, the glass transition temperature for the organic material within the raw oil shale is likely to be systematically lower than for the isolated kerogen and the present sample is likely to contain a greater fraction of rubber-like organic component than found in isolated kerogen. (29) Mavko, G.; Mukerji, T. Geophysics 1995, 60, 1743–1749.
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