Article pubs.acs.org/EF
Porosity of Coal and Shale: Insights from Gas Adsorption and SANS/ USANS Techniques Maria Mastalerz,*,† Lilin He,‡ Yuri B. Melnichenko,‡ and John A. Rupp† †
Indiana Geological Survey, Indiana University, Bloomington, Indiana 47405-2208, United States Biology and Soft Matter Division, Neutron Scattering Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393, United States
‡
ABSTRACT: Two Pennsylvanian coal samples (Spr326 and Spr879-IN1) and two Upper Devonian-Mississippian shale samples (MM1 and MM3) from the Illinois Basin were studied with regard to their porosity and pore accessibility. Shale samples are early mature stage as indicated by vitrinite reflectance (Ro) values of 0.55% for MM1 and 0.62% for MM3. The coal samples studied are of comparable maturity to the shale samples, having vitrinite reflectance of 0.52% (Spr326) and 0.62% (Spr879-IN1). Gas (N2 and CO2) adsorption and small-angle and ultrasmall-angle neutron scattering techniques (SANS/USANS) were used to understand differences in the porosity characteristics of the samples. The results demonstrate that there is a major difference in mesopore (2−50 nm) size distribution between the coal and shale samples, while there was a close similarity in micropore ( 0.04 Å−1, which corresponds to pores smaller than 63 Å) increases upon fluid injection. This effect is especially pronounced for Spr879-IN1 (see inset in Figure 9) and is related to strong condensation and densification of CD4 in small pores.35 The fact that scattering curves at vacuum and ZAC in the limit of high Q are almost parallel for Q > 0.04 Å−1 indicates that pores smaller than 63 Å−1 in Spr879-IN1 are approximately equally accessible to CD4 molecules; however, absolute evaluation of the accessibility cannot be performed based on the available data because of the unknown extent of the pore filling with condensed fluid.. Using analysis procedures described in Melnichenko et al.23 the fraction of porosity accessible to CD4 (ϕAC) in coals was
sin(Qr ) dr Qr (2)
where γ0 is the normalized correlation function of the SLD fluctuations; (ρs* − ρf*)2 is the neutron contrast between the SLD of the solid matrix (ρ*s ) and the SLD of the fluid in the pores (ρ*f ); c is the volume fraction of pores in the sample (total porosity); V is the volume of sample illuminated by the neutron beam; and Q = 4πλ−1 sin θ in which 2θ is the scattering angle. As can be seen in eq 2, the intensity varies with the contrast in SLD between the matrix and the fluid in the pores. At low pressures (P < PZAC), ρs* ≫ ρf* and the intensity decreases as pressure increases. If the pores are all accessible to 5115
dx.doi.org/10.1021/ef300735t | Energy Fuels 2012, 26, 5109−5120
Energy & Fuels
Article
Figure 8. Scattering intensity ∑(Q) (number of neutrons per 100 s) from Spr879-IN1 coal at a fixed value of Q = 3.9 × 10−5 Å−1 (pore size ∼65 000 Å) as a function of pressure. The minimum (zero average contrast condition) is observed around P = 274.2 bar, close to the precalculated PZAC ∼ 265 bar (see arrow).
Figure 10. Scattering from Spr326 coal in the USANS and SANS domains under vacuum and at P = 274.2 bar ≈ PZAC. The inset shows a blowup of the SANS as a function of pressure.
Figure 9. Scattering from Spr879-IN1 coal in the USANS and SANS domains under vacuum and at P = 274.2 bar ≈ PZAC. The inset shows a blowup of the SANS as a function of CD4 pressure.
Figure 11. Variation of the volume fraction of pores accessible to CD4 as a function of Q and pore sizes in the coals studied.
accessible porosity explains why both coals having relatively similar total porosity (9.8% in Spr879-IN1, 11.3% in Spr326, Table 2) adsorb significantly different amounts of CO2 at high pressures. A separate experiment was performed to monitor possible changes in CO2 adsorption with temperature. The scattering from Spr879-IN1 coal saturated with both sub- and supercritical CO2 is independent of temperature in the SANS domain corresponding to scattering from small pores (Figure 13). These data demonstrate that the temperature of a coal seam has a lesser effect on adsorption compared to the poresize distribution, accessibility, and pressure. SANS from Shale Samples. SANS scattering intensities of MM1 and MM3 with CD4 and CO2 as a function of pressure are shown in Figures 14−17. We note that the data on the variations of USANS with CD4 and CO2 were not acquired
calculated as a function of Q/pore size in Figure 11. The fraction of accessible porosity is similar in both coals in the micrometer size range. It decreases gradually in smaller pores and reaches a minimum for Spr879-IN1 (ϕAC ∼ 0.3) for pore sizes around 700 Å and increases to ∼0.7 for the majority of pores with sizes 50−130 Å corresponding to the maximum in the pore-size distribution (5−13 nm, Figure 2). The fraction of accessible pores of this range in Spr326 is only ∼0.2. The accessibility of smaller pores further decreases for Spr326. Thus, the accessibility of pores having widths of 100 Å differs by almost an order of magnitude between both coals (∼0.7 in Spr879-IN1 and ∼0.07 in Spr326). Much higher accessible porosity in the pore-size range of 50−130 Å may be responsible for the higher adsorption capacity of Spr879-IN1 coal compared to that of Spr326 (Figure 12). This difference in 5116
dx.doi.org/10.1021/ef300735t | Energy Fuels 2012, 26, 5109−5120
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Figure 12. High-pressure CO2 adsorption isotherms for the coals and shales studied. Note significantly higher adsorption capacity for coals (Spr326 and Spr879-IN1) than for shales (MM1 and MM3) and also higher adsorption capacity for deeper coal Spr879-IN1.
Figure 14. SANS from shale sample MM1 with CD4 (21 °C) as a function of pressure/density.
Figure 13. I(Q) from Spr879-IN1 coal as a function of temperature. This figure shows that the scattering does not change within the temperature range between 25 °C (subcritical CO2) and 60 °C (supercritical CO2), which indicates that the adsorption in small pores is temperature independent in this range of temperatures.
Figure 15. SANS from shale sample MM1 saturated with subcritical CO2 (21 °C) as a function of pressure/density. Abrupt decrease in scattering at elevated pressures is due to condensation of CO2 in pores (gas−liquid phase transition).
because of the lack of beam time. Also for SANS, the highest pressure of CD4 is much lower than the one corresponding to the calculated value, PZAC = 624 ± 80 bar (MM1) and 496 ± 70 bar (MM3). Because of these two factors, reliable determinations of inaccessible porosity of these samples are not possible. Figures 14−17 demonstrate, however, that scattering from both shales vary little with CD4 and CO2 pressure, suggesting very low penetrability of their matrices to the fluids, consistent with much lower than for coals adsorption capacity (Figure 12). The abrupt decrease of scattering at pressures above 40 bar is due to the condensation of CO2 in pores, which occurs for bulk fluid at P = 64.36 bar at T = 25 °C. For shale MM1, SANS data (Figure 18) show that accessibility to CD4 in the range of pore sizes from 25 to 40 Å is approximately 30%. For other pore ranges, accessibility of pores cannot be determined at this point. Implications of Pore Accessibility in Coals and Shales. Recent studies have demonstrated that there is size-specific
accessibility of pores in coals of varying ranks.35,36 This study shows that accessibility of pores can vary significantly even within the same coal bed (Figure 11). The Springfield Coal Member samples studied here are of similar rank (high-volatile C bituminous for Spr326 and high-volatile B bituminous for Spr879-IN1), and they both are vitrinite-rich coals (Table 1). The main difference between them is depth (Spr879-IN is 2.5 times deeper). Except for the largest pores (>1 μm) that have almost total accessibility for both coals, accessibility of pores smaller than 1 μm is greater for the deeper coal. Accessibility of pores in the micropore and mesopore range (smaller than 500 Å) dramatically increases for Spr879-IN1 (deeper coal) compared to the shallower coal Spr326 (Figure 11). Higher accessibility of pores indicates improved interconnectivity of pores; therefore, one would expect increased permeability with 5117
dx.doi.org/10.1021/ef300735t | Energy Fuels 2012, 26, 5109−5120
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Article
Figure 16. SANS from shale sample MM3 saturated with CD4 (36.7 °C) as a function of pressure/density.
Figure 18. Volume fraction of pores in the size range 25−40 Å accessible to CD4 in MM1 shale sample.
reported here for the coals, could not be accomplished. SANS analysis alone gives a suggestion that pore accessibility within the pore size of 25−40 Å for sample MM1 is approximately 30%, which is significantly less than Spr879-IN1 in this size region, but still measurably higher than Spr326 (Figure 11). Detailed analysis of other pore-size regions is necessary to further understand size-specific pore accessibility and its relationship to shale permeability. It has been already suggested that the percentage of open porosity may be an important factor controlling permeability in shales.37
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CONCLUSIONS 1. Mesopore (2−50 nm) size distribution varies greatly between the coal and the shale samples. For the coals, the dominant size is between 6 and 13 nm, with a volumetric maximum at 9 nm for Spr326 and at 7 nm for Spr879-IN1. In contrast, the shale samples do not have a distinct suite of pores forming the maximum within the mesopore-size range of 6−13 nm. Instead, the pore volumes gradually increase in the size range of 2 to 50 nm. Pores sizes of 6−13 nm are characteristic of vitrinite, and the lack of an abundant pore population within the 6−13 nm size in shales results from their very low vitrinite contents. 2. In contrast to the differing mesopore-size distribution between the coal and the shale samples, the micropore (
