ct = cf + Swcw + Sgcg + cs (2.29) where
ct = total compressibility, psia–1, cf = cleat compressibility, psia–1, Sw = water saturation,
cg = gas compressibility, psia–1, Sg = gas saturation, and
cs = sorption compressibility, psia–1.
Coal cleat compressibility is important if no free gas is present, such as an undersaturated coal above the desorption pressure. Total compressibility of the system is now
ct = cf + cw ≈ cf
Tpc Pseudocritical temperature, °R or K
Tpc΄ Pseudocritical temperature adjusted for CO2 and H2S, °R
Tpc΄΄ Adjusted pseudocritical temperature corrected for N2 and H2O, °R Tpr Pseudoreduced temperature
Tr Reduced temperature
tD Dimensionless time, equation (2.2)
V Desorbed gas volume, cm3, or gas content, scf/t or cm3/g, or volume, ft3 or m3 Vi Volume fraction inertinite
Vl Volume fraction liptinite
VLdaf Dry, ash-free Langmuir volume constant, scf/t or cm3/g Vp Cleat volume, ft3 or cm3
Vt Total desorbed gas volume, cm3 Vv Volume fraction vitrinite
w Equilibrium moisture fraction, wt % yj Mole fraction of component
Z Gas deviation factor or compressibility factor Greek
γg Gas gravity
γh Hydrocarbon gas gravity
ξ Constant in Wichert and Aziz correlation ρ Coal bulk density, g/cm3
ρa Ash density, g/cm3 ρi Inertinite density, g/cm3 ρl Liptinite density, g/cm3
ρo Organic fraction density, g/cm3 ρr Pseudoreduced density
ρv Vitrinite density, g/cm3 ρw Water density, g/cm3 ϕ Porosity, fraction
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39. Gas Research Institute. 1996. A Guide to Coalbed Methane Reservoir Engineering. GRI-94/0397. Chicago: Gas Research Institute.
40. Bustin, R. M. 1997. Importance of fabric and composition on the stress sensitivity of permeability in some coals, Northern Sydney Basin, Australia: Relevance to coalbed methane exploitation. AAPG Bulletin. V. 81 (no. 11). p. 1,894.
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43. Ibid.
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45. Laubach, S. E., et al. 1998.
46. Ibid.
47. Close, J. C. 1993.
48. Laubach, S. E., et al. 1998.
49. Ibid.
50. Ibid.
51. Close, J. C. 1993; and Laubach, S. E., et al. 1998.
52. Reiss, L. H. 1980. The Reservoir Engineering Aspects of Fractured Formations. Paris: Gulf Publishing Company.
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54. Reiss, L. H. 1980.
55. Ibid.
56. Gan, H., et al. 1972.
57. Ibid.
58. Ibid.
59. Ibid.
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63. Ibid.
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3
Introduction
Coal gas exploitation is difficult, if not impossible, without understanding the geology of a given deposit.
Geology strongly influences the nature of a coal deposit, the gas resource it may hold, and its producibility. Coals have been mined for centuries and studied extensively in recent decades. Excellent studies covering different aspects of coal geology abound, such as those of Stach et al. and Galloway and Hobday.1 However, since coals are mined, these studies have historically focused on shallow, high-purity seams that could be accessed with mining technology available at the time. Realization of coals as an unconventional gas resource has led to consideration of a much larger population of carbonaceous rocks from a much different perspective. Coal mines develop a deep understanding of at most a few seams on length scales much smaller than typically encountered in