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Modeling Early Methane Generation in Coal Y. Tang,*,† P. D. Jenden,† A. Nigrini,‡ and S. C. Teerman† Chevron Petroleum Technology Company, La Habra, California 90631, and TerraSpec Associates, La Habra Heights, California 90631 Received July 31, 1995. Revised Manuscript Received December 6, 1995X
Thermogenic methane yields can be estimated indirectly from the average elemental composition of coals of different rank or inferred from the results of coal pyrolysis experiments. Unfortunately, most published studies have been insufficiently detailed to estimate gas contents in lignite, subbituminous coal and high-volatile B bituminous coal. In addition, we note that the theoretical coalbed methane generation curves of Juntgen and Karweil and other commonly quoted papers overestimate methane yield because they do not consider hydrogen loss from coal in the form of water. In order to place better constraints on the economic potential of methane in lowrank coals, anhydrous, sealed-tube pyrolysis experiments were carried out on a Paleocene lignite from North Dakota. Experiments were conducted at heating rates of 10 °C/h and 10 °C/day between temperatures of 100 and 454 °C. With increasing final pyrolysis temperature, mean random huminite/vitrinite reflectance values increased from 0.31 to 1.61%, atomic H/C values of the extracted coal decreased from 0.88 to less than 0.56, and methane yields increased to a maximum of 46 mL/g initial lignite, or approximately 1560 cf/ton (dry, ash-free basis) (cf ) cubic feet). Based on these results, coalification to high-volatile A bituminous rank or higher (Ro g 0.8%, atomic H/C e 0.72, and NAI [log(n-C16/n-C30)] g 0.03) appears required to achieve a modest in situ economic threshold of 300 cf/ton methane. Pyrolysis yields were used to model early methane generation with a series of parallel, first-order reactions with activation energies between 41 and 54 kcal/mol and a single frequency factor of 9.88 × 1011 s-1. Extrapolation of these parameters and a modified version of the EASY%Ro vitrinite reflectance model to geologic heating rates suggests that T > 120 °C and Ro g 0.9% are required to exceed the 300 cf/ton threshold. We conclude that while methane concentrations greater than 300 cf/ton may be found in highvolatile B bituminous and lower rank coals, in most cases they must be attributed to migrated gas or to near-surface (e3000 ft) microbial activity.
Introduction An important constraint governing the success of coalbed methane exploration efforts in the U.S. Rocky Mountains is the amount of thermogenic methane which has been generated by low-rank coals. Current Rocky Mountain coalbed methane production is primarily from high-volatile A bituminous and higher rank seams of Cretaceous age5 (Figure 1). Tertiary seams are generally shallower than Cretaceous seams and have received comparatively little attention, although they reach thicknesses up to 200 ft in the Powder River basin.6 Most Tertiary seams are composed of lignite and subbituminous coal6,7 and few gas content data have been published. * To whom correspondence should be addressed. † Chevron Petroleum Technology Co. ‡ TerraSpec Associates. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Karweil, J. In Advances in Organic Geochemistry; Schenk, P. A., Havenaar, I., Eds; Pergamon Press: New York, 1969; pp 59-84. (2) Hunt, J. M. Petroleum Geochemistry and Geology; W. H. Freeman: San Francisco, 1979; 617p. (3) Meissner, F. F. In Hydrocarbon Source Rocks of the Greater Rocky Mountain Region; Woodard, J., Meissner, F. F., Clayton, J. L. Eds.; Rocky Mountain Association of Geologists, 1984; pp 401-431. (4) Sweeney, J. J.; Burnham, A. K. Am. Assoc. Pet. Geol. Bull. 1990, 74, 1559-1570. (5) Rightmire, C. T. In Coalbed methane resources of the United States; Rightmire, C. T., Eddy, G. E., Kirr, J. N. Eds.; American Association of Petroleum Geologists, 1984; pp 1-13. (6) Wood, G. H. Jr.; Bour, W. V. III. Coal Map of North America, 1:5,000,000. U.S. Geological Survey. 1988.
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In order to assess the economic potential of methane in low-rank Tertiary coal seams, studies were undertaken to (1) evaluate indirect estimates of methane generation based on average elemental analysis data for coals of different rank, (2) conduct nonisothermal sealed-tube pyrolysis experiments to obtain laboratory relationships between gas yield and rank, and (3) use chemical kinetic models developed from these experiments to estimate rates of methane generation at geologic times and temperatures. Preliminary results were reported earlier.8 The following account is a detailed treatment of our findings and includes modeling results for methane generation from Upper Cretaceous Cameo coal at Chevron’s Skinner Ridge prospect in the Piceance basin of western Colorado. It is important to point out that our work specifically addresses the occurrence of indigenous thermogenic methane, or methane generated within the coal seam of interest due to thermochemical reactions largely occurring at temperatures in excess of 50 °C. We do not address potential alternative sources of methane related to indigenous bacterial activity9,29 or to the (7) Lyons, P. C.; Rice, C. L. Geol. Soc. Am. Spec. Pap. 1986, 210, 200. (8) Tang, Y.; Jenden, P. D.; Teerman, S. C. Organic Geochemistry, Advances and Applications in the Natural Environment; Manning D. A. C., Ed.; Manchester University Press: Manchester, U.K., 1991; pp 329-331. (9) Rice, D. D. Hydrocarbons from coal; Law, B. W., Rice, D. D., Eds.; Studies in Geology No. 38, American Association of Petroleum Geologists, 1993; pp 159-184.
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Figure 1. Coal rank classification based on heating value and volatile matter content34 showing estimated vitrinite reflectance values3 and the occurrence of hydrocarbon gases and liquids.
sorption of migrated gas. The significance of these alternative sources is debatable and may best be appreciated by screening published gas content data to identify gas-rich coal seams whose maturities fall below the threshold values for thermogenic gas formation reported here. In the opposite situation, when low gas contents are encountered in mature coal seams, one can infer that expulsion has occurred. Gas expulsion is an inevitable consequence of coalification to mediumvolatile bituminous rank3 but may also occur in lower rank coals in response to regional uplift and erosion of overburden. Indirect Methods of Estimating Methane Generation Karweil,1 Hunt,2 and Meissner3 have published theoretical curves for methane generation versus coal rank which are widely cited in the literature. Their curves can all be traced back to the original work of Juntgen and Karweil10,11 which suggested an onset for thermogenic methane generation between 40 and 35% volatile matter (0.6-0.9% Ro) and an ultimate yield of 6-7 Mcf methane per short ton of coal. [The short ton, commonly used in reporting coal gas contents, is equal to 2000 lb or 907.18 kg. 1 Mcf ) 103 standard cubic feet (60 °F and 1 atm).] In their 1966 paper, Juntgen and Karweil11 offered two indirect methods to calculate methane from the variation in elemental composition of coals of different rank. The simplest is based on two assumptions: (1) methane and carbon dioxide are the only significant coalification products for coals with less than 40% volatile matter, and (2) methane yield, carbon dioxide (10) Juntgen, H.; Karweil, J. Brennst.-Chem. Inkohl. Steinkohlenberg., Brennst.-Chem. 1964, 45, 15-18. (11) Juntgen, H.; Karweil, J. Erdol Kohle, Erdgas, Petrochem. 1966, 19, 251-258.
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yield, and coal weight loss can be calculated from massbalance equations expressing the abundance of carbon, hydrogen, and oxygen in “average” coals of different rank. Because this method of calculation assumes that hydrogen in coal is not released as water, the results indicate the maximum amount of methane which can be formed (in excess of 6 Mcf/ton; Appendix 1). The second method offered by Juntgen and Karweil11 allows for the loss of hydrogen as water by assuming that the weight loss during coalification can be independently calculated from laboratory measurements of volatile matter. This is a very poor approximation and, in many cases, produces unrealistically high methane yields (Appendix 2). By injudicious application of this method, Meissner3 (his Figure 6) calculated a methane yield through anthracite rank of more than 9 Mcf/ton, a figure in error by at least 35%. Models using a different approach to estimate methane generation have been proposed by Higgs12 and Burnham and Sweeney.13 Higgs exhaustively pyrolyzed U.S. Carboniferous and German Tertiary coals of subbituminous to anthracite rank (Ro ) 0.4-3%) and estimated the rate of methane generation from the measured differences in their hydrocarbon generation potential Higgs’s results suggest that thermogenic methane formation may begin in subbituminous coals at vitrinite reflectance values between 0.4 and 0.5% Ro. Burnham and Sweeney,13 in contrast, developed a kinetic model for changes in vitrinite composition and used material balance calculations similar to those of Juntgen and Karweil, but with different constraints, to calculate product yields. They then related product yields to vitrinite reflectance through correlations with vitrinite composition. Their results indicate that the onset for thermogenic methane formation occurs in high-volatile bituminous coals between 0.6 and 0.7% Ro. In Figure 2, the curves of Higgs12 and Burnham and Sweeney13 are compared to maximum methane generation curves recalculated from elemental compositions reported by Mott14,15 and Juntgen and Karweil.11 For illustrative purposes, we have taken 300 cf/ton as an economic threshold for prospects with net coalbed thicknesses of a few tens of feet at average depths >2000 ft. Given such a constraint, Figure 2 suggests that subbituminous coals and lignites (Ro < 0.5%) are likely to be uneconomic whereas medium-volatile bituminuous and higher rank coal (Ro > 1.1%) are likely to have commercial potential. This finding compares favorably with the generalized empirical relationships between coal rank, seam depth, and maximum gas content reported by Eddy and Rightmire.16 The work of Eddy and Rightmire16 suggests that at moderate depths (e.g.
