Medina, JC
1998
Mineral-Catalyzed Formation of Natural Gas During Coal Maturation
Bartholomew, C.H.; Butala, T.Q.; Medina, J.C.; Lee, M.L.; Taylor, S.J. and Andrus, D.B.
Proceedings of the International Conference on Coal Seam Gas and Oil, Brisbane, Australia, March 23-25, 1998.
Coal seam reservoirs are important worldwide commercial sources of natural gas. It is commonly assumed that hydrocarbon gases are formed in coal seams by thermolysis (cracking) of coal organic matter. Recently, however, the reliability of this geologic process model has been questioned. In fact, results of artificial maturation experiments indicate that raw (mineral-containing) coal generates hydrocarbon gas at substantially higher rates than demineralized coal. This difference suggests that mineral catalysis could be a critical variable affecting hydrocarbon gas formation during coal maturation.
The objective of our combined literature and experimental study is to evaluate potential roles of minerals in catalyzing coal-bed methane formation. In the first phase of this study, rate and product selectivity data for hydrocarbon thermolysis and mineral-catalyzed cracking or synthesis reactions were compiled in a comprehensive review of technical literature sources. Kinetic models were used to predict conversion rates and product yields at typical low-temperature conditions of coal maturation. It was found that under these conditions hydrocarbon thermolysis reactions would be too slow to generate, even over geologic times, large, self-sourced coal seam natural gas deposits. By contrast, acid-mineral- and transition-metal-catalyzed reactions would occur at sufficiently high rates in geologic time and at geologic conditions to generate large quantities of natural gas, although the product distribution over acid-mineral catalysis is very different than for natural gas. Two potentially viable catalytic routes involving naturally occurring transition metal species and capable of forming large natural gas deposits within hours to several years are: (1) hydrogenolysis of alkanes and/or alkenes over iron and nickel and (2) CO2 methanation on iron and nickel. Selectivities of these catalysts in both reactions for methane are high, and the product distributions are similar to those of natural gases. We were also able to identify several geologically viable catalytic and noncatalytic routes for production of H2, a reactant typically found in coal gas and important in the catalytic production of methane from hydrocarbons or CO2.
In the second phase of this work, potential methane-forming reactions were conducted for 100-hour periods at 180°C in 1 atm of H2 in the presence or absence of reduced or unreduced iron-silica catalysts. Carbon dioxide and 1-dodecene, both found in coal beds, were utilized as model substrates. A computer-automated batch reactor system with Pyrex reactor, glass stirrer, and on-line GC analysis was used to measure reactant and product concentrations as a function of time.
Significant rates of methane formation are observed in both reactions in the presence of the prereduced catalyst after just a few hours. However, induction time and methane yield vary with substrate. In carbon dioxide methanation, the induction time is 1 h compared to 17 h for olefin hydrogenolysis, and the rate of methane production is an order of magnitude higher in CO2 methanation relative to olefin hydrogenolysis (262 and 32 mmol gFe^-1 d^-1 respectively). The latter rate compares favorably with data reported for C8 olefin hydrogenolysis. Production rates of light alkanes other than methane (i.e., ethane, propane, and butane) are also significant, although an order-of magnitude lower than for methane; thus the product distributions are characteristics of natural gas. On the other hand, no products are observed over 100 h for either reaction if no catalyst or the unreduced catalysts (Fe2O3/silica) is present.
These data suggest that natural gas may be formed in coal seams by either CO2 methanation or liquid hydrocarbon hydrogenolysis on reduced iron minerals present in the coal. An important implication of our analysis is that iron-mineral catalysis rather than homogeneous thermolysis leads to natural gas formation during coal maturation. This, in turn, suggests using coal minerals rather than currently used coal thermal maturity parameters for gas resource assessment and exploration.
1997
Catalytic Effects of Mineral Matter on Natural Gas Formation During Coal Maturation
Butala, S.; Medina, J.C.; Bowerbank, C.R.; Lee, M.L.; Felt, S.A.; Taylor, T.Q.; Andrus, D.B.; Bartholomew, C.H.; Yin, P. and Surdam, R.C.
Gas Research Institute, GRI-97/0213, July 1997. Funded in part by ACERC.
Coal seam reservoirs are important commercial sources of natural gas in the U.S. It is commonly assumed that coals function as self-sourced reservoirs for hydrocarbon gases formed by temperature-controlled thermolysis (cracking) of the bulk coal organic matter. However, this geologic process model may be an unreliable exploration guide. Artificial maturation results indicate that raw coal generates more hydrocarbon gas than demineralized coal. This difference suggests that mineral catalysis merits evaluation as a critical variable affecting hydrocarbon gas formation during coal maturation.
Kinetic modeling of temperature-controlled hydrocarbon thermolysis reactions using coal maturation geologic times and temperatures indicate that thermolysis reaction rates would be too slow to generate large, self-sourced coal seam natural gas deposits. By contrast, acid mineral, transition metal, and metal oxide mineral catalyzed reactions would occur at rates sufficiently fast under geologic time and temperature conditions to generate large quantities of natural gas. The unavailability of suitable benchmark coal reactivity data preclude assessment of whether catalytic reactions actually control hydrocarbon gas formation during coal maturation.
Medina, JC
1998
Mineral-Catalyzed Formation of Natural Gas During Coal Maturation
Bartholomew, C.H.; Butala, T.Q.; Medina, J.C.; Lee, M.L.; Taylor, S.J. and Andrus, D.B.
Proceedings of the International Conference on Coal Seam Gas and Oil, Brisbane, Australia, March 23-25, 1998.
Coal seam reservoirs are important worldwide commercial sources of natural gas. It is commonly assumed that hydrocarbon gases are formed in coal seams by thermolysis (cracking) of coal organic matter. Recently, however, the reliability of this geologic process model has been questioned. In fact, results of artificial maturation experiments indicate that raw (mineral-containing) coal generates hydrocarbon gas at substantially higher rates than demineralized coal. This difference suggests that mineral catalysis could be a critical variable affecting hydrocarbon gas formation during coal maturation.
The objective of our combined literature and experimental study is to evaluate potential roles of minerals in catalyzing coal-bed methane formation. In the first phase of this study, rate and product selectivity data for hydrocarbon thermolysis and mineral-catalyzed cracking or synthesis reactions were compiled in a comprehensive review of technical literature sources. Kinetic models were used to predict conversion rates and product yields at typical low-temperature conditions of coal maturation. It was found that under these conditions hydrocarbon thermolysis reactions would be too slow to generate, even over geologic times, large, self-sourced coal seam natural gas deposits. By contrast, acid-mineral- and transition-metal-catalyzed reactions would occur at sufficiently high rates in geologic time and at geologic conditions to generate large quantities of natural gas, although the product distribution over acid-mineral catalysis is very different than for natural gas. Two potentially viable catalytic routes involving naturally occurring transition metal species and capable of forming large natural gas deposits within hours to several years are: (1) hydrogenolysis of alkanes and/or alkenes over iron and nickel and (2) CO2 methanation on iron and nickel. Selectivities of these catalysts in both reactions for methane are high, and the product distributions are similar to those of natural gases. We were also able to identify several geologically viable catalytic and noncatalytic routes for production of H2, a reactant typically found in coal gas and important in the catalytic production of methane from hydrocarbons or CO2.
In the second phase of this work, potential methane-forming reactions were conducted for 100-hour periods at 180°C in 1 atm of H2 in the presence or absence of reduced or unreduced iron-silica catalysts. Carbon dioxide and 1-dodecene, both found in coal beds, were utilized as model substrates. A computer-automated batch reactor system with Pyrex reactor, glass stirrer, and on-line GC analysis was used to measure reactant and product concentrations as a function of time.
Significant rates of methane formation are observed in both reactions in the presence of the prereduced catalyst after just a few hours. However, induction time and methane yield vary with substrate. In carbon dioxide methanation, the induction time is 1 h compared to 17 h for olefin hydrogenolysis, and the rate of methane production is an order of magnitude higher in CO2 methanation relative to olefin hydrogenolysis (262 and 32 mmol gFe^-1 d^-1 respectively). The latter rate compares favorably with data reported for C8 olefin hydrogenolysis. Production rates of light alkanes other than methane (i.e., ethane, propane, and butane) are also significant, although an order-of magnitude lower than for methane; thus the product distributions are characteristics of natural gas. On the other hand, no products are observed over 100 h for either reaction if no catalyst or the unreduced catalysts (Fe2O3/silica) is present.
These data suggest that natural gas may be formed in coal seams by either CO2 methanation or liquid hydrocarbon hydrogenolysis on reduced iron minerals present in the coal. An important implication of our analysis is that iron-mineral catalysis rather than homogeneous thermolysis leads to natural gas formation during coal maturation. This, in turn, suggests using coal minerals rather than currently used coal thermal maturity parameters for gas resource assessment and exploration.
1997
Catalytic Effects of Mineral Matter on Natural Gas Formation During Coal Maturation
Butala, S.; Medina, J.C.; Bowerbank, C.R.; Lee, M.L.; Felt, S.A.; Taylor, T.Q.; Andrus, D.B.; Bartholomew, C.H.; Yin, P. and Surdam, R.C.
Gas Research Institute, GRI-97/0213, July 1997. Funded in part by ACERC.
Coal seam reservoirs are important commercial sources of natural gas in the U.S. It is commonly assumed that coals function as self-sourced reservoirs for hydrocarbon gases formed by temperature-controlled thermolysis (cracking) of the bulk coal organic matter. However, this geologic process model may be an unreliable exploration guide. Artificial maturation results indicate that raw coal generates more hydrocarbon gas than demineralized coal. This difference suggests that mineral catalysis merits evaluation as a critical variable affecting hydrocarbon gas formation during coal maturation.
Kinetic modeling of temperature-controlled hydrocarbon thermolysis reactions using coal maturation geologic times and temperatures indicate that thermolysis reaction rates would be too slow to generate large, self-sourced coal seam natural gas deposits. By contrast, acid mineral, transition metal, and metal oxide mineral catalyzed reactions would occur at rates sufficiently fast under geologic time and temperature conditions to generate large quantities of natural gas. The unavailability of suitable benchmark coal reactivity data preclude assessment of whether catalytic reactions actually control hydrocarbon gas formation during coal maturation.