Relationship of Methane Content of Coal Rank and Depth: Theoretical vs. Observed

Abstract

Prediction of gas content of coalbeds, and therefore potential producibility, has relied primarily on its observed relationship to coal rank, pressure, and the methane adsorption capacity of a given coal. This relationship has been illustrated by the construction of adsorption isotherms for various coals from experimental data. These adsorption isotherms were developed for sized, clean coal particles. The value and limitations of these adsorption isotherms are addressed by relating them to observed coalbed methane gas content data.Coalbed gas content determined by the "Direct Method" includes three separate figures: lost gas, desorbed gas, and residual gas. The residual gas and its relationship to the total gas content is discussed. The ratio of lost and desorbed gas to residual gas has been determined for some ranks of coal.The ratio is very dependent on rank. Residual gas increases as rank increases, reaches a maximum at the rank of high-volatile A bituminous and decreases rapidly as rank increases to medium-volatile.The general conclusion is made that the cause of the pattern of correlation of residual gas to rank is a change in the internal structure of the coal and the moisture content of the coal.

Controls on Methane Adsorption Capacity of Lower Cretaceous Coals from Northeastern British Columbia, Canada: Part 1—Geology, Rank Variation, and Adsorption Isotherms

“…This is not surprising, and is in agreement with the high rank of the Gething coals. However, the adsorption capacities are slightly higher than those obtained based on the Eddy et al (1982) curves and those estimated using the Ryan (1992) equation, which is a theoretical equation based on empirical relationships. Maximum gas-holding capacity of Gething coals at 1,000 m depth and 1.7% Ro,max ranges from 22-27 cc/g (daf) (700-870 scf/ton), whereas gas content estimated using the Eddy curves and the Ryan Equation is about 17 cc/g (daf) (545 scf/ton).…”

Extension of the Paraná Basin to offshore Brazil: Implications for coalbed methane evaluation

“…This has significant implications for the regional CBM potential, since it is known from early artificial coalification experiments (Karweil, 1956) that coal starts to generate substantial volumes of thermally derived methane once the high volatile bituminous rank is achieved, corresponding to a vitrinite reflectance level of approximately 0.7% Rrandom (Choate et al, 1986). It is also known that the methane adsorption capacity of coals increase with coal rank (Kim, 1976;Eddy et al, 1982;Meissner, 1984;Dawson and Kalkreuth, 1994), making the northeastern part of the Santa Terezinha Coalfield the prime candidate for onshore CBM exploration.…”

“…Thermally derived methane in coal starts to form at a rank of high volatile bituminous coal (vitrinite reflectance between 0.6 and 0.7% Ro) and methane content can reach 400 cc/g at the rank of anthracite (Karweil, 1956). The methane storage capacity of coals is likewise influenced by coal grade, coal type and coal rank, but is also affected by other parameters such as reservoir depth, temperature, coal moisture content and structural setting (Eddy et al, 1982;Kaiser et al, 1992;Dawson and Kalkreuth, 1994).…”

O potencial de geração CBM (Coalbed Methane) na jazida Sul Catarinense: 1. Características petrográficas e químicas das camadas de carvão da Formação Rio Bonito, Permiano da Bacia do Paraná

“…A geração do metano está relacionada diretamente com o grau de carbonificação da matéria orgânica, que, por sua vez, dependerá das condições de temperatura e pressão às quais a sucessão estratigráfica foi submetida (Murray 1991apud Bustin & Clarkson 1998. O metano gerado termicamente nos carvões começa a se formar a partir do rank betuminoso alto volátil (com refletância da vitrinita entre 0,6 e 0,7% Ro) (Karweil 1956 estrutural (Eddy et al 1982;Kaiser et al 1992;Dawson & Kalkreuth 1994). As análises petrográficas e químicas e a interpretação dos resultados obtidos no presente estudo indicaram que o carvão da Formação Rio Bonito da jazida Sul Catarinense tem as características necessárias para a geração de gás, principalmente as camadas Barro Branco e Irapuá, por apresentarem o rank necessário para a geração de gás, alto conteúdo de vitrinita e baixo teor de matéria mineral, enquanto a camada Bonito apresenta baixo potencial devido ao alto conteúdo de matéria mineral.…”