High-pressure methane and carbon dioxide sorption on coal and shale samples from the Paraná Basin, Brazil

An experimental study has been conducted to assess the potential for coalbed methane production and carbon dioxide storage in coals, carbonaceous shales and source rocks in the Paraná Basin in Brazil. High-pressure sorption tests with methane and carbon dioxide were performed on coal and carbonaceous shales from the Santa Terezinha Coalfield and samples from two principal petroleum source rocks. Measured excess sorption capacities ranged from 0.03 to 0.47mmol/g for methane and 0.14 to 0.81mmol/g for carbon dioxide, showing a decrease with decreasing organic matter content. Linear regression lines for methane sorption capacity vs. TOC extrapolated to approximately zero, whereas for carbon dioxide the intercept of the regression line indicated a residual sorption capacity of ~0.2mmol/g on the mineral matter. Present-day gas contents of coals collected from the first CBM well in the Santa Terezinha Coalfield correspond to 13-38% of the measured maximum sorption capacities. Carbon dioxide sorption capacities exceed methane sorption capacities by a factor of 1.9 to 6.9 for these coals. Free sorption capacities of the under-saturated coals in combination with preferential sorption of carbon dioxide could favour CO₂-enhanced methane recovery and CO₂ storage in coals and shales of the study area. Based upon the calculated coal reserves, gas contents and measured sorption capacities, a total storage potential of 15.4Gt CO₂ was estimated for an area of 20×40km² in the Santa Terezinha coal field, assuming a combined CO₂ enhanced coalbed methane (ECBM) production and CO₂ storage operation. To fully evaluate the potential for carbon dioxide storage and coalbed methane production, further studies are required to assess producibility of methane and efficiency of long-term CO₂ storage in the study area.