The Coalbed Methane (CBM) Potential and CO2 Storage Capacity of the Santa Terezinha Coalfield, Paraná Basin, Brazil – 3D Modelling, and Coal and Carbonaceous Shale Characteristics and Related Desorption and Adsorption Capacities in Samples from Exploration Borehole CBM001-ST-RS

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Abstract
This study investigates the CBM generation and CO2 storage capacity of the coal-bearing Permian age Rio Bonito Formation in the Santa Terezinha Coalfield, Paraná Basin, Brazil. In order to assess the volumetrics of the coal seams a fourth-order sequence stratigraphic framework of the coal-bearing succession was established based on parasequence boundaries. A 3D geometric model based on data from 36 well logs was developed, and enabled subdivision of the study area into regions with very different CBM potential, due to variable cumulative coal thickness, depths and presence/absence of diabase. Strata of the coal-bearing Rio Bonito Formation were penetrated in well CBM001-ST-RS at a depth of 575.70 to 638.35 m, from which twelve coal seams and carbonaceous shales, ranging in thickness from 0.15 to 1.77 m, were retrieved for analysis. Gas desorption values range from 0.3 to 2.2 cm3/g. Gas composition is dominated by methane (> 94 Vol%). Gas isotopic analyses suggest a mixed biogenic and thermogenic origin. Vitrinite reflectance indicate a rank of high volatile A bituminous coal, with semi-anthracite occurring in contact with igneous rocks. The majority of the samples has vitrinite group contents > 50 Vol%, mineral matter-free basis. Ash yields range from 42.1 to 92.8 wt.%, whereas sulphur contents range from 0.1 to 7.7 wt.%. Volatile matter and calorific values (dry ash-free) range from 37.6 to 79.4 wt.%, and 2352 to 8107 kcal/kg, respectively. Mineral composition of the samples is dominated by quartz and kaolinite, whereas major element analyses show that SiO2 is most abundant. The 3D model allowed volumetric calculation for the coal seams and formed the basis for calculations of gas content and CO2 storage capacity. According to the 3D model the four coal-bearing parasequences of the study area contain a coal volume on the order of 3212 × 10 6 m3 suggesting a resource base of 5.5 × 109 m3 of methane associated with the coal seams and carbonaceous shales of the study area. The methane and carbon dioxide sorption capacities show a high positive correlation with TOC content. The coal seams appear to be undersaturated, since sorption capacity is significantly higher than the desorbed gas volume. The CO2 sorption capacity at mean reservoir pressure was up to 7 times higher than the methane sorption capacity. Based upon sorption capacities and reservoir conditions, a storage potential of 13.8 Gt CO2 in coal seams and carbonaceous shales of the study area is estimated.

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