Coalbed Methane (CBM)

Im Vortrag wird das erste Flözgasprojekt Brasiliens vorgestellt. Flözgas ist ein fossiler Energieträger, der sorptiv an die Oberfläche der Kohle gebunden ist. Das methanreiche Gas entsteht beim Inkohlungsprozess aus der pflanzlichen Substanz und wird durch Bohrungen freigesetzt. Flözgas gehört zu den nicht-konventionellen Gasen, welche 2007 bereits mit 6 % einen relativ moderaten Anteil an der Welt-Erdgasförderung hatten.

Der Schwerpunkt des Vortrages liegt in der Darstellung der Ergebnisse der verschiedenen organisch-geochemischen und organisch-petrographischen Methoden, sowie der CH4 and CO2 Hochdruck-Sorptiontests. Gezeigt werden Beispiele aus dem abgeschlossenen Projekt im Bereich der Exploration auf Flözgas in Brasilien.

Zusammenfassung:
The first coalbed methane (CBM) test well in Brazil was drilled in the Santa Terezinha Coalfield, Rio Grande do Sul in early 2007, with the recuperation of 12 coal seams and carbonaceous shales from the Permian age Rio Bonito Formation. The coal bearing interval (cumulative coal thickness= 7.41 m) was penetrated at a depth range from 605 to 638 m and was retrieved by wireline coring techniques to keep lost gas volumes to a minimum.

Analytical techniques included determination of desorbed gas volumes, gas composition, stable C-isotopes, petrological and chemical analysis of the coal samples, followed by high pressure CH4 and CO2 sorption tests on a selected number of samples.

Twenty-four samples were collected in canisters for gas desorption measurements. The results indicate a variation from 0.38 to 2.18 cm3/g. The variations of the gas content associated to the coal seams are related to the content of ashes. The gas composition has a high content of methane (94,26 to 99,47 %) with minor contributions of C2, C3 and non-hydrocarbon gases (CO2 and N2). The stable isotope values of CH4 (-50,85 to -55,54 δ13C) indicate thermogenic origin of methane. The vitrinite reflectances indicate high-volatile C/A bituminous coal, except for some coal seams at the top of the succession, which are in contact with volcanic intrusions (semi-anthracite). The maceral analysis indicate vitrinite and inertinite predominance. Chemically the seams and carbonaceous shales show enrichment in ash yields varying from 35.2 to 92.7wt.%. The mineral matter is predominated by quartz, kaolinite and mica, with less contribution of pyrite, ankerite, calcite, dolomite, albite and alunite. The sulfur content shows a large variation ranging from 0.12 to 7.68 wt.%.

Three samples were selected for high-pressure CH4 and CO2 sorption tests. The sorption capacities range for CH4 from 0.15 to 0.27 mmol/g, whereas the sorption capacities for CO2 range from 0.45 to 1.21 mmol/g. From the preliminary results presented here it appears that CH4 sorption capacities are not controlled by rank, but are strongly influenced by ash yields and inertinite content of the samples. CO2 sorption capacities show also the lowest adsorption capacity for the sample highest in ash yield, whereas the other trends in CO2 sorption capacities are at this point difficult to explain.

However, in each of the three samples analyzed, the sorption capacities for CO2 are much higher than those for CH4 (by a factor of 3.0 to 4.5). This observation has important implications for the potential of the Santa Terezinha Coalfield seams to eventually serve as a CO2 reservoir, while at the same time liberate CH4. Currently, further studies are needed to enhance our knowledgement on the CBM generation and CO2 storage capacity of the Santa Terezinha Coalfield, based on additional test wells and sample analyses.