Coalbed methane (CBM) is a growing resource for "clean" natural gas and is becoming of great interest for academic research. Despite much research already be done, geochemical investigations, especially with focus on the stable carbon and hydrogen isotope composition of CBM, are rare.
For this study, over 1,000 CBM samples were analyzed. The samples were collected during 10 different sample campaigns from seven different coal bearing basins worldwide. Seven sample sets were collected during desorption experiments following drilling of exploration wells and three sample sets were collected from CBM producing wells. The considered coals range in maturity from sub bituminous A (min. 0.57%Ro) to anthracite (max. 4.55%Ro), cover a wide range of different maceral compositions and were accessed in depths between 10m and 1312m beneath surface. Samples cover a production time of up to 6312 hours and a desorption time of up to 2773 hours.
The geochemical analyzes were carried out using GC—IRMS. Analyzes include gas composition (methane, ethane, propane, n—butane, i—butane and carbon dioxide) and the proportions of the different gas species, as well as stable carbon isotope ratios for all gas species and stable hydrogen isotope ratios for methane.
The analyzed averaged and normalized gas composition of the considered samples reveals average proportions for methane between 44.3% and 98.7%, for ethane between 0% and 9.98%, for propane between 0% and 1.15%, for n–butane between 0% and 1.09%, for i–butane between 0% and 0.003% and for carbon dioxide between 1.34% and 53.9%. The gas composition does not show conclusive trends with increasing production/desorption time. Methane stable carbon isotope ratios vary with production/desorption time. Samples from production scenarios show a general depletion in 13C for methane with increasing production time and isotope shifts between -1.6% and -35.8%0. Samples from desorption experiment scenarios show mostly enrichment in 13C for methane with increasing desorption time and isotope shifts of up to -43.4%0, but also 12C enrichment was observed in some sample sets with isotope shifts of up to +32.1%0. Overall, the magnitudes of the observed isotope shifts vary considerably between different sample sets, but also within samples from the same source.
The stable carbon isotope composition of methane does not display the expected composition of methane generated from coal. This indicates the influence of secondary processes. The secondary processes mixing, adsorption, desorption and diffusion/migration cannot be separated and considered individually, because they overlap and have identical directions for compositional and isotope alteration. The significant alteration of the considered gases generated from coal has therefore to be considered as a combined effect of the mentioned secondary processes. Because of multiple complicating factors it was not possible to delineate the presence of primary and secondary microbial gas, but due to the geologic context and the retentive ability of coal the presence of these gases is reasonable to assume. Indications were found in samples for the presence of gases generated from a Kerogen Type II shale in CBM gases. The unaltered molecular and isotope composition of CBM gases is unknown. This parameter cannot be estimated, because during the physical history of a coal seam, the coal probably had experienced the loss of gas accompanied by molecular and isotope effects of unknown extent and magnitude. Based on the current knowledge and the data available from this and other studies, a classification of CBM is unreasonable and more work is required for the establishment of a stable isotope systematic for CBM, including the separate experimental evaluation of molecular and isotope effects of adsorption, desorption and diffusion/migration of gases in coal.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/1917 |
| Date | 30 November 2009 |
| Creators | Niemann, Martin |
| Contributors | Whiticar, Michael J. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
Page generated in 0.0019 seconds