Analysis of Gas and Water Production Pathways in Coal Seams

Emma Kinnon (2010). Analysis of Gas and Water Production Pathways in Coal Seams MPhil Thesis, School of Earth Sciences, The University of Queensland.

       
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Author Emma Kinnon
Thesis Title Analysis of Gas and Water Production Pathways in Coal Seams
School, Centre or Institute School of Earth Sciences
Institution The University of Queensland
Publication date 2010-07
Thesis type MPhil Thesis
Supervisor Prof. Joan Esterle
Prof. Suzanne Golding
Total pages 93
Total colour pages 1
Total black and white pages 92
Subjects 04 Earth Sciences
Abstract/Summary Understanding gas flow pathways in coal seams will assist in optimising well placement and design for gas drainage in underground coal mines, coal bed methane (CBM) production in commercial fields and, in the future, enhanced CBM recovery with CO2 geosequestration. In this study, drilling, production and geological data from a CBM field in the Bowen Basin, Queensland, were analysed to determine how geology affected production rates over time. The study field produced from medium radius surface to in-seam wells, consisting of a vertical production well intersected most commonly by two lateral wells. Basic geological and gas data were provided by the company and analysed in this thesis, along with the development of well performance indicators, well path analysis, and an investigation into the origins of the gas and water determined using stable isotopes. Daily gas production data were averaged over yearly intervals and contoured for two coal seams across the field to illustrate the variability in gas production rates over a four year period relative to the geological setting. Gas production is variable with distinct domains of high and moderate production rates for the examined period, which reflects geological features across the field, in particular faults, folds and seam splits. The seams dip basinward, and are gently folded along strike into a syncline and anticline, which is separated by a large normal fault that marks a boundary between production domains. For one of the target seams, a master seam split occurs, resulting in elevated mineral matter content and separation of the seam reservoir. A number of smaller normal faults also occur across the anticline. The high production domain occurs north of the fault where gas reservoir size is greater due to higher gas contents in the syncline combined with greater net coal for both seams; the moderate gas production domain occurs south of the fault where gas contents are slightly lower across the crest of the anticline. Little communication occurs between these two distinct areas, and this compartmentalization was established by examining the geochemistry of the water and gas. Stable isotope analysis and water quality tests, including anions, cations and trace elements, were conducted on water samples from the two producing coal seams across the study field to assess zones of recharge, water mixing and pathways. The stable isotope analysis showed that production waters with higher δD and δ18O compositions were associated with areas of higher water production and shallower depths. Lower δD and δ18O compositions, and in particular those that lie above the GWL on the δD vs δ18O graph are generally associated with low water production and high gas production. The water quality analysis showed that waters with lower sodicity and bicarbonate concentrations were associated with high water production and waters with higher sodicity and bicarbonate concentrations were associated with high gas production. The presence of higher bicarbonate concentrations and lower δD and δ18O compositions in the high gas production domain suggests that biogenic gas could have been generated in this area, and is confirmed by the isotopic analysis of the methane. Stable isotope and composition analyses were undertaken on gas samples collected from selected wells in the study field. Gas compositions were consistent across the field with all CH4 composition measurements over 95% and CO2 compositions 1.6% or less. The results from the stable isotope analysis showed that biogenic gas was produced at shallow depths, and the ratio of thermogenic to biogenic CH4 increased with depth. Wells with the highest gas production have a well depth of between 200m and 300m, produce gas that is a mixture of biogenic and thermogenic gas and have larger CO2-CH4 carbon isotopic fractionations from 60.8‰ to 62.9‰. Areas of recharge had smaller isotopic fractionations ranging from 48.6‰ to 57.6‰.
Keyword Coal bed methane (CBM)
Coal seam gas (CSG)
Production performance
reservoir compartmentalisation
Stable Isotopes
gas isotopes
Gas origins
Thermogenic
Biogenic
Additional Notes Colour page: 20 Landscape pages: 61, 90-93

 
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