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Address : 1Arrow Global CBM Pty Ltd, Australia; 2Department of Geological Sciences, New Zealand
Celebration date : 28 Sunday September 2008
Author name : Moore, Tim1; Mares, Tennille2; Butland, Carol1

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  • Article title : Gas saturation: Controls and uncertainty in biogenically-derived coalbed methane, examples from New Zealand coal fields
    Article type : Other
    Location : International Geological Congress,oslo 2008

    Fulltext :

    Gas saturation is probably second only to permeability in determining whether a coalbed methane reservoir will yield gas at rates deemed commercial. Yet, the controls on gas saturation are not well understood. In New Zealand, the Greymouth, Ohai and Huntly coalfields were investigated and all found to be biogenic to mixed biogenic/thermogenic in origin. There was also a wide variance in the degree of gas saturation within and between deposits. The Ohai coalfield has the highest δ13C isotopes values but is intermediate in rank between the Greymouth and Huntly coals. It is postulated that it’s relatively high gas saturations (>75%) are the result of gas migration up-dip from more thermally mature coal beds. It should be noted, however, that potentially large uncertainties can exist around gas saturation values. To assess this, one coal core from the Huntly coalfield was sampled from top to bottom (10 samples) for desorption and adsorption gas properties. Desorption analyses for measured gas (average in-situ basis) ranged from 2.69 to 3.35 m3/t (standard deviation (sd) = 0.25) and gas adsorptive capacity at 4MPa (average in-situ basis) from 2.50 to 4.19 m3/t (sd = 0.46) resulting in saturations ranging from 68% to 123% (sd = 15). Allowing for a ±7 % error in adsorption capacity, saturation was calculated for each sample using measured gas contents with ±5, 10, and 15 % error estimates. This resulted in a ±11, ±16 and ±21 % difference in saturation values respectively, ranging from 54 % to 152 %. To assess how many samples are required to make a realistic assessment of reservoir properties, random samples were averaged in lots of 2, 3, 4 etc and compared to the overall mean for the 10 samples. It was found at least three adsorption and desorption samples would have been required in the Huntly core to bring the average within to one standard deviation of the overall mean for measured gas content, adsorption capacity and saturation.

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