Assessing coalbed methane prospects: How reliable are your numbers?

Mares, T. E. and Moore, T. A. (2008). Assessing coalbed methane prospects: How reliable are your numbers?. In: New Zealand Petroleum Conference proceedings 2008. New Zealand Petroleum Conference 2008, Auckland, New Zealand, (). 10-12 March 2008.

Author Mares, T. E.
Moore, T. A.
Title of paper Assessing coalbed methane prospects: How reliable are your numbers?
Conference name New Zealand Petroleum Conference 2008
Conference location Auckland, New Zealand
Conference dates 10-12 March 2008
Proceedings title New Zealand Petroleum Conference proceedings 2008
Place of Publication Wellington, New Zealand
Publisher Ministry of Economic Development
Publication Year 2008
Sub-type Fully published paper
Total pages 10
Language eng
Formatted Abstract/Summary
To assess the commercial viability of a coalbed methane prospect two of the key geological parameters
measured are gas content (desorbed gas) and gas holding capacity (adsorption capacity). These two
measures, together with reservoir pressure, give an estimate of the saturation of a reservoir. Typically
saturation has been assessed by collecting one adsorption isotherm sample and assuming it is
representative of the whole seam reservoir conditions. This study addresses that assumption.
To understand the level of variation, and thus the inherent uncertainty in saturation, one core from the
Huntly coalfield in New Zealand was analysed in detail. Ten canisters of coal (representing the whole
seam) were desorbed for ten days and then analysed for adsorption capacity. Desorption analyses for
measured gas (average in-situ basis) ranged from 2.69 to 3.35 m3/t (standard deviation (sd) = 0.25
m3/t) and gas adsorptive capacity at 4 MPa (average in-situ basis) from 2.50 to 4.19 m3/t (sd = 0.46
m3/t) 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 total
gas contents with ±5, 10, and 15 % error estimates. This resulted in a ±12, ±17 and ±23 % 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 samples are
required to bring the average within to one standard deviation of the overall mean for total measured
gas content, adsorption capacity and saturation.
Keyword Coal bed methane
Gas adsorption capacity
Q-Index Code E1
Q-Index Status Provisional Code
Institutional Status Non-UQ

Document type: Conference Paper
Collection: School of Earth Sciences Publications
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Created: Tue, 20 Sep 2011, 09:55:16 EST by Tennille Mares on behalf of School of Earth Sciences