Uncertainty of gas saturation estimates in a subbituminous coal seam

Mares, Tennille E., Moore, Tim A. and Moore, Catherine R. (2009) Uncertainty of gas saturation estimates in a subbituminous coal seam. International Journal of Coal Geology, 77 3-4: 320-327. doi:10.1016/j.coal.2008.07.002


Author Mares, Tennille E.
Moore, Tim A.
Moore, Catherine R.
Title Uncertainty of gas saturation estimates in a subbituminous coal seam
Journal name International Journal of Coal Geology   Check publisher's open access policy
ISSN 0166-5162
1872-7840
Publication date 2009-01-01
Year available 2009
Sub-type Article (original research)
DOI 10.1016/j.coal.2008.07.002
Open Access Status Not yet assessed
Volume 77
Issue 3-4
Start page 320
End page 327
Total pages 8
Place of publication Amsterdam, The Netherlands
Publisher Elsevier BV
Language eng
Abstract 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 gas saturation of the reservoir. Typically gas 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.
Formatted abstract
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 gas saturation of the reservoir. Typically gas 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 (Jasper-1) from the Huntly coalfield in New Zealand was analysed in detail. Ten samples (representing the whole coal seam) were placed into gas desorption canisters and desorbed for ten days and then analysed for adsorption capacity. Desorption analyses for total measured gas content (average in-situ basis) ranged from 2.32 to 2.89 m3/t (standard deviation (sd) = 0.18) and gas adsorptive capacity at 4 MPa (average in-situ basis) from 2.11 to 3.51 m3/t (sd = 0.38) resulting in saturations ranging from 66% to 120% (sd = 15).

Determination of how many samples are required to make a realistic assessment of average reservoir properties requires a consideration of: (i) the level of accuracy desired, (ii) the limit of accuracy possible, which is governed by the magnitude of experimental error, and (iii) the innate variability of the seam. It was found that a minimum of five samples each for adsorption and desorption were required in order to significantly decrease the uncertainty in gas saturation estimates for a subbituminous coal.
Keyword Coal bed methane
Gas adsorption capacity
Desorption
Gas saturation
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ
Additional Notes Available online 19 July 2008

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Earth Sciences Publications
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