Influence of moisture content and temperature on methane adsorption isotherm analysis for coals from a low-rank, biogenically-sourced gas reservoir

Samples of subbituminous coal from the Huntly Coalfield, New Zealand, were analyzed for methane adsorption at a variety of moisture contents and temperatures. Density and moisture determinations as part of the procedure were used to experimentally confirm that the packing density of water adsorbed to the coal's surface is equivalent to that of normal liquid water. Moisture content was found to be critical in evaluating the storage capacity of these coals. Low moisture was associated with elevated adsorption isotherms. The relationship between moisture content and gas adsorption is non-linear. Temperature over the range we tested was found to play only a small role in storage capacity for this coal. A series of algorithms were developed to correct for moisture and temperature variations between samples. Testing of the algorithms by using an additional sample indicated that they are close to predicting experimental data but that the constants still require further refinement. These algorithms only apply to this coalfield and are not intended to be widely applied. In addition, it was found that methane adsorption isotherms determined on samples collected after desorption were elevated in comparison to those determined on samples which were collected immediately after drilling.