Earth Sciences Report 2003-03
Alberta has vast coal resources that may be a potential source of coalbed methane (CBM). Exploration and research are currently underway in the province to quantify gas potential, identify key geological factors that maximize CBM potential, and identify the 'most favourable' areas for CBM production potential.
There are four main coal zones within the Plains and Foothills of Alberta. The Ardley Coal Zone of the Plains and the correlative Coalspur Coal Zone of the Foothills are undergoing limited CBM exploration and production piloting. Much of the effort is centred in the Pembina area of the Plains. Horseshoe Canyon Formation coals of south-central Alberta were initially thought to have gas concentrations too low to be economic CBM producers. It is these coals, however, that host Alberta's first commercial CBM production project. Although similar in both geographic distribution and coal quality to Horseshoe Canyon coals, not much is known regarding the gas potential of underlying coals of the Belly River Group. The deeper Mannville coals have some of the highest gas concentrations of Alberta coals; however, they are also relatively deep and generally have lower permeability than the overlying Belly River, Horseshoe Canyon and Ardley coals.
Maximum gas-in-place for the Plains and Foothills has been estimated to be greater than 1.42 x 10 mï¿½ (>500 trillion cubic feet [Tcf]). Although this number is very large, little is known about the proportion of this vast resource that is actually producible. A key challenge to producibility in Alberta has been the generally low permeability of coals with the highest gas concentrations (Mannville coals), and the moderate to low gas concentrations of higher permeability coals (Horseshoe Canyon, Ardley coals).
Regionally, coal distribution and average gas-in-place concentrations are well established for Alberta. Identifying and explaining local areas with favourable CBM production characteristics within the regional setting is necessary to establish economic CBM plays within Alberta. There are currently several pilots and numerous exploration efforts underway in the province.
This study integrates existing data with new data collected from key areas that show favourable CBM potential. In the Pembina area, increased gas-production potential from simulated wells in the Ardley Coal Zone is indicated by increasing flow via permeability enhancement (up to 7mD) to potentially economic levels. A cost-effective, gas-content screening method of using cuttings rather than cores for gas desorption and analysis has shown much potential, providing that cuttings results are calibrated and corrected to baseline data derived from core work.
Coal from the Coalspur Formation in the Foothills has been shown to be a potentially attractive CBM exploration target. New data obtained from a shallow exploration hole in the Coal Valley area had saturated gas concentrations averaging 4.3 cc/g, more than double the previously reported gas concentration results from the same area. More detailed studies into geology and reservoir characteristics are warranted in this area.
Coals of the Horseshoe Canyon Formation are currently undergoing CBM production. Seams are typically thin, discontinuous and difficult to correlate. Adjacent wells have significantly different production rates. This study indicates that different seams within a wellbore contribute differently to overall production of that well and, furthermore, that the same (correlatable) seams between adjacent wells have different contributions to well production.
Mannville coals are generally thick, deep and of low permeability. Local areas of the Mannville Coal Zone are reported to have enhanced permeability, and several of these areas are undergoing production pilots. This study recognized a large region in east-central Alberta with favourable CBM gas-in-place; however, no permeability data were available. Two tests from this area were conducted. Although significant elevated permeability was not encountered, a slight increase over regional values was indicated. Furthermore, differences were encountered in permeability between the two seams tested.
This study indicates great potential for CBM producibility in Alberta. Local areas have enhanced characteristics favourable to production. Ongoing geological investigations are needed to explain these anomalies, and to identify characteristics that will act as an exploration tool for future CBM discoveries.
Beaton, A.P. (2003): Production potential of coalbed methane resources in Alberta; Alberta Energy and Utilities Board, EUB/AGS Earth Sciences Report 2003-03, 68 p.