Evaluating the coalbed methane potential of the Gething coals in NE British Columbia, Canada: An example from the Highhat area, Peace River coalfield

摘要

Lower Cretaceous coals from the Gething Formation in the Highhat area of NE British Columbia were analyzed for gas content as part of an exploration program. Samples of continuous wireline core and cuttings were processed. Vitrinite reflectance and dry ash-free volatile matter both indicate that the rank of the coal is low volatile bituminous. The coals are well cleated and contain shear fractures, as seen in the core description and in the FMI logs. Coal cuttings were screened to remove a very fine fraction (-200 mesh) and then treated in heavy liquid (SG 1.75 g/cm~3) to remove inorganic lithologies. For the cores, total gas content ranges from 516.9 to 876.8 scf/t (16.1-27.4 cm~3/g; adb) with a mean value of 671.1 scf/t (20.9 cm~3/g) at 20.5 percent ash (adb). For the cuttings, all desorbed gas was assigned to the +200 mesh/float 1.75 SG weight fraction. On this recalculated basis, total gas content for the processed cuttings ranges from 651.8 scf/t (20.4 cm~3/g) to a high value of 2083.6 scf/t (65.1 cm~3/g; adb). Weight yields after screening/floating are generally low. For samples yielding very low values (<19 percent by weight), the recalculated gas content for the cuttings is strongly influenced by its yield. Cuttings with yielding above approximately 19 percent appear to provide gas contents that are reasonably aligned with those obtained from cores. For these samples the recalculated gas content ranges from 651.8 to 852.7 scf/t (20.4-26.6 cm~3/g; adb) with a mean value of 760.4 scf/t (23.8 cm~3/g) at 15.5 percent ash (adb). The desorbed gas is predominantly methane (98-99 percent; air-free and acid-free basis) and average carbon dioxide is about 6 percent. Methane is thermogenic based on isotopic analysis. Methane adsorption isotherm analysis at reservoir conditions shows that the Gething coals are at least 95 percent gas saturated. Using 1300 m as a depth cut-off and an average gas content of 672 scf/t (21 cm~3/g), GIP (gas-in-place) in the Highhat prospect area is estimated to be 270 bcf (billion cubic feet). This translates to 24.5 bcf/section (a section is equal to a square mile or 640 acres) for the 11 sections of land in the vicinity of the corehole or to 3 bcf/m/mile~2. The low absolute permeability of the coals (0.5 md) is the main limiting factor for producibility and commercial development of the resource. Even at today's high natural gas prices, these coals are not economically viable.