Continuously cored Horseshoe Canyon-Belly River (HSC-BR) coal measures in four wells in the middle of the coalbed methane producing fairway in south-central Alberta have been evaluated in order to quantify the total original gas-in-place. Gas contents were determined in all lithologies by a combination of canister desorption experiments and log and laboratory analyses that included adsorption isotherms, measurement of effective matrix porosity to gas, and calculation of fracture porosity and solution gas.
The gas-in-place in coals, following the Alberta Energy Resources Conservation Board guidelines (ERCB), only quantifies the desorbed gas, which underestimates the gas-in-place in the coals by 18 to 28%. The balance of the gas occurs as free gas in pores and about 1% to 3% of the gas is calculated to be in solution at reservoir conditions. Inasmuch as the coals are dry (no mobile water), any effective porosity not occupied by adsorbed gas will be occupied by free gas in equilibrium, with the adsorbed and solution gas at the reservoir pressure and temperature. The average effective porosity of most coals ranges from about 4 to 8%, with the duller coals having higher porosity. Gas in fracture porosity, estimated based on measured system permeability and assuming a match stick fracture model, contributes less than 0.4% of the total porosity of the coal seams.
With increasing reservoir pressures, there is a proportionally greater amount of gas stored in the free state than the adsorbed state. This is due to flattening of the adsorption isotherm with pressure and thus coal desorption tests result in a greater underestimation of total gas-in-place.
Gas also occurs in non-coal lithologies throughout the coal measures. The gas content measured and calculated for each reservoir facies varies primarily because of varying organic content. Canister desorption tests show that adsorbed gas comprises between about 60% and 85% of the total gas-in-place in fine-grained lithologies and that the amount is correlated with organic content (r2 = 0.68). The highest average adsorbed gas contents from desorption experiments occur in the black mudstone (0.42 to 0.52 cc/g; 13.4 to 16.7 scf/ton), followed by the dark grey mudstone (0.23 to 0.32 cc/g; 7.3 to 10.3 scf/ton), siltstone (0.21 to 0.29 cc/g; 6.6 to 9.4 scf/ton) and medium grey mudstone reservoir facies (0.18 to 0.21 c/g; 5.9 to 6.8 scf/ton). The organic matter in most lithologies is particulate carbonaceous material derived from vascular plants (Type III kerogen). Effective porosity to gas in the fine-grained lithologies ranges from about 3.1% to 7.6% and contributes between about 14% to 34% of the total gas-in-place in the coal measures. Solution gas contributes between 1% and 6% to the gas-in-place. Gas storage in the sandstones determined by conventional core analyses indicates effective porosity values ranging from 0.7% to 6.3%, which represents up to 10% of the gas-in-place in the coal measures.
Gas in the coal (desorbed plus free and solution gas) represents between approximately 10% and 19% (5.09×107 m3 to 8.34×107 m3/259 ha; 1.8 to 2.95 BCF/section) of the total gas in the coal measures. In the interval from the Upper Horseshoe Canyon coal zone through to and including the Lethbridge coal zone of the Dinosaur Park Formation, the total gas-in-place of the coal measures ranges from 4.32×108 m3 to 4.88×108 m3/259 ha 15.28 to 17.25 BCF/section).
Gas contents determined solely by desorption analyses using ERCB guidelines for coal in the HSR-BR results in significant underestimation of the total gas resource, which in turn leads to errors in calculated recovery efficiencies and thus non-optimal well spacing and development strategies.