Desorbed gas volume estimation using conventional well-log data for the Montney Formation, Deep Basin, Canada

This study focuses on desorbed gas volume estimation of the Montney Formation, Deep Basin, Canada. Because desorbed gas has significantly contributed to shale gas production, and ranges from 10% to 40% of the total production, an accurate desorbed gas volume estimation is important to evaluate and develop shale gas fields. The desorbed gas volume was calculated using two methods - the adsorption capacity (cf. Langmuir test) and canister volume measurements. Both methods were originally adopted from coal-bed-methane production and applied to shale gas reservoirs after the commercial production of shale gas. The Montney Formation contains less than 2% of total organic carbon, with an average of 0.7% primarily derived from the secondary cracking of migrated oil. This suggests the absence of the adsorption capacity and organoporosity of organic matter. Conversely, the Montney Formation contains more than 20% of clay minerals, and the canister volume test shows that the formation has 22 scf/ton of desorbed gas volume. Based on a mineralogical analysis, this desorbed gas is primarily adsorbed to clay minerals, such as illite, mica, smectite, kaolinite, among others. Because illite is the major component among the clay minerals and has a positive correlation to the canister volume, the illite volume was calculated from conventional well logs to estimate exact desorbed gas volume. We assumed that the clay volume primarily consisted simply of illit The and the rest clay minerals and figure out the volume of illite by using neutrondensity cross plot. The calculated illite volume has a positive correlation to the canister volume, which can be used to infer the desorbed gas volume of the Montney Formation. The estimated desorbed gas volume in the Montney Formation ranges from 19 to 30 scf/ ton, which is lower than the free-gas volume.