CO2 Permeability in Shale Gas Reservoirs: Insights from the Montney Formation

Abstract The Montney Formation, in north–eastern British Columbia and western Alberta, is a widely developed, low porosity and permeability shale gas and oil reservoir. Due to existing midstream infrastructure, it is an ideal candidate for CO2 sequestration which can potentially be coupled with CO2 enhanced hydrocarbon recovery (EHR). Extensive petrophysical analyses of representative Montney wells and cores validate that the characteristics of supercritical CO2 are more suitable for sequestration compared to either liquid or gas properties. The producing Montney reservoir has absolute permeabilities to helium in the order of 10−2 to 10−5 millidarcies and porosity ranging from 2.9 to 11.1%. At reservoir pressure and temperature conditions, sequestered carbon dioxide will be in the supercritical state. The measured apparent permeability of representative Montney cores matrix to supercritical CO2 is approximately 3.8×10−4 to 3.4×10−2 mD higher than either gas or liquid CO2 values (apparent supercritical CO2 permeabilities range between 4.0×10−4 and 1.4×10−2 mD). The difference between liquid and gas CO2 permeabilities ranges between 3.2×10−5 and 3.0×10−3 mD. Absolute permeabilities to helium were found to be higher than any of the three CO2 phases. The higher apparent permeability to supercritical CO2 compared to the gas or liquid phase is attributed to the higher molecular kinetic energy and the smaller impact of adsorption compared to gas CO2. Permeability data of gas CO2 show both volumetric and adsorption effects, resulting in a lower apparent permeability compared to both liquid and supercritical CO2. Helium data show the highest permeabilities since helium is a non-adsorbing gas and He molecular diameter is 74 pm smaller than the molecular diameter of CO2. The results of this study show that carbon dioxide in the supercritical state has favourable characteristics for the utilization and sequestration in depleted shale gas and oil plays compared to CO2 in either the liquid or gas phase. The relatively high density of the supercritical state – around 750 kg/m3 – will minimize leakage to adjacent formations. Upon reaching reservoirs’ minimum miscibility pressure, supercritical CO2 interfacial tension will approach zero and thus mixing with the residual liquid hydrocarbons will occur. The CO2 will cause the oil or condensate to swell, reducing the viscosity and thus improving the mobility and production rate of the remaining hydrocarbons in place.