Natural gas in shales exists as free and adsorbed gas subject to prevailing pore pressures and confining stress conditions. Accordingly, to estimate/predict the shale gas recovery potential accurately, a central requirement is to represent gas transport (including viscous flow and Knudsen diffusion) and sorption behavior (including sorption kinetics and isotherms) under varying stress conditions. The objective of this work is to facilitate the interpretation of laboratory-scale experiments, at relevant conditions, in an attempt to bridge the gap in scales between laboratory- and field-scale observations.
We have conducted a series of high-pressure experiments on a full-diameter core sample from the Marcellus play. These include gas loading (pressure-decay) and depletion (production) experiments with pure Methane at variable stress conditions, to characterize transport and sorption behaviors close to the reservoir conditions. We have formulated and applied a novel integral formulation for mass transfer/storage in multi-porosity shale systems that allows us to separate transport and sorption phenomena effectively: We delineate gas transport by interpreting Helium pressure-decay experiments and demonstrate how to translate the relevant transport coefficients to Methane (and other gases). A separate measurement of Methane sorption isotherms, on a smaller sample (shale cube), was interpreted and combined with the transport description to predict the production behavior of Methane from the full-diameter core.
Our experiments demonstrate that a representation of sorption hysteresis is crucial to predict and guide shale gas production. We demonstrate that our integral triple-porosity model provides an effective approach for the interpretation and prediction of gas transport and sorption behaviors during loading and production experiments on shale cores under variable net stress conditions.
In summary, our work combines measurement and modeling of mass transfer and sorption in shale, to validate an integral characterization approach that facilitates an improved understanding of shale gas production, Furthermore, the dimensionless groups used in our modeling define a pathway for upscaling of laboratory-scale experimentation.