A Multiscale Study of Sorption and Mass Transfer in Shale using Helium and Argon

Abstract:

In this work, we characterize mass transfer and sorption at different length scales in shale using Helium (He) and Argon (Ar) as probe gases. Thermogravimetric analysis (TGA) with a shale cube (1 cc) and gas expansion experiments with a full-diameter core (3.5” diameter) are used to characterize sorption kinetics/isotherm and mass transfer, respectively.

Ar excess sorption isotherms were initially generated via the TGA steady-state technique, while sorption kinetics were measured via dynamic TGA experiments. The TGA experiments demonstrate that Ar, which has a similar sorption potential as Methane, adsorbs onto the surfaces of the mesoporous and microporous regions of the shale samples according to a Langmuir-type behavior.

Helium expansion experiments were performed with the full-diameter core to measure the overall porosity, on the basis that He is a non-sorbing and inert gas as compared to Ar. The He expansion experiments, furthermore, allow us to delineate the mass transfer of gas across the inherent hierarchy of pore sizes, including macropores (macro- and microcracks), mesopores and micropores. Equivalent expansion experiments were performed with Ar to study the combined behavior of mass transfer and sorption.

A triple-porosity model (TPM) was utilized to interpret the He expansion experiment with the shale core and to extract relevant transport parameters. Assuming that the shale cube is representative of the matrix region of the core, the Ar sorption kinetics from the cube experiments were then combined with transport parameters extracted from the He experiments to predict the behavior of the Ar expansion experiments with the full-diameter core. An excellent agreement is observed between the model predictions and the experimental data.

The experimental observations and their interpretation indicate that one must be cautious when using Ar to estimate the true porosity and permeability of shales. In addition, we demonstrate that He and Ar probe gases, when used in tandem, can be used effectively as a tool to characterize shales in terms of mass transfer and sorption dynamics across scales.