Modeling Dynamic Helium Release as a Tracer of Rock Deformation

TL;DR

This study models helium release during shale deformation to understand how fracture development and material property changes influence gas transport, providing a new quantitative tool for assessing stress and strain in geological materials.

Contribution

It introduces a dynamic dual-permeability model that quantitatively links helium release signals to fracture evolution and matrix property changes during deformation.

Findings

Helium signals are sensitive to fracture development and evolution.

Increases in matrix permeability explain pre- and post-failure gas flow.

Increases in matrix porosity match long-term post-failure gas flow.

Abstract

We use helium released during mechanical deformation of shales as a signal to explore the effects of deformation and failure on material transport properties. A dynamic dual-permeability model with evolving pore and fracture networks is used to simulate gases released from shale during deformation and failure. Changes in material properties required to reproduce experimentally observed gas signals are explored. We model two different experiments of $^4$He flow rate measured from shale undergoing mechanical deformation, a core parallel to bedding and a core perpendicular to bedding. We find that the helium signal is sensitive to fracture development and evolution as well as changes in the matrix transport properties. We constrain the timing and effective fracture aperture, as well as the increase in matrix porosity and permeability. Increases in matrix permeability are required to explain gas flow prior to macroscopic failure, and the short-term gas flow post failure. Increased matrix porosity, is required to match the long-term, post-failure gas flow. Our model provides the first quantitative interpretation of helium release as a result of mechanical deformation. The sensitivity of this model to changes in the fracture network, as well as to matrix properties during deformation, indicates that helium release can be used as a quantitative tool to evaluate the state of stress and strain in earth materials.