Modeling Dynamic Helium Release as a Tracer of Rock Deformation

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 He-4 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 postfailure. Increased matrix porosity is required to match the long-term, postfailure 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. Plain Language Summary This paper describes an effort to model gas released during mechanical deformation of rock. In two recent papers, we have described laboratory experiments where we measured noble gas release as a result of mechanical deformation of rock. We find that rocks release noble gases in a repeatable manner during deformation. These releases are extremely sensitive to the amount and type of rock deformation. Thus, we postulate that noble gas release could be used as a tracer of mechanical deformation. Up to this point, we have only described our experimental results and have not attempted to interpret the noble gas signal during deformation. In this paper, we develop a model, which allows us to change the properties of rock controlling gas release during deformation. We then use this model to recreate observed gas release from a deformed shale. This modeling exercise shows that gas release is quite sensitive to the evolution of transport parameters and that we can use the gas release signal to explore the type and extent deformation. These results set the stage for using naturally released gases as a tool to monitor rock deformation and failure.