Modeling Dynamic Helium Release as a Tracer of Rock Deformation

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.Comment: 8 figure

In situ observation of helium and argon release during fluid-pressure-triggered rock deformation

“…The temporal scales of the processes involved not only require a high-frequency monitoring system 24,32 but also a multi-tracer approach, i.e., He, Ar and N 2 in the case of the GTS. Complementing the findings of previous experiments performed in the laboratory [17][18][19][20] , as (2020) 10:6949 | https://doi.org/10.1038/s41598-020-63458-x www.nature.com/scientificreports www.nature.com/scientificreports/ well as those advanced from large-scale observations in the context of earthquakes 5 , we interpret these anomalies as the release of radiogenic He (and Ar) stored in the rock mass that could be transported within the newly created and pre-existing fracture system. While the identification of the source of the mobilized fluids is still speculative, two complementing hypotheses on its origin are proposed: (1) radiogenic He (and Ar) accumulated in the rock matrix is released by the formation of new fracture areas; and/or (2) the shearing of pre-existing fractures induces the remobilization of stagnant fluids, with higher residence times enriched in radiogenic He and Ar, trapped in lower transmissivity zones of the fracture network.…”

“…Chemical anomalies created by earthquakes have been explained by the mixing of fluids from different sources (with different chemical compositions) inhibited by changes in flow pathway geometry and hydraulic properties during stress build-up and failure 15,16 . Laboratory studies have also observed the emergence of concentration anomalies in noble gases during rock deformation experiments [17][18][19][20] . The authors argued that the progressive increase in microcrack surfaces during increasing stress allows the remobilization of accumulated radiogenic gases and even prior macroscopic failure.…”

Noble Gas Release from Bedded Rock Salt during Deformation

“…These data provide information on the mechanics and timing of gas release. Previous studies have shown that helium is released during elasticity and brittle deformation from both sedimentary and crystalline rocks [2,3,35]. In this study, we show that helium and argon are released from rock salt during mechanical deformation.…”

Investigating Fracture Network Deformation Using Noble Gas Release

“…These nobles gases are likely released as a result of episodic fracturing and brittle deformation in the subsurface [2,3]. Recent research has shown that radiogenic noble gases released from rocks as they are deformed can be observed [4][5][6][7][8]. Thus, monitoring noble gas release and isotopic composition could provide a new method of measuring and inferring subsurface deformation mechanics.…”