Helium release during shale deformation: Experimental validation

Abstract

This work describes initial experimental results of helium tracer release monitoring during deformation of shale. Naturally occurring radiogenic 4He is present in high concentration in most shales. During rock deformation, accumulated helium could be released as fractures are created and new transport pathways are created. We present the results of an experimental study in which confined reservoir shale samples, cored parallel and perpendicular to bedding, which were initially saturated with helium to simulate reservoir conditions, are subjected to triaxial compressive deformation. During the deformation experiment, differential stress, axial, and radial strains are systematically tracked. Release of helium is dynamically measured using a helium mass spectrometer leak detector. Helium released during deformation is observable at the laboratory scale and the release is tightly coupled to the shale deformation. These first measurements of dynamic helium release from rocks undergoing deformation show that helium provides information on the evolution of microstructure as a function of changes in stress and strain.

Modeling Dynamic Helium Release as a Tracer of Rock Deformation

“…The remaining matrix volume, modified to a lesser extent by dilation and damage, then controls the late‐time, postfailure gas release. Examination of cores after the experiment did show the existence of smaller fractures intersecting large failure planes in both experiments, which lends qualitative support to this hypothesis (Bauer, Gardner, & Heath, ).…”

“…In this simulation, we increase matrix porosity linearly from 3% to 7% during the prefailure dilation period, with a subsequent jump to 9% at the time of macroscopic failure. These changes in volume are larger than the total volume strain or dilation observed (Bauer, Gardner, & Heath, ); thus, the increase in matrix porosity results from increasing effective porosity due to damage. We have no other estimates of the increase in effective porosity other than the late‐time gas release signal.…”

“…Radon emanation decreases during early elastic deformation as porosity is decreased during compaction, begins to increase at about one third of the elastic yield stress due to the production of microfractures, and remains permanently higher after failure (Holub & Brady, 1981;Mollo et al, 2011;Nicolas et al, 2014;Tuccimei et al, 2010). Bauer, Gardner, and Lee (2016) and Bauer, Gardner, and Heath (2016) show that accumulated helium in porespace and mineral grains is also liberated during deformation. Radiogenic helium follows a repeatable sequence during deformation similar to radon, with decreasing helium flow rate during early compaction, increase in helium flow rate at about one third of the yield strength, a sharp increase during macroscopic failure, and a subsequent long-term decline in gas flow.…”

Noble Gas Release from Bedded Rock Salt during Deformation

“…Transport of gases occurs within the rock grain, along grain boundaries, in the pore fluid, and within the micro-to macrofracture network. Their release during natural and manmade stress and strain changes represents a signal of deformation (e.g., [2][3][4][5][6]).…”

“…Noble gas emission and its relationship to crustal processes have been studied for many years in the geologic community including correlations to tectonic velocities [7], origin of crustal fluids [8], qualitative estimates of deep permeability from surface measurements [7,9], study of hydrothermal systems (e.g., [10]), and fingerprints of nuclear weapon detonation [11]. Increases in radiogenic gas at lab and field scales have been observed and related to preseismic stress, dilatancy, and/or fracturing of the rock and as potential precursory signals to earthquakes attributed to gas release due to preseismic stress [2,3,12,13]. Radon release has been observed relative to earthquake activity [14,15].…”