Abstract
A helium leakage detection system was modified to measure gas permeability on extracted cores of nearly impermeable rock. Here we use a Helium - Mass - Spectrometry - Permeameter (HMSP) to conduct a constant pressure, steady state flow test through a sample using helium gas. Under triaxial stress conditions, the HMSP can measure flow and estimate permeability of rocks and geomaterials down to the nanodarcy scale (10-21m2). In this study, measurements of flow through eight shale samples under hydrostatic conditions were in the range of 10-7to 10-9Darcy. We extend this flow measurement technology by dynamically monitoring the release of helium from a helium saturated shale sample during a triaxial deformation experiment. The helium flow, initially extremely low, consistent with the low permeability of shale, is observed to increase in advance of volume strain increase during deformation of the shale. This is perhaps the result of microfracture development and flow path linkage through the microfractures within the shale. Once microfracturing coalescence initiates, there is a large increase in helium release and flow. This flow rate increase is likely the result of development of a macrofracture in the sample, a flow conduit, later confirmed by post-test observations of the deformed sample. The release rate (flow) peaks and then diminishes slightly during subsequent deformation; however the post deformation flow rate is considerably greater than that of undeformed shale.
1. INTRODUCTION
Understanding fluid flow in shale and or mudstone may prove important if that rock were to be considered as an extensive seal in a CO2 storage scenario, as a top seal in an oil and gas reservoir, or similarly as the closure barrier in a compressed air energy storage reservoir. In the above "applications" the low shale permeability is relied upon to create a flow barrier. It is reasonable then to study the flow characteristics of shale at reservoir conditions, and conditions where geologic and manmade conditions may have prompted the rock to fracture. We are studying a different method to measure flow, and estimate shale permeability; we are also, as an extension of that method, determining helium release during shale deformation.
