Triaxial Rock Deformation Cell for Inside Micro-CT Scanner
Authors: Houben A. Maartje , Coorn A. Ab , van der Linden A. Arjan 
Published: August 2024

ABSTRACT:
In general, geomechanical experiments are conducted to measure the impact of changes in pressure (axial, radial and pore pressures) due to production or injection of fluids for instance. The change in reservoir rock strength can then be used to quantify for example the risks of subsidence, loss of seal integrity, fault reactivation and other effects when an oil or gas field is developed. In standard geomechanics tests we make a CT image of a plug (e.g., 1.5-inch diameter and 3-inch length) before and after a geomechanical experiment using a 50 μm voxel size. Hence, we get a snapshot in time of what the plug looked like before deformation (without pressures on the sample) and what the plug looks like after deformation (without pressures on the sample). We have developed a new triaxial rock deformation cell that can be operated inside a CT scanner and enables us to conduct triaxial tests on rock mini plugs (6 mm diameter and 12 mm length). This newly developed cell enables us to image the plugs at different stages during the deformation experiment without having to mount and unmount the sample in-between giving us snapshots of rock deformation during the experiment.

1. INTRODUCTION
Geomechanical experiments are often conducted to measure the impact (in strains) of changes in axial, radial and pore pressures on a plug. Typically, in a triaxial compression test (σ1 &gt; σ2 = σ3) the plug will be brought to its reservoir pressures for instance, where after the axial pressure will be increased until brittle or ductile failure of the plug. In general, one can describe the deformation in a triaxial test in three different stages: 1. Initial closure of pre-existing cracks and elastic deformation, 2. Linear combination of inelastic and elastic deformation, 3. Brittle deformation and localization (e.g. Pijnenburg et al., 2019a). In standard geomechanical tests (including but not limited to triaxial compression tests) applied stresses are known and induced strains will be measured during the test. We make X-ray computed tomography (CT) scans of the plug before and after test using typically a voxel resolution of 50 μm to get a snapshot of what the plug looked like before deformation and what the plug looked like after deformation (as described by: Baud et al, 2015). Both CT scans are performed without any stresses acting on the sample, using a high-resolution Bruker Skyscan 1275 scanner. From these CT scans before and after a geomechanical experiment at a 50 μm resolution it is, however, hard to infer evidence which deformation mechanisms are at play during deformation (e.g., inter-granular cracking/slip versus intra-granular cracking, e.g. Brantut et al., 2013; Tengattini et al., 2014; Hol et al., 2018; Pijnenburg et al., 2019b). Momentary phenomena in geomechanics like the onset of deformation and how this continues can be imaged in 4D (3D image, time-lapsed) using a CT translucent triaxial setup inside a CT scanner. The design of CT translucent triaxial devices is an ongoing field of development that started with imaging soil and loose sand packs (low pressures, e.g.: Viggiani et al., 2004), but continued into rock deformation (e.g.: Renard et al., 2016; Glatz et al., 2018; Voltolini et al., 2019; Butler et al., 2020; Cartwright-Taylor et al., 2020; Fusseis et al., 2022; Van Stappen et al., 2022; Freitas et al., 2024). We have developed a new triaxial rock deformation cell that can be operated inside one of our laboratory CT scanners (ZEISS Xradia versa 520). The mini-triaxial deformation cell has been designed and built in collaboration with MetaRock laboratories. This paper describes the operational parameters of this mini-triaxial cell, and we discuss the first results.