Advances_247_253_Baue+ - Eawag-Empa Library

Measuring Local
under Stress
Tomography
Strains
with
Andreas Bauer*, Eelco Verhulp**, Stan Schoofs*
in Sandstones
Micro-computed
*Shell International Exploration & Production, Kesslerpark 1, 2288 GS Rijswijk,
The Netherlands
a.bauer@shell.com
stan.schoofs@shell.com
**Department of Biomedical Engineering, Eindhoven University of Technology,
Den Dolech 2,5600 ME Eindhoven, The Netherlands
e. verhulp@tue.nl
ABSTRACT: We report on micro-computed tomography (peT) measurements of Bentheim
sandstone under stress using a specially designed triaxial pressure cell. First results
demonstrate the feasibility of measuring local deformations and strains in sandstones with
mstrain sensitivity. Data analysis includes segmentation of three-dimensional peT images,
grain partitioning, and application of a cross-correlation technique for determining
displacements and rotations of single grains.
KEYWORDS: Micro-computed tomography, sandstones, stress-strain relationship.

Micro structure 249
Figure 1 a shows a drawing of the pressure cell. The sample has a diameter of 5
mm and a length of 14 mm and is clamped between two pistons, by which axial
stress is applied to the sample. The upper piston is moveable and driven
hydraulically. The lower piston is fixed. The sample is surrounded by a relatively
thick poly-ether-ether-ketone (PEEK) mantle (wall thickness 7 rom, outer diameter
21 mm) that can sustain stresses up to 400 bar both along the symmetry axis and
radially. PEEK is a machinable and rather strong material, and, more importantly for
f.LCT applications, it shows only little x-ray absorption as compared to the rock
sample (for typical x-ray energies of 70 keY, the x-ray absorption of the PEEK
mantle is less than 5%). The small annulus between sample and mantle can be filled
by hydraulic oil for radial confinement. Here, a thin rubber sleeve forms a seal
between sample and confinement oil. Small holes in the upper and lower piston
allow for adjustment of pore-fluid pressures and fluid flow (drainage, etc.). The tube
that connects the pore-fluid line in the upper piston slides within the piston, with an
O-ring between tube and piston acting as a seal.
00=20 bar
Figure 1. (A) Schematic drawing of the cylindrical triaxial pressure cell for micro-CTand
(B) resulting J1CT images of a Bentheim sandstone sample at different uniaxial stresses.
The rubber sleeve around the sample is also visible in the images. Before failure at 200
bar, grains and pores can easily be tracked (see circles) although there might be overall
displacements from one image to the other
B

Micro structure 253
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