Micro-scale effects of stylolite orientation on the motion of tensile failure

Abstract

Many laboratory tests of samples where the rock fractures are based on
meso-scale (cm) characterization of effective ‘intact’ strength parameters neglecting the microstructure effects. Understanding fracturing processes at
micro-scale (mm) will require models with microstructure data. However,
data is lacking on micro-scale.
Stylolites are natural rock-rock interlocked interfaces which form by a localized
dissolution process and their interface contains minerals and material
different from that in the surrounding host rock. Microstructures such as
stylolites influence the tensile stress behaviour in a rock formation. We are
interested in stylolites because they can act as drains or as barriers to flow.
Therefore, we introduce a study where we investigate on micro-scale failure
mechanisms of limestone samples with stylolites in diverse orientations.
In preceding study (Pluymakers et al., pers. comm.) a series of Brazilian
Disc Tests was performed on eleven samples all including stylolites of microscale
carbonate samples from the ”Treuchtlinger Marmor” formation from
the Molasse Basin (Munich, Germany). All experiments were filmed using a
DSLR camera.
In this study we aim to develop the use of the Particle Image Velocimetry
method to analyze such type of movies. We use the developed Particle Image
Velocimetry method to analyze three of the movies of the preceding study,
which contained samples where the stylolite is at different angles to the
horizontal axis of the sample, so to s3. Two samples has an angle of 90°
between the stylolite and s3 and one with an angle of 40°.
In this open-source software the pixel displacement is analyzed in frames
(i.e. consecutive images, ’before’ and ’after’ image) and it calculates the velocity
distribution within the framepairs, but it is also used to derive, display
and export multiple parameters of the flow pattern. In this study we derived
and displayed the strain rate on the image pairs.
The results show that the strain field of the two limestone samples of a 90°
stylolite behaves identical in the two major stress drops (i.e. DStress) from
the stress-displacement curve. However, the sample with a stylolite of 40° behaves differently. For the two 90° oriented stylolites random extension and
compression takes place at the first highest negative stress drop and symmetric
extension takes place at the second highest negative stress drop from
the stress-displacement curve. However, for the 40° oriented stylolites asymmetric extension takes place at the first highest negative stress drop from
the stress-displacement curve. And random extension at the second drop.
Another result of this study is that the strain rate obtained from PIVlab, the
strain rate per stress drop (MPa) (i.e. in the stress displacement-curve) in
both the two 90 ° stylolite samples and the 40 ° sample were different.
To put this work in the broader context of the energy transition, we consider
the rapid development of the geothermal sector. Nowadays geothermal
energy is considered as one of many alternative sources of energy using
the hot water in the Earth’s surface to generate electricity and power heating/
cooling systems. The Dinantian carbonates in the Netherlands and the
”Treuchtlinger Marmor” carbonates from Germany are of interest for Ultra
Deep Geothermal wells, because of their high geothermal potential. To enhance
the porosity/permeability in a formation where hot water needs to be
extracted hydraulic fracturing can be an option and this study gives insight
about how the rock formations on micro-scale behave when tensile failure
occurs. Tensile is one of the most important properties to be evaluated for
any textile material (i.e. rocks). Tensile failure is important because it occurs
when the stress on a component exceeds the strength of the material thus it
determines the strength of a rock and consequently is influenced by single
plane of weakness such as stylolites.