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.

Geothermal energy is one of the more sustainable alternatives to fossil fuels that could facilitate the energy transition. The production of geothermal energy incorporates the use of corrosion inhibitors to protect the steel well-parts from the corrosive production fluid. The injection of inhibitors leads to a risk of exposing the reservoir rock to these solutions. Up until now no data has been published to describe the effects of the corrosion inhibitor on the mechanical reservoir rock properties. This thesis aims to remedy this situation by performing compressive triaxial experiments on rock samples which have been saturated with inhibitor solutions. Two rock types which are representative of geothermal reservoir rocks were tested using two different inhibitors. The results were compared to a set of control experiments performed on water-saturated samples. The sandstone experiments show no discernible difference between the different fluid-types. The limestone experiments do show a number of differences: The limestone samples that were saturated with the first inhibitor type saw an increase in rock cohesion and a decrease in internal angle of friction. In contrast limestone samples that were saturated with the second inhibitor saw a decrease in rock cohesion and an increase in internal angle of friction. The lack of difference between the sandstone samples could be attributed to the inert nature of the constituent minerals. Limestone minerals are chemically more reactive and even though the exact nature of the chemical processes which lead to the observed differences remains unclear, a number of potential explanations are provided. We suggest that the change in mechanical rock parameters in the limestone rock are a result of the adsorption of the inhibitor components onto the particle surface. This adsorption alters the particle surface charge which leads to a change of the electrostatic repulsive forces. Additionally the same adsorption may affect the interparticle friction of our rock samples.

The interaction between fractures and the associated effects are studied in fields like geothermal engineering, seismology, volcanology and geo-engineering. Fractures can massively influence the permeability and porosity in a rock formation, reducing resistance to flow. However, to improve permeability, multiple fractures must connect to each other. Therefore, it is important to understand the effects of a stress field under varying orientations and how it influences new and existing fractures. Brazilian disc tests were filmed and performed on 18 Indiana limestone samples, after which 13 samples were fractured a second time under orientations varying from 20° to 90°. Afterwards, video footage of the tests was used to study fracture propagation and fracture roughness. Analysis of the results showed that four distinct types of fracture behaviour occurred. Each type was generally displayed between certain angles. Case 1, under 30° shows reactivation of the original fracture. Case 2, between 30 and 45°, shows largely reactivation of the primary fracture but new secondary fractures towards the ends of the sample. Case 3, between 45 and 60°, shows the primary fracture closing and formation of secondary fractures near the centre of the disk. Case 4, from 60° onwards, shows the primary fracture close completely while a new fracture forms perpendicular and independent of the first. The results imply that initiating a stress field in a certain orientation has differing consequences. A stress field more parallel towards the original fracture causes reactivation of the fracture, without much impact on the permeability. However, a stress field initiated perpendicular to the primary fracture causes a new fracture to form, independent of and straight through the primary fracture. This is likely to increase permeability and therefore reduce resistance to flow.