The increasing global demand for energy has necessitated the exploration of untapped potential energy resources. Unconventional oil reservoirs present a significant opportunity for development through various enhanced oil recovery (EOR) methods. Among these methods, the huff’n’puff scheme using Carbon Dioxide (CO2) EOR has emerged as a potent technique, considering both productivity and its impact on global warming. Unlike water injection, CO2 EOR exhibits high injectivity potential in tight formations, making it a promising approach.
This thesis focuses on investigating the performance of CO2 EOR in unconventional oil reservoirs using the huff’n’puff scheme through a comprehensive reservoir simulation study based on an open-source geological model. The study evaluates the effects of key parameters on oil production, including injection rate, injection time, soaking time, diffusion coefficient, and well completion.
The simulation results demonstrate that CO2 EOR can effectively enhance oil recovery from unconven- tional oil reservoirs. Specifically, the study reveals that implementing CO2 EOR increases cumulative oil production by 27% after 20 years compared to natural depletion cases. Furthermore, when incorporating a two-year primary recovery period, cumulative oil production is further elevated to 45%. By implementing the findings from the sensitivity study, cumulative oil production can be increased up to 67% in the optimum case.
In terms of the influence of the tested parameters on oil production, the study identifies that higher injection rates and longer injection, soaking, and production times for each cycle improve oil recovery until reaching an optimum value. Additionally, extending injection period by 10 times in each cycle with the same CO2 injected volume improved cumulative oil production by 7% compared to fixed injection rate as a result of longer diffusion and dissolution time. Moreover, implementing bottom CO2 injection targeting bottom layers improves vertical sweep efficiency due to buoyancy forces. The study also highlights the importance of the diffusion coefficient, as higher values facilitate faster CO2 transport, resulting in higher oil recovery and reduced CO2 reproduction.
This thesis provides valuable insights into the design and implementation of CO2 EOR projects in tight oil reservoirs. It emphasizes the need for careful selection of optimal operating conditions based on specific reservoir properties and highlights the significance of the diffusion mechanism in CO2 EOR performance. The findings contribute to the development of more effective CO2 EOR strategies for unconventional oil reservoirs, addressing gaps in previous research. Moreover, the study’s outcomes have practical implications for improving the design and implementation of CO2 EOR projects in the field, ultimately leading to increased oil production.

Stylolites are rough dissolution surfaces ubiquitous in sedimentary rocks, which locally exercise a strong influence on porosity and cementation, resulting in a change in strengths and permeabilities of the host rock. Thus, these structures are important in many applications in geosciences. Yet, little research has been done to test the effect stylolites have on the strength of limestones, due to their seemingly obvious role as stress concentrators. Furthermore, there has been no research into whether the morphology of stylolites plays a role into this weakening, and to what extent. Ultrasonic monitoring using coda-wave decorrelation has shown to be effective in monitoring temporal changes in homogeneous rocks during loading tests in the laboratory. However, no research has shown that this technique can detect small changes in rock deformation for rocks with stylolites. To investigate the effect stylolites and their morphology has on the mechanical and acoustic behaviors of limestones experimentally, uniaxial compression tests with active source ultrasonic testing were performed on Treuchtlinger Marmor samples containing sedimentary stylolites and Meyrargues Limestone samples containing tectonic stylolites. Samples were grouped based on stylolite morphology using a novel classification
scheme utilizing the geometric features of 2D traces of the stylolites. Acoustic data was analyzed in conjunction with stress-strain curves, by utilizing coda-wave decorrelation, which allowed small perturbations such as microcracks and pore-collapse in the rocks to be detected. Additionally, X-ray micro-CT experiments were conducted on some samples pre and post-deformation, for greater insight into the weakening effect of stylolites. These experiments found that stylolites, for almost all samples, weaken the rock, however, this weakening effect depends on the degree of stylolite morphology. In these cores, the larger the stylolitized zone, the greater the strength of the rock, when compared to samples containing stylolites. Additionally, stylolite morphology impacts the frequency content of ultrasonic waves, whereby rougher stylolites attenuate higher frequencies during loading.

Global energy transition is crucial in mitigating climate change. Numerous initiatives are being taken globally to achieve a carbon-neutral energy system. These goals require a complete shift from non-renewable energy sources to renewable sources like geothermal energy. These renewable sources of energy have to be constantly improved in order to maximise their efficiency and to ensure the cleanest possible source of energy. One of the challenges in geothermal energy is degassing of geothermal fluids. These fluids contain a lot of minerals and chemicals dissolved in them that are at equilibrium at a certain pressure and temperature. When there is change in this pressure and temperature during extraction, the equilibrium condition changes and the gases dissolved in the fluids degas. These gases then get trapped in pores and decrease the efficiency of extraction of these fluids. They might also prove harmful to the environment due to release of greenhouse gases from the geothermal fluids. The aim of this experimental research project is to visualise the degassing of geothermal fluids and understand the conditions and affect of degassing on the core.
The experiments were carried out by injecting either tap water or brine at a set flow rate of 15ml/min and different concentrations of 6B2. Each experiment had a specific concentration of 6B2 which was injected simultaneously with brine at a certain back-pressure. Then the back-pressure was lowered gradually and when the fluid de-gassed it was allowed to reach steady state and a scan of the core was taken. Throughout the experiments pressure measurements were taken along the core.

In the energy transition from fossil fuels to less polluting renewable energy sources geothermal energy are considered as a promising technology. Gasses such as CO2 are often dissolved in geothermal waters. With the extraction of these fluids from the reservoir, a change in pressure will occur towards the extraction well which may cause dissolved gas to exsolve. The exsolved gas may clog the pores of the reservoir rock near the extraction well and therefore reduce the effective permeability, which can result in reduced production of geothermal waters. This project is aimed at experimentally investigating the conditions of the onset of the degassing process and what the influence of the degassing process is on the permeability. Therefore Bentheimer and Berea sandstone core flood experiments were performed, using either tap water or brine with different CO2 concentrations (0.2 1.3 mol/L) and at temperatures between 30 and 90°C. Flow rates were varied/kept constant, between 90 2 bar and between 5 and 22 bar respectively.

At 30 °C and up to 50 bar the onset of the degassing process is controlled by Henry’s law, i.e. it is governed by the solubility of gaseous CO2. The onset of the degassing process is not influenced by the pore size and initial permeability. At these conditions the effective permeability decreases by a factor 2 to 5 in the Bentheimer sandstone core and by about a factor 10 in the Berea sandstone core. This change in effective permeability is gradual in the Bentheimer sandstone while in the Berea sandstone the change is nearinstant. For rocks with small pore sizes and low initial permeability, the reduction in effective permeability is larger and the rate of permeability decrease is faster.

Experiments at temperatures between 30 and 90°C show that with increasing temperature Henry’s law becomes increasingly inaccurate to find the onset of the degassing process. The onset degassing pressure increases with temperature but are significantly lower than Henry’s law. This inaccuracy can partly be explained by the fluid in the core not reaching the right temperature or by the values used for extrapolating Henry’s law for different temperatures. In experiments performed with a 1M NaCl brine the pressure at which the degassing process starts is higher than for experiments performed with tap water. The increased salinity does not influence the change in effective permeability due to degassing. The reason for this is that the change in interfacial tension between CO2 does not change sufficiently between tap water and Brine to cause differences in the degassing process.

The results obtained from this research can be used for successful management of geothermal projects. It shows at what conditions the degassing process starts and what the effect of this process is. Once the conditions are known they can be avoided. The research also shows the degassing process and its effects can be reversed.

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.

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.

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.