Accurate estimation of the minimum horizontal stress is essential for well design, drilling safety, and predicting induced seismicity. In many regions of the Netherlands, however, only formation integrity tests (FIT) and leak-off tests (LOT) are available, raising the question of whether these drilling tests can be used to reliably infer the minimum horizontal stress. This study evaluates the reliability of stress estimation from hydraulic fracturing data, with a particular focus on the differences between fracture initiation and closure pressures.A comprehensive dataset of extended leak-off tests (XLOT), micro-fracture tests, and conventional LOT/FIT data is analysed. Multiple closure interpretation methods are compared, and their variability, bias, and applicability are quantified. In addition, initiation-based pressures are evaluated within a mechanical framework, including sensitivity analyses and stress consistency checks.The results show that closure-derived pressures provide the most reliable estimate of the minimum horizontal stress, whereas initiation-based pressures are strongly influenced by near-wellbore effects and operational conditions. Closure interpretation is inherently method-dependent, with inter-method differences up to 25~bar, significantly exceeding the repeatability within a single method. Among the evaluated techniques, the semilogarithmic derivative method demonstrates the best balance between robustness and applicability.Pressures from FIT and LOT tests often exceed closure-derived minimum horizontal stress and exhibit large variability, including non-physical results in a significant fraction of cases. While these pressures can be interpreted as upper or lower bounds within a mechanical framework, their quantitative reliability is limited. Furthermore, an offset data case study from nearby wells did not provide a reliable predictor the minimum horizontal stress.This study establishes a hierarchical interpretation framework in which closure-derived pressures form the primary estimate of the minimum horizontal stress, while initiation-based pressures provide only supplementary constraints.
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In the Netherlands, geothermal energy is considered a major contributor to achieving climate and energy goals. The success of geothermal projects depends strongly on understanding the reservoir. Knowing the thermo physical and mechanical properties of reservoir rocks, which govern heat transfer and mechanical stability, is therefore essential. However, these properties are often poorly constrained due to high measurement costs, the substantial time required for laboratory testing, and limited availability of suitable rock material. In addition, reliable rock property prediction is challenging because of strong heterogeneity in lithology, mineralogical composition, and diagenetic history.
This thesis generates a comprehensive database of thermal, acoustic, and mechanical properties for key Dutch geothermal formations. Based on measured data and their integration with downhole petrophysical logs, several predictive equations and models were developed, including machine learning approaches. These models improve property prediction tailored to the Dutch subsurface and enhance geothermal reservoir characterisation in general.
The research begins with a comprehensive study of Permian Rotliegend sandstones, a key geothermal reservoir in the Netherlands. More than 1100 core plugs were analysed to determine porosity, density, acoustic velocities, thermal properties, and mineralogy. The results confirm that porosity is the primary control on most rock properties. Higher porosity corresponds to lower density, acoustic velocity, thermal conductivity, and diffusivity. Systematic deviations from porosity trends were linked to mineralogical and diagenetic factors. For example, nacrite and other kaolinite group minerals enhanced thermal conductivity beyond porosity based predictions, whereas other clay types reduced it. Porosity dominates, but mineralogy and texture impose measurable secondary effects.
The analysis was extended to the Triassic Main Buntsandstein Subgroup, with more than 700 core plugs studied and compared directly to the Rotliegend dataset. Similar porosity dependent trends were observed, but systematic inter formation differences emerged. At equal porosity, Buntsandstein samples show lower thermal conductivity than Rotliegend samples. This difference is attributed to variations in clay type and distribution, as well as mineralogical features such as dolomite cementation and replacive clays. The lower Cretaceous Delft Sandstone Member was investigated to assess coupled mechanical and thermal behaviour. Laboratory tests included ultrasonic velocity measurements, thermal properties, and mechanical loading. Dynamic elastic moduli derived from ultrasonic data were systematically higher than static moduli measured during loading. A lithology specific workflow was developed to convert dynamic to static Young modulus, enabling continuous static modulus logs. Sandstones follow trends comparable to Permian samples, while clay rich intervals exhibit distinct but explainable behaviour due to low porosity.
The final part focuses on machine learning based prediction of thermal properties using laboratory and well log data. Ensemble models and regularised regression achieved promising results for thermal conductivity prediction, even in unseen wells. Thermal diffusivity remained poorly predictable, reflecting its sensitivity to mineralogical and microstructural factors. Density and acoustic features dominate conductivity prediction, whereas no single parameter controls diffusivity.
Overall, this thesis establishes a coherent framework for predicting thermo physical and mechanical properties of Dutch geothermal sandstones. It combines laboratory measurements, petrophysical analysis, and machine learning to improve reservoir characterisation and support reliable geothermal resource assessment.
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Understanding the behaviour of rocks subjected to high temperatures and pressures is essential for a wide range of subsurface applications. This thesis aims to develop an image analysis methodology to measure the porosities and permeabilities of rock samples. These samples were created as part of the doctoral thesis of K.H.A.A. Wolf by compacting rock rubble in a copper tube and subjecting it to various high temperatures and pressures. Despite earlier image analysis on these samples, there was an opportunity for improvement using higher-resolution imaging and more advanced processing techniques. UsingaDSLRcameraand2typesoflight (normal and UV), 33 samples were photographed and then analysed using Adobe Photoshop and ImageJ. Additionally, three samples were selected to be scanned using a µCT scanner. The image processing included the conversion to greyscale, binarisation and the use of the morphological ”open” operation. Porosity was directly measured, while permeability was estimated using a modified Kozeny-Carman equation. The quality of the impregnation of the sample with UV dye differed, yet for 28 out of the 33 samples, imaging with UV light worked better than with normal light. Using temperature data from when the samples were formed, a clear trend emerged showing that the higher the temperature, the lower the porosity and permeability become. This trend was also visible in the work by Wolf (2006) and the values were generally in agreement, except for the samples with extremely low porosities and permeabilities. While CT and image analysis gave similar porosity values, permeability results differed, highlighting the need for simulation-based permeability estimates from CT data. Further research into impregnation techniques is also recommended to enhance future image analysis workflows. ... Read more

This thesis explores the occurrence of salt dry-out and hydrate formation when injecting CO2 into porous media. In large-scale CCS projects, injecting CO2 can potentially lead to salt precipitation or hydrate formation. These processes diminish injectivity and negatively alter reservoir rock properties. To gain deeper insight, experiments were conducted utilizing microfluidic setups, which allow for visual observation of salt-crystal or hydrate formation. Using microfluidic chips, ten salt dry-out experiments were conducted with varying pore sizes and six hydrate experiments were conducted with varying pulse trigger times. For the dry-out experiments, it was shown that salt crystals form mostly at the outlet side and that heterogeneity has a large impact on the precipitation process. A heterogeneous pattern results in a shift in salt distribution to the small pores, with results showing salt saturation at 7% in the small pore section of the medium-small pore chip, exceeding the 3% in the larger pore section. This shows the significant role of capillary action on salt precipitation. Results highlight that higher CO2 flow rates accelerate water evaporation and salt formation, yet final salt precipitation levels remain similar across varied flow rates. For instance, in the small pore size chip, final salt saturation was observed at 7% with a flow rate of 4.38 mm/s, decreasing to 4% at 0.78 mm/s. Additionally, the importance of high water saturation for salt dry-out and the impact of water backflow is shown. For hydrate formation, the importance of temperature and pressure was noted in these experiments. Three different pressure pulses were employed: manual control, a 0.5-second electronic pulse, and a 0.2-second electronic pulse, which all showing great effect on hydrate formation, yet no correlation could be determined between pulse length and hydrate saturation. Specifically, manual control yielded a 15% hydrate saturation with a 9.4% conversion factor, while the 0.5-second pulse achieved a 7% saturation and 9.1% conversion factor. The 0.2-second pulse resulted in 8% saturation and a 5.9% conversion factor. For the dissociation, the large effect of temperature was observed. All experiments showed a stable hydrate concentration, and a dissociation temperature between 5 and 9°C where temperature differences as small as 0.1°C were shown to be the difference between no dissociation and complete disappearance of all hydrates. Next to this kinetics, an interesting observation regarding the hydrate morphology was made. In addition to the five hydrate morphologies found in literature, a sixth, ‘sheet’-like type was observed. ... Read more

Addressing climate change and transitioning to renewable energy will involve subsurface activities like carbon storage, geothermal exploitation, and underground energy storage. However, fluid injection and extraction in the subsurface can alter the pressure, temperature, stress, and rock geochemistry, potentially leading to seismicity and subsidence. Understanding the mechanisms of fault reactivation and the geomechanical response of intact reservoir rock to variations in pore fluid pressure from injection and depletion operations is thus crucial. In this thesis, we integrate our findings from multiple studies to explore the impact of parameters related to injection and depletion, such as pattern (monotonic, cyclic) and rate, on the deformation of intact reservoir rock, slip behaviour in faulted reservoir rock, and the evolution of microseismicity, with a focus on how we can mitigate induced seismicity.

Our experimental investigations employ uniaxial compressive tests on intact Red Felser sandstone samples, subjecting them to cyclic recursive (CR), cyclic progressive (CP), and monotonic stress patterns at varying stress rates. The recording of Acoustic Emission (AE) waveforms revealed that cyclic stress patterns, especially CP, are characterized by lower maximum AE amplitudes compared to the monotonic pattern. By reducing the stress rate, the maximum AE energy and final mechanical strength both decrease significantly. Moreover, high-stress rates were found to alter the AE signature of events, suggesting that cyclic stress patterns combined with low-stress rates may mitigate induced seismicity in subsurface injection operations.

For underground energy storage, we investigate the geomechanical response of Red Felser sandstone to cyclic loading, crucial for safe and efficient underground porous reservoir operations. Experimental results, complemented by constitutive modeling, revealed various deformation mechanisms, including linear elastic, viscoelastic, and inelastic responses. Our study shows that the magnitude of inelastic deformations is influenced by mean stress, amplitude, and frequency of the stress waveform, with our models closely fitting the experimental data.

As part of our investigation into mitigating induced seismicity, we examine how stress and sliding patterns affect fault slip behaviour and seismicity evolution. To achieve this we carry out displacement-driven fault reactivation experiments on saw-cut Red Felser sandstones. Our results indicated that cyclic sliding, compared to continuous sliding, reduces seismicity but can accelerate slip velocity during the reloading phase due to the healing of gouge material on the fault plane. Additionally, under-threshold cycling effectively prevents seismicity and shear slip but poses a risk of increased seismicity if shear stress exceeds critical levels.

Furthermore, we explore the influence of injection pattern and rate on fault reactivation in porous Red Felser sandstone. High injection rates were linked to increased slip velocity and seismicity. Furthermore, our results from samples subjected to various injection patterns demonstrate that the cyclic recursive pattern exhibits a higher maximum slip velocity, more episodes of slow slip, and greater radiated AE energy than a monotonic pattern. A proper injection strategy must consider fault drainage, critical shear stress, injection rate, and injection pattern. Our results demonstrate that a monotonic injection pattern and low pressurization rate may mitigate seismicity on pre-existing faults in a highly permeable porous reservoir.

Finally, we investigate the fault slip nucleation within a displaced fault system. Our triaxial experiments on displaced faults reveal that differential compaction intensifies from the top of the sample towards the internal corner at the centre of the fault, indicating a variation in the stress field surrounding the fault plane. Our direct measurements near the displaced fault plane confirm the anomalies and peaks in stress observed in previous numerical and analytical studies.

This thesis offers new insights into the mechanical behaviour and seismicity evolution of intact and faulted reservoir rocks under variations in stress patterns and rates. These findings may contribute to mitigating injection-induced seismicity in intact and porous faulted rock settings. Furthermore, they enhance our understanding of the behaviour of deep geo-reservoirs subjected to diverse injection strategies, thereby expanding our knowledge of reservoir-related phenomena. ... Read more

Activities underground, such as gas extraction or fluid injection, can disturb the natural stresses present and can cause human-induced earthquakes along pre-existing faults. Even though they are related to engineering, these earthquakes are currently unpredictable. Monitoring and understanding how these earthquakes occur are essential for a safe use of the subsurface and to progress with mitigation measures and earthquake forecasting.
Current monitoring relies on post-failure seismic recordings, emphasizing the need for advancements in monitoring and forecasting techniques. Detecting stress changes before seismicity (pre-failure) occurs allows for the timely implementation of mitigation measures. Active seismic monitoring methods have the potential to detect stress changes early and as such precursory information that can improve the forecasting methods and models. However, there is still much to discover regarding the relationship between precursors and the underlying physics. In general, the common fault mechanisms during the seismic cycle are well known. Initial stress build-up is followed by first slip instabilities where the local stress exceeds the fault strength, leading up to a seismic event, during which stress on the fault is released. However, robust and reliable predicting of fault failure and the resulting earthquake has proven to be challenging even for reactivating experimental faults in a controlled laboratory setting.... ... Read more

Depleted-gas sandstone reservoirs in the Dutch North Sea are being evaluated for offshore carbon storage under the ARAMIS project. The variability in geomechanical properties urges an investigation of the elastic parameters. This study involved cored samples and logs from five Slochteren and Solling formations wells. It aims to characterize seventeen cored samples’ static and dynamic elastic behaviour experimentally via triaxial compression tests with active acoustics. Substantial variability was shown in wave velocities, moduli, Poisson and velocity ratios highly correlated to porosity, density and confining pressure variations. Therefore, empirical correlations were established between elastic moduli, confining pressure and porosity from the lab measurements. Another empirical correlation was established between the dynamic and the static Young’s modulus with a high correlation coefficient of 0.8. To enable elastic moduli log predictions with the absence of S-wave velocity log measurements, a workflow was developed to predict S-wave velocities using empirical correlations from lab measurements, nearby wells, and literature to evaluate the estimation quality. Comparisons between measured and predicted dynamic moduli showed agreement, validating the physics of the different datasets. However, it is important to mention that discrepancies existed for parameters like Poisson Ratio, indicating that factors like porosity affect predictions. Although there is limited data from logs and experiments, this study provides important geomechanical insights and predictive capabilities for this initial stage of geomechanical reservoir characterization for the ARAMIS CCS project. ... Read more

Measuring rock stress is a difficult process, especially in the Stjernøya Nepheline-Syenite mine in Northern Norway. The complex nature and topography of the shallow deposit makes it difficult to conduct industry standard tests. Therefore, this thesis aims at answering how different stress measurement techniques and their results compare to each other in a shallow low-stress hard-rock mine. To do this, an elaborate laboratory work is done to find a relation between stress (UCS) and acoustic properties and velocities of the nepheline syenite. Using that information, a similar combination of tests is done in the field, namely a flat jack test and acoustic velocity measurements with a hammer and geophones. To give further insight in the gathered field-data, visual classification methods, like RMR are done to verify and aid the tests and the results. The laboratory tests gave more insight in the relation between stress and acoustic velocity. There exists a positive, somewhat logarithmic relation between confining pressure and velocity through the sample, until the end of the elastic domain. Depending on the sample, this becomes more apparent after an initial loading phase between 0 and 10-20 MPa. In this low-stress zone, the wave arrival times could not be accurately observed and tend to give very low results. This problem translated to the field. Despite the flat-jack provided an understandable vertical pressure of 9.8 MPa, the acoustic measurements in the field did not always coincide with this pressure, according to the laboratory relationship, generally giving lower velocities than expected. However, a relation between rock quality and wave-velocity could also be present, as the cracks in the heterogeneous rock could heavily influence acoustic velocity. Therefore, this research concludes that both the flat jack and the acoustic investigative methods have their use in understanding the stress-profile of the shallow deposit. Given the low stresses in the mine, the acoustical investigative method could not be used directly to measure stress, but it can be used as a control for the flat jack tests and have use in monitoring of the rock wall. ... Read more

Following the 2015 Paris Agreement, many countries are switching from fossil fuel to a more renewable energy supply. However, the CO$_2$ concentration in the atmosphere keeps increasing which makes difficult to accelerate the transition to a net-zero emission. In 2020, a new record of CO$_2$ concentration at 412.5 ppm was achieved, the highest concentration seen in the last 800,000 years. For this reason, several technologies have been in the focus to help reducing the amount of CO$_2$ in the atmosphere as well as keeping it from increasing.

One effective technique in the mitigation of these emissions is called Carbon Capture and Storage (CCS). It is not a new technology, in fact, it has been used in the Oil \& Gas industry since the early 1970s for the purpose of Enhanced Oil Recovery (EOR). In very general terms, when CO$_2$ is mixed with petroleum in the subsurface, its viscosity decreases making it easier to extract more oil. In addition, some of the CO$_2$ gets trapped in the rock which introduced the idea to use the mechanism to reduce CO$_2$ concentrations in the atmosphere.

However, this technique is not absent of risks and it is one of the main drawbacks experienced with activities that involved the subsurface, the surrounding uncertainties are too great sometimes. Some of the risks that could be encountered when producing, injecting or drilling a well in the subsurface are: under or over pressurization, leakage, uplift or subsidence, induced seismicity, and fault reactivation. In order to reduce the mentioned risks, geomechanical studies help with the understanding of the rock behavior and their response to applied stresses.

In this thesis, UCS and Triaxial laboratory experiments were carried out to test the geomechanical behavior of different rocks coming from Norway, Denmark and Germany. For the triaxial tests, five different cycles at different and increasing $P_c$ were performed. Doing so, some main insights were obtained: the modulus of elasticity increases with depth and confining pressure more significantly with $P_c$ lower than 30 MPa, and plateauing at higher $P_c$; the Young's modulus decreases with increasing porosity and increases with increasing cohesion; the P and S waves velocities change when the yield point is surpassed; with low porosity and high cohesion the acoustic velocities are not dependent on the $P_c$, and vice versa; the SYM and DYM follow similar trends; and the elastic modulus is negatively dependent on the Poisson's ratio.

The information and data obtained from this geomechanical study will be of use to model predictions involving CCS projects to avoid reaching the risks factors mentioned. An example of the lack of knowledge and understanding about rock behavior could be the induced seismicity caused by the extraction of gas from the Groningen gas field. These seismic events acted as a turning point, sparking a necessary shift in perspective and paving the way for a more informed and balanced perception of activities involving the use of the subsurface. ... Read more

The level of advancement in the understanding of the mechanical properties of volcanic rocks is comparatively lower than that of sedimentary rocks. As part of the SUCCEED Project (Synergetic Utilisation of CO2 Storage Coupled with Geothermal Energy Deployment), which aims to investigate the feasibility of injecting captured and produced CO2 into the reservoirs to enhance geothermal production and achieve permanent CO2 storage at the Hellisheiði Geothermal Field in Iceland, this experimental research provides significant insights into the petrophysical and mechanical properties of the volcanic rocks collected from surface outcrops. The subsurface in Hellisheiði is mainly built up of hyaloclastite formations and interglacial basaltic lavas. During a field campaign samples were collected in different outcrops, ensuring that the samples were of high quality and sufficiently diverse to enable comprehensive analysis. Four samples per block and rock type have been prepared from the collected blocks, and they have been subjected to different laboratory tests to evaluate their petrophysical properties, such as porosity, density, and permeability, and their geomechanical behavior, using Unconfined Compression Test (UCS), Active-Source Acoustic Test, and Splitting Tensile Strength Test. Additionally, laboratory experiments have been conducted to investigate the impact of rapid cooling on rock damage due to thermal fracturing. The results show that there are interdependent relationships between porosity, bulk density, ultimate strength, Young's modulus, and wave velocities that can be observed when considering average values per rock. The rocks studied showed a negative correlation between porosity and other parameters and a direct correlation between ultimate strength and Young's Modulus. When examining individual rock samples, no significant correlations were observed between porosity and other parameters, however, those correlations where evident when comparing between different rock types, emphasizing the importance of analyzing rock properties from a broader perspective. The rocks studied could be classified into five units based on their petrophysical and mechanical properties. Ordered from higher porosity and lower mechanical parameters, these units are: unit 1 consists of hyaloclastite HH-1, unit 2 includes porous basalts HBA-18 and HPB-23, unit 3 consists of low-porosity basalts HB-4 and HBimp-9, unit 4 is made up of dike ND-6, and unit 5 comprises gabbro G. This implies that there is a notable variation in the properties of rocks between different units, but the properties of rocks within the same unit do not differ significantly. Volcanic rocks have a significant amount of unconnected porosity, which, if connected, can enhance the storage capacity of the reservoir and improve the reactive surface area of the rocks in contact with the reinjected fluid, leading to a more efficient mineral storage process. The results of a thermal shock conducted to simulate reservoir and injection temperatures (270ºC and 60ºC) have shown no significant changes in the petrophysical and mechanical properties of the rocks, indicating that this temperature difference does not increase the effective porosity nor compromise the integrity of the reservoir. This study validates the potential use of certain rocks collected from surface outcrops in Hellisheiði as reservoir analogs for future geological models, particularly those with lower porosity. ... Read more

Underground energy storage (UES) in porous and cavity reservoirs can be used to balance the mismatch between the production and demand of renewable energy, as well as for securing gas and oil supply during shortage or high demand periods. Understanding the geomechanical behavior of these reservoirs under different storage conditions, i.e., storage frequency and fluid pressure, is key in defining their capacity and effective lifetime. This thesis work presents a rigorous analysis performed on sandstones to unravel their geomechanical response under cyclic loading. This study includes, importantly, both experimental and numerical investigations under several conditions which are relevant to UES. The rock response was studied considering cyclic stress states above and below the onset of dilatant cracking, under different frequencies and amplitudes. Within the number of cycles studied, measurements of axial strains and acoustic emissions indicated that inelastic strains accumulated cycle after cycle following an exponentially decreasing rate. Five types of deformations were interpreted: elastic, plastic, viscoelastic, cyclic-plastic and brittle creep. Based on these novel experimental results and observations, Nishihara's constitutive model was used for simulating viscoelastic and brittle creep deformations, while dilatant plastic strains were modeled using a Hardening-Softening model. Finally, an extension of the Modified Cam-Clay model was proposed to account for cyclic-plastic compaction. This approach can be extended and improved to study cyclic sandstone deformation's implications on subsidence, fault reactivation and cap rock flexure, among other physical phenomena impacting a reservoir's storage capacity. ... Read more

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. ... Read more

The Southern Chotts and Jeffara Basins are situated within the Saharan Domain of Central Tunisia, North Africa. The Southern Chotts Basin hosts reservoirs within the Triassic, Permian and Ordovician units that contain significant hydrocarbon accumulations whilst the Jeffara Basin contains outcrop analogues of the same hydrocarbon­bearing formations. The basins experienced a late Hercynian shortening phase which involved the uplift of a major topographic high (Tebaga de Medenine). This high, in conjunction with a older regional high, the Telemzane Arch influenced the deposition and geometry of the Permian and Triassic units across both basins. This shortening event is characterised at the scale of hundreds of meters by E­W striking folds into which the mid­late Triassic and early Jurassic units are deposited. The folding is also observed at field scale (10’s meters) through small fault­related folds in the Permian deposits of the Tebaga de Medenine. This late Hercynian phase occurs between the late Permian and early Jurassic in the basins. Fracture data collected from the upper Permian and lower Triassic units (Jeffara Basin) provides an analogue to the fracture networks at depth (Southern Chotts Basin) in the Paleozoic reservoirs. A conjugate fracture system observed in the field (from fracture pavements) corroborates with the interpretation of regional shortening in the basins. Seismic attribute analysis on depth slices in the Paleozoic reservoirs also shows the conjugate system at depth. This analysis is integrated with outcrop fracture data and FMI data from wells to create an open fold distributed fracture model of the system in the basins. This model indicates the main driver for fracture generation in the region is folding and is used to predict the fracture networks at depth. This is undertaken using discrete fracture network (DFN) modelling of the subsurface. This model is integrated with a deterministic model from the seismic time slices to create a hybrid predictive fracture model of the early Paleozoic reservoirs. Analytical aperture modelling of the fracture model demonstrates the fractures varied in openness depending on orientation and fracture length. The conjugate set orientated at 240∘and longer joints detected from seismic attributes presented the widest aperture size. These fractures in the subsurface at implications on the transmissibility of the reservoirs, especially Permian units which have low bulk rock permeability and the lower Triassic (TAGI) sequences which are susceptible to compartmentalisation. ... Read more

Mudstones play an important role in hydrocarbon exploration and production, carbon capture and storage, and nuclear waste disposal. The high concentration of clay minerals contribute to the high intrinsic anisotropy (e.g., velocity, strength, permeability, and resistivity changes with direction) of mudstones. This high anisotropy complicates, among other things, seismic interpretation for hydrocarbon exploration and production, as well as predictions on the mechanical behaviour of these clayrich rocks. Mudstones are also characterized by a low-permeability matrix, which makes it difficult for fluids to flow through the rock. This impermeable character of mudstones makes them a potential natural seal for long-term CO2 storage and a potential host rock for nuclear waste disposal. For hydrocarbon production, open fractures are needed to enhance the productivity of oil and gas reservoirs, whereas the presence of such fractures can result in unwanted leakage of CO2 or nuclear waste in the subsurface. Fracture formation depends on, among other things, the mechanical properties of the mudstone. It is thus important to understand the elastic anisotropy and mechanical properties of mudstones for successful hydrocarbon exploration and production, and to safely store CO2 and radioactive waste in the subsurface. Although mudstones are important in the energy sector, the understanding of their elastic anisotropy and deformation behaviour under various physical conditions is limited, due to their complex character and the lack of laboratory experiments performed on well-preserved samples. ... Read more

Irreversible damage induced by stress in brittle rock is accompanied by the formation of micro-cracks. The strain energy released during the fracturing process is released in the form of acoustic emission. This thesis applied the non-intrusive method of acoustic emission monitoring to assess the deformation process of brittle rock during cyclical loading and loading to failure in a standard uniaxial compressive strength test set-up. It is confirmed that the stress-strain curve is clearly separated into five phases and that the cumulative hits recorded throughout the failure process correspond with these five phases. Furthermore it is found that there is an obvious rise in trend in the amplitude of the acoustic events as the rock nears failure, but that individual events of high amplitude should not be considered indicative for the damage in the rock. Additionally it is found that high amplitudes characteristic for near failure stress are recorded at 25\% of the failure stress if the rock has been under high stresses, indicating a change in fracture mode. The Kaiser Effect, the phenomenon defined as the absence of detectable acoustic emission events until the load imposed on the material exceeds the previous applied level, was confirmed during uniaxial cyclical loading. Through loading samples from a highly stressed pillar in the Nepheline Syenite Stjernoya Mine in Norway, it is found that the onset of acoustic emission may be indicative of the stress in the pillar. These findings are useful to further develop acoustic emission as a monitoring method in a range of applied earth science applications. ... Read more

The heterogeneity of layered systems leads to variation in rock or soil mechanical properties, influencing the resistance to failure. A formation breaks by fracture propagation when the effective stress surpasses the formation strength. The propagation of a fracture through a mechanical interface depends on whether the formation strength of the second formation is overcome. This critical effective stress level could be trespassed by high natural or industrial induced pore pressures. Understanding fracture propagation in multi-layered systems with variable pore pressure regimes has important implications and applications to many industries such as quarrying and hydraulic stimulation. The slope stability of Westerwald Clay Quarries is influenced by inter-bedding of thin sand layers. Surface and slope fractures within the clay formation originated from a high observed hydrostatic head within the sand layers and a reduced confining stress from mining activities. The slopes of the quarry are key in determining the volume of economically mineable clay, these slopes are in turn controlled by the size/extent of fractures and whether they extend through multiple formations. This study examines the effect of fracture continuation from sand layers into the stiff Westerwald Clay Formation. The soil parameters (cohesion and friction angle) of the different lithologies within a Westerwald Clay Quarry are determined for slope stability analysis by shearbox testing. Soil classification has been done in terms of plasticity, grain size and mineralogy by Atterberg Limits, Sieving and XRD & XRF respectively. The results show that fracture initiation within the Westerwald quarries is a combination of mining activities lowering the confining stress and a constant natural hydraulic head. The hydraulic head within the small sand formation lowers slope stability by causing fracture initiation and water infiltration into the clay formation. Slope stabilisation occurs by artificial water pumping or natural water dissipation lowering the hydraulic head. Slope stability is decreased by embankments of low permeability backfill and increased by high permeability backfill. Inter-bedded systems are common target locations for an unconventional reservoir systems and can be found both within source rocks as well as in conventional geological traps. Improvement in recovery from these tight systems often depends on the extent and continuity of fractures through heterogeneous interfaces. This study examines the propagation and continuity of the fractures in an artificial heterogeneous layered system. The fractures will be initiated by hydraulic fracturing in a dried layered system via water injection in a triaxial cell. Fracture propagation is analysed through Micro-CT scans. The mechanical properties such as acoustic wave velocities, unconfined & confined compressive strength and tensile strength are all determined for the analysed layered systems. The results show that hydraulic fractures initiated within the weakest layer are arrested at the interface between a mechanically weak and strong formation, whereas fractures initiated within mechanically stronger layers prograde through the interface. Hydraulic fractures are initiated when local pressure difference at the interface exceeds the formation’s critical tensile stress, the formations critical tensile strength is dependent on the confining pressure. ... Read more

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. ... Read more

Geothermal energy is a relatively sustainable energy source of which the essence is to extract heat from hot subsurface rocks. Circulating fluids serve as the transport agent of heat. The contact area between the fluids and the rocks is where the relevant heat transfer occurs, i.e., where the water is heated up. In some geothermal reservoirs, this circulation occurs naturally through porous matrix (mainly sediments) or through heavily fractured formations. Enhanced geothermal systems (EGS) are potentially favorable reservoirs where subsurface permeability is increased by means of artificial stimulation techniques. These stimulation techniques often involve hydraulic fracturing where a limited existing fracture network is expanded or “enhanced” by injecting fluids under high pressure conditions. While the geometry of the generated fractures influences permeability, the effect on heat exchange has received less attention. This thesis discusses the effects of fracture geometry on the heat transfer between solid and fluid. Along with laboratory experiments, numerical simulations were conducted. All investigations were performed on igneous granite rocks. Tensile fractures were generated to allow a fluid flow along the otherwise impermeable rock samples. Several parameters were varied throughout the experiments and simulations including volumetric flow rate, fracture aperture, rock temperature and fracture geometry and surface area in order to investigate their impact on heat transfer processes. Flow rate variations in the experiments have shown that higher flow rates cause the fluid to absorb less heat per unit volume and cause the rock to cool down more extensively, therefore thermal depletion of the reservoir is likely to occur within a shorter time frame. The dependency of exchanged heat on fracture aperture variations (in the range of 0.05 to 0.5 mm) did not yield a clear trend within the experiments, but does so in numerical simulations. Aperture variations in the numerical simulations did not cause notable differences in transferred heat as long as the volumetric flow rate is kept constant. However, as the fluid velocity is kept constant the amount of fluid flushed along the fracture per unit time is affected by varying apertures. This causes a difference in heat transfer as well. Increased fracture surface areas alone (more extensive topology/roughness) have shown a minimal impact on the heat production while a more extensive fracture network (additional branches) has shown notable enhancement in the amount of heat produced. Cooling behavior of the rock has shown correlations with Newton’s law of cooling and suggests a limitation of heat production by the heat conduction occurring within the rock.
Experimental findings cannot directly be compared with natural reservoir conditions. The reason for this is a thermal equilibrium that is achieved at each flow experiment, i.e., the heat withdrawn equals the heat resupplied by a heater. In natural reservoirs this is often not the case where a cold front propagates towards the production well and determines the lifetime of how long heat can efficiently be produced from a certain rock mass. This results in an unsteady heat conduction where the heat withdrawn does not equal the heat resupplied. ... Read more

Rapid injectivity decline is frequently observed during injection in unconsolidated sand reservoirs. Field data suggests that hydraulic fracture processes are directly or indirectly related to this injectivity decline. Conventional fracture theories do not apply to unconsolidated sand since this material has little to no cohesion and tensile strength. The main fracture mechanism hypotheses are shear failure of the zone ahead of the fracture tip and fluidization. For both mechanisms, a fluid pressure high enough to initiate and propagate the fracture is required. The fluid pressure is dependent on the injection rate, fluid viscosity and permeability of the formation. Unconsolidated sands have a high permeability, thus under normal waterflood conditions a high injection pressure is not expected. Three main impairment mechanisms leading to the injectivity decline have been identified based on field evidence and previous work:

-Plugging
-Wellbore fill
-Resorting of grains and finer particles

Plugging results from the infiltration of fines originating from the injection fluid, crossflow or drilling mud. The external and/or internal filter cake can locally reduce the permeability of the formation. During surface shut-ins, backflow and/or crossflow can occur leading to the infiltration of solid particles and fluids into the wellbore. This reduces the leak-off area of the well. Lastly, resorting of grains and finer particles can result in a denser packing of the reservoir. The dynamically mixing of particles can lead to lower permeability regions.

Research goal
The main goal of this research is to develop a better qualitative and quantitative description of the fracturing process and the impairment mechanisms causing the observed injectivity decline. This thesis comprises of the first phase of this research, focusing on the capabilities of the equipment to create and detect fractures under waterflooding conditions. What makes this research unique is the use of low viscosity fluids, to mimic field conditions. Other work often involves the use of efficient fracturing fluids that have a high viscosity and/or good filter cake building capabilities to minimalize the leak-off. Next to that, injection of fluids is performed live in a CT scanner. This allows the visualization of fractures or low-density regions through density distributions in three dimensions over time.
Equipment
Injection takes place in a high-strength aluminium vessel with a sample volume of 3.84 dm3. See images 2.1 and 2.2 of an overview of the setup and pressure vessel. Axial and radial pressure can be controlled independently up to 20 MPa. A pore fluid system records the outflow mass and provides a fluid pressure on the sample. The sample consists of a very fine, very well sorted sand with a permeability around 5 Darcy. The main injection fluids are water and Fluorinert FC-770. This is a high density, low viscosity fluid that is used to visualize the preferential flow path of the injection fluid in the CT scanner.
Results
Fractures have been successfully created using a high viscosity fluid during the first experiment. The goal of that experiment was to test the setup and the equipment. The fractures were created at an injection pressure of 38 MPa and were up to 1 cm long and 2 mm wide. Experiments 2 and 3 were performed in the CT scanner with the use of Fluorinert as the injection fluid. The infiltration zone of this fluid was clearly visible but no fractures were created. Sand infiltration in the injection tube leaded to a number of problems during the experiments. Experiments 4 and 5 added fines to the injection water. Quartz powder was used in experiment 4 and bone meal in experiment 5. The fines leaded to a gradual increase in injection pressure, but did not lead to a higher density in the CT scans. No fractures were observed but low-density regions in front of the perforations were created during both experiments as a result of backflow. Experiment 6 introduced the use of internal pressure sensors in the sample and used a sample created with two sands and Kaolinite, a non-swelling clay. During two high flowrate injection cycles, the clays migrated away from the near wellbore region, leaving behind lower density regions.
Conclusion & future work
Creating fractures with low viscosity fluids in a laboratory environment has proven to be difficult. No fractures have been created throughout the low viscosity experiments. Several impairment mechanisms that were identified in the field have also been observed in the experiments. This thesis forms a solid basis for the next research phase to investigate these impairment mechanisms more closely. By lowering the confining stresses, increasing the flow rates and decreasing the sample permeability, there is a good probability that fractures can be created with this equipment in future work.
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The induced seismic activities and subsidence in the Groningen region urges for deeper investigation of the mechanical elastic parameters and lithological facies. A recently (2015) drilled well in the area of Zeerijp has provided a rich dataset from the Permian and Carboniferous to be analyzed and eventually help to understand and characterize the penetrated intervals.
The well was cored and logged extensively, providing a wide and diverse database that includes well logs, computed tomography (CT) scans, x-ray diffraction (XRD), petrography, routine core analysis (RCAL), scratch test, unconfined compression test (UCS) and triaxial compression test (TCS). These data were integrated using the disciplines of petrophysics, rock physics, geology and geomechanics, in order to analyze and build one lithology- and one mechanical- data based model that describe the Permian and Carboniferous section.
Each lithology- and mechanical- model consisted of six different facies; four sandstones and two shales facies were classified using the data and the understanding of the geological depositional model. The generated geology-reflected lithology facies model with the proposed workflow can aid into building a more reliable 3D geological model. The benefits of this methodology can be extended to assist in a more robust dynamic modeling. Additionally, the mechanical model can be used to provide granularity in previous mechanical models, not only for the reservoir, but also for the over- and under-burden. The two models (lithology- and mechanical-facies model) correlate 70% in general. ... Read more