Author: Patrick Lutz

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Cementing is a well-established practice in the oil and gas industry. Its main purpose is to protect the wellbore from the surrounding downhole environment which includes prevention of unwanted communication of formation fluids with the wellbore or other permeable horizons. During a cement job the existing mud in the wellbore will be displaced by the spacer, to clean the pipe and borehole wall, followed by cement and a displacement fluid which usually is a mud. The intermixing between these fluids (spacer, cement, and mud) could arise during the placement phase which tends to affect the specified cement properties and hence jeopardize the quality of a cement job. Thus, a better understanding of intermixing during the fluid displacement phase is required to improve the fluid compatibility in mitigating this problem. The main goal of this thesis is to generate ultrasonic data for several commonly used materials in the oil and gas industry to prepare muds, spacers, and cements. A baseline study is conducted to measure the variation in sonic velocity of individual materials dispersed in water. The generated baseline database will serve as a reference point to predict the sonic velocity in the mixed fluid. A feasibility study is conducted to determine the practicality of ultrasonic sensors to determine the sonic velocity of different fluids. The result of this study poses new questions which have been answered in the static single additive experiments. A total of thirteen (13) commonly used drilling and cementing additives are analyzed using a custom-made ultrasonic setup. Therefore, fluids of different concentration of each additive are mixed and the average sonic velocity determined. The results of this study give an intrinsic insight into the effect of each additive on the sonic velocity. Finally, a proof-of-concept experiment is presented to display how the acquired knowledge can be applied in the field. Therefore, two (2) muds of different density are mixed and displaced on a benchtop setup. Fluid discrimination, density evaluation, degree of intermixing calculation and required volume for full displacement prediction is successfully conducted and presented. Most of the objectives of this thesis are successfully achieved and are presented in detail.

Author: Ilia Filippov

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Many E&P Operators are today assessing the viability of converting existing assets designed for production of hydrocarbons into assets designed for injection and permanent monitoring of CO2. This tendency is mainly driven by laws and regulations aiming at reducing CO2 emissions, and permanent storage in depleted reservoirs is seen as a great enabler to achieve just that. The number of CCS projects is growing because of the prospect to reduce the carbon footprint of oil and gas operations by converting existing production wells to CO2 storage wells. The use of existing wells has certain advantages. On the one hand, there is no need to drill new wells, thereby reducing potential costs, which will be particularly significant when drilling offshore. On the other hand, there is a safety aspect: the fewer wells that penetrate the caprock, the better the integrity of the storage is. The main objective of this work is to create a workflow of converting existing oil/gas production wells into injector/monitoring wells in CCS projects and adapt it to one or several company’s existing wells in the Dutch sector of the North Sea.

Author: Stefan Erakovic

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Casing leaks are a common issue in the oil and gas industry that might lead to severe production problems. Corrosive fluids often cause casing leaks, as well as prolonged exposure to corrosive gases, casing fractures under pressure, or casing wear from extended periods of drilling work. While drilling, the cementing process is done between the formation and the casing or between two casings to provide zonal isolation and well protection. If the cementing process is done poorly, a well barrier can fail, and cracks and micro-fractures can appear, allowing corrosive fluids to migrate. This can slowly corrode tubing and casing over time. Another possibility for casing corrosion to occur is from fluids flowing inside the wellbore, leading to the same consequences. Thus, casing leaks must be detected early to prevent these losses, which can result in substantial expenses. This master¿s thesis is focused on casing leak remediation by using conventional and ultrafine cement, as well as epoxy resin. Each fluid was tested according to API standards, and later, the injectivity tests were done for that fluid in the casing leak setup, which was developed in the previous master¿s thesis. Upon completing the API and the injectivity tests for one fluid, the data was analyzed, and the next fluid was tested. As a reference point, injectivity tests for water were also conducted. The injectivity tests of cement and epoxy resin were done using three different internal tubing diameters, which include 4,572 mm, 1,753 mm, and 0,774 mm. The results of the experiments demonstrated that all fluids successfully passed through 4,572 mm internal tubing diameter without problems. The injectivity tests for the 1,753 mm internal tubing diameter showed the influence of solids inside the liquid, with the conventional cement slurry starting to plug and the pressure spikes starting to appear. Other fluids passed through successfully without issues. The injectivity tests for 0,774 mm indicated that only solid-free liquids like epoxy resin can guarantee a successful casing leak remediation. However, due to the high cost of epoxy resin, an attempt should be first made with the ultrafine cement slurry.

Author: Emanuel Hofer

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This thesis covers the methodology of the development of a data analysis tool for designing kick-off plugs as well as laboratory-based simulations and experiments in order to validate the prediction quality. The data analysis tool can be used to design cement plugs and to simulate the consequence of specific fluid rheological parameters as well as distinctive selected parameters on the outcome of the plug job. The goal of this thesis is the implementation of a simple, field applicable and intuitive program that enables the engineer to design a kick-off plug that fulfils all requirements for a successful placement of the plug on the first attempt. The thesis describes the development of the data analysis tool starting with a detailed literature review where the most prominent industry related cement plug issues are described in more detail. Based on the assessment, a root cause analysis is implemented that reduces the common plug problems to four distinctive elements. Following the root cause analysis, the development of the design software and its individual modules are explained in detail. All four elements as well as the basic workflow and their structure are illustrated properly. In order to validate the outcome and the prediction quality of the software, laboratory-based simulations are executed. Prior to executing lab simulation runs, they were mathematically simulated using the data analysis tool. Afterwards predicted parameters and observed laboratory results are compared and rated. In addition, computed tomography images (CT scans) support the assessment and enable a direct look into the laboratory produced kick-off plugs. In a last step, a novel compressive strength enhancing material is tested. Therefore, the compressive strength behaviour of a neat Class G cement and fibre reinforced cement cubes are compared and benchmarked. Recommendations as well as results and future work steps can be found in the appropriate sections as part of the discussion and conclusion chapters at the end of this master thesis.

Author: Stefan Lebwohl

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Carbonation of Portland-based cement systems poses a significant risk to well integrity in the harsh environment of CO2 injection wells. Due to the corrosion potential of CO2 in conjunction with water, the cement matrix is attacked, resulting in cement degradation. Therefore, a fundamental knowledge of the ongoing changes in physical, mineralogical, and mechanical properties within the cement matrix and the propagation rate of the carbonation front has the highest priority to ensure well safety throughout its entire life cycle. In this thesis, an innovative, in-situ, computed tomography (CT) -scannable test cell will be presented, enabling unprecedented research on the carbonation propagation over time. Furthermore, with this pressure cell, a system permeability indication, real-time monitoring, and cement curing within the cell under simulated downhole conditions were realized. The carbonation resistance of the specially designed cement was additionally investigated by an autoclave experiment, where the cement samples were exposed to a supercritical CO2 environment for 28 days. Mineralogical investigations, such as optical microscopy, scanning electron microscopy (SEM), element mapping, and X-ray diffraction (XRD) analysis provided insights into the cement matrix alteration and the distribution of cement phases. Compressive strength tests were conducted on the untreated and CO2-exposed samples to evaluate the cement¿s strength alteration. The results of the mineralogical investigation show that the cement matrix was significantly chemically restructured by the influence of CO2. Both the autoclave and in-situ test cell experiment verified a self-healing effect due to calcium carbonate precipitation. Impressive results were obtained by the CT-scan analysis. These show that the propagation rate of the carbonation front reached a maximum after CO2 injection and declined steadily with time since a protective layer was formed.

Author: Paya Roknian

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Well integrity is one of the most important concepts in the upstream oil and gas industry. No matter whether a well is in drilling phase or has been already completed, the well integrity should be maintained. It is a multidisciplinary approach and has always been a serious challenge for major oil companies, regardless of operator or contractor. Therefore, all various departments in an oil company including drilling, completion, production, and plug and abandonment must cooperate and apply different methods, software and hardware, in order to maintain the well integrity for each operation of each section of each well. This research work aims to perform ultrasonic measurement as a non-destructive evaluation method (NDE) to estimate the compressive strength of oil well cement with different densities and recipes at several ages. Therefore, the main project’s target is to find a correlation between these two mechanical properties of oil well cement, the uniaxial compressive strength and the ultrasonic wave velocity, by inducing ultrasonic waves into the specific cement with unique composition at the known age. Using the correlation allows estimating the compressive strength of the cement in various densities and formulations other than the ones used in this project. As to the methodology, six cement compositions with different recipes have been chosen for the linear measurement tests in which the ultrasonic wave velocity is correlated to the compressive strength of oil well cement. Additives used to prepare a wide density range from 11.0 to 16.0 ppg include barite, bentonite, and 3M glass bubble. Different regression methods have been tested only best, with the highest R2 of 0.96 chosen to estimate the strength for 2D models. The factors considered in the model as input include water-to-cement ratio, density, ultrasonic wave velocity, and age. The result shows that the composition with higher density, made by barite, is more resistant against being fractured and can convey an ultrasonic wave faster, while the lightweight cement, bentonite, and glass bubble made, has a lower compressive strength and a slower ultrasonic wave velocity. Moreover, a function model is created based on the correlations between compressive strength, ultrasonic wave velocity, age periods, and density. The model's inputs are ultrasonic wave velocity, age, and cement density, while the compressive strength is the model’s output. Moreover, by using the acquired data, different correlations among (UCS, density, age) and (UCS, UWV) and (UCS, age) and (UWV, age) have been shown all of which have accuracy above 0.96 R2. Among the 2D correlations, UWV showed the best power fit for all cement samples. Eventually, by employing the 2D correlations and their generated coefficients, a rigorous analytical approach was given to define a generic 3D model of UCS as a transcendental function of several variables, including the density, age, and UWV. The function obtains the accuracy of R2=0.77. The modeling was a rigorous analytical approach. In case more data is generated in future work, the accuracy of the model can be improved. Another approach to improve the inherent accuracy could be employing machine learning techniques to get the best fit in transcendental functions rather than algebraic functions.

Author: Sharen Leon

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Wellbore instability problems are frequently encountered in drilling operations. Large cavings, which are recovered on the shale shakers, are the most prominent indicators and often give an abundance of information. For instance, the evaluation of the micro-fractures and the interaction between the drilling fluid and the shale itself, which are key factors to draw a better conclusion of the possible cause and prescribe solutions to prevent such problems in the future. By analyzing the samples of cavings obtained from a caprock shale of an oil field in Mexico, this master thesis aims to propose a methodology to better understand the root cause of the wellbore instability problems in this type of formations. The wellbore instability problem presented in the Mexican onshore wells included in this master thesis is associated with micro-fractured shale, anisotropic failure and weak bedding planes. This is evident by the appearance of three to four centimetres tabular cavings which causes a main problem when controlling the well and handling the cavings on the surface. This study covers in an integrated manner, real-time monitoring data analysis, geomechanical analysis, micro-CT scanning, shale characterization as well as an experimental set up of the HPHT (High Pressure High Temperature) filter press use for permeability plugging tests. The proposed setup of the HPHT filter press is designed to analyze the pore plugging effects in shale as well as the interaction between the drilling fluid and the actual rock. This is achieved by developing a replacement of the conventional ceramic disk with the shale samples obtained from the cavings. A methodology to prepare disk for the permeability plugging test from shale cavings was developed and is presented. The results of the laboratory tests, the geomechanical analysis and the shale characterization give us a better understanding of the behaviour of the shale under borehole conditions, the in-situ stress state of the area and the possible causes of the problem. The methodology applied in this thesis can be beneficial to optimize the selection of the LCM (lost circulation material) by analyzing the micro-fracture width in relation with the pore plugging effects.

Author: Karin Gurtner

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Casing leaks are a major problem in the entire oil, gas and geothermal energy industry all over the world. The reasons for casing leaks are multiple, but the majority of the failures is due to corrosion and mechanical wear, because of reciprocating or vibrating artificial lift systems. Additionally, wellbore treatments such as acidizing or fracturing are well known to be another major source for casing failures.
The impact of casing failures is usually phenomenal and in the worst case scenario the wellbore has to be plugged and abandoned. In this Master Thesis several options are discussed in order to repair the casing failures. The most common methods in the industry in order to overcome this problem are mechanical or chemical solutions. QHSE aspects are considered for every application. Mechanical solutions are most reliable, however the inside diameter of the borehole is reduced and therefore another barrier for the installation of artificial lift systems is created. In such a case the operator must be aware that after such an installation the original productivity of the wellbore cannot be achieved anymore. Repair methods like casing patches, expandable tubulars, and swaging operations are discussed in detail. Chemical solutions like the injection of specially designed cement slurries or resins or a combination of both of them have been investigated. Several methods like low pressure squeeze, high pressure squeeze and running squeeze have been discussed. The design of the cement slurry and all the relevant rheological and chemical parameters has been investigated.
The Master Thesis comprises a comprehensive literature review and analysis of 244 real case scenarios from the Vienna basin. The data sets were supplied by a business partner who operates the wellbores in the Vienna basin for more than 80 years. Therefore, it is obvious after such a long period of production the data material submitted is ideal for investigating in the subject of the Master Thesis.
Furthermore, numerous lab tests on various cement slurry designs have been carried out in accordance to the API standard “API recommended practice 10B-2”, and an optimized slurry design has been sorted out. The optimization criteria have considered Conventional-Cements, Microfine-Cements and Ultrafine-Cements. The type of cement to be chosen predominantly depends on the boundary conditions given by a particular wellbore problem and design. During the execution of the lab test it has turned out that especially the particle size of the cement grains, the density and the rheology of the cement slurry are the main criterion for choosing the right cement for the individual application. Additionally, quality checks on cement slurry samples have been carried out and crosschecked with the data sheets delivered by the cement manufacturer. It has turned out that the data sheets from the manufacturer are most reliable and of crucial relevance for the operators.
The specific test setup for the laboratory test are discussed in detail and documented and calibrated. The existing infrastructure is therefore ideally tailored for future lab tests and master theses.

Author: Nico Masching

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In the course of this master thesis, high-speed camera images were analysed to draw direct conclusions on the efficiency of fluid displacement processes as they occur in primary cementing operations in the oil and gas industry. These are about ensuring the integrity of wells, using cement as a well barrier element, which is inherently difficult due to complex trajectories and great depths. In addition to efficiency, it was also an objective to determine the degree of intermixing by means of optical spectrometry. An individual experimental setup, consisting of several components, first of all a transparent Plexiglas tube simulating a well, was created to visualize the displacement process. The generated images were analysed by means of a specially created Python program code using thresholding methods. All spectrometer data was processed and analysed. In all experiments, non-Newtonian drilling fluids with additives commonly used in the industry were utilized. The code used to analyse the displacement process has proven itself and the spectrometer has turned out to be a promising novel technique for investigating the degree of intermixing and its impact on cement integrity.

Author: Adel Ahmed Ibrahim Eid

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Ordinary Portland cement (OPC) is the primary material, which is used in oil well cement, isolation formation and plug and abandonment. OPC has many advantages and some limitations reviewed by different authors. The limitations associated with cement cause well integrity issues, risking humans, and environment. Several studies and experiments are conducted to evaluate different materials, which could be an optimal alternate to OPC. Geopolymer is one of these materials, which has been tested in lab scale to find its potential to replace OPC. Geopolymers are inorganic materials based on rock sources, which are rich in aluminum silicates. Many pieces of research have conducted on the geopolymer to assess its characteristics and properties. Studies showed that geopolymer is a ductile and low shrinkage material. It develops sufficient bond strength, high compressive strength and less fluid loss comparing to OPC. However, studies showed some shortcomings of geopolymer, which should be enhanced to allow using the geopolymer in the oil field. The pumpability at elevated temperatures, for a certain period, is one of the current limitations of the geopolymer. Several experiments have been performed to get the proper compositions of the geopolymer and the appropriate retarding admixture, which help to increase the pumping time of the geopolymer paste. Chemical S&H revealed its potential to retard the setting time by 80 mins. It is proved that there many parameters, which control the setting time of the sample. The modular ratio, weight of the admixture and composition of the precursors, have an impact on delaying the thickening time. BS2 is more pumpability than BS1, and S7 is the sample which has the most significant pumping time among the others. Results from the uniaxial compressive test (UCS) and ultrasonic cement analyzer (UCA) show the compressive strength value is in an acceptable range for utilization in oil well cementing. Rheology properties of geopolymers were characterized as non-Newtonian shear-thinning fluid and its density within the permissible range (1.95 sg). These characteristics promote the geopolymer to be a good quality alternative material to be applied in downhole applications.

Author: Felix Hibler

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Drilling operations and production facilities of the oil and gas industry are spread around the globe, also in remote locations, offshore or in the desert. In several cases, it is impossible to make the right component available at the right time to the right location, without enormous additional costs or effort. Manufacturing the required component to the exact specification directly at the location certainly adds huge benefits. Other industries such as automobile, aerospace have applied this just-in-time strategy very effectively by using the fast-developing additive manufacturing technologies. This thesis is embedded in an overall project which is performed by the Chair of Drilling and Completion Engineering together with OMV E&P GmbH. It investigates the usage of additive manufacturing in the oil and gas industry. The content of the thesis is divided into three main phases: testing of additive manufactured parts, an oil and gas specific SWOT-Analysis and a methodology describing the workflow for spare part manufacturing. During the first phase of the thesis the additive manufactured parts, which were produced from the selected material, 1.4542 (17-4 PH), are evaluated and compared to a conventionally manufactured part and the metal grade API C-110, which is a controlled yield strength casing or tubing grade. This phase includes the preparation of the specimens, testing and analysis of the results. The static behavior of the material in hardness, tensile and Charpy-V notch impact tests is evaluated. Sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC) tests were conducted in an external lab. For the second phase a SWOT-Analysis is performed to evaluate the general opportunities and shortcomings of this manufacturing method, as well as the specific chances for embedding it into the supply chain of an oil and gas production or service company. During the third phase of the thesis, a methodology or workflow to produce an additive manufactured part is evaluated and established. The workflow starts at the point, where it is recognized that a specific component is needed at the rig or the production facility and ends when the manufactured part can be delivered to this location. Therefore, different methods are investigated and researched to create a 3D model where a blueprint may not be available for a variety of reasons. The main objectives of the thesis are to gain knowledge about the properties, particularities and limitations of additive manufactured parts, especially for the application in the oil and gas business. Furthermore, the benefits of integrating this technology in certain areas are shown, to get one step closer to a safe and efficient way to use it in the oil and gas industry.

Author: Fabian Fasching

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To ensure well integrity during carbon capture and storage (CCS) operations, it is essential to understand the effects of carbon dioxide (CO2) on the cement at high-pressure and temperature conditions. There are several providers of CO2-resistant types of cement for the oil and gas industry, promising the integrity of their products. However, there are no standardized testing procedures to test their cement. This thesis aims to create a new testing methodology for CO2-resistant cement types and to investigate the quality of a CO2-resistant cement using the proposed methodology. The new testing methodology uses two types of high-pressure vessels. Smaller autoclaves are used to condition cement specimens (2" height x 1" diameter) with CO2. These specimens are used to analyze the compressive strength, mineralogical composition, and inhomogeneities in the cement matrix at different conditioning stages. The second pressure vessel is the CT-Scannable CO2 Cell, in which the propagation of the carbonation front is measured using a medical CT scanner. By combing the results of both procedures, an estimation of the material's behavior under downhole conditions can be made. The results from this study can assist in the prevention of well integrity problems due to CO2 leakage, therefore mitigating health, safety, and environmental risks and increasing the efficiency and economic success of CCS projects.

Author: Anass Al Didi

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Cement plays an integral role in maintaining well integrity throughout the life cycle of a well. Successful cementing jobs provide good zonal isolation and ensure strong bonding of cement to the casing and formation. The cementing job success is mainly governed by the fluid displacement efficiency and the degree of contamination with other wellbore fluids. However, displacing fluids downhole over long distances is a complex task that requires understanding of mud-spacer-cement interactions, their rheological behavior, as well as frictional pressure losses and flow regimes. Computational Fluid Dynamics (CFD) has been proven to be a powerful tool for modelling fluid behavior in numerous industries. The use of CFD allows us to model these complexities in a precise and reliable manner, and it can provide tailored solutions for individual cementing jobs to ensure maximum job efficiency and safety. In this study, a state-of-the-art CFD model was created using Ansys Fluent software to examine the displacement efficiency of a cementing job under different conditions in eccentric annuli. The CFD model was validated in single phase simulations using two sets of experimental data. The parameters studied include fluid density and rheology, casing eccentricity, flow rate, wellbore deviation, and casing rotation. The effect of each parameter was analyzed and the data was compiled to provide guidelines for efficient fluid displacement. This study stressed the importance of maintaining density and viscosity hierarchies between the displacing and displaced fluids. The drastic effect of eccentricity on the displacement process was shown, as well as possible solutions to counteract this effect by optimizing fluid properties and flowrates. Furthermore, casing rotation proved to be a valuable tool that enhances the displacement efficiency and can partly mitigate the negative effects of high eccentricity. The CFD model proved to be an invaluable resource for optimizing the cement placement process and can be utilized in a variety of ways to provide specialized solutions for each cementing job.

Author: Elias Johannes Benedikt

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Well Integrity is defined in two different ways. According to the standard NORSOK D-010, which defines the most widely accepted definition, well integrity means “Application of technical, operational and organizational solutions to reduce risk of uncontrolled release of formation fluids throughout the life cycle of a well”. Another definition gives the ISO TS 16530-1: “Containment and the prevention of the escape of fluids (i.e. liquids or gases) to subterranean formation surfaces.” To prevent the escape of fluids it is crucial to maintain the integrity of the well throughout the whole life cycle of the well. By establishing and implementing a Well Integrity Management System (WIMS) the well operator has an efficient management tool to cover and ensure the different aspects of well integrity. To fulfil the objective a set of operational Key Performance Indicator (KPIs) help to measure the effectiveness and the development of the WIMS. Well Integrity Management requires a multidisciplinary approach thinking which leads to the integration of aspects such as risk management, monitoring, organizational structures etc. In this thesis, the local WIMS at Gas Nord of Wintershall Dea will be reviewed and an overview of the current aspects that are in place will be given. According to the 117 Norwegian guidelines O&G, the current risk categorization system at Gas North has associations with risk, however it is not an absolute measure of the total risk exposure of the well. This categorization system does not replace risk assessment. Therefore, a measurement tool for the risk exposure has to be developed. By having a closer look at the well construction, intervention and production history of the well through a performance assessment with a scorecard, the well operator is able to identify future problematic wells in terms of risk exposure. Well integrity events are not only linked to constraints in the well operation phase but also to shortcomings in the well construction and intervention phase . By means of scorecards, 11 wells will be evaluated not only based on the operation phase but also on basis of the well construction phase. The scoring results will help to predict probable weak points in the well that can cause future well integrity issues. Further possible correlations of current well integrity incidents with the scorecard evaluation results can be identified and analysed based on probable origins. This thesis will show a pathway on how to use the holistic approach for a risk assessment. The identification of the critical wells and well sections in a risk assessment will help to better predict and solve the well integrity issues before they are become a major risk.

Author: Sven Curis

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In this thesis, the overview of the potential of carbon capture and storage projects is presented. As carbon dioxide is one of the main contributors to total greenhouse gas emissions, this could be a way to mitigate global greenhouse gas emissions in the future. The main problem of carbon dioxide injection is the carbonation process, which causes the increase of cement’s permeability and reduction of its compressive strength. This could ultimately lead to the loss of well integrity. One of the methods which could monitor the carbonation in real-time is the acoustic method. The sonic velocity of ultrasonic waves which go through a certain cement density is in correlation with its compressive strength. This is of a great importance as the compressive strength of the cement may be monitored with the acoustic method. This method is already widely applicable as the Ultrasonic Pulse Velocity method to determine the quality of the concrete. Together with the correlation, the crack investigation also took place to determine the depth of cracks and correlate them with sonic velocity. Finally, the overall experimental procedure with the necessary apparatus is presented in this thesis along with the results and steps which were so far made.

Author: Elisabeth Csar

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Primary cementing is critical to zonal isolation and well integrity. Besides, cement plugs are used for several operations in the oil and gas industry, such as lost circulation control, formation testing, directional/sidetrack drilling, zonal isolation, and well abandonment. Set cement may be too soft or not in the planned location and hence fail to provide hydraulic seal and lead to well integrity and safety issues. This could be due to a number of issues, including contamination with wellbore fluids, ineffective displacement, and casing eccentricity. This thesis focuses on continuous monitoring real-time data during the hydration process of cement to be able to determine the phase of hydration process, to define the exact location of the cement, to establish contaminated parts of cement and prove a competent/imperfect well barrier. Electrical conductivity measurements could be an indication of how the cement slurry is hydrating and the degree of contamination. Several tests with different levels of mud contamination were performed and analyzed. In addition, experiments with fiber-optic sensoring to measure strain and temperature changes during cement slurry hydration were conducted. Both the electrical conductivity and fiber-optic measurements are able to identify degree of hydration and also level of contamination. The lab setup, test procedure, data, analysis, and results are presented and discussed. These measurements, analysis, and application would significantly improve the accuracy of cement jobs, and operational performance in the oil and gas industry. Benefits to the industry from continuous monitoring and evaluation are discussed. In addition, recommendations for future work and conclusions can be found in the last chapter of the thesis.

Author: Paul Wagner

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Global warming is one of the most significant issues the world is facing. Capturing carbon dioxide from the atmosphere or industrial processes and storing it in geological formations can help counteract climate change. Nevertheless, the interaction between well barrier elements such as cement, casing, tubulars, packers, and valves can lead to possible leakages. To accomplish successful carbon dioxide sequestration, injecting the carbon dioxide in its supercritical state is necessary. The supercritical carbon dioxide can corrode steel and elastomers and react with the calcium compounds in the cement, dissolving them and forming calcium carbonate and bicarbonate in the process. This carbonation can lead to channels forming on the cement-to-rock interface or cracking due to the carbonate precipitation, resulting in a loss of well integrity. This study focusses on finding ways that enable the continuous monitoring of well integrity under in-situ conditions. The construction of an autoclave, capable of withstanding supercritical conditions of carbon dioxide, facilitates the in-situ monitoring. This autoclave also makes CT-scans of the pressurized sample possible, as well as acoustic measurements, using state-of-the-art piezo elements. The first tests will establish a baseline using neat Class G Portland cement to verify the design and sensors. The set up consists of a rock core in the middle of the autoclave cell surrounded by a cement sheath. Drilling a channel in the middle of the core expedites the distribution of the carbon dioxide. Once the ability of the sensors to monitor the integrity is verified, different cement compositions and their interaction with supercritical carbon dioxide can be studied. The experimental setup and the procedure discussed here closely simulate the downhole condition. Hence, the results obtained using this setup and procedure is representative of what could be observed downhole. The direction is not to remove the sample from the cell and analyze it under in-situ conditions. Digitalization is powering the in-situ analysis in this experiment. After the carbonation, samples from the autoclave undergo a thorough chemical and physical analysis. The correlation of the data from the sensors and chemical analysis aids in further developing real-time monitoring. The results from this study can lead to the prevention of leakage of carbon dioxide to the environment and other formations, which defeats the purpose of carbon dioxide sequestration. These results should improve the economics of these wells as well as the health, safety, and environmental aspects.

Author: Andreas Liegenfeld

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Maintaining the integrity of a well throughout its whole life cycle is very crucial in well construction and operation. Many risks are waiting along that chain of events to threaten the well’s integrity. Recent studies and research have long tried to link well integrity events to constraints in the well operation phase, with emphasis on stresses and their limits in the cement sheath. However, this thesis aims to have a closer look at the well construction process and how it might impact well integrity events. Through a simple performance assessment of the well construction process, a score between 0 and 100 is assigned to each casing/liner section to asses wellbore quality. Responses and parameters during drilling, final BHA pull, casing-running and cementing were used as input for the scoring process. The performance assessment output of the scorecard is precious in detecting shortcomings in the well construction process to help determine root causes of well integrity events. The scoring results of 29 casing/liner sections on 11 OMV Vienna Basin wells already indicate the tool’s effectiveness in detecting problematic well sections. The lowest-scoring ones face incidents like failing casing pressure tests, proven lack of zonal isolation or cementing losses during cementing. Furthermore, the scorecard’s ability to trouble-shoot problematic well sections has been proved with actual and expected/calculated casing pressure test bleed-off volumes. Gaining knowledge about the performance of the well construction process helps to enhance wellbore quality and avoid future well integrity events. The proposed wellbore scorecard tool only covers and captures what is happening in the well construction process, but to study and understand well integrity along the whole life cycle of the well, a more holistic approach will be necessary. Therefore, a stress model is proposed to understand the impact of well operations on the cement sheath in terms of stresses and failure of the cement. Manufacturing a test cell according to the proposed design to verify the stress model is highly recommended. This piece of work covers one essential part of the holistic approach to better understand well integrity events and aims towards a more environmental-friendly, safer and cost-effective way to operate hydrocarbon wells.

Author: Pouya Ziashahabi

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Well integrity is one of the main issues during all life cycles of the well and if any barrier element fails and compromises the integrity of well, remedial strategy may have to be performed to restore the safety and economics of the well. Cement is conventionally used as the isolation material in remedial and primary jobs. However, due to several inherent limitations in cement characteristics, such as cement shrinkage or poor cement slurry compatibility with downhole fluids, it is not always the best solution. Besides these, cement slurry is a particle laden fluid which prevents it penetrating in to tight cracks/pathways to ensure isolation during remedial treatments. This project illustrates the effort to develop a customized polymer as cement alternative and/or cement additive to address challenges posed by conventional cement. Experimental studies are designed to evaluate the characteristics such as rheology, injectivity and mechanical properties of the customized polymer. Furthermore, the compatibility of these polymers with drilling fluids contamination and shrinkage behaviour upon cure are evaluated. Afterward, based on the results of measurements, the limitations of each different formulation are determined and the best formulation is optimized. In addition, the application of the polymer as an additive to conventional cement is studied to optimize some specific properties, such as mechanical properties and permeability. The results of experiments prove that beside the appropriate rheological properties, the polymer provides excellent mechanical properties and much lower shrinkage rate upon cure compared to cement and other systems available in the market. The customized polymer withstands and maintains its properties after being contaminated with considerable volume of drilling fluids and its compressive strength is not much affected by contamination. Also, combination mixtures of customized polymer with conventional cement slurries provide enhanced properties, such as lower permeability and higher compressive strength. The unique characteristics of customized polymer as cement alternative could provide benefits to industry by solving several current challenges in achieving secure isolation. Particularly due to its ability to penetrate in to tight cracks, this customized polymer affords an effective method in to mitigate sustained casing pressure or as a remedial solution for casing leakages which would not be achieved easily by conventional methods.

Author: Christian Scheurer

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Nanoparticles have gained close attention over the recent years in many industries but especially so in the oil and gas. Various researches have been investigating, for instance, the use of surface-modified silica nanoparticles in reservoir rock applications. In this work, the interaction of silica nanoparticles and sandstone rock was investigated using a combination of various experimental approaches. Among others, fluid-fluid and rock-fluid interactions were assessed by means of fluid compatibility, batch sorption experiments and single-phase core floods. The underlying task was to gain a better understanding on the factors influencing nanoparticle adsorption to the rock material. In the experimental approach, diol and polyethylenglycol (PEG) surface-modified silica nanoparticles were tested using two brines differing in ionic strength, plus sodium carbonate (Na2CO3) and Berea and Keuper outcrops (core plug and crushed form). Core flood effluents were analysed to define changes in concentration and a rocks retention compared to a tracer. Field Flow Fractionation (FFF) and Dynamic light scattering (DLS) in selected effluent samples were performed to investigate changes in size distribution. Adsorption was evaluated using UV-visible Spectroscopy and scanning electron microscopy (SEM). Highest adsorption was observed in brine with high ionic strength whereas the use of alkali reduced the adsorption. Crushed material from Berea rock showed slightly higher adsorption compared to Keuper rock whereas temperature had a minor effect on adsorption behaviour. In single phase core-flood experiments no effects on permeability have been observed. The used nanoparticles showed a delayed breakthrough compared to the tracer and bigger particles passed the rock core faster. Nanoparticle recovery was significantly low for PEG-modified nanoparticles in Berea, suggesting high adsorption. SEM images indicate, that adsorption spots are defined via surface roughness rather than mineral type. Despite an excess of nanoparticles in the porous medium monolayer adsorption was the prevailing type observed. Investigation of nanoparticle interactions with rocks required the development and improvement of methods to evaluate concentration history and recovery. The understanding obtained is crucial for further research in this area and application in a possible field trial.

Author: Anastasiia Fedorova

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Well abandonment is a major challenge in the industry from different perspectives, including costs, the amount of effort and technical difficulties involved with permanent wellbore isolation. In recent years there has been an increased focus on plug and abandonment (P&A) operations. Oil and gas producing companies have an enormous asset of wells to be decommissioned due to mature fields all over the world. The North Sea region is not an exception, as it is the mature play experiencing the wave of well decommissioning activity for the last 15 years.
In the North Sea area, nearly two thousand wells are intended to be plugged and abandoned in the upcoming 5-10 years. P&A of wells of North Sea assets is estimated to reach 50 % of decommissioning costs, including platform removals, which compromise 20 % (Vrålstad, et al. 2018). Operators are searching for new technologies that could minimize expenditures on P&A operations as there are no financial benefits from them.
Well abandonment must fulfil local government regulations, which require long-term well integrity and isolating formations between each other and from the surface. A conventional well decommissioning process might be time-consuming due to remedial cementing, casing milling and casing pulling operations with the rig installation.
The main objective of the master thesis is to investigate the ´shale-as-a-barrier´ (SAAB) technique as the alternative solution to conventional well barriers. Naturally occurring barriers may extend over hundreds of meters along the well and eliminate the need for additional sealing of the annulus. Such a technique could simplify the plugging operations and imply significant cost reductions during P&A operations.