Numerical modeling of shear stimulation in naturally fractured geothermal reservoirs

Shear-dilation-based hydraulic stimulations are conducted to create enhanced geothermal systems (EGS) from low permeable geothermal reservoirs, which are initially not amenable to energy production. Reservoir stimulations are done by injecting low-pressurized fluid into the naturally fractured formations. The injection aims to activate critically stressed fractures by decreasing frictional strength and ultimately cause a shear failure. The shear failure leads to a permanent permeability enhancement of the fractures, which contributes to the overall reservoir permeability, owing to the damage in fracture surface characteristics during the shear failure. Shear stimulation is considered a key for geothermal energy development; however, seismicity is a critical by-product, which has to be controlled. Numerical modeling can provide a deeper understanding on governing mechanisms, which is essential for reservoir assessments and the control of seismicity. The primary goal of this thesis is to aid further development of EGS by contributing to the current state-of-the-art for numerical modeling of shear-dilation-based hydraulic stimulations. Numerical modeling of shear-dilation-based hydraulic stimulations requires mathematical modeling of flow and mechanical deformation in fractured formations. The initial focus of the thesis is the modeling of the mechanical deformation of naturally fractured rock. The deformation and stress state of the rock are controlled by the deformation of pre-existing fractures, which is governed by different equations than the deformation of the surrounding formation. A cell-centered finite-volume approach is developed where the fractures are represented as co-dimension one inclusions in the domain. The method is capable of modeling deformation considering open and closed fractures with complex and nonlinear relationships governing the displacements and tractions at the fracture surfaces. The method aims to provide benefits for studies including flow and deformation couplings in a discontinuous rock. Hydraulic stimulations are essentially coupled hydro-mechanical processes, where the deformation of fractures has an impact on the permeability as well as on the stress state of the rock. We develop a computational model, which has the capability to capture these interrelations in two- or three-dimensional domains. Considering the significance of the pre-existing fractures, we model the reservoir as a network of explicitly represented large-scale fractures immersed in a permeable rock matrix. The model can forecast the permeability evolution of geothermal reservoirs with complex fracture networks. To be able to mitigate the seismic hazards, the contributing processes and the interaction between them should be examined. The computational model developed here also has the capability to investigate the induced seismicity. By using the developed model, a novel hypothesis regarding the induced seismicity generated after the termination of injections has been tested. During the fluid injections, the pressure builds up inside the fractures, which causes normal deformation and increases the void place between the fracture surfaces. The termination of the injections reverses the void increase; i.e., the fracture starts to close owing to the pressure decrease. We identify that the fracture closure is one of the mechanisms that are responsible for the induced seismicity generated after the termination of injections

Modelling of the Shear Dilation Based Hydraulic Stimulation in Enhanced Geothermal Systems Considering Fractures in Different Scales

A numerical approach for modelling of shear dilation of existing fractures in hydraulic stimulation of geothermal reservoirs at low elevated pressures is presented. The fractured rock in the reservoir is modelled as a combination of explicitly represented fractures and the rock matrix surrounding these fractures. The efficient modelling of slip-induced permeability enhancement requires coupling of the fluid flow in fractured rock with the mechanical deformation of the rock matrix and the shear dilation of the fractures. For flow simulations, conductive fractures are represented in the domain as high-permeable discontinuities; therefore they dominate the overall flow behaviour. The rock matrix is represented by a low permeability, capturing the effect of small-scale fractures. For the mechanical deformation problem, the rock matrix is assumed to be a linear elastic material, while the fractures in the rock matrix are introduced as internal boundaries. The shear dilation of the fractures is calculated by a joint deformation model (JDM), which connects the shear slip in the fracture surfaces and additional permeability caused by shear displacement. The flow simulations and the mechanical deformation of the rock matrix are both obtained by finite volume discretizations. Several numerical experiments designed by resembling realistic reservoir parameters are conducted to provide better understanding of the shear dilation mechanism. Moreover, fractures present in different scales in a geothermal reservoir. Ignoring the effect of small-scale fractures to the fluid flow in the matrix may result in an overestimate of the permeability enhancement. Hence, the influence of rock matrix permeability on fracture aperture and the overall flow behaviour of the reservoir are examined.publishedVersio

Solution methods of instrumentation related complications in endodontic treatment

Background: Endodontics is a skill that requires the use of precision instruments in tight spaces and is a complex discipline often underestimated by clinicians and patients. Inevitably, this will lead to complications. Complications encountered during endodontic treatment occur during diagnosis, anesthesia, cleaning, and shaping of the root canals and filling the root canals. Complications can occur at any stage of treatment. The various procedures associated with root canal treatment can be divided into three treatment phases: Pre-operative, operative, and post-operative. Since endodontic complications may occur at any of the pre-operative, intraoperative, and post-operative stages, complications that may develop are also classified in the same way. Aim: In this review, operative complications including instrument fracture, ledge formation, canal obstruction, apical transportation, and strip perforation that may occur during canal preparation in endodontic treatment are discussed. Conclusion: Technological developments in endodontics have revealed methods and instruments enable successful treatment of calcified root canals, severe canal curvatures, ledges, resorption defects, perforations, and broken canal instruments without complications. Clinical Significance: Therefore, the physician should always be ready to manage complications. The task of the physician is to know how to avoid potential complications and how to manage complications that may arise during treatment

Effect of differently functionalized carbon nanotubes on the properties of composite nanofibres

Effect of differently functionalized (carboxyl, amine and hydroxyl functionalized) multi-walled carbon nanotubes (MWCNTs) on the structure and properties of composite polyacrylonitrile nanofibres produced by electrospinning has been studied. Fourier transform ınfrared spectroscopy has been used to confirm the successful functionalization of carbon nanotubes while mechanical testing, electrical conductivity, scanning electron microscopy, differential scanning calorimetry and X-ray diffraction analysis have been used to characterize the composite polyacrylonitrile nanofibre webs. The addition of MWCNTs, either pristine or functionalized, results in slight increases in the diameter of nanofibres. The tensile strength, crystallinity, thermal properties are all found to be affected by the functional groups of the carbon nanotubes, while the conductivities of the nanowebs seem to be insensitive to the different functional groups of the carbon nanotubes

Heart Rate Variability in Children with Tricyclic Antidepressant Intoxication

The aim of this study was to evaluate HRV in children requiring intensive care unit stays due to TCA poisoning between March 2009 and July 2010. In the time-domain nonspectral evaluation, the SDNN (P<0.001), SDNNi (P<0.05), RMSDD (P<0.01), and pNN50 (P<0.01) were found to be significantly lower in the TCA intoxication group. The spectral analysis of the data recorded during the first 5 minutes after intensive care unit admission showed that the values of the nLF (P<0.05) and the LF/HF ratio (P=0.001) were significantly higher in the TCA intoxication group, while the nHF (P=0.001) values were significantly lower. The frequency-domain spectral analysis of the data recorded during the last 5 minutes showed a lower nHF (P=0.001) in the TCA intoxication group than in the controls, and the LF/HF ratio was significantly higher (P<0.05) in the intoxication group. The LF/HF ratio was higher in the seven children with seizures (P<0.001). These findings provided us with a starting point for the value of HRV analysis in determining the risk of arrhythmia and convulsion in TCA poisoning patients. HRV can be used as a noninvasive testing method in determining the treatment and prognosis of TCA poisoning patients

Evaluación de parámetros de estrés oxidativo y actividades metabólicas de enfermeras trabajando en turnos diurnos y nocturnos

Se objetivó evaluar estrés oxidativo y actividades metabólicas de enfermeras en turnos diurnos y nocturnos. Participaron enfermeras de Unidad de Terapia Intensiva (UTI, n=70) y del servicio común (SC, n=70). Se obtuvieron muestras sanguíneas al inicio y al final de los turnos para medir parámetros de estrés oxidativo. Las actividades metabólicas también fueron analizadas utilizando brazaletes SenseWear. Los parámetros de estrés oxidativo aumentaron hacia el final de los turnos de todas las enfermeras SC y UTI, comparados con la medida de inicio. Los niveles de TAS, TOS y OSI no eran significativamente diferentes entre enfermeras SC y UTI al cierre de los turnos diurnos y nocturnos. Las actividades metabólicas de enfermeras SC y UTI se mostraron similares. Consecuentemente, los parámetros de estrés oxidativo y las actividades metabólicas de enfermeras SC y UTI no resultaron diferentes, y todas las enfermeras sufren efectos semejantes en sus turnos, tanto diurnos como nocturnos.O objetivo deste estudo foi avaliar o estresse oxidativo e as atividades metabólicas das enfermeiras em turnos diurnos e noturnos. Enfermeiras da Unidade de Tratamento Intensivo (UTI) (n=70) e do serviço comum (SC) (n=70) participaram do estudo. Logo no início e ao final dos turnos, amostras de sangue foram obtidas para medir parâmetros de estresse oxidativo. Atividades metabólicas também foram analisadas com o uso da braçadeira SenseWear. Parâmetros de estresse oxidativo aumentaram no fim dos turnos de todas as enfermeiras SC e UTI quando comparados ao início dos turnos. Comparados às enfermeiras SC, os níveis de TAS, TOS e OSI das enfermeiras de UTI não eram significativamente diferentes no final dos turnos diurnos e noturnos. Além disso, as atividades metabólicas das enfermeiras de SC e UTI se revelaram como sendo similares. Assim, os parâmetros de estresse oxidativo e as atividades metabólicas das enfermeiras SC e UTI não eram diferentes, e todas as enfermeiras sofrem efeitos semelhantes dos turnos, tanto no dia quanto na noite.The aim of this study was to evaluate the oxidative stress and metabolic activities of nurses working day and night shifts. Intensive care unit (ICU) (n=70) and ordinary service (OS) nurses (n=70) were enrolled in the study. Just before and the end of the shifts, blood samples were obtained to measure the participants' oxidative stress parameters. Metabolic activities were analyzed using the SenseWear Armband. Oxidative stress parameters were increased at the end of the shifts for all OS and ICU nurses compared to the beginning of the shifts. Compared to the OS nurses, the ICU nurses' TAS, TOS, and OSI levels were not significantly different at the end of the day and night shifts. The metabolic activities of the OS and ICU nurses were found to be similar. As a result, the OS and ICU nurses' oxidative stress parameters and metabolic activities were not different, and all of the nurses experienced similar effects from both the day and night shifts

Evaluation of oxidative stress parameters and metabolic activities of nurses working day and night shifts

The aim of this study was to evaluate the oxidative stress and metabolic activities of nurses working day and night shifts. Intensive care unit (ICU) (n=70) and ordinary service (OS) nurses (n=70) were enrolled in the study. Just before and the end of the shifts, blood samples were obtained to measure the participants' oxidative stress parameters. Metabolic activities were analyzed using the SenseWear Armband. Oxidative stress parameters were increased at the end of the shifts for all OS and ICU nurses compared to the beginning of the shifts. Compared to the OS nurses, the ICU nurses' TAS, TOS, and OSI levels were not significantly different at the end of the day and night shifts. The metabolic activities of the OS and ICU nurses were found to be similar. As a result, the OS and ICU nurses' oxidative stress parameters and metabolic activities were not different, and all of the nurses experienced similar effects from both the day and night shifts

Numerical modeling of shear stimulation in naturally fractured geothermal reservoirs

Shear-dilation-based hydraulic stimulations are conducted to create enhanced geothermal systems (EGS) from low permeable geothermal reservoirs, which are initially not amenable to energy production. Reservoir stimulations are done by injecting low-pressurized fluid into the naturally fractured formations. The injection aims to activate critically stressed fractures by decreasing frictional strength and ultimately cause a shear failure. The shear failure leads to a permanent permeability enhancement of the fractures, which contributes to the overall reservoir permeability, owing to the damage in fracture surface characteristics during the shear failure. Shear stimulation is considered a key for geothermal energy development; however, seismicity is a critical by-product, which has to be controlled. Numerical modeling can provide a deeper understanding on governing mechanisms, which is essential for reservoir assessments and the control of seismicity. The primary goal of this thesis is to aid further development of EGS by contributing to the current state-of-the-art for numerical modeling of shear-dilation-based hydraulic stimulations. Numerical modeling of shear-dilation-based hydraulic stimulations requires mathematical modeling of flow and mechanical deformation in fractured formations. The initial focus of the thesis is the modeling of the mechanical deformation of naturally fractured rock. The deformation and stress state of the rock are controlled by the deformation of pre-existing fractures, which is governed by different equations than the deformation of the surrounding formation. A cell-centered finite-volume approach is developed where the fractures are represented as co-dimension one inclusions in the domain. The method is capable of modeling deformation considering open and closed fractures with complex and nonlinear relationships governing the displacements and tractions at the fracture surfaces. The method aims to provide benefits for studies including flow and deformation couplings in a discontinuous rock. Hydraulic stimulations are essentially coupled hydro-mechanical processes, where the deformation of fractures has an impact on the permeability as well as on the stress state of the rock. We develop a computational model, which has the capability to capture these interrelations in two- or three-dimensional domains. Considering the significance of the pre-existing fractures, we model the reservoir as a network of explicitly represented large-scale fractures immersed in a permeable rock matrix. The model can forecast the permeability evolution of geothermal reservoirs with complex fracture networks. To be able to mitigate the seismic hazards, the contributing processes and the interaction between them should be examined. The computational model developed here also has the capability to investigate the induced seismicity. By using the developed model, a novel hypothesis regarding the induced seismicity generated after the termination of injections has been tested. During the fluid injections, the pressure builds up inside the fractures, which causes normal deformation and increases the void place between the fracture surfaces. The termination of the injections reverses the void increase; i.e., the fracture starts to close owing to the pressure decrease. We identify that the fracture closure is one of the mechanisms that are responsible for the induced seismicity generated after the termination of injections