Auswahl der wissenschaftlichen Literatur zum Thema „Tri-Axial Cell Stress“

Based on the developed low permeability test instrument, gas transport properties of typical Jin-Ping marbles are studied under steady flow of nitrogen. A low permeability test instrument and its tri-axial cell structure were introduced in detail, and this test instrument was used to study transport properties of dense rocks under different temperatures and stress conditions, the Test results showed that intrinsic permeability of Jin-Ping marble is about 10－20 m2. Comparisons are made between test results and an exact method which considers Klinkenberg effect in gas flow equation. Fitting results using the exact method show better agreement with laboratory testing results and the transport parameters gained are more convincible.

Hydraulic fracturing has been widely applied to enhance the conductivity in tight sandstone reservoirs, i.e. reservoirs with low porosity and permeability. The interaction between a hydraulic fracture (HF) and a natural fracture (NF), including crossing, arresting and opening (tensile or dilation), are crucial for controlling the fracability of a reservoir. Previous studies have elucidated that shear dilation is the main mechanism for enhancing the permeability of an unconventional reservoir. Moreover, the brittleness index (BI) is considered another critical parameter that controls the fracability of candidates. However, the fracability of candidates with respect to both shear dilation and BI have not been fully investigated. We performed a practical fracability evaluation by integrating the shear dilation mechanism and BI quantification. We obtained the mechanical parameters from mechanical tests conducted on synthetic tight sandstone samples, and we manufactured specimens with different friction coefficients and shear strengths. Next, we performed scaled hydraulic fracturing experiments on 15 identical 10 cm cubic samples using a true tri-axial stress cell, and the interaction mechanisms between HFs and NFs were investigated. We also evaluated the brittleness of each specimen based on a previous BI model and our own novel BI model. We found that a weak interface cohesion with an interaction (between HF and NF) angle of 60° exhibited a shear dilation (or reactivation) mechanism and a higher BI. We thus conclude that such conditions are more favourable for reservoir stimulation (i.e. hydraulic fracturing) in the field.

Les injections de fluides liées à l’exploitation de réservoirs géothermiques entraînent bien souvent la réactivation de failles, sous la forme d’un glissement lent ou asismique, déclenchant à son tour des séismes dits induits. Cette thèse est consacrée à une étude numérique du glissement asismique déclenché par injection de fluide. Un modèle FEM y est développé afin de simuler des expériences d’injection effectuées en presse triaxiale. Les simulations présentées dans ce travail permettent de quantifier l’effet du scenario d’injection, de la diffusivité de la faille, des propriétés de frottement et de l’état de contrainte initial sur la dynamique d’expansion du glissement asismique, fournissant un nouveau regard sur les lois d’échelles caractérisant la vitesse de rupture et le moment maximum libéré. L’approche présentée permet de fournir des pistes de réflexion pour améliorer l’évaluation de l’aléa lié à l’exploitation géothermique. Le modèle numérique développé est également validé sur un jeu de données expérimentales, ce qui ouvre des perspectives importantes pour approfondir l’interprétation mécanique des expériences d’injection menées en laboratoire
Fluid injections performed in the framework of geothermal exploitation can reactivate slip on preexisting crustal faults, leading to aseismic slip transients in turn triggering so-called triggered earthquakes. This PhD thesis is a numerical study dedicated to the physical control on the fluid-induced aseismic slip. A hydromechanical FEM is developed to simulate injection experiments performed in a tri-axial cell in the laboratory. The simulations presented allow to quantify the effect of the injection scenario, the hydraulic diffusivity, the fault friction and pre-stress on the dynamics of induced aseismic slip, providing new insights into the scaling laws commonly used to characterize this phenomenon, in particular the rupture speed and the maximum moment released. The approach presented here is thus of importance in the perspective of improving hazard mitigation in the context of geothermal exploitation. The model predictions are also validated on a real experimental dataset, which opens a new avenue to improve the mechanical interpretation of injection experiments in the laboratory

Abstract New methods for long-term performance evaluation based upon tri-axial load cell measurements are presented, quantitatively analyzing the reliability of new polyaramid-reinforced cement material by measuring cumulative fatigue damage to predict long-term failure. Compressive strength, Young’s modulus, and Poisson’s ratio measurements under dynamic force loading conditions result in the overall design of innovative polyaramid additives applied as non-metallic alternatives for the improvement of mechanical properties of high sulfate resistant Portland cement (HSR). In this study, strain-controlled cyclic tests to measure mechanical properties showed aramid-reinforced cements with 1.6% to 4.5% and 11% permanent strain (maximum) when compared to neat cement (with no admixture) at 30% permanent strain. Enhancement to cement mechanical properties is attributed to the unique response by novel polyaramid-reinforced samples under extreme strain-stress cyclic conditions up to 40 MPa. From this data, various new polyaramid-reinforced cements showed energy storage (low YM) and low strain rate, potentially prolonging the lifespan of HSR Portland cement without sacrificing compressive strength.

Hydraulic fracturing is a fracturing processes initiated from a pressurized open borehole section into solid formations. The process is characterized by solid-fluid interaction. On the solid side, the formation is deforming with the propagation of the fracture front and pressurization of the fracture face. At the same time the fluid is driven by the borehole pressure to flow into the narrow fracture cavity. At same time, the fluid may also infiltrate into the porous rock media. The fracture cavity is supported by the fluid pressure. In turn, the fluid distribution depends on the fracture conductivity related to the cavity width (aperture). The mass balance is maintained amongst the injection rate, fracture volume increment and leak-off A series of experiments have been performed to initiate fracture in a number of natural rock samples confined by a tri-axial cell. As the tight and shale gas and oil reservoirs are found global-wise, stimulation techniques such as polymer flooding and fracturing start to draw attention. We have employed X-link gel as the fracturing material in contrast with the Newtonian viscous fluid. From the results, we found that the ability to sustain certain yielding stress made the gel unlikely to infiltrate into the porous media as leak-off and likely to create a unanimous fracture. This paper is about the tests results and analysis that describe the gel behavior.

Abstract Horizontal wells with multi-stage longitudinal fractures have been widely used to develop low-to-moderate permeability sandstone formations in enhanced oil recovery schemes. A long-held misconception exists that high fracture conductivity is not essential to well productivity for longitudinal fractures in openhole completions because the pressure drop through the fracture is small due to the geometry. This study provides workflows to optimize longitudinal fracture design for horizontal well completions on the North Slope of Alaska and presents practical considerations that challenge this misconception. Mechanical properties of core samples across the reservoir interval were evaluated by hardness-based strength calculators and tri-axial compression tests. Based on the rock strength profile, candidate intervals were selected for fracture conductivity measurements. Both summer (low salinity) and winter (high salinity) seawater based fracturing fluids were injected through the propped fracture cell containing both sandstone and mudstone lithologies. Numerical fracture models were built and matched to bottomhole pressures acquired during project appraisal well stimulations. Proppant concentration distribution along the fracture was generated for different design scenarios by varying proppant volume, proppant size, pump schedule, etc. The impact of various design scenarios on well production was also investigated. Full alignment between the fracture plane and the wellbore results in the highest productivity for fractured horizontal wells with openhole completions. However, calculations demonstrate that even a few degrees of misalignment between horizontal well orientation and the maximum horizontal principal stress results in fracture deviation from the open hole. Due to flow along the fracture and convergence from the fracture to the wellbore, fracture conductivity dominates the pressure drop and completion skin factor for this geometry. Since actual fracture conductivities in wells on the North Slope of Alaska are not infinite, it is therefore inappropriate to use "infinitely-acting" fracture assumptions as has often been used historically for longitudinally fractured horizontal wells with openhole completions. Fracture conductivity tests show severe conductivity loss due to gel residue as well as mudstone and seawater interactions. Realistic discount factors for fracture conductivities in the targeted shallow sandstone-mudstone formations were developed for subsequent reservoir studies. Modeling results suggest that larger job sizes and bigger proppant are needed to achieve desirable skin factors and well inflow performance when the fracture becomes misaligned from the wellbore.

Freight trains all over European Countries are equipped with mechanical couplers, which are using two buffers and a screw coupler in the centre. This configuration needs an extensive study in case of pulled mass exceeding 1600 t: actually National Association would limit train mass to 2000/2400 tons, in order to avoid excessive stresses on couplers. This challenging operative condition should become really severe in presence of switches or sharp radius curve, especially considering that freight trains are able to sustain deceleration in emergency braking condition over 1 m/s2. As well known, in order to investigate the safety issue of heavy freight trains under severe braking/traction conditions (i.e. emergency braking) also negotiating turnouts or sharp curves, an experimental approach can be performed, using a suitably instrumented trainset. Unfortunately, this approach is usually very expensive and does not provide a full understanding of the problem: the information gained with experimental tests would apply only to the particular trainset composition and to the specific track considered. On the other hand, the adoption of numerical simulations, in connection with the experimental tests, can be a useful approach to extend obtained results to a wider range of conditions, allowing an easier variation of the different test parameters. The paper will deal with the investigation performed by Politecnico di Milano together with R.F.I. (Rete Ferroviaria Italiana, the Italian Railway Network operator) on the heavy freight train dynamics. An experimental approach has been used in order to investigate the typical operative condition of a freight train; a freight wagon has been equipped with load cells, displacement transducers on the buffers and tri-axial accelerometers on the wagon frame. Moreover the traction/braking torque applied by the locomotives have been measured. The experimental trainset was composed by two heading locomotives, a series of freight wagons (pulled mass 1600 t), and then another locomotive at the end of the train. The results of the test allowed a better comprehension of the behaviour of the complete trainset, on a steep line, especially during sharp curve negotiation (R = 200 – 300 m): particular attention was paid on the buffer behaviour, because of its fundamental importance for the running safety of the wagons. It was highlighted that the operative condition typical of a sharp radius curve negotiation leads to a stiffer buffer: the increased stiffness of the buffer cannot be neglected for the investigation of the running condition. These tests were used to update an existing numerical software for the analysis of the longitudinal dynamics of heavy freight train, named T.S.Dyn. (Trainset Dynamic Simulator): its numerical model is able to reproduce forces/displacement in the coupling between two adjacent vehicles (buffers and draw gear) of a trainset. Moreover the model is able to consider the dynamic behaviour of the pneumatic braking system of the entire trainset and for this reason it can find proper application even for the simulation of severe braking condition (i.e. emergency braking). The numerical model has been updated taking the advantage of the experimental activity performed and it was implemented in T.S.Dyn code. A comparison between numerical and experimental results will be described in the full paper.