Stress sensitivity of velocities in anisotropic rocks - Rock models comparison and true-triaxial apparatus description
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The purpose of this study was to investigate the effects of anisotropy on elastic parameters of rocks, and to compare various types of anisotropic rock models with the use of ultrasonic data obtained with a true-triaxial loading apparatus.In the first part of the thesis, general concept of anisotropy is presented. Anisotropy sources and quantification methods are characterised, and its influence on seismic waves velocity is described. Grain pack, inclusion and microscopic classes of models are introduced. The thesis examines two of the presented models, an inclusion proposed by Fjær (2006) and a macroscopic model published by Prioul et al. (2004) - their general features, characteristics of modelling procedure, results and their limitations. Due to technical difficulties, the ultrasonic measurements on transversely isotropic Berea and Colton sandstones subjected to hydrostatic and biaxial loading used to test the models are taken from Coyner (1984) and Cruts et al. (1995), respectively.The results of the tests show that both models are well suited to reproducing elastic stiffness parameters estimated from ultrasonic measurements. However, their characteristics entail significantly different methodologies and suggest that the models may be used interchangeably, depending on modelling conditions, interpreter expectations and amount of available experimental data.The second part of the thesis focuses on the true-triaxial loading apparatus owned by SINTEF Petroleum Research and Norwegian University of Science and Technology. It describes performance tests and modifications introduced to the apparatus, and discusses possible causes of equipment malfunctions. Despite all the changes, performance of the apparatus has not been sufficiently improved. The author recommends further modification to the apparatus before any other uses are considered.