Evaluation of local porosity changes in limestone samples under triaxial stress field by using X-ray computed tomography
C. O. Karacan, A. S. Grader, P. M. Halleck, 2003. "Evaluation of local porosity changes in limestone samples under triaxial stress field by using X-ray computed tomography", Applications of X-ray Computed Tomography in the Geosciences, F. Mees, R. Swennen, M. Van Geet, P. Jacobs
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The character of reservoir rocks is uncertain and variable at the depths where they are subjected to different tectonic forces and pressure changes due to drilling, well stimulation and production operations. The stress- and time-dependent deformation of the porous structure is expected to change the behaviour of the most important properties of the rock, such as porosity and permeability, which in turn changes the reservoir production predictions.
In this study, we demonstrate the use of X-ray computed tomography (CT) to investigate the porosity and permeability changes of Cordova Limestone samples during deformation in a triaxial cell. The experiments were performed in a specially designed X-ray transparent triaxial test cell, which enables applying stress as well as making flow measurements. Because the presence and value of confining pressure changes the deformation behaviour of the rock, different constant confining stresses were applied to the samples to change the deformation regime. As the axial load was increased, samples were scanned at different locations to determine the stress-dependent local changes in porosity. Absolute permeability during the deformation was also measured.
Results show that the stress condition applied to the porous medium changes the rock and fluid transport properties, compared to measurements taken without stressing the sample. X-ray CT enabled the local porosity changes to be quantified, to locate compaction bands and places where shear location occurs, and to evaluate how the inner structure of the rock changes during different modes of deformation.
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Applications of X-ray Computed Tomography in the Geosciences
X-ray computed tomography (CT) is a technique that allows non-destructive imaging and quantification of internal features of objects. It was originally developed as a medical imaging technique, but it is now also becoming widely used for the study of materials in engineering and the geosciences. X-ray CT reveals differences in density and atomic composition and can therefore be used for the study of porosity, the relative distribution of contrasting solid phases and the penetration of injected solutions. As a non-destructive technique, it is ideally suited for monitoring of processes, such as the movement of solutions and the behaviour of materials under compression. Because large numbers of parallel two-dimensional cross-sections can be obtained, three-dimensional representations of selected features can be created. In this book, various applications of X-ray CT in the geosciences are illustrated by papers covering a wide range of disciplines, including petrology, soil science, petroleum geology, geomechanics and sedimentology.