Abstract
Reservoir rock near wellbore experiences substantial changes in the poroelastic stress field due to presence of the well itself and later to changes in effective stress conditions as reservoir pore pressure is depleted. Hydraulic fracturing or other stimulation treatments may introduce further stress changes. Although such stress changes may lead to macroscopic rock failure, even under less severe loading, substantial changes in the pore structure. Such stress- and time-dependent deformation of the porous structure is known to change important properties of the rock such as porosity, and permeability.
In this study we investigate the porosity and permeability changes in deforming porous rock using X-ray CT imaging. We used a specially designed X-ray-transparent tri-axial cell enabling fluid flow at the same time. We scanned the samples at successively higher differential stress to observe stress-dependent changes in porosity and permeability. Since the presence and value of confining pressure changes the deformation behavior of the rock, we applied different constant confining stresses on the samples to change the deformation regime and map the porosity in the failure regions.
Results showed that the stress condition applied to the porous medium changes the rock and fluid transport properties compared to the sample measurements done without stressing the sample. Medical and industrial high resolution X-ray scanners enabled us to quantify and map local porosity changes, locate compaction band and where shear location occur and how the inner structure of the rock changes during different modes of deformation. We also compared the properties of sandstone with limestone under deforming stresses.
