Permeability evolution of chalk under different stress states

Abstract

This thesis reports the results obtained from triaxial tests on Kansas outcrop chalk flooded with inert NaCl brine. Permeability measurements are conducted at 50ºC under different stress states simulated by five test sequences of hydrostatic and deviatoric loadings. The loading behaviors like shear failure, dilatancy, compression of chalk and their influences on permeability evolution are studied and discussed in detail. The results show that permeability is strongly dependent on stress and strain. It generally decreases with increasing confining pressure by compressing the pore spaces. Deviatoric loading performed prior to hydrostatic loading has minor effect on permeability. As the confining pressure continues to increase, permeabilities of all the tested samples tend to converge. Shear failure occurring in the deviatoric loading phase contributes to the enhancement of permeability in chalk with porosities ranging from 38 to 40%. Single shear-band failure is exhibited with a symbol of the critical point where radial strain rate exceeds axial strain rate. This behavior is accompanied with an increase in permeability. Despite the profound influences of the loading phases, short-term creep and unloading cycles only have minor effects on permeability evolution. Experimental results from deviatoric loadings indicate that confining pressure has a great influence on permeability since chalks tend to be more brittle at lower confining pressure, thus resulting in greater increase in permeability when shear failure occurs.

These experimental results can provide the fundamental approaches to establish the basic understanding of the stress states impact on permeability evolution and can be employed as a foundation and illustration for the future work.