Poroelastic properties of carbonates: a case study on Indiana limestones

Abstract

This thesis studies the poroelasticity of carbonate rocks and investigates the dependence of various physical properties on confining pressure and pore pressure. These physical properties include porosity, permeability, static strain, ultrasonic P- and S-wave velocities. In a process, I estimate dynamic and static stiffnesses of carbonates as they often differ due to various effects, such as velocity dispersion, heterogeneity of the rock, and amplitudes of applied strain. The studied materials include carbonate rocks with similar mineralogy and porosity, but varying permeability to see the effect of hydraulic properties on the elastic properties. The samples with higher permeability had larger stiffness values and faster velocities, which I explain through a lower specific surface area per grain volume and hence, better cementation of grains. Estimated elastic properties were then used to produce dynamic and static effective stress coefficients, which were found to have a linear relationship. I then characterize the effective stress law for strain for an Indiana limestone rock by measuring the volumetric strain as function of confining and pore pressures. Biot’s coefficient decreased from 0.70 to as low as 0.59 in response to a 35 MPa change in differential pressure. Such a strong pressure sensitivity implies the importance of accurate estimations of ? with respect to pressure rather than using a constant value. Finally, I experimentally demonstrate the interaction of the rock with deionized water as a pore fluid; during exposure to aggressive intrinsic fluid, the rock’s solid matrix becomes softer. This type of rock-fluid interaction affects fluid substitution models, such as Gassmann’s model, which fail to predict the actual stiffness of the saturated rock.