Mechanical and hydraulic properties of carbonate rock: The critical role of porosity

Abstract

Carbonate rocks are extensively used in civil infrastructure and play a critical role in geoenergy geoengineering, either as hydrocarbon reservoirs or potential repositories for CO 2 geological storage. Carbonate genesis and diagenetic overprint determine the properties of carbonate rocks. This study combines recent data gathered from Madison Limestone and an extensive dataset compiled from published sources to analyze the hydraulic and mechanical properties of limestone carbonate rocks. Physical models and data analyses recognize the inherently granular genesis of carbonate rocks and explain the strong dependency of physical properties on porosity. The asymptotically-correct power model in terms of (1- ϕ / ϕ ∗) α is a good approximation to global trends of unconfined stiffness E and unconfined compressive strength UCS , cohesive intercept in Mohr-Coulomb failure envelopes, and the brittle-to-ductile transition stress. This power model is the analytical solution for the mechanical properties of percolating granular structures. We adopted a limiting granular porosity ϕ ∗ = 0.5 for all models, which was consistent with the loosest packing of monosize spheres. The fitted power model has exponent ( α = 2) in agreement with percolation theory and highlights the sensitivity of mechanical properties to porosity. Data and models confirm a porosity-independent ratio between unconfined stiffness and strength, and the ratio follows a log-normal distribution with mean ( E/UCS ) ≈ 300. The high angle of internal shear strength measured for carbonate rocks reflects delayed contact failure with increased confinement, and it is not sensitive to porosity. Permeability spans more than six orders of magnitude. Grain size controls pore size and determines the reference permeability k ∗ at the limiting porosity ϕ ∗ = 0.5. For a given grain size from fine to coarse-grained dominant carbonates, permeability is very sensitive to changes in porosity, suggesting preferential changes in the internal pore network during compaction.