Stress sensitivity of porosity and permeability of Cobourg limestone

We have analysed the stress sensitivity of porosity and permeability of macroscopically intact limestone samples, from the Middle Ordovician Cobourg formation, Ontario, Canada. This limestone is a potential host formation for geological storage of low- and intermediate-level nuclear waste. Porosity and permeability were measured with gas (helium) on dry cylindrical plugs under defined isostatic stress conditions. Measurements were performed only during the second loading and unloading paths (5-20 MPa) to minimize the impact of fissures induced by sample preparation. All samples have porosity values of less than 2%. The Klinkenberg-corrected permeability coefficients of three adjacent samples drilled from one cubic block in different orientations span one order of magnitude (2.3 x 10(-19) -2.0 x 10(-20) m(2)). The transport porosity of these samples is dominated by slit-shaped pores and micro-fractures within the clay matrix. Much higher permeability coefficients (1.5 x 10(-17) -6.2 x 10(-18) m(2)) were measured on another sample containing open stylolites, as identified by X-ray Computed Tomography (micro-CT) and broad ion beam milling-scanning electron microscopy (BIB-SEM). Stress sensitivity coefficients of permeability range from 3.0 x 10(-2) to 7.1 x 10(-2) MPa-1 , those of porosity from 3.9 x 10(-3) to 9.8 x 10(-3) MPa-1 . A loading hysteresis effect was observed with permeability coefficients along the unloading path, up to 47% lower than values on the loading path. This is taken as an indication for creep. Due to the occurrence of stylolites in this limestone sequence, the matrix permeability is not considered as a critical factor for the sealing efficiency.