Abstract 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 × 10−19–2.0 × 10−20 m2). 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 × 10−17–6.2 × 10−18 m2) 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 × 10−2 to 7.1 × 10−2 MPa−1, those of porosity from 3.9 × 10−3 to 9.8 × 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.

中文翻译：

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科堡灰岩孔隙度和渗透率的应力敏感性

摘要 我们分析了来自加拿大安大略省中奥陶统 Cobourg 地层的宏观完整石灰岩样品的孔隙度和渗透率的应力敏感性。这种石灰岩是中低放核废料地质储存的潜在宿主地层。孔隙度和渗透率是在规定的等静压条件下用气体（氦气）在干圆柱塞上测量的。仅在第二个加载和卸载路径 (5-20 MPa) 期间进行测量，以最大限度地减少样品制备引起的裂缝的影响。所有样品的孔隙率值都小于 2%。从一个立方体块体钻出的三个相邻样品的 Klinkenberg 校正渗透系数跨越一个数量级（2.3 × 10−19–2.0 × 10−20 m2）。这些样品的输运孔隙度以粘土基质内的狭缝状孔隙和微裂缝为主。通过 X 射线计算机断层扫描 (micro-CT) 和宽离子束铣削扫描电子显微镜鉴定，在另一个含有开放式缝合线的样品上测得更高的渗透系数（1.5 × 10-17-6.2 × 10-18 m2）。 BIB-SEM）。渗透率应力敏感系数为3.0×10-2～7.1×10-2 MPa-1，孔隙度应力敏感系数为3.9×10-3～9.8×10-3 MPa-1。沿卸载路径的渗透系数观察到加载滞后效应，比加载路径上的值低 47%。这被视为蠕变的指示。由于在该石灰岩序列中存在缝合线，基质渗透率不被认为是密封效率的关键因素。