The safe storage of CO₂ in deep reservoir units requires an efficient sealing of the overlaying caprock. The acidic environment caused by the dissolution of CO₂ into the pore fluid can induce changes in the microstructure of the material over the long term and might consequently impact its retention capabilities through changes in the pore size and pore connectivity. In the first part of this paper, the impact of a low pH environment on some of the physical properties, such as grain density, void ratio and dominant entrance pore size, and on the retention capacity of a shaly caprock representative material, i.e., Opalinus Clay shale, is investigated by using an HCl solution inducing a pH = 3. The results show that grain density, dominant entrance pore size and void ratio are not significantly affected by the contact with a low pH environment in the considered period. Similar conclusions can be drawn for the retention capacity because the air entry value appears to be the same for treated and non-treated material.

Subsidence and mechanical failure of the caprock are among the issues related to CO₂ storage technology. The second part of the paper is dedicated to the analysis of the impact of CO₂ injection on the mechanical behaviour of the Opalinus Clay shale. CO₂ injection under constant stress conditions with consideration of different overconsolidation ratios is conducted to take into account different loading paths that could be experienced in situ by the caprock. The results show that the injection of CO₂ induces the development of volumetric strains of less than 0.1%. Lower strains are measured when the material is overconsolidated; this result can be related to the fact that the material structure is more prone to collapse when it is found in normally consolidated conditions. The observed vertical displacements can be partially caused by desaturation effects induced by the differential pressure between CO₂ and pore water, together with double layer effects induced by the CO₂ diffusing along the height of the specimen. The impact of CO₂ injection on the hydro-mechanical properties of the material is also analysed. The findings suggest that the diffusion of CO₂ into the shale does not impact the hydro-mechanical properties of the material because no significant change in oedometric modulus, coefficient of consolidation, secondary compression coefficient, poromechanical parameters and hydraulic conductivity are highlighted after the injection of CO₂.

在深层储层单元中安全存储CO ₂要求有效覆盖上盖岩层。CO ₂溶解引起的酸性环境进入孔隙流体会长期引起材料微观结构的变化，因此可能会通过孔径和孔隙连通性的变化而影响其保留能力。在本文的第一部分中，低pH环境对某些物理性质（如颗粒密度，孔隙率和主要入口孔径）的影响以及对页岩盖层代表性物质Opalinus的保留能力的影响通过使用HCl溶液诱导pH = 3，对粘土页岩进行了研究。结果表明，在所考虑的时间段内，与低pH环境的接触不会显着影响颗粒密度，主要入口孔径和空隙率。

盖层的沉陷和机械故障是与CO ₂储存技术有关的问题。本文的第二部分致力于分析CO ₂注入对Opalinus粘土页岩力学行为的影响。考虑到不同的超固结比，在恒定应力条件下进行CO ₂注入时要考虑到盖层可能在原地经历的不同加载路径。结果表明注入CO ₂引起小于0.1％的体积应变的发展。当材料过度固结时，可测得较低的应变。该结果可能与以下事实有关：在正常固结条件下发现材料结构时，材料结构更容易崩溃。观察到的垂直位移可能部分由CO ₂和孔隙水之间的压差引起的去饱和作用，以及由CO ₂沿试样高度扩散引起的双层效应引起。还分析了CO ₂注入对材料的流体力学性能的影响。研究结果表明，CO ₂的扩散注入页岩中不会影响材料的水力机械性能，因为注入CO ₂后突出显示了测渗模量，固结系数，二次压缩系数，孔隙力学参数和水力传导率没有明显变化。