The geological storage of carbon dioxide (CO 2 ) is a well-studied technology, and a number of demonstration projects around the world have proven its feasibility and challenges. Storage conformance and seal integrity are among the most important aspects, as they determine risk of leakage as well as limits for storage capacity and injectivity. Furthermore, providing evidence for safe storage is critical for improving public acceptance. Most caprocks are composed of clays as dominant mineral type which can typically be illite, kaolinite, chlorite or smectite. A number of recent studies addressed the interaction between CO 2 and these different clays and it was shown that clay minerals adsorb considerable quantities of CO 2 . For smectite this uptake can lead to volumetric expansion followed by the generation of swelling pressures. On the one hand CO 2 adsorption traps CO 2 , on the other hand swelling pressures can potentially change local stress regimes and in unfavourable situations shear-type failure is assumed to occur. For storage in a reservoir having high clay contents the CO 2 uptake can add to storage capacity which is widely underestimated so far. Smectite-rich seals in direct contact with a dry CO 2 plume at the interface to the reservoir might dehydrate leading to dehydration cracks. Such dehydration cracks can provide pathways for CO 2 ingress and further accelerate dewatering and penetration of the seal by supercritical CO 2 . At the same time, swelling may also lead to the closure of fractures or the reduction of fracture apertures, thereby improving seal integrity. The goal of this communication is to theoretically evaluate and discuss these scenarios in greater detail in terms of phenomenological mechanisms, but also in terms of potential risks or benefits for carbon storage.

Capacitation is the final maturation step spermatozoa undergo prior to fertilisation. The efflux of cholesterol from the sperm membrane to the extracellular environment is a crucial step during capacitation but current methods to quantify this process are suboptimal. In this study, we validate the use of a BODIPY-cholesterol assay to quantify cholesterol efflux from spermatozoa during in vitro capacitation, using the boar as a model species. The novel flow cytometric BODIPY-cholesterol assay was validated with endogenous cholesterol loss as measured by mass spectrometry and compared to filipin labelling. Following exposure to a range of conditions, the BODIPY-cholesterol assay was able to detect and quantify cholesterol efflux akin to that measured with mass spectrometry. The ability to counterstain for viability is a unique feature of this assay that allowed us to highlight the importance of isolating viable cells only for a reliable measure of cholesterol efflux. Finally, the BODIPY-cholesterol assay proved to be the superior method to quantify cholesterol efflux relative to filipin labelling, though filipin remains useful for assessing cholesterol redistribution. Taken together, the BODIPY-cholesterol assay is a simple, inexpensive and reliable flow cytometric method for the measurement of cholesterol efflux from spermatozoa during in vitro capacitation.

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CO 2 uptake by smectites can cause swelling and self-stressing in shallow clay-rich caprocks under CO 2 storage P-T and constrained conditions. However, little data exist to constrain the magnitude of the effects of CO 2 -H 2 O-smectite interactions on the sealing properties of clay-rich caprocks and faults. We performed permeability experiments on intact and fractured Opalinus Claystone (OPA) cores (~ 5% smectite), as well as on a simulated gouge-filled faults consisting of Na-SWy-1 montmorillonite, under radially constrained conditions simulating "open" transport pathways (dry and variably wet He or CO 2 ; 10 MPa fluid pressure; 40 °C). Overall, the flow of dry CO 2 through intact OPA samples and simulated smectite fault gouge caused a decrease in permeability by a factor of 4-9 or even by > 1 order, compared to dry He permeability. Subsequent to flow of dry and partially wet fluid, both fractured OPA and simulated gouge showed a permeability reduction of up to 3 orders of magnitude once flow-through with wet CO 2 was performed. This permeability change appeared reversible upon re-establishing dry CO 2 flow, suggesting fracture permeability was dominated by water uptake or loss from the smectite clay, with CO 2 -water-smectite interactions play a minor effect. Our results show that whether an increases or decreases in permeability of clayey caprock is expected with continuous flow of CO 2 -rich fluid depends on the initial water activity in the clay material versus the water activity in the CO 2 bearing fluid. This has important implications for assessing the self-sealing potential of fractured and faulted clay-rich caprocks. Highlights• Permeability of potential clay-rich caprock (Opalinus Claystone) was systematically measured using variably wet CO 2 versus He. • Through-flow of dry CO 2 caused a decrease in permeability of intact Opalinus Claystone and simulated smectite fault gouge by up to > 1 order, as composed to dry He. • Fractured Opalinus Claystone and simulated gouge showed a permeability reduction of up to 3 orders of magnitude with through-flow of wet CO 2 . • Permeability decrease by through-flow of variably wet CO 2 appeared reversible upon re-establishing dry CO 2 flow. • The initial water activity in the clay material versus the water activity in the CO 2 bearing fluid determines change in permeability upon CO 2 flushing. Keywords Shale permeability • CO 2 storage • Clay swelling • CO 2 -H 2 O-smectite interactions * M. Zhang