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It is generally challenging to predict the postabandonment behaviour and integrity of wellbores. Leakage is, moreover, difficult to mitigate, particularly between the steel casing and outer cement sheath. Radially expanding the casing with some form of internal plug, thereby closing annular voids and fractures around it, offers a possible solution to both issues. However, such expansion requires development of substantial internal stresses. Chemical reactions that involve a solid volume increase and produce a force of crystallisation (FoC), such as CaO hydration, offer obvious potential. However, while thermodynamically capable of producing stresses in the GPa range, the maximum stress obtainable by CaO hydration has not been validated or determined experimentally. Here, we report uniaxial compaction/expansion experiments performed in an oedometer-type apparatus on precompacted CaO powder, at 65°C and at atmospheric pore fluid pressure. Using this set-up, the FoC generated during CaO hydration could be measured directly. Our results show FoC-induced stresses reaching up to 153 MPa, with reaction stopping or slowing down before completion. Failure to achieve the GPa stresses predicted by theory is attributed to competition between FoC development and its inhibiting effect on reaction progress. Microstructural observations indicate that reaction-induced stresses shut down pathways for water into the sample, hampering ongoing reaction and limiting the magnitude of stress build-up to the values observed. The results nonetheless point the way to understanding the behaviour of such systems and to finding engineering solutions that may allow large controlled stresses and strains to be achieved in wellbore sealing operations in future.

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Subsurface mineralization of CO 2 by injection into (hydro-)fractured peridotites has been proposed as a carbon sequestration method. It is envisaged that the expansion in solid volume associated with the mineralization reaction leads to a build-up of stress, resulting in the opening of further fractures. We performed CO 2 -mineralization experiments on simulated fractures in peridotite materials under confined, hydrothermal conditions, to directly measure the induced stresses. Only one of these experiments resulted in the development of a stress, which was less than 5% of the theoretical maximum. We also performed one method control test in which we measured stress development during the hydration of MgO. Based on microstructural observations, as well as XRD and TGA measurements, we infer that, due to pore clogging and grain boundary healing at growing mineral interfaces, the transport of CO 2 , water and solutes into these sites inhibited reaction-related stress development. When grain boundary healing was impeded by the precipitation of silica, a small stress did develop. This implies that when applied to in-situ CO 2 -storage, the mineralization reaction will be limited by transport through clogged fractures, and proceed at a rate that is likely too slow for the process to accommodate the volumes of CO 2 expected for sequestration.