[1] It is widely accepted that the structure of a grain boundary undergoing pressure solution can have a strong influence on the rates at which diffusive transport in the grain boundary occurs. However, the influence of grain boundary structure on internal grain boundary dissolution rates has received little attention, despite evidence that dissolution controlled pressure solution in quartz is slower than expected assuming dissolution kinetics appropriate for free surfaces. In this paper, three hypothetical steady state grain boundary structures are defined and the influence of these structures on dissolution controlled pressure solution rates in an elastic solid are considered by deriving simple models based on internal grain boundary mass and energy balances. It is found that average dissolution rates in a rough grain boundary (island-channel network) are slowed down by up to 13% compared to dissolution in a flat grain boundary containing a thin fluid film. This can only partly account for the discrepancy between models and experiments reported in the literature. In addition a model is derived providing a criterion or ''yield stress'' for pressure solution, below which the process is prevented by surface energy driven grain boundary healing (progressive reduction of the contact area filled by connected fluid). This ''yield stress criterion'' for pressure solution offers a further explanation for reduced rates or cessation of pressure solution at low effective stresses in nature and experiment. Using this criterion, limiting porosity depth curves are predicted for sandstones compacting by pressure solution, which show favorable agreement with porosity-depth data for quartz sandstones.Citation: van Noort, R., H. J. M. Visser, and C. J. Spiers (2008), Influence of grain boundary structure on dissolution controlled pressure solution and retarding effects of grain boundary healing,

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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.

[1] We report isostatic compaction experiments performed on granular quartz under hydrothermal conditions (3-129 mm of initial grain size, 300-600°C, 200 MPa of fluid pressure, and 25-100 MPa of effective pressure). From microstructural evidence, it was determined that, whereas microcracking controlled precompaction at room temperature, pressure solution was the main mechanism during hydrothermal compaction, although a role of microcracking could not be excluded entirely. Our mechanical data, together with theoretical pressure solution rate models, further indicated that pressure solution was controlled by interface kinetics, dissolution being the most likely rate-controlling mechanism. An empirical relation of the form _ e = 10 À7.8 (f/f 0 ) 10.0 s 3.4 d À1 exp (À105,000/RT) was fitted to our data to describe experimental compaction rates. Electron backscatter diffraction (EBSD) analysis performed on one sample showed limited evidence for plastic deformation (Dauphiné twinning and lattice bending) at grain contact points under high stress. Contact microstructures formed during compaction were studied using quartz single crystal discs or a mica plate as reference surfaces. This showed that, in all contacts, microstructures were rough, with a micrometer scale roughness. Many contacts also showed internal microcracking, and it is inferred that microcracking is likely an important mechanism for creating and maintaining rough contacts during quartz pressure solution, at least under the present experimental conditions. Extrapolation of our empirical equation for compaction rates to natural conditions is consistent with previous observations that, during burial and diagenesis of a sandstone, pressure solution starts to operate at depths of about $1.5-2 km, becoming the dominant compaction mechanism at depths greater than $2.5-3 km. The extrapolation gives good agreement with porosity-depth trends reported for natural, arenitic sandstones.

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.

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