title: The kinetics and mechanics of a dehydrating system and the deformation of porous rock
Creators: name: Bedford, JD
Publisher: name: University of Liverpool Repository
Year: 2017

description: This thesis aims to address two distinct areas of Earth Sciences that are linked by common processes. The first is dehydration reactions at intermediate depths that lead to seismicity, and the second is the mechanics of the deformation of porous sandstones which is important for the development of sub-surface reservoirs. The breakdown of hydrous minerals in subduction zones is often invoked as the cause of seismicity at intermediate depths (50-200 km). The release of high pressure fluid is thought to reduce effective stress allowing brittle deformation at pressures where instabilities are typically supressed. Pore fluid pressure (Pf) evolution is dependent on the feedback between reaction rate, fluid flow and deformation. Reaction generates fluid which, if unable to drain, will lead to an increase in Pf. However dehydration reactions are also typically associated with (i) solid volume reduction which produces porosity, enhancing fluid flow and allowing high Pf to dissipate, and (ii) compaction of this pore space that can restrict fluid flow enabling Pf to build up. This thesis aims to constrain the processes that control the reaction rate, and hence the fluid production rate, and also determine the deformation behaviour of the porous reaction product. This is done by investigating experimentally the reaction gypsum → bassanite + H₂O. Reaction processes are investigated by imaging a dehydrating gypsum sample using real time 4D X-ray synchrotron microtomography. The datasets acquired allow the evolving pore structure and connectivity to be analyzed during reaction. The growth of bassanite grains is tracked and the kinetics are shown to be intimately linked to the spatial evolution of porosity. New pores wrap around bassanite grains producing moat-like structures; generating diffusion pathways along which the transport of chemical constituents to the growing grains occurs. As the moats grow in width, diffusion and hence reaction rate slow down. Individual moats become interconnected early in the reaction allowing efficient drainage and dissipation of locally high Pf. Identifying the dominant chemical transport pathway is important for modelling of dehydrating systems to constrain better the feedback between reaction, fluid flow and deformation. The mechanical behaviour of the porous reaction product bassanite is investigated by mapping the yield curve evolution along different loading paths. Yield curves are typically plotted in P-Q space where P is the effective mean stress and Q is the differential stress. They are typically considered to be elliptical in shape with the low pressure side being associated with localized brittle faulting (dilation) and the high pressure side with distributed ductile deformation (compaction). A new stress-probing methodology is used to map in high resolution the yield curve and its evolution with continued deformation. This reveals that the yield curve is not perfectly elliptical with the high pressure side comprised partly of a near vertical limb. The yield curve evolution is dependent on the nature of inelastic strain, with deviatorically compacted samples having considerably larger yield curves than hydrostatically compacted samples of similar porosity. This is associated with the formation of a heterogeneous microstructure during deviatoric loading, showing sets of conjugate shear fractures. The same stress-probing methodology is applied to two high porosity sandstones to see if the yield curve evolution observed for porous bassanite is applicable. Both sandstones show a similar near vertical limb on the high pressure side of the curve as observed with bassanite. The yield curve evolution for sandstone is also more sensitive to deviatoric loading, like bassanite, although no localized deformation features are observed. The data highlight that future studies of porous rock deformation should consider the effect of the nature of inelastic strain on the mechanical and microstructural evolution of porous rock.