Microcrack growth and healing in deformed calcite aggregates

Abstract

Modelling of fluid flow in the Earth's crust depends on our understanding of the physical processes responsible for porosity-production and porosity-reduction. This study concerns the role of thermally-activated processes on the evolution of porosity and permeability in rock samples containing microcracks. Experimentally deformed Carrara marble and a synthetic marble were heat-treated at effective pressures of 50 and 150 MPa and temperatures up to 923 K with argon as pore fluid. In situ measurements of the connected porosity and the permeability during heat-treatment indicate a complicated evolution of porosity and permeability. During heating up to 600/700 K, the porosity changes very slowly with increase in temperature and time, decreasing by 0.01–0.02, while the permeability increases by an order of magnitude. At higher temperatures, the porosity reduction rate accelerates, and both the connected porosity and the permeability decrease with increasing temperature and time. The permeability is proportional to the cube of the connected porosity. Microstructural changes during heat-treatment of Carrara marble include (a) the replacement of the finely-crushed fragments within the intergranular damage zones by rounded or polyhedral grains 5–20 μm in diameter; and (b) the partial to complete healing of twin boundary cracks and tips/sub-branches of transgranular cracks. In the finer grain-size synthetic marble, heat-treatment blunts the tips of grain boundary cracks; partial healing of grain boundary cracks is associated with the formation of ridge-channel structures on many grain interfaces. The evolution of porosity, permeability and microstructure points to the roles of thermal cracking, plastic compaction, and diffusional crack healing in controlling fluid transport properties of cracked rocks at elevated temperature. Significantly, the porosity–permeability relationships during isostatic crack healing are very different from those associated with crack growth during deformation.