Microcrack growth and healing in deformed calcite aggregates

Modelling of fluid flow in the Earth's crust depends on our understanding o f the physical processes responsible for porosity-production and porosity-r eduction. This study concerns the role of thermally-activated processes on the evolution of porosity and permeability in rock samples containing micro cracks. Experimentally deformed Carrara marble and a synthetic marble were heat-treated at effective pressures of 50 and 150 MPa and temperatures up t o 923 K with argon as pore fluid. In situ measurements of the connected por osity and the permeability during heat-treatment indicate a complicated evo lution of porosity and permeability. During heating up to 600/700 K, the po rosity changes very slowly with increase in temperature and time, decreasin g by 0.01-0.02, while the permeability increases by an order of magnitude. At higher temperatures, the porosity reduction rate accelerates, and both t he connected porosity and the permeability decrease with increasing tempera ture and time. The permeability is proportional to the cube of the connecte d porosity. Microstructural changes during heat-treatment of Carrara marble include (a) the replacement of the finely-crushed fragments within the intergranular d amage zones by rounded or polyhedral grains 5-20 mum in diameter; and (b) t he partial to complete healing of twin boundary cracks and tips/sub-branche s of transgranular cracks. In the finer grain-size synthetic marble, heat-t reatment blunts the tips of grain boundary cracks; partial healing of grain boundary cracks is associated with the formation of ridge-channel structur es on many grain interfaces. The evolution of porosity, permeability and microstructure points to the ro les of thermal cracking, plastic compaction, and diffusional crack healing in controlling fluid transport properties of cracked rocks at elevated temp erature. Significantly, the porosity-permeability relationships during isos tatic crack healing are very different from those associated with crack gro wth during deformation. (C) 2001 Elsevier Science B.V. All rights reserved.