Finite element study on the closure of thermal microcracks in feldspar/quartz-rocks-11. Intra-, transgranular and mixed cracks

The formation of microcracks is studied in a planar composite material using the finite element method. Intraganular (IGC), transgranular (TGC) and mixed-type cracking is modelled in a system containing angular-cornered quartz (QTZ) grains surrounded by feldspar (FSP). Physical rock properties (elastic moduli, seismic velocity) show a gradual increase with increasing confining pressure due to the closure of cracks as expected from experiments only in the case of IGC. The collapse closure of grain-boundary cracks (GBC) and the TGC is documented by a jump in the pressure-dependent rock properties. Strictly parallel aligned flaws should be replaced by preferred oriented ones or by crack populations with different closing pressures to provide a gradual closure curve. The thermal microcracks investigated can be categorized with respect to their closure process quantified in terms of crack length, aperture and aspect ratio versus confining pressure. Major crack types (GBC I, IGC and TGC) close at pressures ranging from 30 to about 70MPa after cooling the composite by 400°C. For mixed-type cracks (GBC & IGC), the GBC closes at 30MPa before the remaining IGC starts to shorten at 50 MPa. The friction coefficient p along the re-closed GBC-planes is responsible for whether the IGC closes totally ( p = 0.1) or degenerates into a residual pore located at the grain boundary (pr0.3). For prO.3 significant in situ porosity of QTZ/FSP-rich rocks may be found as deep as 10 km within the crust of the earth. Lowering the friction coefficient residual porosity can disappear at shallower depths (e.g. p = 0.1 for the mixed type crack results in a closing pressure of 50 MPa corresponding to a depth of about 2 km)