#Geophysical zond crack crack

Mixtures of water and glycol therefore allow for a systematic study of the effect of water concentration.

Glycol is fully miscible with water, and its effect on chalk strength is similar to that of oil. showed that the strength of chalk decreases systematically with water content when saturated with water-glycol mixtures.

This so-called water weakening effect has been the subject of extensive study, but the underlying mechanism is still not fully understood. The mechanical strength of water saturated chalk is significantly lower than that of dry or oil saturated chalk. Chalk, a highly porous rock composed almost entirely of calcite, has been extensively studied due to its importance as an oil and gas producing reservoir rock. Unlike many other rocks, calcitic rocks display both plastic and brittle behavior at low stresses and temperature, and we therefore expect the fracture behavior of calcite to be more complex than the more commonly studied minerals such as mica and quartz.

#Geophysical zond crack crack

Various mechanisms including dissolution and microplasticity have been proposed to explain subcritical crack growth in calcite. Experiments on single calcite crystals have shown a nontrivial dependency on pH and ionic concentrations, while studies on calcitic rocks have indicated that these rocks display a complex behavior, different to quartz bearing rocks and glasses. The physical processes controlling subcritical cracking in calcite are still, however, poorly known.

Experimental studies show that subcritical cracking plays an important role during the compaction of carbonate sediments. Faulting and fracturing of carbonate reservoirs have major implications for hydrocarbon fluid pathways and carbonates host many active seismic fault zones. Calcitic rocks are abundant in many hydrocarbon bearing environments and other settings where reactive fluid transport is important. Subcritical crack growth has thus been used as the underlying mechanism in models of slow earthquakes and aftershocks of major earthquakes. Subcritical cracking is argued to be the main mechanism of brittle creep of rocks, and to control the time to failure at constant stress. This phenomenon in of importance in the Earth's crust, where water and other active species are present and displacements and stresses are small. Subcritical crack growth, refers to the phenomenon of slow crack propagation in brittle materials at stresses below the nominal failure stress. The slope of the crack velocity curve remains unaffected, something which strongly suggests that the mechanism for subcritical cracking in calcite does not depend on the water concentration. The effect of increasing water on crack velocity is to lower the threshold energy release rate required for crack propagation. The rate of subcritical crack growth in calcite is well described by a reaction rate model. The relation of surface energy to water concentration can explain water weakening in chalks. We find upper bounds for the surface energy of calcite that decrease with increasing water concentration and that are systematically lower than values obtained from molecular dynamics simulations. In this paper, we present experimental data on subcritical cracking in single calcite crystals exposed to glycol-water mixtures with varying water content. Subcritical cracking behavior and surface energies are important factors in geological processes, as they control time-dependent brittle processes and the long-term stability of rocks.