Stress corrosion crack growth in porous sandstones.

Abstract

Stress corrosion crack growth occurs when the chemical weakening of strained crack tip bonds facilitates crack propagation. I have examined the effect of chemical processes on the growth of a creack population by carrying out triaxial compression tests on Clashach and Locharbriggs sandstones at temperatures of 25-80 degrees C and at strain rates of 10-5 to 10-8/s. The axial strain, permiability, acoustic emission (AE) activity and the pore fluid chemistry were monitored continuously during these tests. Rock strength is reduced in the presence of water and on the application of a slower strain rate. Elastic modulus also decreases with decreasing strain rate. Microstructural observations indicate that microfracturing is more pervasive in the slow strain rate tests in comparison to the high strain rate tests. Damage parameters derived from the AE data predict the stress-strain curves adequately. The accumulation of damage is more rapid in the slow strain rate tests than in the high strain rate tests. The exit pore fluid silica (Si) concentrations correlate with the main microfracturing domains of the stress-strain curve. In the strain hardening phase of the Locharbriggs tests the Si concentrations and AE damage increase exponentially. The small reactive surface area and the temperature dependance of the Si concentration in the Locharbriggs tests suggest that silica is dissolving actively from the growing crack tips and that reaction rates contribute towards this signal. the Locharbriggs Si signal and damage parameters are strongly correlated by a power law relationship. the obseved strain rate and environment dependance of mechanical properties of Locharbriggs sandstone can be uniquely attributed to crack growth by the stress corrosion mechanism. In the Clashach tests the damage accumulation is best described by a powe-law. The AE activity of both sandstones exhibits clear fore- and aftershock sequences that are well modeled by the Omori law with a power law exponent that is close to unity. The Clashach Omori decay parameter correlates with test temperature, indicating a faster decay of aftershock activity at a higher temperature. The permeability evolution also displays a distinct strain rate dependence. At high strain rates permeability correlates with microcrack damage. At slow strain rate the fluid flow properties correlate with mean effective stress or pore fluid ion concentrations. These observations suggest that brittle fracturing, chemical reaction and hydraulic properties of porous sandstone are strongly coupled processes in the crust.