To investigate the relationship between crack damage evolution and permeability changes we conducted a series of experiments where different amounts of isotropic crack damage were introduced into rock samples through heat treatment. Three different materials were used; Westerly granite, Ailsa Craig microgranite and a basalt from Iceland. Microcracking was monitored by means of acoustic emission measurements, and characterized by acoustic wave velocity measurements on the heat treated samples. Subsequent to heat treatment the fluid permeability of the rocks was measured under hydrostatic stress conditions using the steady state flow method. Increasing crack damage results in permeability enhancement although no simple relationship between permeability and crack density or permeability and porosity exists over the whole range of heat treatment temperatures. In all three rock types, three phases exist between permeability evolution and amount of crack damage. Two levels of critical changes in pore network connectivity, or percolation thresholds, are identified in permeability as heat treatment temperature is increased. Comparison of the three rock samples shows that either large changes in crack density leading to increased crack linkage, or small changes in crack density leading to increased crack linkage, is responsible for critical changes in pore connectivity. These results emphasize that changes in crack interconnectivity, and not crack density or porosity alone, is the critical factor in controlling permeability evolution.
1. INTRODUCTION
Permeability in rock is critically dependent upon interconnectivity of the pore network. Thus a high porosity material may exhibit negligible permeability until a critical threshold of pore interconnectivity, the percolation threshold, is achieved. Such critical changes in crack network connectivity need not necessarily result from large changes in crack density. This has important implications for modeling of fluid flow in which damage parameters derived from more easily measurable rock properties (velocities, porosity, quantitative microstructural analysis) are used to calculate permeability.
To investigate the relationship between crack damage evolution and permeability changes we conducted a series of experiments where isotropic crack damage was introduced into rock samples through heat treatment. Samples were heated at a rate of 1°C/min to various peak heat treatment temperatures up to 900°C at ambient pressure. A low rate of heating was established to ensure that cracking events were the result of temperature alone and not due to thermal gradients across the sample. Once the selected peak temperature was reached the samples were held at peak temperature for an hour before being cooled at 1°C /min to room temperature. Thermally-induced microcracking was monitored through recording the acoustic emissions (AE) during heat treatment. Data from acoustic emission (AE) monitoring was used to relate the main periods of thermal cracking to heat treatment temperature. Before and after heat treatment, ultrasonic compressional and shear wave velocities were measured on the Westerly granite and the Iceland basalt. The changes in velocities were interpreted in terms of a crack density parameter using the method of [1]. Subsequent to heat treatment the fluid permeability was measured under hydrostatic stress conditions using the steady state flow method.
