The stress dependent elastic properties of thermally induced microfractures in aeolian Rotliegend sandstone

The impact of thermally induced microfractures on the stress-sensitive elastic wave properties of aeolian Rotliegend sandstone samples is analysed. It is found that to identify the effects of the microfracture contribution accurately, a correction must first be made to account for water loss (representing a mass loss of 4-6%) from the pore throats and clays due to the heating process, despite care being taken to ensure that the thermally fractured samples re-adsorb room moisture. Both the original and thermally fractured rocks are stress-sensitive at the ultrasonic wave frequencies of the laboratory. However, a distinct shift in the estimated distribution of internal rock compliance indicates that the population of thermal microfractures differs in nature from that caused solely by core-plug extraction damage. In particular, the ratio of normal to tangential compliance is observed to be higher for the thermally generated microfractures than for the broken grain-grain contacts created by extraction unloading. This can be explained by the intragranular thermal-fracture surfaces being smoother when compared to the intergranular boundaries. Mechanical hysteresis is observed between the up- and downgoing test cycles for both the original and, to a greater extent, the thermally fractured rock. This indicates that there is compaction-induced movement of the fractures in the samples during application of stress in the laboratory. © 2007 European Association of Geoscientists and Engineers.