Using the Q factor to detect closed microfractures

Abstract

Fractures are ubiquitous within the subsurface and play an important role in the fluid flow, elasticity, and strength of rocks. Because they are essential to geologic systems such as hydrocarbon and geothermal systems, they need to be properly imaged and monitored. We used the spectral-ratio technique to measure the P-wave Q factor ([Formula: see text]) and the S-wave Q factor ([Formula: see text]) of dry and water-saturated crystalline rock samples that were thermally fractured by heating to 250, 450, 650, and 850°C. Increasing the temperature during thermal treatment produces an increased fracture density that is isotropically distributed. The samples were subjected to hydrostatic pressures up to the pressure at which all fractures are closed, and they were axially loaded to 25% of the failure strength. Axial loading of the samples further closes fractures that are oriented perpendicular to the loading direction, but it opens those that are oriented parallel and oblique to the loading direction. Attenuation measurements were made in the frequency range of 0.8 to 1.7 MHz. At the fracture closure pressure, the sample with the highest fracture density showed a reduction in [Formula: see text] and [Formula: see text] when compared to the sample with the lowest fracture density of 84% and 24%, respectively, under dry conditions and by 33% and 5%, respectively, under saturated conditions. The [Formula: see text] and [Formula: see text] increase as the samples were axially loaded. The increase is more pronounced in [Formula: see text] because it is less influenced by the opening of parallel and obliquely oriented fractures. The opening of these fractures mostly affects the propagation of the S-wave and therefore reduces the increase in [Formula: see text]. The results suggest that [Formula: see text] is very sensitive to fracture closure at intermediate and high effective mean stresses. We determine that [Formula: see text] is a useful measure of the fracture density even at high pressures at which fractures might be expected to be fully closed.