Ultrasonic properties of sedimentary rocks: effect of pressure, saturation, frequency and microcracks

Abstract

A B S T R A C TSeismic velocities and attenuations are influenced by lithology, porosity and permeability as well as the kind and quantity of the pore fluids. The microstructure of rocks is another important factor influencing seismic properties. This influence can be used to investigate the pressure-dependent closure of microcracks. For this purpose, velocities and attenuations of P and S waves have been determined at ultrasonic frequencies in three different rocks (vacuum dry and partially saturated with water) as a function of hydrostatic pressure up to 200 MPa. A new combined model has been developed. With this model an explicit description of the microstructure [local fluid flow (LF)] and macrostructure (Gassmann effect, global fluid flow) is possible. Assuming a patchy saturation (i.e. inhomogeneous distribution of the fluid in the pores) and modulus reduction, both the saturation-dependent measurements and the pressuredependent data can be explained. The modelling and the phenomenological interpretation of the data gained under increasing hydrostatic pressure yields consistent results concerning the amount of crack closure as well as the range in which crack closure appears: in high-porosity sandstones (Bentheimer and Obernkirchner Sandstone) a strong closure of cracks already with a small increase of pressure must be concluded. In a low-porosity Harzer Greywacke a significantly lower closure of cracks can be concluded within the pressure range of 0-100 MPa. It is shown, however, that there must be open cracks in all rocks even under elevated hydrostatic pressure, resulting in a measurable effect due to LF.

Integrated interpretation of physical properties of rocks of the borehole Yaxcopoil‐1 (Chicxulub impact structure)

“…This may be due to the different kind of drying: In oven‐dried samples a small amount of moisture is always remaining in the rock. This might have led to modulus reduction (see, e.g., Mayr and Burkhardt [2006] and section 5). …”

“…Acoustic compressional ( v p ) and shear ( v s ) wave velocities in approximately 80 vacuum dry and in approximately 50 fully water saturated samples were determined with a pulse transmission technique in the ultrasonic frequency range (0.4–1.0 MHz) [ Wulff and Burkhardt , 1996, 1997; Mayr , 2002; Mayr and Burkhardt , 2006]. Representative samples (homogeneous mineralogy and without visible cracks) were chosen for these measurements.…”

Ultrasonic logging across unconformities — outcrop and core logger sonic patterns of the Early Triassic Middle Buntsandstein Hardegsen unconformity, southern Germany

“…In lithologically uniform and conformable strata, sonic velocity is mainly a function of porosity, thus a gradual increase can be expected with depth (cf. Mayr and Burkhardt, 2006). Abrupt offsets of sonic velocities then provide reliable proxies for the positioning of unconformities and even for the calculation of associated stratigraphic loss (Henk, 1992).…”

“…Common factors are: a variable degree of rock water content (due to precipitation), variable (non-standardized) contact pressure and transmitting quality between rock and transmitter/receiver probes, influence of rock surface weathering processes, and rock disintegration. The behaviour of ultrasonic velocities in sediments under different pore fluid saturation stages and variable hydrostatic pressures was previously examined in laboratory experiments (Schütt, 1992;Mayr and Burkhardt, 2006).…”

Clay squirt: Local flow dispersion in shale-bearing sandstones

“…Several authors have ascribed the dispersion in sandstone to "squirt flow": pore-pressure gradients caused by deformation of compliant pores embedded in the stiffer framework of the sandstone (i.e., microcracks, grain boundaries, etc.) (Mayr and Burkhardt, 2006;Gurevich et al, 2010;Paula and Peruvhina, 2012), and according to this concept, the stress dependency of the elastic moduli of dry sandstone should delimit the possible dispersion in the water-saturated state (Mavko and Jizba, 1991;Shapiro, 2003;Gurevich et al, 2010;David and Zimmerman, 2012). In spite of this theory, dispersion in shaly sandstones has been frequently found to exceed the bounds for dispersion based on the measured stress dependency (Marketos and Best, 2010;Paula and Peruvkhina, 2012).…”