The relationships between the velocities, attenuations and petrophysical properties of reservoir sedimentary rocks¹

Abstract

We have measured the velocities and attenuations of compressional and shear waves in 29 water‐saturated samples of sandstones and shales at a confining pressure of 60 MPa and at frequencies of about 0.85 MHz. The measurements were made using a pulse echo method in which the samples (diameter 5 cm, length 1.5 cm to 2.5 cm) were placed between perspex buffer rods inside a high‐pressure cell. The velocity of each seismic wave was determined from the traveltime difference of equivalent phase points (corrected for diffraction effects) of the signals reflected from the top and from the base of each sample. Attenuation was determined in a similar way by comparison of the diffraction corrected amplitudes of the signals. The attenuation data are presented as ‘quality factors’: Qp and Qs for compressional and shear waves respectively. The results show that Qs is strongly correlated with Vs, that Qp is weakly correlated with Vp, and that Qp is strongly correlated with Qs. Qp is strongly dependent on the volume percentage of the assemblage of intra‐pore minerals, whether they are clays or carbonates. It is concluded that the attenuation mechanism is due to the local fluid flow arising from the differential dilation of the solid rock frame and the intra‐pore mineral assemblage, which is a result of their very different elastic moduli.

Thin-layer effects in glaciological seismic amplitude-versus-angle (AVA) analysis: implications for characterising a subglacial till unit, Russell Glacier, West Greenland

“…Although lower quality factors have been recorded for sea-floor sediment (e.g. Best et al, 1994;Ayres and Theilen, 2001;Riedel and Theilen, 2001;Sain and Singh, 2011), we allocate the highest values to minimise the Q-contrast (and hence its reflectivity contribution) across the glacier bed.…”

Clay squirt: Local flow dispersion in shale-bearing sandstones

“…We affix a range of isoframe values to sandstone by calculating the isoframe value from a collection of dry elastic moduli for clean consolidated and unconsolidated sandstones. The data are compiled from the literature (King, 1966;Han, 1986;Jizba, 1991;Best et al, 1994). Figure 4 shows the dry bulk modulus for clean consolidated sandstones and unconsolidated sands versus porosity.…”

“…Here, an obvious example is pore space among clay minerals in shaly sandstone because shale can be pore filling and isolated from the applied stress, so that it does not participate in the load-bearing frame (Anstey, 1991). Accordingly, Best et al (1994) find dispersion and attenuation of fluidsaturated sandstone to increase with shale fraction and suggest that the dispersion is caused by pore-pressure gradients induced in the intrashale porosity, a mechanism they coined "clay squirt." Pore-pressure gradients in shale porosity, clay squirt, could therefore explain the extra dispersion in shaly sandstones.…”

“…The elastic moduli of shale bearing sandstones exhibit different porosity trends from those of clean sandstone (Tosaya, 1982;Han, 1986;Klimentos, 1991;Best and McCann, 1995), and the frequency dispersion of elastic moduli is larger in shale-bearing sandstones than in clean ones (Han, 1986;Best et al, 1994;King et al, 2000;King and Marsden, 2002). 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.)…”

Non Invasive Acoustic Measurements for Faults Detecting in Building Materials and Structures

“…It has long been believed that attenuation is more suitable than velocity (or travel time) to study the inner properties of materials, because an anomaly has a greater effect on the attenuation of a signal than on the propagation time (Best et al, 1994;Hudson, 1981). In fact, as previously mentioned, wave's characteristics such as attenuation, scattering and frequency content may allow one to get relevant information about the material, because of the reliance of the propagation on the properties of the medium through which waves travel.…”