Compressional velocity, seismic attenuation and permeability relationships for sandstones from WOSPP

Some experimental results measuring P-wave phase velocity and seismic attenuation in laboratory under dry and water-saturated conditions at ultrasonic frequencies and atmospheric pressure on sandstone samples of the Milk River Formation in the Writingon-Stone Provincial Park (WOSPP), southern Alberta, are presented and are correlated with petrophysical data obtained from these samples as clay content, porosity and permeability. These sandstone samples present transverse anisotropy (TI) after analyzing its measured phase velocities under dry conditions along three orthogonal axis. In addition, these sandstone samples present different degree of permeability anisotropy. The principal objective of this study is try to estimate how these petrophysical properties and permeability anisotropy affect the observed behavior of P-wave phase velocities and attenuations on sandstone samples from WOSPP. Additionally, from this analysis is expected to sight which geophysical parameter, P-wave velocity or seismic attenuation, is more important for predicting permeability and its behavior from ultrasonic seismic data. INTRODUCTION For most of this century, oilfield theory and practice considered that rocks exhibits isotropic wave velocities, that is, the measured velocities are no direction-dependent. But actually it is known that seismic waves travel through some rocks with different velocities in different directions due to a spatial ordering of crystals, grains, cracks, bedding planes, joints or fractures essentially an alignment of strengths or weaknesses on a scale smaller than the length of the seismic wave. This phenomenon is called elastic anisotropy and is represented by the anisotropic elastic stiffness tensor (Anderson et al., 1974; Crampin, 1978; 1981; 1984a,b; Crampin et al., 1984). If there exists any elastic anisotropy caused by horizontal fine layering or fractures, it implies that, in addition to an anisotropic elastic stiffness tensor, the material will show an additional dynamic effect due to anisotropic permeability (Gelinsky and Shapiro, 1994a,b; 1995). For materials showing transverse isotropy, due to the presence of horizontal fine layering, the permeability the ease with which fluids flow through rock measured parallel to the layers of porous sedimentary rocks can be greater than the permeability measured vertically (kh > kv). On the other hand, for rocks that are azimuthally anisotropic due to parallel fracture planes, the permeability measured perpendicular to the fracture planes is smaller than the permeability of the rock measured parallel to the fractures (kh < kv) (Gelinsky and Shapiro, 1994a,b; 1995). Several approaches have been used, principally at ultrasonic frequencies in laboratory, for estimating permeability anisotropy from other rock measured properties. Gibson and Toks&& oz (1990) predicted how the permeability would vary with direction in fractured rocks based on seismic velocity anisotropy. But, the ultrasonic experimental data of Han et al. (1986) and Klimentos and McCann (1990) acquired on