Attenuation anisotropy in shale at elevated confining pressures

Abstract

Shale has been estimated to comprise approximately two-thirds of all sedimentary rocks [1]. Nevertheless, studies of the physical properties of shale are relatively uncommon compared to other sedimentary lithologies. Especially rare are studies of shale attenuation anisotropy at elevated confining pressure. An understanding of attenuation and attenuation mechanisms is particularly important for seismic exploration. VSP and cross well experiments have used attenuation information to help determine the lithology of subsurface formations [2-4]. Attenuation is also believed to have a significant affect on AVO responses [5], and may affect lithological interpretations based on AVO analyses. Measurements of attenuation have been correlated with fluid content (e.g. [6]) and porosity (e.g. [7]), and may be used to determine in-situ hydraulic properties. Seismic attenuation has also been linked to rock strength [8,9], and may be useful in relation to blasting, and the location of landslide material. Accurate values of attenuation are also necessary to estimate ground motion associated with earthquakes [10,11]. Shale has been shown to have significant velocity anisotropy (e.g. [12]) that persists at elevated confining pressure. It is unclear, however, whether attenuation displays the same anisotropic relationships with pressure. Previous studies of attenuation in shale have involved either measurements in a single direction, measurements at atmospheric pressure, or measurements on unsaturated samples only (e.g., [13-15]). This study provides the first measurements of shale attenuation in multiple directions, at elevated confining pressure, under both saturated and unsaturated conditions. Laboratory measurements have been made on four samples, up to 250 MPa. Velocity and attenuation have been measured parallel and perpendicular to