The physical properties of a set of sandstones—part III. The effects of fine-grained pore-filling material on compressional wave velocity

Abstract

Aspect ratio modelling is employed to explain the measured compressional wave velocities of 20 different dry sandstone samples with varying clay contents at a single confining pressure of 50 MPa. Velocities of the sandstones range between 3.1 and 5.7 km/sec. Measured porosities are between 6 and 33%, clay contents between 2 and 30%. Pores were described using three simple type classifications. The pore-type distributions of the samples were quantified by point counting polished impregnated thin sections using a scanning electron microscope. A representative aspect ratio was assigned to each of the three categories of pore-type. Velocities were modelled using these aspect ratios weighted by the observed distribution of the porosity types. Agreement between theoretical and measured velocities is generally within 10%. The modelling suggests that the effect of clays in sandstone pores is to reduce the sample porosity without reducing the non-framework (void + clay) volume. Thus, for a given porosity, clay-rich samples contain greater non-framework volume, which in turn lowers velocity. The model derived from the dry measurements can be used to approximate empirical relations between velocity, porosity and clay content for saturated samples using data taken from the literature.