Equivalent pore radius and velocity of elastic waves in shale. Skjold Flank-1 Well, Danish North Sea

Abstract

Abstract Equivalent pore radius links permeability and porosity of a porous medium. This property can be calculated from specific surface and porosity data measured in the laboratory. We can obtain porosity information from logging data but specific surface information can only be obtained from laboratory experiments on cuttings or core samples. In this study we demonstrate that elastic moduli as calculated from bulk density and velocity of elastic waves relate to equivalent pore radius of the studied shale intervals. This relationship establishes the possibility of calculating equivalent pore radius from logging data. We used cuttings samples and available well logs to characterize Cenozoic, Cretaceous and Jurassic shale sections in the Skjold Flank-1 well of Danish North Sea. Logging data and well reports were used to select 31 shale cuttings samples and experimental data for porosity, grain density and BET specific surface were obtained from these samples using kaolinite and smectite as reference. The cuttings samples were also characterized with respect to mineralogical composition, content of organic carbon and cation exchange capacity. Equivalent pore radius was calculated from porosity and BET data. It varies from 5 nm for some Cretacous and Jurassic shale samples to about 25 nm in some Cenozoic samples. Pore radius is controlled by shale mineralogy and the degree of compaction. We found exponential relationships between equivalent pore radius and elastic moduli, and these empirical relationships were used to calculate equivalent pore radius for the Cenozoic, Cretaceous and Jurassic shale sections in Skjold Flank-1 well from elastic moduli. Elastic moduli were calculated from sonic velocity and density logs. The calculated equivalent pore radius logs vary from 27 nm at 500 m to 13 nm at 2000 m within Cenozoic shale and from 12 nm to about 6 nm in the deeper Cretaceous and Jurassic shale intervals. Cross plots of the equivalent pore radius with neutron porosity and gamma ray data separate the Cenozoic shale section with high equivalent pore radius from Cretaceous and Jurassic sections.