Summary
The origin of the difference in the relationship between permeability and porosity for Danian and Maastrichtian chalk from the Gorm field offshore Denmark has been investigated. The investigation was based on 300 sets of core data (He-expansion porosity and air permeability) from Well Gorm N-22X. On 24 of the core plugs, the specific surface was determined by BET and, on 14 of these samples, image analysis was made.
The data were rationalized by the use of the Kozeny equation and it was found that each geologic unit had a characteristic relationship among porosity, permeability, and specific surface. Furthermore, it was found that the nature of porosity (intrafossil, intergranular, etc.) had no significant influence on the air permeability, so that the permeability of the chalk can be calculated from total porosity and specific surface.
Kozeny's empirical constant, c, was determined analytically from a simple porosity model and Poiseuille's law.
Introduction
Experience has illustrated that to a first approximation, porosity and the logarithm of permeability are well correlated in the North Sea chalk. However, different stratigraphical units have distinctly different porosity-permeability relations. For a given porosity, the permeability of chalk of Maastrichtian age is, e.g., on average, larger than the permeability of chalk of Danian age. The cause for the differences in porosity-permeability relationships between the chalk units is not well documented in literature, but the size of pores and particles, and the surface characteristics of the particles are likely controlling factors.
This study was undertaken to obtain a simple model for the relationship among porosity, permeability, and specific surface for reservoir chalk irrespective of stratigraphic unit, and to evaluate the usefulness of image-analysis data relative to laboratory data. To meet this purpose, we have studied core data from a chalk field (the Gorm field) applying the Kozeny equation.