Anisotropy in chalk studied by rock mechanics

Chalk formations in the southern part of the North Sea are important oil reservoirs, where seawater injection, as an IOR fluid, has been used with great success to improve oil recovery in high temperature reservoirs. Compaction is observed due to water weakening of the chalk matrix, which has initiated an extensive research to study the mechanism of this weakening effect. In most cases, outcrop chalk was used as a model sample. In this paper, the effect of anisotropy on mechanical properties within two outcrop chalks is studied by drilling samples horizontally and vertically from the same chalk block. It is of interest when performing experimental research to have comparable test material. In that way, large test series can be performed on test material with similar mechanical properties. The results showed that all mechanical properties, except permeability and tensile strength, displayed anisotropic behaviour. The difference in compressive strength was a factor 1.20 to 1.45 for the different drilling directions, while for the bulk modulus the difference was a factor 1.45 to 1.50. Drilling direction had only minor affect on the permeability measurements, the reduction in permeability by increasing hydrostatic stress appeared however to be controlled by the total volumetric strain. These results indicate that mechanical properties from outcrop chalk are only directly comparable when the chalk samples are collected and drilled in the same direction from the same block of chalk.