Sensitivity study of chemical effects on the compaction behavior of reservoir chalk (Dan field, Danish North Sea)

Experimental evidence has shown that water-saturated chalk specimens are weaker than oil-saturated samples. The extent of water-induced weakening does not rely only on the stress state and degree of saturation but also on temperature, brine composition, and mineralogy. Our goal is to assess chalk weakening induced by chemical interactions by performing 1-D compaction simulations under three different waterflooding scenarios in the Dan field. While scenario 2 assumes Na2SO4 brine injection, scenarios 1 and 3 simulate MgCl2 brine injected in pure and impure chalk. These scenarios are compared to a base case scenario that is representative of true reservoir compaction. In this purpose, we collect published experimental data on the yield strength of outcrop and reservoir samples. Deemed complementary with laboratory works, these modelling scenarios consider simplistic fluid compositions to assess the impact of single ions on rock compaction at the reservoir scale. Sulfate adsorption on calcite surface is the most influential parameter that could increase the total vertical strain by a factor of three. The dissolution of Opal-CT by MgCl2 also enhances compaction by promoting the plastic deformation of low-porosity rocks. Sensitivity studies at the reservoir scale of the chemical effects on chalk compaction observed in laboratory represent an important step in developing advanced chemo-mechanical models.