Anhydrite (CaSO4) is a chemically reactive evaporite mineral that readily hydrates to gypsum (CaSO4.2H2O) when exposed to fresh water, absorbing water and swelling, which can severely disrupt drilling fluid rheology and destabilize the wellbore; drilling through thick anhydrite sequences therefore requires a saline (salt-saturated or high-salinity) drilling fluid to suppress this hydration reaction, formulated with rheological properties (a controlled, time-dependent decrease in viscosity/shear-thinning behavior) that allow efficient hole cleaning and circulation while minimizing chemical interaction with the reactive evaporite formation.