Does the chemical interaction between seawater and chalk affect the mechanical properties of chalk?

Chalk petroleum reservoirs are characterized as fragmentary parts of calcite skeletons produced by plankton algae, known as coccolithophorids, and successful improved oil recovery from this type of reservoir is obtained by injection of seawater. In general, the mechanical strength of chalk is weakened by substitution of oil by water, which is known as water-weakening of chalk. Recent creep studies at 130 degrees C, after flooding 9 pore volumes (PV) of water, have shown that chalk cores flooded with distilled water are considerably stronger than the cores flooded with artificial seawater, a factor of 2.2. Seawater contains potential determining ions (Ca2+, Mg2+ and SO42-) which have influence on the charge of the chalk surface. Core flooding tests at high temperatures showed that Mg2+ ions substituted Ca2+ ions at the chalk surface, and the degree of substitution increased as the temperature increased. Thus, the substitution reaction increases with temperature, while the dissolution of CaCO3(s), i.e. chalk, decreases with increasing temperature. The following hypothesis was suggested: The effect of temperature on the mechanical properties of chalk flooded with seawater is dictated by the degree of substitution of Ca2+ ions by Mg2+ ions and the dissolution of chalk. The processes have an opposite behavior regarding the temperature. This hypothesis has later been confirmed by mechanical tests.