The Impact of Surface Charge on the Mechanical Behavior of High-Porosity Chalk

We present rock mechanical test results and analytical calculations which demonstrate that a negative surface charge, resulting from sulfate adsorption from the pore water, impacts the rock mechanical behavior of high-porosity chalk. Na₂SO₄ brine flooded into chalk cores at 130 °C results in significantly reduced bulk modulus and yield point compared with that of NaCl brine at the same conditions. The experimental results have been interpreted using a surface complexation model combined with the Gouy-Chapman theory to describe the double layer. The calculated sulfate adsorption agrees well with the measured data. A sulfate adsorption of about 0.3 μmol/m² and 0.7–1 μmol/m² was measured at 50 and 130 °C, respectively. Relative to a total sites of 5 sites/nm² these values correspond to an occupation of 4 % and 8–13 % which sufficiently explains the negative charging of the calcite surfaces. The interaction between charged surfaces specifically in the weak overlaps of electrical double layer gives rise to the total disjoining pressure in granular contacts. The net repulsive forces act as normal forces in the grains vicinity, counteracting the cohesive forces and enhance pore collapse failure during isotropic loading, which we argue to account for the reduced yield and bulk modulus of chalk cores. The effect of disjoining pressure is also assessed at different sulfate concentrations in aqueous solution, temperatures, as well as ionic strength of solution; all together remarkably reproduce similar trends as observed in the mechanical properties.