Water weakening of soft and stiff outcrop chalk induced by electrical double layer disjoining pressure

Abstract

In this work we assessed the strength and stiffness of high and low porosity outcrop chalk plugs saturated with three different fluids. Plugs were triaxially tested in a Hoek cell at varying confining stress to probe different parts of the failure envelope as described in the p-q’ space (brittle, transition and yield failure modes). Stiffness of the plugs was derived from bulk density and P and S-wave ultrasonic velocities measured throughout the mechanical test. Selection of the saturating fluids was aimed at testing the hypothesis that water weakening of chalk arises due to a physico-chemical process acting on the vicinity of the chalk grain contact, the electrical double layer (EDL) disjoining pressure. This pressure depends both on the salinity and ionic species dissolved in the saturating water. Fluids used were designed to exert varying levels of disjoining pressure between the chalk grains: calcium chloride brine (highest), calcite equilibrated water (moderate) and as reference Isopar-L or Ethylene glycol (no induced disjoining pressure). We found that both strength and stiffness parameters are affected by the level of the EDL disjoining pressure: The higher the pressure between grains induced by the saturating fluid, the weaker and softer were the core plugs. Water related reduction in strength and stiffness were more prevalent in the soft chalk samples and for the samples mechanically tested at low confining stress. We attribute this selectivity to the smaller cementation area between grains (assessed by Biot's coefficient) for the softer chalk.