The effect of divalent ions on the elasticity and pore collapse of chalk evaluated from compressional wave velocity and low-field Nuclear Magnetic Resonance (NMR)

The effects of divalent ions on the elasticity and the pore collapse of chalk were studied through rock-mechanical testing and low-field Nuclear Magnetic Resonance (NMR) measurements. Chalk samples saturated with deionized water and brines containing sodium, magnesium, calcium and sulfate ions were subjected to petrophysical experiments, rock mechanical testing and low-field NMR spectroscopy. Petrophysical characterization involving ultrasonic elastic wave velocities in unconfined conditions, porosity and permeability measurements, specific surface and carbonate content determination and backscatter electron microscopy of the materials were conducted prior to the experiments. The iso-frame model was used to predict the bulk moduli in dry and saturated conditions from the compressional modulus of water-saturated rocks. The effective stress coefficient, as introduced by Biot, was also determined from density and ultrasonic velocities measured on core plugs. Low-field NMR spectroscopy was used in addition to the mechanical testing to prove any changes observed after the saturation related to the surface-to-volume ratio of the pore space in each of the samples or to surface relaxivity. Backscatter-electron (BSE) images were recorded in order to identify the texture of the core plugs under investigation. The experimental results revealed that both elasticity and pore collapse are influenced by the presence of divalent ions in distinct ways. Compressional wave velocities indicate that saturation with water rich in magnesium and calcium ions softens the contact among the mineral grains. Pore collapse strength is deteriorating after the saturation of chalk with water rich in divalent ions. The presence of calcium and sulfate ions in the saturating fluid results in pore collapse at lower stresses than in the case when samples are saturated with deionized water or sodium chloride solution. Low field NMR spectrometry revealed precipitation of crystals in the pore space of chalk saturated with Mg-rich brine. The precipitation of Mg-carbonates was not used to explain the deteriorating pore collapse strength and effects on the elasticity after the saturation since none of the other plugs saturated with divalent ions (Ca2+ and SO₄²⁺) experienced it.