Investigations into the factors influencing long range matrix diffusion rates and pore space accessibility at depth in granite

Abstract

Investigations in granite systems have been conducted on (1) diffusion through fissure surfaces, (2) the effect of confining pressures on diffusion rates, (3) the extent of pore interconnectivity over metre-distances, and (4) the accessiblity of the pore space to colloidal particles in the size range 0.3−0.09 μm. Diffusion rates through fissure surfaces were found to be greater than or equal to those measured on samples taken well away from such surfaces. No evidence for pore blocking by alteration products was found. Confining pressures of up to 16 MPa reduced diffusion rates. At the highest pressures reductions by factors of between 2 and 2.5 compared with diffusion rates measured at ambient pressure were recorded. Pore connectivity over distances of up to 125 cm on granite cores saturated with 3 M NaCl was demonstrated by the measurement of formation factors as a function of core length using a conductivity technique. Increases in the formation factor for short core lengths (∼ 1 cm) were ascribed to sample size and pore geometry effects which converted some of the dead-end porosity into transport porosity. Colloidal particles in the size range investigated neither entered the pore structure nor blocked pore apertures to any significant extent.