POROSITY AND PERMEABILITY EVOLUTION DURING HOT ISOSTATIC PRESSING OF CALCITE AGGREGATES

Porosity, permeability, and storativity were measured during isostatic hot-pressing of fine-grained calcite aggregates at temperatures of 633 to 833 K, confining pressures of 200 to 300 MPa, and argon pore pressures of 100 to 250 MPa. The progressive changes in total porosity were measured in situ by monitoring the sample length changes. The connected porosity and the permeability and storativity were measured in situ by incrementing and oscillating pore pressure techniques, respectively. In a given test, there was a decrease with time in the rate of reduction of porosity, the rates being higher at higher temperature and effective pressure. The permeability k was nonlinearly related to the total porosity phi in the form k is-proportional-to phi(n). The exponent n was approximately equal to 3 and thus consistent with the prediction of the ''equivalent channel'' model, in the range of porosity from 0.18 down to 0.07. Below 0.07, n became much larger (around 14), an effect that can be attributed to loss of connectivity and which is qualitatively similar to that observed by Bernabe et al. [1982] in post-hot-pressing measurements. However, a cube law continues to apply below 0.07 total porosity if the permeability is related to the connected porosity itself. The storativity is also nonlinearly related to the porosity. Model analyses of the permeability and storativity results indicate both that the pore apertures decrease and that the pore shapes become more equant as the porosity decreases. The marked downturn in the permeability-porosity relationship at total porosities below 0.07 appears from microscopical observation to correspond to a change in pore geometry from largely connected, irregular pores between grains to isolated, tubular pores at junctions of several grains. Application of the ''Swiss-cheese'' continuum percolation model indicates a percolation threshold of about 0.04 porosity. Microstructural evidence, the apparent activation energy for densification, and the stress dependence of densification rate suggest that porosity reduction has occurred mainly by dislocation creep.