Permeability modifying processes : thermal cracking and crack healing of geomaterials

Abstract

We explored two complementary mechanisms for change in porosity and permeability of geomaterials: thermal cracking and crack healing by diagenesis. A suite of thermal cracking experiments was performed on andesite plugs from the geothermal field in Awibengkok, Indonesia. Permeability (k) and specific storage capacity were measured by the oscillating flow method in a wide range permeameter, at room temperature, with effective pressures between 15 MPa and 95 MPa, before and after thermal cracking. The samples were cracked at 150 and 300 °C and ambient pressure. Andesite samples have low permeability, on the order of 10-²⁰ M² . With increased pressure, permeability is reduced by a factor of two. Contrary to expectations, thermal cracking reduced the permeability of this material by an order of magnitude. We also examined a set of samples from crack healing experiments performed on Sioux quartzite by M. Messar. In these experiments the quartzite permeability fell by three orders of magnitude within a few days when the samples were saturated with water and heated to temperatures from 300 to 500 °C and pressures from 25 to 200 MPa. In order to correlate Messar's permeability measurements and experimental conditions with visual observations of the pore structure (mainly consisting of grain boundary cracks), we took scanning electron microscope micrographs of the samples. We then counted the intersections of test lines with healed and unhealed cracks. This yielded a set of measurements of the crack area per volume of the quartzite. We found that the final permeability of the samples was related to the area per volume of unhealed cracks by a power relation. Combining the findings from the two sets of experiments, it seems that cracking and healing effects due to the ambient temperature and pressure in geothermal fields such as Awibengkok could eliminate any permeability through grain boundary scale cracks within a matter of days.