Crack linkage and enhanced fluid permeability in heat-treated Icelandic basalt

In these experiments, the fluid permeability of basalt samples 40 mm in diameter by 40 mm in length were measured both prior to and following heat-treatment. All permeability measurements were carried out using a new wide-range permeameter system that makes use of two servo-controlled fluid pressure intensifiers to enable permeabilities from 1 darcy to lower than 1 nanodarcy to be measured, using water as the pore fluid. The mean permeability of the basalt prior to heat-treatment (k0) was 9.4 nanodarcy ( $9.4 \cdot 10^{-21} m^2$), and the change in normalized permeability (k/k0) as a function of heat-treatment temperature is plotted on Figure 22.

  

Figure 22: Increase in basalt permeability as a function of heat-treatment temperature.

Similar to the previous results, the normalized permeability remained essentially constant after heat treatment to temperatures up to 300$^\circ$C, and showed only a slight increase after treatment to 400$^\circ$C. At higher temperatures, however, the normalized permeability changed dramatically, increasing by an order of magnitude at 700$^\circ$C and by a factor of 40 by 800$^\circ$C.

It is clear that such a large increase in permeability is unlikely to result merely from the increase in the number or size of thermally-induced cracks. Note, for example that over the same temperature interval the rock strength (as measured by fracture toughness and Brazilian tensile strength) decreased by only about 30%. We believe, therefore, that these permeability results show the profound effect of crack linkage processes above some percolation threshold to form extensive sample-spanning permeable pathways for fluid flow.

Due to the high temperature gradient and low lithostatic stress, thermal cracking may be an important process in controlling fracture in Icelandic crust. In the absence of confining stress, such cracking starts in the temperature range 300$^\circ$C to 400$^\circ$C in fresh basalt. This leads to significant decrease in mechanical strength and resistance to crack propagation at higher temperatures. Thermal cracking also leads to increased fluid permeability above about 300$^\circ$C, with the permeability increasing very non-linearly with temperature. The decrease observed in rock strength is likely to be considerably enhanced by the presence of a chemically active pore fluid (e.g. water), especially when its activity is increased by elevated temperatures.

In the next phase of the project, we will measure the same key parameters under elevated temperature and pressure, and modify our experimental apparatus for the measurement of compressional and extensional strength.