Permeability and thermal cracking at pressure in Sioux Quartzite

We heated specimens of Sioux Quartzite to 683, 783 and 830 K at confining pressures (Pc) of 0.1, 100 and 200 MPa. Following the heat treatments, we measured the permeability under confining pressures that varied from 170 MPa to 60 MPa, with pore-fluid pressures of 10, 16 or 22 MPa, resulting in tests having effective pressures ranging from 36 to 160 MPa. When measured under those effective pressures, the permeability of samples with microstructures formed by cracking at 683–830 K and atmospheric pressure increased by a factor of 3–4. Scanning electron microscopy (SEM) shows that thermal cracking increases the number of long, narrow cracks, qualitatively explaining the increased sensitivity of permeability to low effective pressure. Fracture mechanics models suggest that confining pressures lower the stress intensity factors to well below the critical value for crack propagation, and the absence of many long cracks is consistent with that prediction. At Pc ¼ 200 MPa, thermal cracking during the heat treatment is suppressed. Although the heating experiments were performed on nominally dry samples, in samples heated at the highest temperatures SEM revealed microstructural features that are similar to those seen in crack healing experiments.