Thermally Induced Compaction of Shales and Its Impact on Acoustic Velocities

Thermoplastic effects in consolidated, rather competent subsurface shales is often ignored in geomechanical modeling. However, in laboratory tests with several subsurface and outcrop shales, we have found sizable, non-reversible compaction upon heating to temperatures up to 120 degrees C, in some cases exceeding 1% volumetric strain. Such large strains could potentially result in rock failure, compromising the integrity of the shale formation, and would have to be accounted for in, e.g., caprock integrity assessments for thermal enhanced oil recovery or CO2 storage projects. Seismic monitoring could help to map thermal strains and stresses in shale formations but the interpretation of seismic data would require calibrated thermal rock-physics models that are not available yet. In order to get a better understanding of thermally induced compaction in shales and its impact on acoustic velocities, we carried out some dedicated laboratory thermal compaction tests with Pierre shale. Heating resulted in timedependent volumetric strains of up to 5%, accompanied by a large increase in ultrasonic p-wave velocity that was explained by the reduction in porosity.