Alteration's Control on Frictional Behavior and the Depth of the Ductile Shear Zone in Geothermal Reservoirs in Volcanic Arcs

The majority of geothermal energy is produced in tectonically active volcanic-arc regions due to their high geothermal gradients. Reservoirs in these settings are often stratified with smectite/kaolinite-, illite-, and chlorite-rich zones, in order of increasing depth and temperature. Eighteen andesitic core and surface samples were taken from five geothermal fields in the Lesser-Antilles and Cascade volcanic arcs. The collected samples have experienced various degrees of alteration and can be considered, in their ensemble, to be representative of the previously mentioned alteration zones. The influence of the alteration was assessed through biaxial rate-and-state friction experiments on prepared gouge. The samples were each tested at 10, 30, and 50 MPa normal stress in both nominally dry and nominally wet conditions. While significant water-induced frictional-strength reduction was observed, phyllosilicate content dominates frictional behavior, with increased phyllosilicate content reducing frictional strength, promoting velocity-strengthening behavior, and reducing frictional healing. Negative frictional healing is observed and likely related to the presence of expandable clays, leading to frictional weakness over long time periods. It is suggested that, by controlling frictional strength, phyllosilicate content influences the depth of onset of ductile shear zones, which often underlie these reservoirs and are critical for the horizontal advection and vertical sealing of geothermal fluid. Further, as these types of reservoirs are likely critically stressed, varying degrees of alteration within different reservoir zones can give rise to the formation of stress jumps. Overall, the frictional behavior depended to a first order on overall phyllosilicate content, potentially simplifying engineering studies.