EFFECT OF CONFINING PRESSURE ON PERMEABILITY OF A HYDROTHERMALLY ALTERED AND FRACTURED BALLISTIC FROM THE CONDUIT OF WHAKAARI VOLCANO, NEW ZEALAND

The effective pressure experienced by rock in a volcanic system changes with depth and by time-variable pore pressure. The permeability of a rock is affected by changing pressure and characteristics such as rock type, level of hydrothermal alteration, and micro and macro structures. This study investigates the permeability of hydrothermally altered and fractured ballistic blocks from Whakaari under changing confining pressure. The permeability of fractured ballistics is several orders of magnitude higher than unfractured ballistics and increases as porosity and alteration increases. When compared to previous experiments these findings highlight the effect of a tensile fracture on permeability of hydrothermally altered rock. The data show that the relative increase in permeability caused by the fracture is less in the hydrothermally altered samples.

The permeability of two samples with the low and intense hydrothermal alteration experienced were tested under increasing followed by decreasing confining pressure. The permeability of partially altered and altered lava decreases with increasing confining pressure and increased again as confining pressure was reduced. The permeability at 1 MPa confinement during increasing pressure is over an order of magnitude higher than at 1 MPa after decreasing of confining pressure, which shows significant hysteresis of each lava. The permeability-confining pressure curve for the partially altered and altered lava is steeper than for the unaltered lava, indicating the hysteresis is from progressive modification of the microstructures as confining pressure increases that permanently reduces the permeability. The data presented in this study provide insight into how fractured hydrothermally altered rock affects permeability at different confining pressures in response to increasing and decreasing confining pressure, and help characterize rocks to better understand how a hydrothermal system may fracture and seal.