HR: 17:00h
AN: V54C-05    [Abstracts]
TI: Evolution of vertical permeability in Coso Geothermal Well 58A-10
AU: * Davatzes, N C
EM: ndavatzes@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS977, Menlo Park, CA 94025, United States
AU: Hickman, S H
AF: U.S. Geological Survey, 345 Middlefield Rd., MS977, Menlo Park, CA 94025, United States
AB: Static temperature (T) profiles in geothermal systems record the relative role of conductive and advective heat transport. Under near steady-state conditions these profiles can reveal the long-term average permeability (k) along the T profile to an order of magnitude by solving for the advective component. Well 58A-10 in the undisturbed margin of the Coso Geothermal Field, CA, offers the opportunity to quantify vertical variations in k that provide a critical insight into the life cycle of geothermal systems. This well has three distinct zones of T gradient: a shallow zone from 400-1100 m depth of 100 ° C/km, a near-isothermal intermediate zone from 1100-2900 m, and a basal zone >2900 m of 100 ° C/km. Simple analytical solutions for 1-D heat transport and convective instability can be fit to the observed temperature profiles in the upper two zones. From these solutions the shallow zone requires k<10^-17 m² whereas the intermediate zone requires k>10^-13.5 m², indicating a >10³ k difference between these intervals despite similar host rocks and fracture populations. The difference in 1-D, vertical k between the upper two zones appears to arise from the minerals that "heal" fractures as revealed by surface mapping, cuttings and core analysis, and geophysical logs. Initial brittle fracture and frictional slip in low porosity crystalline rocks causes dilation owing to surface roughness along fracture walls, brecciation, and micro-cracking. Yet active precipitation and alteration in geothermal areas implies rapid healing that requires fracture generation or reactivation to maintain k. Fault rocks enriched in neoformed clays minimize dilation during slip and can reduce k to lower than 10^-19 m². Reduced frictional strength in clay-rich faults also promotes their reactivation over the generation of new faults. Since the stability of clays is restricted to relatively low temperature, clay-rich faults are confined to shallow depths. Thus, a fault core enriched in clays at shallow depths becomes a persistent barrier to cross-fault flow that effectively reduces vertical k. Conversely precipitation of calcite or silica dominates healing at greater depth. These minerals retain dilatant behavior during slip that regenerates k as revealed by crack-seal textures. Thus k remains high enough to support convection and isothermal T profiles.
DE: 5114 Permeability and porosity
DE: 8010 Fractures and faults
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8045 Role of fluids
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting