ÐOÀN Mai Linh (Grenoble University), with Corinth Rift: François Cornet (IPG-Strasbourg)

1. In situ Study of Fault zone Hydrology ÐOÀN Mai Linh (Grenoble University), with Corinth Rift: François Cornet (IPG-Strasbourg) TCDP (Taiwan): Emily Brodsky(UCSC), Kuo-Fong Ma(NTU), Yasuyuki Kano(Kyoto) Rapid Response Drilling Workshop November 2008

2. Hydraulic properties of faults Host rock High pressure reduces effective normal stress and facilitates slip Damage zone Fault core During rupture, hydraulic diffusivity controls pressure buildup Between earthquakes, hydraulic properties control the fault recovery Seismic cycle • We focus on direct in-situ measurements, • which capture: • Mesoscale • Original fluid and confining pressure • Time evolution

3. 2 major methods • Intermittent large scale testing • Permeability • Hint on pressure • Long term monitoring • Pressure • Poroelastic behavior • Hint on permeability

4. Example 1 • AIG10 borehole • in Corinth Rift Laboratory • Long term pore pressure monitoring • Complete poroelastic determination (Earth tides) • Permeability determination (Oceanic tides) Other examples: PBO, PFO (both not in faults), soon SAFOD

5. 0.9±0.1MPa karst Corinth Rift Laboratory South North Conglomerates Clays Limestone 0.5±0.1MPa Radiolarites Impermeable fault Limestone

6. Typical high precision record Pressure Resolution better than 1% Pressure (Bar) UT Time The data is dominated by the karst, not the fault Tides are recorded. We get the response of the aquifer to a known input

7. Pore recording on tides Two effects 1-Poroelastic response of the formation 2- Hydraulic properties of the formation  Tides deform the Earth and the formation surrounding the well Water flows to the well to equilibrate the pressure 1 2 In a confined medium this induces variation in pressure

8. Major results Full poroelastic determination Notably, strain-pressure conversion coefficient BKu=27GPa pore pressure ≈ strainmeter Estimate of the hydraulic diffusivity D=20m2/s A value that direct pumping tests were unsuccessful to provide.

9. Dynamically triggered event 5 minutes with D=20m2/s 110m  Local  Fault ? November, 17th 2003 06:43 UTC Drop of 60 Pa (equivalent to 3.5nstr with BKu=27GPa) 30min 5min Doan et Cornet, EPSL, 2007

10. Example 2 • Taiwan Chelungpu Fault Project • Pumping test • Hydraulic diffusivity and permeability • Rough estimate of pressure Other examples: Nojima (Kitagawa,99), Talwani

11. Chichi earthquake Sept. 21 1999 Mw=7.6 Large movement Small acceleration High fluid pressure ? Is the diffusivity low? (Ma et al. 2003)

12. Principle of the test Hole A Hole B Diffusivity along the fault Perforation 1111m ~1137m Chelungpu fault Perforation 40m

13. Major results • Weconstrain the hydraulicdiffusivityof the damage zone • D=(7±1)×10-5 m2/s • Thermal pressurization probable • No direct measurement of permeability. • Using lab data and theory for the missing parameters, • k between 10-18m2 and 10-16m2 • M pattern of permeability • Damage zone not so permeable. Premature sealing after 6 yr ? Fault Core < Host rock < Damage Zone 10-21-10-19m2 10-19-10-18m210-18-10-16m2 (Lockner2006) (Chen2005) (Doan et al.2006) Rough overpressure estimate from inflow rate and k between 0.06MPa and 6MPa  Not highy pressurized (<20% lithostatic pressure only)

14. Conclusions • Direct assessment of • pressure, permeability and hydraulic diffusivity • within active fault zone yielded • by pore pressure monitoring and hydraulic tests. • Parameters • The faults studied were not much overpressurized • They are poor fluid conduits. • Methods • Both methods are complementary • Pumping test are more spatially resolved • Permanent pore pressure monitoring yields refined time evolution •  Dynamic triggering and other transients can be detected • in addition to postseismic healing.

15. Perspectives of the RRDB Contribution of Rapid Response Drilling to fault hydrology Still missing : no precise pore pressure measurement within the fault zone of a seismogenic fault. A rapid response borehole could also test fault valve mechanism: pore pressure buildup and permeability decrease during healing. Contribution of fault hydrology to Rapid Reponse Drilling Borehole One major objective of the borehole is to resolve heat flow paradox. Fluid flow may advect heat. Fault hydrology helps to estimate natural flows and flows induced by the borehole drilling.

16. Remarks 1) The studies presented were impeded by leaks, borehole collapse, bad cementing… 2) Faults have different permeability patterns. Each pattern requires a different testing strategy. « Geotechnical model » Fault is a hydraulic conduit  Chelungpu fault « Petroleum model » Fault is a hydraulic barrier Corinth Rift Laboratory Preserve money for clean borehole completion Not evident since completion is the last drilling operation, when funding is all spent… I am in the shallow-but-well-done-(why-not-1+1) borehole side