Tuesday, April 1, 2008 : 2:00 p.m.
Magnetotellurics (MT) and Microearthquake (MEQ) in High Enthalpy Geothermal Resource Exploration – The Search for Fracture-Related Permeability
Typical andesite-hosted, high enthalpy geothermal systems depend largely on permeability from open fracture systems to sustain the necessary convective regime in the exploitable reservoir. The reservoir seal above and to the sides, is a clay altered zone created by the same geothermal system, characterized by a low permeability, steep gradient conductive thermal regime. Reservoir delineation and characterization is routinely mapped with surface geophysics using both:
1) Magnetotellurics (MT) – providing a 3D resistivity volume to >5km depth
2) Microearthquake (MEQ) – providing fracture and fluid path locations, velocity parameters such as Vp/Vs ratios related to fluid types.
a) coherent host rock capable of sustaining open fractures and the development of convective reservoir, and
b) high temperature (electrically resistive) propylitic alteration rank in the thermally convective reservoir zone.
1) Magnetotellurics (MT) – providing a 3D resistivity volume to >5km depth
2) Microearthquake (MEQ) – providing fracture and fluid path locations, velocity parameters such as Vp/Vs ratios related to fluid types.
The high-temperature (convective heat flow) reservoir zone is characterized by propylitic alteration mineralogy, and is electrically resistive compared to the overlying lower temperature cap zone (conductive heat flow) characterized instead by electrically-conductive argillic alteration. The change to the most exploitable, fracture-dominated reservoir conditions, therefore, is marked by the transition from electrically conductive to resistive conditions, imaged during 3D inversion modelling of MT data. The deep resistor indicates:
a) coherent host rock capable of sustaining open fractures and the development of convective reservoir, and
b) high temperature (electrically resistive) propylitic alteration rank in the thermally convective reservoir zone.
MEQ surveys are used in both exploration and monitoring of production and injection tests to map typical active fault failure on shear zones and steam/liquid expansion/compression. Accurate event locations are achieved through master and slave event coherency analysis. Moment tensor analysis on coherent events provides the fracture mechanism and direct window on the stress regime of individual fracture zones. Events are also used to illuminate the reservoir and derive not only the Vp and Vs 3D velocity field, but the Vp/Vs ratio – a critical indicator of fluid type. Monitoring in Time Lapse surveys shows pressure drawdown from the most productive (= most permeable fractures) reservoir zones.
Stephen Hallinan, Geosystem Srl, Western GECO to be submitted later