Geothermal energy resources are sustainable if the rates of energy extraction and recharge are balanced, and renewable over 100s of years if not. Electric power generation and district heating have been supported commercially by producing hot water from permeable geothermal reservoirs at 200 to 4000 m depth from 200 to 380°C magmatically heated systems for 100 years, from 80 to 220°C deep-circulation fault-hosted systems for 40 years, and from 100 to 150°C deep sedimentary systems for 20 years. Research is ongoing to develop power from >380°C supercritical magmatic systems and from enhanced geothermal systems (EGS) created in low permeability hot rocks that are found within drillable depths worldwide. The geophysical applications that are relevant to geothermal exploration and development depend on the geoscience and practical context. Magnetotelluric (MT) surveys are the default exploration method for imaging the low resistivity impermeable smectite clay that caps almost all volcano-hosted reservoirs. MT is typically more effective than reflection seismic for imaging shale caps of geothermal reservoirs hosted in sediments. Less reliably, MT supplements microseismic monitoring of shallow magma that can limit rift-hosted geothermal reservoirs. Transient electromagnetic, gravity and magnetic surveys are sometimes relevant in narrower contexts. For deep sedimentary geothermal targets, high quality 3D seismic reflection survey is essential for targeting wells on permeable, water saturated rock. Well logs are important in modeling the petrophysics of potential permeable zones. After a >230°C field is developed, repeat microgravity surveys are used to monitor boiling and phase change. Subsidence surveys, where feasible using InSar, support the repeat microgravity and help characterize reservoir pressure and temperature changes. Because injecting cold water into very hot rock induces microseismic events, seismic monitoring arrays are routinely installed to track injection, characterize the structural compartments of reservoirs, and predict the depth to the base of the permeable reservoir. Microseismic monitoring surveys are also crucial to understanding the dimensions and properties of both EGS and supercritical systems. New geophysical technology of particular relevance to the geothermal industry includes 3D joint inversion, greater integration with well logs, improved characterization of uncertainty in 3D inversions, and adapting geophysical methods to higher risk geothermal prospects less analogous to the archetypes for which the current methods have proven most useful.
Presentation Date: Monday, October 12, 2020
Session Start Time: 1:50 PM
Presentation Time: 3:30 PM
Location: 361F
Presentation Type: Oral
