Reviewing the relations of seismic velocities and electrical resistivity with the temperature of high-enthalpy geothermal reservoirs with an example from an East African rift volcano

Wu, Xiaoyang; Baptie, Brian; Huebert, Juliane; Beggan, Ciaran. 2023 Reviewing the relations of seismic velocities and electrical resistivity with the temperature of high-enthalpy geothermal reservoirs with an example from an East African rift volcano. Nottingham, UK, British Geological Survey, 30pp. (OR/23/030) (Unpublished)

Before downloading, please read NORA policies.

Preview

Text
OR23030.pdf
Download (1MB) | Preview

Abstract/Summary

High-enthalpy geothermal reservoirs have been widely explored as they can potentially provide exceptional energy productivity and efficiency. However, there remain unresolved questions about how to characterise these reservoirs from surface measurements, which at present preclude a better understanding of the physical properties of such heat-bearing rocks with saturated/injected fluids. This report aims to summarise the available rock physics modelling approaches for elucidating the electrical and seismic properties of geothermal reservoirs and high-temperature fluids, and to provide solutions for estimating the temperature distribution of high-enthalpy reservoirs, particularly those related to magma storage reservoirs in volcanic regions. We revisit the empirical relations including the Horner plot method and the commonly used resistivity-temperature relation to define the empirical relationship between temperature and electrical or seismic properties observed with geophysical sounding techniques. The report also introduces the methods that relate conductivity to elastic properties through porosity and the cross-properties Differential Effective Medium (DEM) method, which establishes the conductivity/resistivity–seismic velocity relationship directly, allowing the calculation of one physical property from another more easily measured one. We use Burgers model to numerically model the frequency-dependent shear modulus of linear viscoelastic media, where the steady-creep effect in the rock frame is caused by high temperatures and the Arrhenius equation is used to link viscosity with temperature. Finally, we demonstrate the use of rock physics modelling techniques to estimate geothermal gradient from 1-D seismic velocities obtained from inversions of earthquake data and study the resistivity and velocity relations at Aluto, a stratovolcano in the Central Main Ethiopian Rift, part of the East African Rift. Geothermal exploration at Aluto started in the 1970s and it is the site of Ethiopia’s first geothermal electric power plant. Both passive seismic and magnetotelluric (MT) surveys have been conducted in the region to help understand the subsurface structure and repeated episodes of volcanic unrest. In the report, we attempt to summarize and combine different rock physics models to characterize high-enthalpy geothermal systems for their elastic and electrical properties. We do this to facilitate joint inversion of geophysical data to better identify zones of high geothermal energy potential that might be economically useful for renewable energy production in the Ethiopian rift and elsewhere.

Item Type:	Publication - Report
Funders/Sponsors:	British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.:	This item has been internally reviewed, but not externally peer-reviewed.
Date made live:	06 Nov 2023 15:40 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/536212

Item Type:

Publication - Report

Funders/Sponsors:

British Geological Survey

Additional Information. Not used in RCUK Gateway to Research.:

This item has been internally reviewed, but not externally peer-reviewed.

Date made live:

06 Nov 2023 15:40 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/536212