Page 105 - Geothermal Energy Systems Exploration, Development, and Utilization

2.5 Geochemistry 81
These geostatistical and simulating methods include Montecarlo simulation, neural
networks, fuzzy logic, and Bayesan methods. Computational time and a priori
distributions of model parameters are the main concerns.
A very recent example (Mu˜ noz et al., 2010) of integrated interpretation of MT and
seismic data sets shows that there is a huge potential for the future in combining
different methods even if fundamental laws linking the parameters investigated
cannot be formulated explicitly.
2.4.4.1 Joint Inversion Procedures
Geothermal exploration research is increasingly turning to joint inversion strategies
in which multiple geophysical data sets and/or geophysical–hydrothermal data sets
are processed simultaneously to produce more realistic estimates of the hydrologic
parameters that satisfy all the available data sets. Thus, joint inversion methods are
configured either as a coupled inversion of geophysical and hydrological data or as
a coupled inversion of multiple geophysical data.
When two data sets are both sensitive to the same physical property, the
simultaneous inversion is achieved by minimizing the misfit of both data sets.
On the other hand, if the geophysical data sets are sensitive to different physical
properties final models will provide complementary information at the same
location point. Joint inversion of hydrogeological, hydrothermal, and geophysical
data is expected to improve the final hydrogeological model. Hydrogeological
and hydrothermal data calibrate the hydrogeophysical variables based on the
assumption that any relevant hydrogeological structure has a geophysical signature.

2.5
Geochemistry

2.5.1
Introduction

In any type of geothermal systems, high temperature fluid is in chemical equilib-
rium with the surrounding rock. This equilibrium controls the natural porosity
and permeability of the fractured rocks and will shift due to forced-fluid circulation
of the exploitation phase.
Geochemical techniques provide information that could support future EGS
developments. During the exploration phase of an EGS resource, fluid and rock
geochemistry is a major tool in the determination of the origin and the quality and
quantity of the fluid resource, helping to build a conceptual model.
Fluid and gas composition, water–rock volume ratio, and reservoir temperature
are important parameters when forecasting the processes taking place in an EGS
reservoir.
The first task is of course to prepare a review of published research and spatial
data on the fluid and rock geochemistry. Data from springs and wells (groundwater,