Page 77 - Geothermal Energy Systems Exploration, Development, and Utilization

2.4 Geophysics 53
2.4.1
Electrical Methods (DC, EM, MT)

Most rocks are poor conductors, but they are usually porous and the pores are
filled with fluids, which means that they are also electrolytic conductors. As
the reservoirs are volumes of rock filled with hot fluids (water, vapor, and gas),
electrical resistivity of subsurface rocks is the most diagnostic parameter for
geothermal resources that can be measured from the surface. In addition, electrical
resistivities are strongly temperature dependent. Bulk conductivity increases by
more than an order of magnitude when temperatures are raised from room
◦
temperature to 200 C(Yokoyama et al., 1983). Up to the critical temperature of
water, a temperature increase of the water in the pores enhances the conductivity
additionally. When we approach the melting point of a rock, even more significant
changes in electrical properties take place. Therefore, electrical methods have
gained the same significance in geothermal exploration as seismic methods have
in oil exploration. They are extensively used to obtain a first approximation of
subsurface conditions, because an area of several square kilometers can be studied
within a short time and the costs are relatively low.
Generally, dense volcanic, igneous, and carbonate rocks have higher resistivities
than clastic sedimentary rocks, while for shales and clays resistivity values are
the lowest (1–10 m). Resistivities for hydrothermal reservoirs are typically lower
than for the surrounding rocks and depend on several factors, for example, the
porosity of the rocks and the salinity of the fluids. These interdependencies can be
described by a formula empirically derived by Archie (1942):
ρ = αφ −m −n (2.7)
S ρ w
where ρ and ρ w are the resistivity of the formation and of the pore water, respectively,
φ is the porosity and S the water saturation. α, m,and n vary for different rock types.
In case of sandstone, the tortuosity, α, ranges from 0.5 to 2.5, the cementation
factor, m, ranges from 1.3 to 2.5, and the saturation index, n, is typically 2, while
for loose sand, typically α = 0and m = 2.15. The ratio of formation resistivity to
water resistivity is often referred to as the formation factor F,suchthat
F = ρ/ρ w = αφ −m (2.8)
In a porous, fluid-filled rock, conductivity is commonly composed of the con-
ductivity of the fluid and of the surface conductivity. Assuming parallel circuit
behavior, the rock conductivity σ is
σ = σ c + σ w /F (2.9)

and as ρ = 1/σ
1/ρ = 1/ρ + 1/Fρ w (2.10)
c
The contribution of the fluid conductivity increases with the amount of dissolved
solids, while surface conductivity becomes important with the presence of clay
minerals, which often occur as a product of hydrothermal alteration and weathering.