Enhanced (or engineered) geothermal systems (EGS) have evolved from the hot dry rock concept, implemented for the first time at Fenton Hill in 1977. This paper systematically reviews all of the EGS projects worldwide, based on the information available in the public domain. The projects are classified by country, reservoir type, depth, reservoir temperature, stimulation methods, associated seismicity, plant capacity and current status. Thirty five years on from the first EGS implementation, the geothermal community can benefit from the lessons learnt and take a more objective approach to the pros and cons of 'conventional' EGS systems.

Electrical spectral induced polarization (SIP) measurements are increasingly being used in environmental studies of the saturated zone. To better understand the mechanisms causing polarization and to extend the range of SIP applications to the vadose zone, it is important to investigate how the SIP response is affected by water saturation. Therefore, sand and sand‐clay mixtures were drained in several steps using a novel measurement set‐up allowing SIP measurements with a high accuracy. The measured SIP spectra were interpreted by Debye decomposition, which provided a relaxation time distribution and a chargeability distribution that was converted to a normalized total chargeability. The results showed that the normalized total chargeability of the fully saturated samples increased with increasing clay content due to the larger specific surface area of the clay minerals. Furthermore, normalized total chargeability first increased and then decreased with decreasing saturation for the pure sand and the 5% sand‐clay mixture. The normalized total chargeability values for the 10% and 20% sand‐clay mixtures only decreased with saturation. The peak relaxation time of the sand‐clay mixtures clearly decreased with decreasing saturation. Existing grain‐size based mechanistic models for SIP are not able to explain the observed behaviour and the observed relationship between relaxation time and saturation suggests that model concepts relying on polarization processes in the pore space are warranted to explain the measurements on variably saturated porous media presented here.

We analyzed the influence of pore fluid composition on the complex electrical conductivity of three sandstones with differing porosity and permeability. The fluid electrical conductivity (σ w ) of sodium and calcium chloride solutions was gradually increased from 25 mS/m to 2300 mS/m. The expected linear relation between σ w and the real component of electrical conductivity (σ 0 ) of the saturated samples was observed. The imaginary component (σ 0 0 ) exhibits a steeper increase at lower salinities that flattens at higher salinities. For a glauconitic sandstone and a high porosity Bunter sandstone, σ 0 0 approaches an asymptotic value at high salinities. Sodium cations result in larger values of σ 0 0 than calcium cations in solutions of equal concentration. Debye decomposition was used to determine normalized chargeability (m n ) and average relaxation time (τ) from spectral data. The behavior of σ 0 0 is comparable to m n as both parameters measure the polarizability. At lower salinity, the relation between m n and σ w approximates a power law with an exponent of ∼0.5. The average relaxation time shows only a weak dependence on σ w . The normalized chargeability of sandstone samples can be described by the product of the pore space related internal surface and a quantity characterizing the polarizability of the mineral-fluid interface that depends on fluid chemistry. We introduce a new parameter, the specific polarizability, describing this dependence. We propose relations between polarizability and fluid chemistry that could be used to estimate pore space internal surface across samples of varying σ w . We observe a consistent maximum polarizability for quartz dominated siliceous material.