Suzanne Hurter

Atlas of geothermal resources in Europe

The geothermal resources of most European countries have been estimated and compiled in the recently published Atlas of Geothermal Resources in Europe, a companion volume to the Atlas of Geothermal Resources in the European Community, Austria and Switzerland. Publication of this Atlas comes at a time when the promotion of a sustainable and non-polluting energy is high on the agenda of local energy suppliers, municipal administrations and all European governments. The participating countries are: Albania, Austria, Belarus, Belgium, Bosnia-Herzegovina, Bulgaria, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Netherlands, Poland, Portugal, Romania, Russia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Ukraine and the UK. A volumetric heat content model for porous reservoirs was the basis for calculating the resources, assuming that exploitation of the geothermal resources would take place in a doublet well system. The geothermal reservoirs are defined in a set of 4 maps, by depth, thickness, temperature and resources. The assessment methodology is simple and is based on a small number of parameters so that regions with very limited data coverage can also be evaluated. An example is given in this paper of the eastern North German Basin. The maps presented in the Atlas permit a first order evaluation of the geothermal potential in terms of technical and economic viability. This uniform procedure applied to all countries and regions allows comparisons and serves as a guide for setting priorities and planning geothermal development. This Atlas also helps in the search for appropriate partners for international cooperation in geothermal exploration in Europe.

Fluid flow in the resurgent dome of Long Valley Caldera: implications from thermal data and deep electrical sounding

Abstract Temperatures of 100°C are measured at 3 km depth in a well located on the resurgent dome in the center of Long Valley Caldera, California, despite an assumed >800°C magma chamber at 6–8 km depth. Local downflow of cold meteoric water as a process for cooling the resurgent dome is ruled out by a Peclet-number analysis of temperature logs. These analyses reveal zones with fluid circulation at the upper and lower boundaries of the Bishop Tuff, and an upflow zone in the metasedimentary rocks. Vertical Darcy velocities range from 10 to 70 cm a −1 . A 21-km-long geoelectrical profile across the caldera provides resistivity values to the order of 10 0 to >10 3 Ωm down to a depth of 6 km, as well as variations of self-potential. Interpretation of the electrical data with respect to hydrothermal fluid movement confirms that there is no downflow beneath the resurgent dome. To explain the unexpectedly low temperatures in the resurgent dome, we challenge the common view that the caldera as a whole is a regime of high temperatures and the resurgent dome is a local cold anomaly. Instead, we suggest that the caldera was cooled to normal thermal conditions by vigorous hydrothermal activity in the past, and that a present-day hot water flow system is responsible for local hot anomalies, such as Hot Creek and the area of the Casa Diablo geothermal power plant. The source of hot water has been associated with recent shallow intrusions into the West Moat. The focus of planning for future power plants should be to locate this present-day flow system instead of relying on heat from the old magma chamber.

Terrestrial heat flow in the Paraná Basin, southern Brazil

We present 56 new heat flow values from the intracratonic Parana Basin in southern Brazil. This large basin is filled with up to 5 km of Paleozoic and Mesozoic sedimentary rocks. In the Late Jurassic-Early Cretaceous a great igneous event capped most of the basin surface with flood basalts up to 1700 m thick. Geothermal gradients computed from 79 deep exploration boreholes range from 20 K km -1 to 30 K km -1 with the lower gradients generally located in the central part of the basin. Thermal conductivities were determined on 247 core samples. The harmonic mean thermal conductivity of the section encountered by the boreholes decreases from 3.0 W m -1 K -1 at the eastern basin margin to 2.0 W m -1 K -1 in the basin center ; this variation is related to the thickening of the basalt cap toward the basin center. Surface heat flow values for the 56 sites range from 40 mW m -2 to 75 mW m -2 , with larger and more variable values (50-70 mW m -2 ) occurring along the eastern margin of the basin in the region without basalt cover. The heat flow in the central part of the basin (40-50 mW m -2 ) is less than that on the basin margin by about 15 mW m -2 and is more uniform. We discount advective effects as an explanation of the heat flow pattern because if a topographically driven flow system existed, it would diminish heat flow in the elevated recharge area along the basin margin and augment heat flow in the discharge area along the basin axis, opposite to what is observed. Wholly conductive models show that larger-scale thermal conductivity contrasts produced by the flood basalts do not refract significant heat into the surrounding higher-conductivity sedimentary section on the periphery of the basalts. Other model calculations show that the heat flow at the surface reflects the heat input from the basement with only minor, if any, redistribution within the basin. We conclude that the thermal data indicate a dominantly conductive thermal regime within the basin and that the observed heat flow pattern is not likely to result from intrabasinal causes. The observed pattern likely reflects the larger-scale thermal structure of the lithosphere of this region, developed at the time the flood basalts were generated and extruded.

Comment on “Numerical investigation of the potential contamination of a shallow aquifer in producing coalbed methane” by Xianbo Su, Fengde Zhou, and Stephen Tyson:

This commentary addresses “Numerical investigation of the potential contamination of a shallow aquifer in producing coalbed methane” by Xianbo Su, Fengde Zhou, and Stephen Tyson. We think the models used in the simulations described in the paper are unrealistic, even as a conceptual worst case scenario. Concerns regarding how the results of these simulations are interpreted and portrayed, and in particular how they are related to previous works are discussed in detail. We believe the original paper uses language which could be misleading, and possibly alarmist, and we suggest references cited in the original paper may have been misinterpreted.

Laboratory and Mathematical Modelling of Fines Production from CSG Interburden Rocks

Twelve clastic core samples from the Walloon Coal Measures, Surat Basin were tested for disintegration in artificially produced fluids varying in ionic strength. XRD data confirm the presence of smectite (water sensitive clay) in the samples. Flow-through rock disintegration experiments demonstrate that the higher the concentration of smectite and soluble plagioclase is, the quicker rock disintegrates in artificial low ionic strength fluid. Pre-soaking of rocks with high ionic strength fluid reduces rock disintegration rate in low ionic strength fluids. This is explained by very strong clay-clay and clay-sand attraction forces, evidenced through zeta-potential measurements, which inhibit rock degradation. For the studied samples it is clear that rock disintegration rate is proportional to fluid velocity. Experimental rock disintegration data are fitted by a power erosion model with two adjusted parameters: fluid ionic strength and Reynolds number. The experimental results satisfactorily agree with theoretical data. Rock disintegration rates are calculated as released particle volume per thickness of interburden layer per day at a fixed Reynolds number and low ionic strength. The laboratory work suggests that keeping wells under strong ionic fluid during shut-in times and a reduction of water production rate will preserve rock integrity for a longer period of time.