EXPLORATION METHODS

Geological and hydrogeological studies are the starting point of any exploration programme, and their basic function is that of identifying the location and extension of the areas worth investigating in greater detail and of recommending the most suitable exploration methods for these areas. Geological and hydrogeological studies have an important role in all subsequent phases of geothermal research, right up to the siting of exploratory and producing boreholes. They also provide the background information for interpreting the data obtained with the other exploration methods and, finally, for constructing a realistic model of the geothermal system and assessing the potential of the resource. The information obtained from the geological and hydrogeological studies may also be used in the production phase, providing valuable information for the reservoir and production engineers. The duration and cost of exploration can be appreciably reduced by a good exploration programme and an efficient coordination of the research.Geochemical surveys (including isotope geochemistry) are a useful means of determining whether the geothermal system is water- or vapour-dominated, of estimating the minimum temperature expected at depth, of estimating the homogeneity of the water supply, of inferring the chemical characteristics of the deep fluid, and of determining the source of recharge water. Valuable information can also be obtained on the type of problems that are likely to arise during the re-injection phase and plant utilization (e.g. changes in fluid composition, corrosion and scaling on pipes and plant installations, environmental impact) and how to avoid or combat them. The geochemical survey consists of sampling and chemical and/or isotope analyses of the water and gas from geothermal manifestations (hot springs, fumaroles, etc.) or wells in the study area. As the geochemical survey provides useful data for planning exploration and its cost is relatively low compared to other more sophisticated methods, such as the geophysical surveys, the geochemical techniques should be utilised as much as possible before proceeding with other more expensive methodologies.Geophysical surveys are directed at obtaining indirectly, from the surface or from depth intervals close to the surface, the physical parameters of deep geological formations. These physical parameters include:
* temperature (thermal survey)
* electrical conductivity (electrical and electromagnetic methods)
* propagation velocity of elastic waves (seismic survey)
* density (gravity survey)
* magnetic susceptibility (magnetic survey).
Some of these techniques, such as seismics, gravity and magnetics, which are traditionally adopted in oil research, can give valuable information on the shape, size, depth and other important characteristics of the deep geological structures that could constitute a geothermal reservoir, but they give little or no indication as to whether these structures actually contain the fluids that are the primary objective of research. These methodologies are, therefore, more suited to defining details during the final stages of exploration, before the exploratory wells are sited. Information on the existence of geothermal fluids in the geological structures can be obtained with the electrical and electromagnetic prospectings, which are more sensitive than the other surveys to the presence of these fluids and to variations in temperature; these two techniques have been applied widely with satisfactory results. The magnetotelluric method, which exploits the electromagnetic waves generated by solar storms, has been greatly improved over the last few years, and now offers a vast spectrum of possible applications, despite the fact that it requires sophisticated instrumentation and is sensitive to background noise in urbanised areas. The main advantage of the magnetotelluric method is that it can be used to define deeper structures than are attainable with the electric and the other electro-magnetic techniques. The Controlled Source Audiomagnetotelluric method (CSAMT), developed recently, uses artificially induced waves instead of natural electro-magnetic waves. The penetration depth is shallower with this technique, but it is quicker, cheaper, and provides far more detail than the classic MT method. Thermal techniques (temperature measurements, determination of geothermal gradient and terrestrial heat flow) can often provide a good approximation of the temperature at the top of the reservoir.
All geophysical techniques are expensive, although some more than others. Nor can they be used indiscriminately in any situation or condition, as a method that produces excellent results in a determinate geological environment may give very unsatisfactory results in another. In order to reduce costs, it is therefore very important that the geophysical method(s) be selected very carefully beforehand by geophysicists working in close collaboration with geologists
Drilling of exploratory wells represents the final phase of any geothermal exploration programme and is the only means of determining the real characteristics of the geothermal reservoir and thus of assessing its potential . The data provided by exploratory wells should be capable of verifying all the hypotheses and models elaborated from the results of surface exploration and of confirming that the reservoir is productive and that it contains enough fluids of adequate characteristics for the utilization for which it is intended. Siting of the exploratory wells is therefore a very delicate operation.