This is a huge and complex topic and only barest essentials will be touched upon here.
It is important to define the objective of a sampling campaign. A focus is required on what exactly needs to be sampled, and when, and also for which parameters the samples should be analysed. Practical procedures for water sampling are described in the United States Geological Survey online Field Guide (and by various national and European standards). Particular attention should be paid to the practice of field filtration of water samples (such that particles and drilling cuttings do not interfere with analyses) and appropriate sample preservation.
It is always beneficial to obtain samples of the geothermal reservoir rock, either by coring of an on-site borehole, or by obtaining cores from an off-site borehole into the same geological stratum. These can be supplemented by samples of disturbed drilling cuttings or samples of the reservoir rock from outcrops in the vicinity (if any). Rock / sediment samples can be analysed by:
- X-Ray diffraction techniques to ascertain which minerals are present.
- X-Ray fluorescence, and other techniques, to ascertain the elemental composition of the rock.
- Water or acid digestion trials to ascertain which elements are readily released to water upon contact and hydrolysis.
- Where acid stimulation is proposed, real samples of the rock can be subject to acid exposure under controlled laboratory conditions to study the effects.
As regards water sampling, samples should be taken at or near the well head (subject, of course, to safety considerations) and determinations of the following characteristics should be made in the field: temperature, electrical conductivity, dissolved oxygen, redox potential, pH, alkalinity, CO2 content and redox-sensitive parameters such as hydrogen sulphide.
Easily operated hand-held meters or kits are available to measure many of these parameters on-site. Fluid samples can be analysed in the laboratory in order to gain information on:
- Fluid composition, by determining major and minor ion contents or gas components
- The age, origin and history of the water by measuring isotopes
Samples of geothermal water should always be analysed (in the laboratory) for concentrations of major ions, namely: sodium, calcium, potassium, magnesium, ammonium, chloride, carbonate/bicarbonate, sulphate and nitrate.
In addition, the following parameters will usually prove invaluable in interpreting the water chemistry and assessing its suitability for geothermal exchange: strontium, barium, lithium, boron, bromide, fluoride, silicon, iron, manganese, arsenic, sulphide, uranium, total dissolved solids, total suspended solids, stable isotopes of oxygen (18O), hydrogen (2H), sulphur (34S) and carbon (13C). Natural radioactive isotopes, such as 14C, can sometimes be useful in estimating the age of the water.
Often, in addition to analysing water from deep geothermal wells, it is common to carry out a campaign to collect water samples from other nearby springs and wells. Comparing the hydrochemistry may help us to understand any hydraulic connection between a deep geothermal reservoir and the shallow groundwater system. Any warm springs near a geothermal operation can be a mixture of deep geothermal fluids and shallow groundwater; - hydrochemistry can assist us in estimating the extent of mixing and the degree of cooling during its journey to the surface.