Exploration risk concerning hydrogeothermal wells is defined as the risk of not achieving a geothermal reservoir by one (or more) well(s) in sufficient quantity or quality.
The term quality in the definition can in general be interpreted as fluid composition (fluid chemistry). Component parts (gas, salinity, oil, etc.) can appear in the fluid, which, if they exceed certain limiting values, hinder or complicate the thermal utilization. The term quantity is defined by the (thermal) power which can be achieved by one well (or more wells). Therefore, the essential parameters regarding the quantity for the exploration risk are flow rate Q and aquifer temperature T. Both parameters are decoupled und independently measurable. The flow rate Q will be determined by production tests, the temperature T can be measured by wireline measurements.
A geothermal well is successful, if minimum level of thermal water production (minimum flow rate) Q at maximum drawdown s and if minimum level of reservoir temperature T are achieved; for that the depth of the aquifer is determined as exactly as possible from seismic reflection surveys.
Information about the hydraulic parameters of the aquifer can mostly be determined in a regional scale only. Information from boreholes nearby or other boreholes having similar conditions can be weighted in a suitable manner. For the temperature prognosis, local conditions must be considered besides regional trends. An area of 1000 km² was normally chosen in the previous assessments. Because of the small data base, the simplest way to calculate the POS of a project is to multiply the single POS of flow rate and temperature.
The composition of all fluids explored in deep aquifers in Central Europe has not stopped geothermal utilization. But sometimes the technical effort can be great and induce additional costs. Nevertheless, there is no approach to assess the possibility of success for the quality.