CHARACTERIZATION OF THE FRACTURED BASALTIC RESERVOIRS USING TEMPERATURE LOGS, ACOUSTIC TELEVIEWER AND PRODUCTION HISTORY DATA, KALDARHOLT GEOTHERMAL FIELD, SOUTH ICELAND

The Kaldkrholt field is one of the most important low-temperature geothermal fields located in S-Iceland. This field is exploited from 2000 and has been used mainly for heating purposes. The production comes mainlyfrom the highly fractured basaltic reservoir rocks. The available well logging and temperature data as well as the acoustic televiewer and production history data, are analyzed as an attempt to estimate the reservoir parameters and identify the possible fracture systems in the study area in relation to the different rock units and feed zones. Aconceptual reservoir model for Kaldkrholt geothermal field is constructed based on the different interpretations of thetemperature data and the available geological information. These interpretations show that there is an oriented upflowof geothermal water with temperature of 67°C.
The different petrophysical parameters of prime interest are estimated (porosity, silica content and permeability, etc) using the conventional logging techniques and calibrated with the actual laboratory analyses. Identification and typing of the different rock units is achieved through interpretation of synthetic seismograms and reflection coefficient as correlated with the created geometrical pattern logs. The mechanical properties of the rocks are estimated and interpreted in relation with the different fractures and feed zones, as well as in matching with the main rock types. The acoustic televiewer amplitude and travel time data and the breakout parameters are analyzed. The different fracture systems prevailing in the area are identified; their azimuth and dip directions are interpreted. A NNE-SSW dipping direction is assigned for these fractures, which exhibit high near-vertical dipping angles; more than 70°.Numerical modelling (Lumped parameter modelling) was applied to simulate the production history of the field and to give some insights into the reservoir properties, especially its volume and permeability. In addition, a geothermal resource assessment is carried out using the volumetric and the probability simulation methods. The volumetric calculations give an average value of 7.4 MWt, while the probability method gives a range of 6.5 to 8.5 MWt for the most likely value for the reserve (with a probability of more than 24%).