Indonesia with its large, but partially unexplored geothermal potential is one of the most interesting and suitable places in the world to conduct geothermal exploration research. This study focuses on geothermal exploration based on fluid-rock geochemistry/geomechanics and aims to compile an overview on geochemical data-rock properties from important geothermal fields in Indonesia. The research carried out in the field and in the laboratory is performed in the framework of the GEOCAP cooperation (Geothermal Capacity Building program Indonesia-the Netherlands). The application of petrology and geochemistry accounts to a better understanding of areas where operating power plants exist but also helps in the initial exploration stage of green areas. Because of their relevance and geological setting geothermal fields in Java (Wayang Windu, Tanguban Perahu) have been visited so far. Mount Salak, Gunung Slamet (Java) and Flores surveys are planned in the near future. Operators, universities and governmental agencies will benefit from this approach as it will be applied also to new green-field terrains. By comparing the characteristics of the fluids, the alteration petrology and the rock geochemistry we also aim to compile an overview of the geochemistry of several geothermal fields in Indonesia. The gathering of this information is the base for the geomechanical experiments on-going at TUD. At the same time the rock petrology and fluid geochemistry will be used as input data to model the reservoir fluid composition along with T-P parameters with the geochemical workbench PHREEQC. The field and laboratory data are mandatory for both the implementation and validation of the model results. If successful, this approach can be applied in many geothermal fields characterized by steep terrain and tropical vegetation, which hampers the classical seismic-geophysical exploration methods.

Thermal fracturing is considered to be a potential mechanism to create additional fractures in geothermal fields. The injected cold water into the hot host rock suddenly cools down the host rock, causing a considerable shrinkage of the material and thus potentially increased local stresses that may potentially lead to the formation of cooling related fractures. This is likely to happen in the near wellbore environment or along existing faults or fractures, ie. areas where the hot rocks juxtaposed to cold fluids. In this research, we experiment with thermal fracturing by exposing heated granitic and basaltic samples with cold water to see the extend of the thermal microfracturing inside the samples at different temperatures. Before and after the heat treatment, the micro CT-scanner is used to get high-resolution 3D images of fracture planes and fracture network connectivity. Moreover, the porosity is measured before and after treatment by using the pycnometer to see the effect of the different temperatures. In addition, the changes in geomechanical behaviour are tested by using an unconfined compressive strength (UCS) apparatus on heat treated and non-heat treated samples. We compare the changes in Young Modulus, Poisson’s Ratio and ultimate strength of the various samples and record the influence of the thermal fractures on the stressdriven fracturing behaviour in the UCS test.