De-risking the Danish geothermal sandstone reservoirs – challenges and solutions

Predicting the occurrence of suitable geothermal sandstone reservoirs in the subsurface will always constitute an uncertainty when geothermal projects are being established. Thus, methods for limiting the geological uncertainty to make the best possible prognosis of reservoir distribution and quality will have a direct influence on minimizing the economic risks. In recent years, various methods, partly implemented from the oil and gas industry, have shown a great potential in improved prediction of the distribution of the reservoir sandstones in the subsurface as well as their reservoir quality. Implementation of these methods may thus further stimulate the growing interest in exploiting the vast geothermal resource in Denmark of which only a minute fraction is utilized at present. Quantitative seismic interpretation has proved useful for making a reliable reservoir characterization with regards to estimation of porosity and sand/clay distribution. Forward stratigraphic modelling is used to predict the distribution and thickness of the reservoirs by modelling the facies distribution through time, constrained by the available seismic and well data. Diagenesis modelling is applied to estimate the reservoir quality and the degree of compaction and cementation of the sandstones by integrating the knowledge about depositional environments, petrography, reservoir properties, provenance, and burial history. The composition of the formation water in existing geothermal plants has been measured and used to establish a brine chemistry database applied in geochemical modelling to evaluate the risk of scaling and corrosion during geothermal production and find ways to avoid it. When combined, the implemented methods and new scientific results provide useful solutions for significantly increasing the chance of success of new geothermal projects when including the best suited measures to minimize the geological uncertainties and prevent problems during drilling and production.