Numerical investigation of reservoir heterogeneity as a risk factor on the size of the protection zone and on the thermal performance of geothermal well doublet

Nowadays, the demand for the utilization of geothermal energy is growing worldwide. Several countries have numerous geothermal projects in the preparatory phase, but in many cases, it is difficult to decide which projects should be financed from a limited budget. To aid decision-making, various risk assessments are carried out to rank the projects, defining various technical and geological risk factors. One from risk factors can be the reservoir quality.

In this study, heterogeneity of porous sedimentary reservoirs is examined as a risk factor, since it (1) can significantly affect the amount of extractable water, (2) can have a strong influence on the thermal breakthrough time, (3) and strongly influences the size of the protection zone (PZ) around the geothermal well. Regarding the PZ, the size of the area associated with both the concentration front (SPZ – solute protection zone related to potential pollution) and the thermal front (TPZ – thermal protection zone) was examined separately. Two- and three-dimensional numerical models were built to study the effects of heterogeneity on geothermal well doublet where stochastic permeability distributions generated by sequential Gaussian simulation represent the heterogeneous reservoir. Detailed comparative simulations were used to reveal how (1) the scale of heterogeneity and (2) the permeability anisotropy influence the size of the PZ and the thermal performance of geothermal well doublet at different well distances and yields.

The results reveal how the required PZ increases with the scale of heterogeneity and provide probabilities for the size of PZ at different times for each heterogeneity scale. On a short time scale (<1 yr), there is no significant difference between the sizes of the SPZ and TPZ, and compared to the homogeneous approach, there is an increase of up to 50–75%. This decreases after 50 yr to 20% in the case of SPZ, while the thermal conduction smears the effects of heterogeneity for the TPZ size. The research also provides the probability of heat power achievable for each heterogeneity scale. Therefore, the research facilitates the more accurate risk assessment, i.e., based on the information about reservoir heterogeneity, it yields an estimate of the recoverable heat power and PZ size by calculating probabilities. Finally, the research applies the findings of synthetic simulations to a real geothermal project in Hungary, where the PZ probability map for geothermal well doublet was completed and the risk of heterogeneity was evaluated on the thermal performance.

Project no. KT-2023-900-I1-00000975/0000003 has been implemented with the support provided by the Ministry of Culture and Innovation of Hungary from the National Research, Development and Innovation Fund, financed under the KDP-2023 funding scheme.