The Application Of Borehole Image Interpretations For Geothermal Well Stimulation A Case Study From Two Deep Geothermal Boreholes In Late Jurassic Carbonates In South-East Bavaria

Abstract

In 2018 Silenos Energy GmbH started a project in South-East Bavaria to produce electricity from a deep geothermal doublet system. Two highly deviated wells targeted the mesozoic cover units of the Alpine Foreland Basin beneath the Bavarian Molasse. The Late Jurassic (Malm) is recognized as the main geothermal reservoir in this area. Exploration activities in other areas of the Bavarian Molasse proved the high complexity of the Malm reservoir due to the interaction of matrix porosity with karstification and fractures (Steiner et al., 2014). Seithel et al. (2015) highlighted the importance of critically stressed fractures for the hydraulic conductivity of the Malm in the Bavarian Molasse Basin as a reservoir. Borehole Image Logs (BHI) were acquired in both wells in order to reduce the risk arising from this complexity. More than 1200m of image were logged around the target interval in each borehole together with standard open hole logs. A quick- look interpretation of the BHI data was performed with short turnaround time of less than 12 hours for fast decision-making before starting the well stimulation processes. After reception of the BHI data from the logging company, the logs were carefully quality controlled and processed to guarantee the completeness and accuracy of the data as well as the correct orientation of the logs. The subsequent analysis was focused on bedding orientation, fracture classification and orientations, fault zones, karstified intervals as well as drilling induced borehole failures. The analyses were performed by two geologists working simultaneously within the tight time schedule in order to ensure the quality of the interpretation. The whole analysis was performed manually, without using any automatic picking algorithms, thus increasing the quality of the analysis. Our interpretation revealed a triple porosity system driven by strong karstification as well as fractured zones in both wells. The boreholes penetrated Late Jurassic limestones and dolostones that contain highly fractured intervals as well as local karst phenomena, i.e. vugs and breccias. The karst features appear to be bound both to certain stratigraphic intervals as well as to the fracture-fault system, thus indicating a strong interaction of the karst system with fractures. Fracture sets strike preferably in E-W/ENE-WSW directions and around N-S direction, where the sets in E-W/ENE-WSW directions show preferably higher apertures. Natural tensile enhanced fractures were encountered in both wells and align with the tensile regions of the boreholes. The fracture density was calculated to identify highly fractured intervals, whereas karst intervals were identified by visual inspection. Small vugs occur along fracture surfaces; bigger vugs and karst holes are seen dispersed in the rocks but are confined to certain intervals in the dolomitic Malm. The BHI analyses suggest, that karstified zones have the strongest impact on reservoir flow, even though the reservoir is highly fractured. Well stimulation was performed through acidization of open fractures and vugs in order to increase the producible volume around the borehole. It is not possible to acidize the entire borehole, as the volume of the stimulation fluid is limited. Hence, the most prospective intervals for stimulation were defined based on the results of the BHI analysis. Pump tests before and after the stimulation from both wells indicate increased flow rates of thermal waters, which proves the success of the stimulation and highlights the value of BHI interpretations for geothermal projects.

AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019