Permeability Enhancement From a Hydraulic Stimulation Imaged With Ground Penetrating Radar

• Published in: Geophysical research letters 2020-09, Vol.47 (17), p.n/a
• Main Authors: Shakas, A., Maurer, H., Giertzuch, P.‐L., Hertrich, M., Giardini, D., Serbeto, F., Meier, P.
• Format: Article
• Language: English
• Subjects: Apertures; Bedrock; Boreholes; Change detection; Crystal structure; Crystalline rocks; Crystallinity; Enhanced geothermal systems; Fluid dynamics; Fluid flow; Fractures; GPR; Ground penetrating radar; Hydraulic properties; hydraulic stimulation; Hydraulics; Imaging techniques; Mapping; Monitoring; Permeability; Properties; Radar; Reflectance; Remote sensing; Remote sensing techniques; Reservoirs; Sensing techniques; Stimulation; time lapse
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL088783

We present evidence of permeability enhancement from hydraulic stimulation experiments in fractured crystalline rock. A total of 9.49 m3 was injected in two fractured intervals of a 300 m long borehole. Repeated Ground Penetrating Radar (GPR) measurements in the same borehole were carried out prior to and following the stimulation. The initial measurements revealed fractures in the vicinity of the borehole that could be traced up to distances of 50 m away. The data measured post‐stimulation were used in a difference‐imaging approach to illuminate changes in the GPR reflections caused by the stimulations. The changes delineate the enhancement of a large and complex fracture network. These changes likely correspond to changes in local aperture, thus permeability. Our results indicate that borehole GPR yields unique information on subtle changes in hydraulic properties within a relatively large volume and provides a new perspective on the characterization and monitoring of deep geothermal reservoirs. Plain Language Summary Deep geothermal reservoirs are a renewable and carbon‐neutral source of energy that is globally underutilized. Their principle is to efficiently extract heat energy from the Earth by circulating a fluid within a deep reservoir. Oftentimes, reservoirs need to be Engineered (or Enhanced), leading to the term Engineered Geothermal Systems (EGS). EGS rely on enhancing a reservoir's permeability, its ability to allow for fluid flow. Geophysical remote sensing techniques are useful in illuminating changes in properties of an EGS, and in monitoring its evolution. In crystalline bedrock environments, borehole Ground Penetrating Radar (GPR) is especially useful in mapping contrasts between rock and water. Here, we present results from a hydraulic stimulation experiment that aimed at enhancing a reservoir's permeability by injecting water in existing fractures. Using GPR single‐hole reflection imaging, we were able to map existing fractures within a relatively large volume. By repeating the measurements after the stimulations, we were able to detect changes in their reflectivity that most likely arise from permeability changes caused by the stimulation. The ability of GPR borehole measurements to image changes of the hydraulic properties in such high resolution offers a new and exciting perspective for characterizing and monitoring EGS. Key Points First‐time direct imaging of stimulation‐enhanced permeability in fractured rock GPR difference imaging