A Slimhole Approach to Measuring Distributed Hydromechanical Strain in Fractured Geothermal Reservoirs

Abstract

Because many fiber optic sensors are fully functional when cemented into boreholes, they are obvious candidates for deployment in boreholes that need to be sealed and/or abandoned. Exploration slimholes that are not converted to production wells, and production wells that are to be abandoned, are obvious candidates for such installations. Many new fiber optic sensors are coming online but in this article the focus is low-frequency dynamic strain sensing through the use of distributed acoustic sensing (DAS). The transfer of strain between formation and borehole is investigated through geomechanical simulations and DAS strain sensing tested in a shallow crystalline bedrock. Simulations indicate that strain transfer between hydraulically impacted fracture zones are localized below the spatial resolution (gauge length) of DAS systems. Field tests demonstrate that strain in a fracture zone can be localized and measured in a fiber optic cable so long as it is mechanically coupled to the formation. The method requires an oscillatory hydraulic forcing to strain the fracture for it to be measured by DAS. In the field experiments, this was accomplished by alternating pumping and injection at a companion borehole 30 m distant from the borehole equipped with the DAS fiber. Strain signals were reliably measured above a noise of 10 picostrain in response to head changes less than 2 mm of water. These results suggest that DAS sensing of a fiber optic cable cemented into a geothermal borehole would be capable of measuring periodic hydraulic responses from an injection or pumping system a great distance from the monitoring borehole, dependent upon the magnitude and frequency of the hydraulic oscillation. Using a slimhole, reservoir connectivity could be imaged over many depths. This could be accomplished using the same fiber-optic cable installed for DAS seismic monitoring or distributed temperature sensing (DTS).