Abstract
Fiber optic sensing has gained importance for wellbore monitoring and reservoir characterization in geothermal fields as it allows continuous, spatially highly resolved measurements. Distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) technologies, among others, enable monitoring of flow regimes and heat transport inside the wellbore to describe the dynamical behavior of the reservoir. The technically challenging installation of a permanent fiber optic monitoring system in a geothermal production well over the entire wellbore length was conducted for the first time at the geothermal site Schäftlarnstraße in Munich, Germany. One cable with two DAS fibers, two DTS fibers, and one fiber for a downhole fiber optic pressure/temperature gauge were clamped to ¾-in. sucker rods and installed to 3.7 km measured depth to collect data from the wellbore after drilling, during testing, and during operations. We present DTS profiles during 3 months of well shut-in and show the results of two cold water injection tests conducted to localize inflow zones in the reservoir and to test the performance of the fiber optic setup. A vertical displacement in temperature peaks of approximately 1.5 m was observed during the injection tests, presumably resulting from thermal contraction of the sucker rod–cable setup. This was verified by analyzing the strain information from the DAS records over 1 h of warm-back after cold water injection with the calculated theoretical thermal contraction of DTS of the same period. We further verified the flowmeter measurements with a gradient velocity analysis of DTS profiles during injection. Intake to the major inflow zone was estimated to 93.5% for the first injection test, respective 94.0% for the second, intake of flowmeter was calculated to 92.0% for the same zone. Those values are confirmed by analyzing DTS profiles during the warm-back period after the well was shut.
Highlights
The role of renewable energy sources gains importance for climate change mitigation strategies today
The feasibility of installing a permanent fiber optic cable for a permanent monitoring along sucker rod into the reservoir of a deep and deviated geothermal production well was successfully demonstrated at the study site
The fiber optic (FO)-monitoring system enables a dynamic monitoring of the thermal distribution in the borehole and the reservoir when the well is set to operation, as well as acoustic studies which can be addressed to the subject of monitoring the pump or microseismicity
Summary
The role of renewable energy sources gains importance for climate change mitigation strategies today. A reliable regionalizing of determined geophysical properties is essential for further geothermal exploration, defining a productive drilling target, and reducing exploration and drilling risks In this framework important geophysical properties are, e.g., permeability and porosity of the reservoir matrix and the structure of the reservoir such as fractures. Suitable conditions of these properties create influx zones that feed the geothermal well and make it productive (e.g., DiPippo 2015). Schlumberger (1997) gives a broad overview of most logging techniques which can be used to interpret feed zones and their application and interpretation All of these logs need well intervention and, in the case of high deviations, a rig during measurements to push the tool into the well. Monitoring in the reservoir is not possible for standard well designs
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