Abstract
Abstract. Fiber-optic distributed acoustic sensing (DAS) data find many applications in wellbore monitoring such as flow monitoring, formation evaluation and well integrity studies. For horizontal or highly deviated wells, wellbore fiber-optic installations can be conducted by mounting the sensing cable to a rigid structure (casing/tubing) which allows for a controlled landing of the cable. We analyze a cold-water injection phase in a geothermal well with a 3.6 km long fiber-optic installation mounted to a 3/4 in. sucker rod by using both DAS and distributed temperature sensing (DTS) data. During cold-water injection, we observe distinct vibrational events (shock waves) which originate in the reservoir interval and migrate up- and downwards. We use temperature differences from the DTS data to determine the theoretical thermal contraction and integrated DAS data to estimate the actual deformation of the rod construction. The results suggest that the rod experiences thermal stresses along the installation length – partly in the compressional and partly in the extensional regime. We find strong evidence that the observed vibrational events originate from the release of the thermal stresses when the friction of the rod against the borehole wall is overcome. Within this study, we show the influence of temperature changes on the acquisition of distributed acoustic/strain sensing data along a fiber-optic cable suspended along a rigid but freely hanging rod. We show that observed vibrational events do not necessarily originate from induced seismicity in the reservoir but instead can originate from stick–slip behavior of the rod construction that holds the measurement equipment.
Highlights
Fiber-optic distributed sensing in borehole applications has gained a lot of attention in recent years
We find strong evidence that the observed vibrational events originate from the release of the thermal stresses when the friction of the rod against the borehole wall is overcome
Besides a detailed analysis based on distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) data of the rod’s dynamics in response to temperature changes during a cold-water injection, we show that the resulting thermal stresses are released by the observed vibrational events indicating stick–slip-like behavior of the rod–borehole wall compound
Summary
Fiber-optic distributed sensing in borehole applications has gained a lot of attention in recent years. The performance of a borehole heat exchanger was monitored with DTS to evaluate the heat input along the wellbore and to measure the regeneration time after a heat extraction period (Storch et al, 2013). While DTS has found its way as a standard tool for wellbore monitoring over the last 2 decades, the utilization of distributed acoustic sensing (DAS) is still subject to many research questions. Johannessen et al (2012) introduced the potential and capabilities for acoustic in-well monitoring applications based on DAS systems which range from, e.g., flow measurements, sand detection, gas breakthrough and leak detection to vertical seismic profiling (VSP). Finfer et al (2014) performed an experiment to study DAS applications for turbulent single-phase water flow monitoring Götz et al (2018) report on a multi-well VSP campaign at a carbon dioxide storage site by using only one single DAS interrogator. Finfer et al (2014) performed an experiment to study DAS applications for turbulent single-phase water flow monitoring
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