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. Fiber-optic distributed acoustic sensing (DAS) data find many applicationsin 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 sensingcable to a rigid structure (casing/tubing) which allows for a controlledlanding of the cable. We analyze a cold-water injection phase in ageothermal well with a 3.6 km long fiber-optic installation mounted to a3/4 in. sucker rod by using both DAS and distributedtemperature sensing (DTS) data. During cold-water injection, we observedistinct vibrational events (shock waves) which originate in the reservoirinterval and migrate up- and downwards. We use temperature differences fromthe DTS data to determine the theoretical thermal contraction and integratedDAS data to estimate the actual deformation of the rod construction. Theresults suggest that the rod experiences thermal stresses along theinstallation length – partly in the compressional and partly in theextensional regime. We find strong evidence that the observed vibrationalevents originate from the release of the thermal stresses when the frictionof the rod against the borehole wall is overcome. Within this study, we showthe influence of temperature changes on the acquisition of distributedacoustic/strain sensing data along a fiber-optic cable suspended along arigid but freely hanging rod. We show that observed vibrational events donot necessarily originate from induced seismicity in the reservoir butinstead can originate from stick–slip behavior of the rod construction thatholds the measurement equipment.

Dynamic motion monitoring of a 3.6 km long steel rod in a borehole during cold-water injection with distributed fiber-optic sensing