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

Abstract

Abstract. Fiber-optic distributed acoustic sensing (DAS) data finds many applications in wellbore monitoring such as e.g. 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 ¾” 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.

Monitoring cold water injections for reservoir characterization using a permanent fiber optic installation in a geothermal production well in the Southern German Molasse Basin

“…This integration entails the risk of systematic error due to a drift of the acquisition unit. Lipus et al (2021) referred to this and relativized the risk of such error as they saw no strain accumulation to the very end of the installation where no thermal contraction is expected.…”

“…DAS has also been used to complement and substitute classical geophones in vertical seismic profiling (Daley et al 2013;Hartog et al 2014;Madsen et al 2016;Götz et al 2018) and has also potential in the field of monitoring production zones (Williams et al 2015;Naldrett et al 2018) and flow characteristics (Bukhamsin and Horne 2016). A broader review of DAS applications in wells is given by Lipus et al (2021).…”

“…Cumulative strain ε DAS covers exactly the 60-min period from 01:08 to 02:08, although the comparison of ε DAS and ε DTS is liable to shift. Further research about the relation of measured DTS and DAS data to friction forces inside the well is discussed in Lipus et al (2021).…”

“…However, it is apparent that the DAS data at the hydraulically active zone at 2830 m MD show high dynamics. As testing time progresses, abrupt and spatially defined acoustic signals occur, mostly below 2800 m, more rarely above from 2000 to 2800 m. Lipus et al (2021) referred to those as 'sucker rod events' and gave an analysis of the complete DAS dataset that was created throughout the entire injection test Inj2. DTS profiles show that the same zone took the injected cold After 24 h of injection, the section from 3500 m MD to TD was the only one unaffected by the cold water injection.…”