Investigating the origin of leaks within the steel casing of deep geothermal boreholes at the Soultz EGS site
Creators: name: Daghmach, Rafat
Publisher: name: ETH Zurich
Year: 2012
Subjects: LEAKAGE MANAGEMENT (PIPELINES)
UPPER RHINE LOWLAND (GERMANY)
VERROHRUNG + ROHRSTRÄNGE (BERGBAU)
GEOTHERMAL EXPLORATION (APPLIED GEOLOGY AND GEOPHYSICS)
OBERRHEINISCHES TIEFLAND (DEUTSCHLAND)
LECKÜBERWACHUNG (ROHRLEITUNGEN)
GEOTHERMISCHE PROSPEKTION (ANGEWANDTE GEOLOGIE UND GEOPHYSIK)
CASING + CASING STRINGS (MINING)
info:eu-repo/classification/ddc/550
Earth sciences

description: The Enhanced Geothermal System (EGS) built at Soultz has three deep wells drilled to 5000 m. The wells form a triplet with two producers (GPK2 and GPK4), and a central injector (GPK3). The reservoir was created by injecting fluid at high pressure into each of the wells after they were drilled in order to establish hydraulic connections between them. Well GPK4 had a relatively poor connection to the reservoir as shown by the comparatively low production rates. To improve this connection, the well was subject to a series of high-pressure injections of various types of acid. Well productivity and injectivity was significantly improved by the treatments. However, it was discovered that most of the improvement came from several leaks that developed in the casing during the treatment program. The leaks are at 4385 m and 4710 m. Subsequently further leaks developed at 4719 and 4725 m. Two explanations for the leaks have been proposed. One is that they reflect corrosion that was accelerated by the transient contact of the steel with the acid during the injections. The other is that the leaks reflect fractures formed by the deformation of the casing, which is forced by shear movement on fracture zones mobilized by the high formation pressures during the injections. Previous work at Soultz has established the history of the development of the leaks, and has related them to ovalization or thinning of the casing inferred from an ultrasonic radial-scanning casing inspection log (i.e. Schlumberger USIT) run in 2005. This work is summarized and some further detail added to the history of leak development. However, the primary contribution of this thesis is analyze a further USIT log run in 2008 and also repeat section logs run in 2005 to evaluate the consistency of the identifications of ovalization and thinning from the 2005 main log. The results suggest that many of the identifications of casing ovalization are fictitious and a probable consequence of eccentering of the sonde. The leak at 4386 m correlates with anomalies in thickness of the casing seen on both USIT logs, and is probably a consequence of corrosion that was perhaps accelerated by the HCL acid treatment. No natural fractures cut the wellbore near this leak. However, the lower leak at 4710 m is located close to a fracture zone that accounted for loss of mud during drilling. Significant ovalization, which is consistent between all three passes of the 2005 log (the 2008 log does not extend to this depth), occurs at this location. Thus shearing is a possible explanation for this leak. No evidence of ovalization was found at the depths of the other leaks in this section at 4719 and 4725 m. However, these leaks did not develop until after the 2005 USIT log, and so it is possible that ovalization which produced the leaks occurred later. The cement in this section is poor, and so once one leak develops for whatever reason, high pressure fluid in the wellbore during subsequent injections could migrate along the annulus and enter other fracture zones, resulting in shearing. There were four high-pressure injections conducted before the leaks at 4719 and 4725 m were first observed in October 2006. It is possible that the first leak at 4710 m was initiated by the HCL acidization, like the leak at 4385 m, and this led to the pressurization of the annulus which in turn initiated shearing.