Recent Advances In In-situ Stress Measurements By Hydraulic Fracturing And Borehole Breakouts

Nous traitons de deux recentes contributions dans le domaine de mesure des contraintes in situ. Nous presentons une etude de cas dans laquelleun puit largement incline a ete fracture hydrauliquement et les donnees interpretees avec succes pour determiner les contraintes tectoniques;les resultats peuvent etre verifies par les informations locales et regionales disponibles sur l"etat de contrainte. Nous presentons aussi les resultats de tests en laboratoire sur des trous fores sous un etat de contraintes simulant l"etat in situ, qui montrent que les dimensions de l"ovalisation peuvent etre utilisees pour evaluer la magnitude d"une composante principale horizontale des contraintes in situ si la valuer de I"autre est connue. Notre analyse ne peut conclure quant a la possibilite d"utiliser les dimensions des ovalisations pour estimer independemment les deux contraintes principales horizontales.

Wir berichten ueber zwei neue Beitrage zur Messung von Gebirgsspannungen. Der erste beschreibt den Fall eines stark einfallenden Bohrlochs, das hydraulisch gefraet worden ist. Die ergebnisse der entsprechenden Messunges sind fuer die Bestimrnungder Krustenspannungen untersucht worden, deren Werte mit Hilfe von sowohl lokalen als auch regionalen Daten bestaetigt werden koennen. Wir legen aussderdem Ergebnissevon laborversuchen vor, bei denen Bohrloecher unter realistischen Spannungsbedingungen belasted worden sind. Die resultierenden Bohrlochausbruechekoennen zur Bestimmungeiner Horizontal- Hauptspannungskomponentebenuetzt werden, wenn die Groesse der anderen bekannt ist. Unsere Analyse fuehrt zu keinem Schluss, ob die Dimensionen von Vohrlochausbruechen fuer eine unabhaengige Abschaetzung beider Horizontal-HauptSpannungen ausreichen.

Recent research in crustal stress measurements at the University of Wisconsin has resulted in two major findings:

1. inclined holes are amenable to hydraulic fracturing for the measurement of the in situ stress, and

2. borehole breakout size appears to be directly related to in situ stress magnitude.

These two findings, although unrelated, constitute major advances in the general area of crustal stress determination at great depths.

Hydraulic fracturing stress measurements are commonly conducted in vertical holes, but testing of inclined holes is also required at times both in civil engineering projects and in oil and gas wells. Conventional plane-strain elastic and poroelastic solutions (Hubbert and Willis, 1957; Haimson and Fairhurst, 1967) are generally not applicable to calculating crustal stresses in inclined holes if the principal in situ stresses are assumed to be in the vertical and the horizontal planes. Attempts to obtain solutions for the principal stress components from hydrofrac test data in arbitrarily oriented inclined holes based on the theory of elasticity (Daneshy, 1973; Richardson, 1983) have not been successful because more information is required than the test data can provide. We have recently conducted an extensive set of hydraulic fracturing stress measurements in an inclined hole for which the prevailing stress regime is generally known, and attempted to verify the suitability of the fracture pressurization method (based on Cornet and Valette, 1984) to interpreting test data in such situations. This paper briefly describes the tests and the analysis conducted.