This paper presents the analyses of the impacts of temperature variations on wellbore stability using an analytical thermoporoelastic model. The model calculates the coupled temperature, pore pressure and stress distributions around a borehole drilled in fluid-saturated porous media, and subjected to a step change in temperature and pressure along the borehole wall due to drilling. The basic mechanisms how the thermoporoelastic process influences wellbore stability are investigated.
Cet article presente les consequences d'une variation de temperature de puits sur sa stabilite en utilisant un modele thermoporoelastique. Le modele calcule de facon couplee la temperature, la pression de pore et les contraintes au roisinage d'un puits fore daus un molieu satures, et soumis à un changement de temperature et de pression la long du forage. Les mecanismes de base affectant la stabilite via des processus thermoporoelastiques ont ete de etudiees et sont presentees.
Dieses Arbeit beschreibt die Analysen der Einfluesse von Temperaturschwankungen auf die Stabilitat von Bohrloechern mit Hilfe eines analytischen thermoporoelastic Modells. Dieses Modell berechnet die Temperatur, den Porendruck und die Druckverteilung urn ein Bohrloch, welches in einem mit Fluessigkeit gesaettigtem poroesen Medium gebohrt wird, und dabei einer vom Bohrprozeß verursachten stufenweisen Aenderung von Druck und Temperatur der Bohrlochwand unterworfen ist. Die grundlegenden Mechanismus der Einfluesse des thermoporoelastic Prozeßes auf die Bohrloch Stabilitat werden untersucht.
During drilling of an oil or gas well, the temperature of the drilling mud is usually quite different from the formation temperature due to the geothermal gradient. Such a difference can result in significant variations of the temperature field around the borehole; hence, the pore pressure and stress fields. The fact that drilling is a non-isothermal activity is common knowledge, but the potential impact of the thermal effects on wellbore stability has basically been overlooked by industry, even though some field cases have been reported blaming temperature for time-delayed borehole failure (Guenot & Santarelli, 1989), and that cooling the mud can effectively reduce wellbore problems (Maury & Guenot, 1995). Part of the reason for industry to believe that thermal effects are a trivial problem is due to the fact that the existing analyses are essentially uncoupled, i.e. temperature is taken into account solely via the thermal expansion coefficient of the rock formation which results in an additional stress term only. The analyses using the fully coupled thermoporoelastic solutions developed recently (Li et al. 1998a, b) revealed that thermal effects can be crucial to wellbore stability, much more significant than the results predicted by uncoupled models, especially when the fluid diffusivity of the formation is small, such as in shales. It is well-known that 95% of the wellbore instability problems occur in shale, and no uniform recognition has been reached so far as to what mechanisms are that control these problems. This study proposes a new potential explanation for the shale instability problems. One of the predicted dominating failure mode conforms to lab observation (Salisbury et al. 1991).
