Constraining the full stress tensor from observations of drilling-induced tensile fractures and leak-off tests: Application to borehole stability and sand production on the Norwegian margin

Abstract

Understanding the interaction between rock strength, in-situ stress, and engineering practice allows one to minimize wellbore failures by designing optimally-stable borehole trajectories and appropriate mud weights. We utilize an interactive software system, Stress and Failure of Inclined Boreholes (SFIB Peska, Zoback 1996), to illustrate how observations of drilling-induced compressive and tensile wellbore failures in vertical and inclined wellbores can be integrated with other routinely-available information to yield an estimate of the full stress tensor. We further illustrate how such information allows one to place bounds on in-situ effective rock strength. We consider a deviated well in the Visund oil field on the flanks of the Viking Graben along the Norwegian margin. A good quality electrical imaging log revealed drilling-induced tensile wall fractures in an orientation consistent with other observations of stress orientation in the region. Interpretation of these fractures indicates that the maximum horizontal stress is 72±6.5 MPa at an azimuth of 100°±10°. No breakouts were detectable in the well, indicating that the uniaxial compressive rock strength is greater than approximately 19 MPa. We demonstrate how we utilized this information to design an optimally-stable wellbore trajectory to minimize sand production.