Bounds on In-Situ Stress Magnitudes Improve Wellbore Stability Analyses

Abstract

Summary Wellbore stability analysis is becoming a necessary tool to improve operational economy and to avoid wellbore problems. Essential elements of this process include estimation of critical fracturing and collapse pressures to obtain an optimal mud-weight window. The input data for this analysis is mainly pore pressure predictions from many sources such as logs and drilling exponents, overburden stresses from logs or drilled cuttings, leakoff tests at casing shoes, and breakout analysis from caliper logs. From these and other data, estimates for in-situ stresses and directions are obtained, which again serve as input for wellbore stability modeling. It is evident that the input data come from many different sources and can therefore not be considered consistent. During wellbore stability modeling, it has often been observed that unrealistic results appear. Sometimes one observes a critical collapse pressure that exceeds the fracturing pressure, clearly a faulty result. There is often an inherent error in some of the input data. This paper addresses this problem and concludes that the relative magnitudes of the in-situ stresses are its main cause. Equations are given that define the bounds on the in-situ stresses for different stress regimes. By using the bounds on the horizontal in-situ stresses on field cases, realistic fracturing and collapse prognosis are always obtained. Because the models also define the minimum permissible anisotropic stress state, these can be used as default parameters when field data are missing. A field case from the North Sea demonstrates the application of the stress bounds and shows that prognoses are obtained that better correspond to observed behavior of the wells.