Well Orientation Effect on Borehole Stability

Abstract

Abstract Using laboratory and field borehole stability data, an empirical correlation of borehole size and stress gradient effects is derived. Using the correlation, the well orientation effect on borehole stability is numerically studied. The study shows that the borehole stability is controlled, under the normal condition, by the largest in-situ stress in the radial borehole direction so that the borehole orientation effect for a horizontal well is not significant if two horizontal stresses are less than the vertical overburden pressure, however, if one horizontal stress is greater than the vertical stress, the orientation significantly affects the borehole stability. Introduction It is well known that well orientation affects borehole stability. However, the magnitude of orientation effect on borehole stability has been overemphasized. Field experiences have showed that when the maximum principal in-situ stress is in the vertical direction, the well collapse pressure is not significantly affected by well orientation although the well angle affects borehole stability. The main problems of previous publications are that most previous works use the Kirsch's solution, which is based on a linear elasticity. The linear elasticity solution indicates that both borehole stability and lost circulation pressures are sensitive to well orientation even if two horizontal stresses are less than overburden pressure. Actual rocks are not a linear elastic material. (A) Before a borehole is collapsed, the non-linearity of rock deformation becomes significant. The significant nonlinearity reduces the stress concentration induced by directional in-situ stresses. (B) In addition, an oriented borehole has a non-uniform stress or stress gradient around a borehole. The stress gradient reduces the stress concentration area, and the smaller size of stress concentrated region is less liable to failure due to the size effect. And (C) A wellbore remains stable after significant borehole breakouts are induced. Well stability is dominantly controlled by the maximum radial stress after local failures rather than the ratio of two radial principal stresses. This paper evaluates the magnitude of well orientation effect on well collapse pressure with the following steps.First, using several laboratory borehole stability tests and field well stability data, the effect of directional in-situ stress on well stability is shown.The well orientation effect induced both by stress gradient and the stress-strain non-linearity is analyzed using borehole stability laboratory experimental results.Since the non-linearity is one of the controlling factors for well collapse, triaxial data on weak and strong sandstones, and limestones are measured (stress strains for shales, and volcanic rocks are also measured but not included in this paper due to the space limitation). Using these data to construct nonlinear constitutive relations, it is shown that well orientation effect on borehole stability depends on magnitude of non-linearity, hence, it depends on lithology, however, it is shown that the well orientation effect upon borehole stability is significantly less than the magnitude predicted by linear elasticity theory.Progressive failure model is constructed to simulate borehole stability after onset of breakout.Finally, using the calibrated model with field and laboratory data, the effect of well orientation on well stability is evaluated for typical field conditions. This paper concludes that the well orientation does not significantly affect wellbore stability when the well direction varies between medium and smallest in-situ stress directions. For example, the orientation of a horizontal well does not significantly affect borehole stability as long as the maximum in-situ stress is in the vertical direction. However, note that if the well angle is varied in the plane of maximum in-situ stress, the well orientation does affect the well stability. For example, a horizontal well drilled in a techtonically active area is significantly affected by the borehole orientation since one of the horizontal in-situ stresses coinsides with the maximum in-situ stress.