Stability of Boreholes Drilled Through Salt Formations Displaying Plastic Behavior

Abstract

Summary This paper presents a mathematical and laboratory analysis that provides solutions to the problem of plastic flow of salt formations into wellbores. The paper predicts how the salt will flow and what type of mud might be used to control such flow for various well conditions. In the mathematical analysis, it is assumed that the incipience of plasticity in the formation is regulated by the level of octahedral shear stress and that the formation is neither permeable nor porous, but homogeneous and isotropic. The stress distribution in the neighborhood of the borehole is studied, and conditions under which this distribution is elastic, elastoplastic, or plastic are determined. Equations are derived that, in terms of two constants of the formation material, yield, and the limit of elasticity. Factors calculated include the radius of the plastic front and the percent of the borehole-diameter shrinkage as a function of mud weight used. This study was prompted by past failures in drilling through the Louann salt formation at depths of 12,500 to 14,000 ft [3810 to 4270 m], a formation that exhibits rapid plastic flow at low mud weights and at the temperatures and pressures encountered. The techniques presented were applied to subsequent drilling of this salt section, and the zone was penetrated without the problem of plastic flow. Salt-property constants were developed at elevated temperatures and pressures for the Louann salt formation by use of the triaxial test cell. Data are presented for elastic, plastic, and creep deformation of salt for pressures to 13,200 psi [91 MPa] and temperatures to 350°F [177°C]. Nomographs are also presented to facilitate the rapid determination of salt behavior. For a given depth and temperature, it is possible to specify a mud weight so that the salt will behave either elastically, in creep flow, or plastically. If plastic flow is predicted, the amount of borehole-diameter reduction is specified. Although this study focuses on the Louann salt section, we believe that the results apply equally well to any other salt section with physical constants similar to those in the Louann salt.