Stability of Boreholes Drilled Through Salt Formations Displaying Plastic Behavior
- E.F. Infante (U. of Minnesota) | M.E. Chenevert (U. of Texas)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- March 1989
- Document Type
- Journal Paper
- 57 - 65
- 1989. Society of Petroleum Engineers
- 1.14.1 Casing Design, 1.6.9 Coring, Fishing, 1.11 Drilling Fluids and Materials, 2.4.3 Sand/Solids Control, 1.10 Drilling Equipment, 5.1.2 Faults and Fracture Characterisation, 1.14 Casing and Cementing, 1.6 Drilling Operations, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties)
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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 degrees F [177 degrees 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.
Problem and Objective. Experiences with drilling operations through salt formations show that the material in the immediate vicinity of the borehole is capable of plastic flow. These indications consist of a reduction in borehole diameter, sticking of bit and drill collars, and collapse of casing set through such zones. Solutions consist of using low-weight salt muds or oil muds with an unsaturated water phase to "control leach" any salt that may flow into the borehole. Another recognized solution is to increase the mud weight when possible, thereby slowing the rate of salt flow. After salt zones are drilled and cemented, salt creep and subsequent casing collapse may still occur. To prevent such collapse, it is recommended that a collapse gradient above 1.0 psi/ft [22.6 kPa/m] be used in the casing design. In each drilling case, the preferred solution is arrived at by trial and error. Although it is often recognized that disruptive salt flow may occur, no available method will predict the degree of flow. The purpose of this study is to present a theoretical and experimental study of the problem of stability of a borehole drilled through salt formations and to provide guidelines for predicting the degree of salt flow that may occur in a given well. To understand this phenomenon and to devise appropriate countermeasures, it is necessary to study the plastic behavior of the material, the possible stress distributions, and the deformations caused by plastic flow. The problem of the stability of an uncased deep well under plastic conditions was first studied by Westergaard, who considered the plastic behavior of the formation material to be described by what is essentially a simple Mohr envelope. His analysis applies to an empty borehole. Serata and Gloyna studied the plastic behavior of the material surrounding underground salt cavities. their interest aroused by the possibility of using such cavities for the disposal of radioactive wastes. In conjunction with this analytical work, Brown and Jessen carried out a set of experiments that were confirmed and extended by others. In 1963, Fekete published a study on the plastic behavior of formations around a borehole that is essentially incorrect. Other more recent studies of salt flow are given by Thomas et al. and Obert. The assumptions made in this study are in close agreement with those of Serata and Gloyna. Various authors have described the elastic and creep properties of salt under reduced stress conditions. Other work on elastic/plastic flow is very informative but is not easily applied to the flow conditions described here. Therefore, a different approach was taken. In the study of the elastoplastic behavior of the formation in the neighborhood of a borehole, it is important to have a good understanding of the elastic/plastic properties of the material. The stresses are strongly dependent on whether the material is elastic or plastic, and the condition of plasticity itself depends on the stress condition. To clarify this relationship, the next two sections are devoted to a brief study of what is believed to be the plastic behavior of salt and its elastic/plastic properties. These sections are then followed by an analysis of the implication of plastic behavior on the state of stress underground before drilling. Subsequent sections deal with the formulation of possible elastic and plastic stresses and of the deformations resulting from plastic flow. The final section presents suggestions on drilling through formations that display plastic flow.
Reason for the Study. This study was motivated by the problem of drilling through a 350 degrees F [177 degrees C] formation of Luann salt found at 12,500 ft [3810 m] in Freestone County, TX. Previous attempts to drill through this massive 500-ft [150-m] section of salt had failed; the bit and collars were stuck and drilling terminated. This work was designed to provide guidance on subsequent drilling of the Louann and other salt formations.
Elastic/Plastic Properties of Salt
Materials, such as the Louann salt, that have a tendency to behave plastically at low stress levels are generally found in formations of extremely low porosity and negligible permeability. Materials of this type at room temperature and under ordinary compression exhibit brittle failure and behave very similarly to consolidated rocks. Under triaxial compression, they lose their brittleness at rather low stress levels, and failure is no longer shear; rather, a decidedly plastic behavior takes place in the form of flow or creep. Temperature is an important factor-the higher the temperature, the more pronounced the plastic flow. Numerous theories exist for describing the incipience of plasticity in a material. The choice of the theory to use is dictated by experimental observations on the particular material. For the Louann salt and other pure salt formations, the octahedral-stress theory best describes plastic behavior and will be applied in this work.
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