Borehole closure in salt.

Abstract

Constitutive laws are developed to predict creep (time-dependent) closure of boreholes in salt specimens subjected to various loading configurations. Rheological models (linear and nonlinear viscoelastic and viscoplastic models), empirical models, and physical theory models have been formulated from the results of uniaxial creep tests, strain and stress rate controlled uniaxial tests, constant strain rate triaxial tests, cyclic loading tests, and seismic velocity tests. Analytical solutions for a thick-walled cylinder subjected to internal and external pressures and for a circular hole in an infinite plate subjected to a biaxial or uniaxial stressfield have been derived from each of the linear viscoelastic models and empirical laws. The experimental results indicate that the salt samples behave as an elastic-viscoplastic material. The elastic behavior tends to be linear and time-dependent while the plastic deformation is time-dependent. The stress increment to strain rate increment ratio gradually decreases as the stress level increases. The transient potential creep law (ε(c) = κσᵝtᵞ) seems to give the simplest governing equation describing the viscoplastic behavior of salt during the transient phase. Variation of intrinsic properties which is mainly contributed by nonuniform distribution of intercrystalline gaps and air voids plays a more significant role upon instantaneous deformation than upon transient deformation. The mechanisms governing the time-dependent deformation are fracture propagation, plastic flow and dislocation of the salt crystals, and healing of the intercrystalline gaps and induced fractures. Different sets of test parameters (strain and stress rates, differential and confining stresses, and testing times) induce different degrees and combinations of deformational mechanisms, which lead to a variation of the fitting parameters of the potential law. The transient potential creep model does not accurately predict the results of triaxial and polyaxial borehole closure experiments, probably due to the predictive capability of the model or the methods used in multiaxial formulation, or both. Since the model parameters apparently depend upon the main mechanisms governing creep rate, prediction of the salt deformation around a borehole subjected to a high stress gradient by using only a set of model parameters may not be accurate.