The convergence characteristics of the borehole in tar sand are described by means of a nonlinear equation of state. The geometrical model of a cylindrical cavity was taken as a basis. The viscoelastic behavior of tar sand was investigated with the use of Burger's rheological model. The rheological constants were determined by means of creep tests on original tar sands. The model thus developed was used to investigate the contraction ofwellbore under different bottomhole conditions. The most unfavorable case thereby observed does not pose a serious problem from the point of practical consideration.