Investigation of Three Modes of Borehole/TWC Failure by Application of the Modified Cam-Clay Material Model

摘要

In this paper we discuss the use of the Modified Cam-Clay material model applied to the problem of borehole stability. We consider the case of a thick walled cylinder (TWC), which is easily extended to a borehole by allowing the outer radius to go to large values. Three mechanisms of failure will be discussed: "Runaway Instability (RAI) when the strains at the borehole wall start growing uncontrollably, "Negative Rate of Work" (NRW) when the rate of work of plastic strains becomes negative, and catastrophic failure when equilibrium conditions cannot be met (Considere instability). The first two of these criteria were discussed in earlier papers related to depleting reservoirs and fault activation/reactivation.Remote horizontal stresses and vertical stresses are initially given. Axisymmetric conditions are assumed. The calculation proceeds by assuming the existence of a plastic zone surrounding the interior wall. As the radius of the plastic zone is increased, the equilibrium interior boundary pressure is calculated. Three mechanism of failure are encountered. The mode of failure depends on the values of remote and vertical stress, material properties, and thickness of the cylinder wall.