Modelling of Diffusion Driven Weakening and Strain of Geomaterials and Applications to Time-Dependant Borehole Failure

Abstract:

Time-dependent borehole failure is a widely recognised problem in borehole geomechanics. Failure may be caused by pressure, temperature and chemical gradients introduced by the drill mud into the formation. The exact coupling between these thermodynamic parameters and the mechanical failure processes is not well understood, because (a) the impact of thermodynamic parameters is not routinely considered in rock mechanical testing and (b) repeat or “time-lapse” runs of borehole imaging tools over the same formation interval are rare. At the same time understanding time-dependant borehole failure can enhance the thermo-hydro-mechanical-chemical (THMC) characterization of a given formation and improve borehole stability predictions.

In this contribution we present a fully coupled material model that allows for diffusion driven weakening and strain. The material model is based on non-equilibrium thermodynamics and implements the material weakening via a damage parameter. This material model is implemented into a Finite Element simulation of near-well bore processes. A parameter study explores the possibilities of using in-situ observations (e.g. time-dependant widening of borehole breakouts in repeat image logs) for the inversion of THMC coupling parameters. We also discuss the implication of the forward modelling for borehole stability predictions.