Abstract The fully-coupled deformation and diffusion behaviour around a boreholedrilled in a fluid-saturated poroelastic medium are modelled using atwo-dimensional finite element method. A variational principle equivalent tothe governing equations in the theory of poroelasticity is derived. Thevariational principle used here to achieve a full coupling is notconventionally employed in porous medium mechanics, therefore verificationprocedures were implemented. The distribution and the history of effective stresses and pore pressurearound the borehole, valuable factors for evaluation of borehole stability andsolids production potential, are evaluated in hydrostatic and non-hydrostaticstress fields, and under different wellbore pressures. For typical cases thatcorrespond to oil field conditions, significant differences are found betweenthe fully-coupled solution and the uncoupled solution. Introduction The basic equations for coupling elastic deformation with pore pressure forporous media subjected to transient fluid flow were first established by Biotin 1941. There have since been numerous restatements given, and Rice and Cleary provide an particularly elegant reformulation of Biot theory byredefining the material constants. The theory of poroelasticity not onlypredicts quantitative differences from uncoupled flow-deformation theory, italso predicts phenomena such as the Mandel-Cryer effect, which are neitherintuitive nor revealed by the uncoupled theories. It has become the preferredapproach in many geomechanical applications, but because of the governingequations' complexity, analytical or closed-form solutions are available onlyfor cases of simple geometry and boundary conditions. For example, a recentdevelopment by Wang and Dusseault provides a solution for fullyporoelastic coupling in an axisymmetric plane strain borehole subjected toinjection or withdrawal. The complexities of the governing equations coupling deformation withdiffusion, combined with the strongly non-linear behaviour of geomaterials, naturally increase the importance and popularity of numerical approaches. The Finite Element Method (FEM) is the most widely used procedure for numericallyapproximating the theory of poroelasticity. In cases involving coupleddeformation-diffusion, there are two formulations of FEM commonly employed tosolve boundary value problems.