Wellbore stresses induced by the nonlinear deformation of an inflatable packer

Abstract

Packers, i.e. inflatable balloons made from elastomere, reinforced with cords, are used by the oil industry for sealing an interval along the wellbore, for open – hole stress tests to be performed. Thiercelin et al. have shown that the packer behaviour is critical to the success of such tests. In particular, the pressure exerted by the packer on the formation needs to be as close as possible to the pressure in the interval, to avoid fracture initiation or fracture growth along the packers. Moreover, axial tension in the rock needs to be kept to a minimum to avoid initiation of transverse fractures at the packer level that would be detrimental to the test. In this paper the theory of membranes, reinforced with inextensible cords, developed by Kydoniefs and its implementation for packers done by Atkinson and Peltier, is used to gain insight into the stresses which are generated in the rock, due to packer inflation. The shape of the packer is first computed for a given net pressure across the packer, via the solution of a nonlinear integral equation. Then the pressure acting on the formation, along the packer/rock contact zone, is found. Results show that the pressure applied by the packer on the rock during stress testing, is almost uniform and close to packer inflation pressure. The formation is afterwards considered loaded with the latter pressure and the stresses in the rock are found by either a finite-element or a semi-analytic method. Axial stresses develop at packer ends but are much smaller than the pressure applied to the formation. Tangential stresses develop at the wellbore wall and are higher at packer level than at interval level.