Stability Analysis for Sidetracked Wells in Poro-Elastoplastic Formations

Abstract

In this study, we formulate the Finite Element model (FEM) of the stress-strain state for a three-dimensional (3D) borehole section with a sidetracked hole in fluid saturated elastoplastic formation. We then investigate the dependence of the safe mud weight range on the build angle while considering the plastic rock deformation behavior. The objective here is to showcase the unique drilling recommendations that can be generated for the sidetrack initiation or junction point, especially when compared to those of the mother-bore. 3D meshes are developed to analyze sidetracking scenarios in terms of orientation and initiation points with respect to the mother-bore. We utilize a 3D poro-elastoplastic model to take into account the pore pressure, using the concept of Biot-Terzaghi effective stresses. The Galerkin weak formulation is used to obtain the equations in discrete form. A robust solution of the static problem for elastoplastic body with stress boundary conditions is ensured by adding the Lagrange multipliers to set the constrains on rotation and translation of the solid as a rigid body. We utilize the Newton-Rapson method to get the solution of the equilibrium equation by providing the non-linear iterations. We demonstrate the ability to determine a safe mud weight range by computing several scenarios for in-situ stresses in a sandstone formation. The scenarios consider changes in the borehole orientation angles and the junction or initiation point. The scenarios also analyze the dependence of the stress-strain state near the sidetracked borehole section on the existing mother-bore. The shape and thickness of the plastic zone strongly depends on the orientation of the borehole and applied pore pressure distribution. Utilizing the elastoplastic model, we implement the procedure to find the optimal build angle for the design of sidetracking sections considering the hole orientation and geometry. The developed code for poro-elastoplastic formations is realistic alternative to conventional geomechanics models that fail to consider the influence of the mother-bore on the drilling conditions of the sidetrack.