Mud weight design for horizontal wells in shallow loose sand reservoir with the finite element method
Conventional mud weight designs are typically developed by using one-dimensional (1D) analytical solutions. These methods, however, are unreliable in shallow, unconsolidated sands, which have very low cohesive strength and pressure-sensitive material properties, such as Young’s modulus and Poisson’s ratio. Using a 1D solution for these reservoirs can result in unnecessarily high mud weights and narrow mud weight windows, especially in deviated wells. In this case, a numerical method with a pressure-dependent material model becomes necessary for predicting a minimum safe mud weight. A fully coupled, nonlinear three-dimensional (3D) finite element model (FEM) was built for a shallow 305 m true vertical depth (TVD) horizontal well in the Tambaredjo NW field of Suriname. The calculation of deformation of loose sand, which has pressure-dependent material properties, including cohesive strength, internal frictional coefficient, and Young’s modulus, was combined with porous flow. We used this model to investigate wellbore stability and the minimum safe mud weight gradient. Core samples from wells in the Tambaredjo NW field of Suriname were analyzed. The results of the analyses were used to develop a nonlinear relationship between material parameters, including cohesive strength, internal frictional coefficient, Young’s modulus, and the mean stress in effective stress space. The pattern of variation of these pressure sensitive parameters and other data were drawn from the multiple disciplines involved in the field development plan. The results of the study show the model-predicted minimum mud weight gradient required to drill the well without instability can differ from the conventional analytical solution by as much as 2348.6 Pa/m (2 ppg). The use of this pressure-dependent material model for predicting a minimum safe mud weight in shallow, unconsolidated sand reservoirs can result in significant savings in field development costs.