Mud Weight Design for Deviated Wells in Shallow Loose Sand Reservoirs with 3D FEM

Abstract

Conventional mud weight designs are typically developed by using a 2D analytical solution. 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 2D 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 3D finite element model for a shallow (1,000 ft TVD) horizontal well in Tambaredjo NW field of Suriname was built. 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 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 mean stress in effective stress space. 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 our 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 2 ppg. The use of this pressure-dependent material model for predicting minimum safe mud weight in shallow, unconsolidated sand reservoirs can result in significant savings in field development costs.

Avaliação Geomecânica De Zonas De Instabilidade Durante a Perfuração De Poços De Petróleo Abaixo De Rochas Evaporíticas

“…Para a previsão e controle da estabilidade de poços é necessário o entendimento, tanto da magnitude quanto da orientação das componentes do tensor de tensões in situ (44). É comum em bacias marginais pressupor que a tensão vertical é uma das tensões principais, logo é necessário apenas definir a orientação das tensões horizontais para o entendimento do estado de tensões.…”