Safe Mud Weight Window Determination Using Log Based Methodology

Abstract

Mud weight controls hydrostatic pressure in a wellbore. Therefore, it is one of the most important drilling fluid properties. Wellbore collapse and tensile fracture are two major instability problems during drilling operation which cause tight hole, stuck pipe, oversize hole, and losses. Therefore, in this work, well logs as well as laboratory test results were used to determine safe mud weight window. First, dynamic properties of rock were extracted from well logs. Static properties of rock are defined using dynamic ones, but there are several relationships between the static and dynamic properties of rock. The relationships between static and dynamic properties were defined based on laboratory test results. After that, pore pressure is computed using a measured value of pore pressure for one point in the studied depth and pore pressure gradient through the depth. In the following, theory of linear poroelasticity is used to calculate in-situ stresses. Leak off test results are used to calibrate the estimated in-situ stresses. finally, Mud Weight Window (MWW) was determined using Mohr-Coulomb and Hoek-Brown criteria. The results showed that both criteria are in good agreement with collapse events. However, they predicted breakout events weakly.