Mohammad Sabah - MSc student of petroleum department of Amirkabir University of Technology, Tehran, Iran,
Mohammad Javad Ameri - Assistant professor of petroleum department of Amirkabir University of Technology, Tehran, Iran
Maryam Sedigh - Exploration, development and production planning specialist, KEPCO, Tehran, Iran
Malek Mohammad Gity - Head of petrophysic department, KEPCO, Tehran, Iran

Wellbore instability, especially in unconsolidated formations is one of the most significant problems during the drilling and causes high costs in drilling industry. Introducing the practical solution to decrease or eliminate these types of problems can significantly reduce the drilling time as well as drilling costs. The aim of this study is to construct a mechanical earth model (MEM) using available data for one of the wells, drilled in the Caspian Sea, to assess wellbore stability. A set of data includes drilling data, logging data, drill stem test (DST), leak off test (LOT), Modular dynamic test (MDT) is used for construction of the MEM in order to estimate the mechanical properties of rocks and in-situ stresses profile in the study well. In this study, since the core data to estimate the rock strength parameters such as uniaxial compressive strength (UCS) are not accessible, the caliper log and two mostly used failure criteria (Mogi-coulomb and Mohr-coulomb) are used to estimate the UCS. Next, by utilizing the mentioned failure criteria, the maximum and minimum allowable mud pressure are determined in the study well. It is observed that, the results obtained by Mogi-coulomb criterion are in good agreement with field observations and caliper log, but the Mohr-coulomb criterion over estimates the minimum mud weight required for safe drilling.

Wellbore stability, Mechanical earth modeling, Caspian Sea