Development of a mechanical earth model in an Iranian off-shore gas field

Abstract

Wellbore instability is a quite common event during drilling, and causes many problems such as stuck pipe and lost circulation. It is primarily due to the inadequate understanding of the rock properties, pore pressure, and earth stress environment prior to well construction. This study aims to use the existing relevant logs, drilling, and other data from offset wells to construct a precise mechanical earth model (MEM) describing the pore pressure, stress magnitudes and orientations, and formation mechanical properties of South Pars Gas field. Since the core test data, MDT/XPT data, and LOT/XLOT data were not available to calibrate the developed model, each component of the model was determined using a range of existing methods and relations, and then the wellbore instability was analyzed based on the developed MEM and the Mogi-Coulomb failure criterion. The predicted incidents such as the lost circulation and tight hole were then compared with the caliper log and reported drilling events to determine the consistency of the model. Since the stability analysis based on the developed MEM had the most agreement with the caliper log and reported drilling events, the equations presented by Eaton and Zoback had good estimations of the pore pressure and rock strengths. Also the estimated horizontal stresses were precise enough to enable the constructed MEM to predict the wellbore instabilities. The stress regime in the field of study was strike-slip, which is frequently specified in the industrial technical reports of the studied field. Finally, it was concluded that the Mogi-Coulomb failure criterion minimized the conservative nature of the mud pressure prediction due to the consideration the strengthening effect of the intermediate stress.