Forecast of Well Drilling Events in Salt Bodies Based on Geomechanical Modeling for Eastern Part of the Orenburg Oil and Gas-Condensate Field

Abstract

The paper is dedicated to problem of forecast of well events drilling through nonproductive reservoirs at shallow depths: mud losses, wellbore instability due to salt tectonics, mud and reservoir temperatures difference and drilling time. Problem of forecast in those zones is complicated because of insufficient amount or complete lack of core tests, well loggings and noisy of seismic data. There is three-dimensional geomechanical modeling was showed with taking into account thermal stresses and time factor in the clay and salt zones on the example of the Eastern part of the Orenburg oil and gas condensate field. Also geological features were analyzed for prediction of mud losses zones. As a result of modeling, a relationship was found between the drilling mud losses and the wells location with respect to the sides of the salt folds and domes, which are accompanied by the development of natural fracturing. While well is drilling in overbalanced conditions in these zones, fractures and faults become critical stressed, which leads to their reactivation and mud infiltration. The trajectory can not be significantly corrected in the 300-800m depths so well schematic recommendations are reduced to a warning about the zones of possible losses. Wellbore stability calculation with taking into account the temperature effect shows an increase or decrease (depending on the drilling season) of the stresses, acting on the wall of the borehole up to 10 MPa; the influence in salts is greater than in clays. Taking into account time factor showed that long-term drilling with high mud density can lead to numerous stuck pipes and tight pulles. The obtained results make it possible to clarify wellbore stability calculations and, accordingly, the intervals of borehole failures. The prediction of critical stressed zones allows to warn the intervals of drilling mud losses. Thus, due to the combination of these approaches, it is possible to reduce the nonproductive time of well drilling and choose candidate wells for an optimized well design.