Stress Condition Modeling to Optimize Mud Weight Window for Efficient Drilling

Abstract

Geomechanical instability refers most significant incidents during drilling operations such as wellbore collapse or failure. In general, wellbore instability is related to drill pipe sticking, tight spots, caving production, wellbore collapse and unscheduled sidetracks. These mechanical conditions are mostly cause by unknown rock stress conditions and lead to increased costs during drilling and completion operations. Wellbore instability issues are important in the success of drilling operations. One of the main goals of any drilling mission is to drill the well economically viable. Wellbore instability can be detrimental to this goal and it is included during well planning stage. Therefore, It is quite difficult to measure the state of stress acting on a borehole wall. The uses of inclined and horizontal wells in the exploitation of natural resources have increased considerably and so the need of wellbore instability while drilling arises. This is particularly true for long reach, highly deviated and horizontal wells where the cost of downtime is very high. During the development phase engineers seek to optimize wellbore instability through determination of optimal well trajectories and safe mud weight window. The response of the wellbore to the drilling process can monitored and compared to the rock failure criterion. Therefore, wellbore instability is a function of several factors such as well bore inclination and azimuth, in situ stresses, mud weight, rock strength properties etc. Some of these factors are controllable and some are not. Among the controllable factors are inclination, azimuth and mud weight. By changing these parameters, one can reduce instability problems significantly. Theoretically, it is possible to design the well trajectory in a way to face least instability problems. Present paper describes that the linear elastic model along with Mohr’s-Coulomb failure criterion can be utilize to calculate optimum mud weight window to avoid wellbore instability problems.