The Importance of Geomechanical Analysis for Well Design and Engineering

Abstract Wellbore instability presents serious challenges to drilling activities, worldwide. It also has its effect when the wellbore is in production mode. Wellbore instability can be one of the greatest pre-conditions for Non-Productive Time (NPT) during drilling operations, if not anticipated and planned against accordingly. The principal stresses in the earth's crust are vertical and horizontal in direction. Our wells are not always exactly vertical or horizontal, and it is the stresses parallel to and perpendicular to the well path that we are most interested in. The knowledge of these principal stresses will help plan a stable wellbore. Some of the factors that affect wellbore stability include, rock strength, mud weight, temperature fluctuations, wellbore geometry, drillstring vibration, stress and strength anisotropy, drilling fluid filtrate invasion, well path orientation and inclination. All these, must be scrutinized and ensured that they in no way affect the safe and efficient delivery of the well. In summary it must be ensured that the total stress redistribution down the subsurface is equal to the summation of the radial stress and hoop stress. Wellbore instability can result in serious operational problems like borehole collapse, lost circulation, stuck pipe etc, which are undesirable and have dire financial implications. It is therefore extremely important that drilling engineers anticipate this problem during planning and well design and introduce measures to curb it. This paper discusses the causes of wellbore instability, underscores the importance of geomechanical analysis as a preventive measure, reviews the various wellbore stability models currently applied in the industry, and presents a well design guideline on how to plan against wellbore instability problems.