The combined effect of drilling fluid, in-situ stresses and inclination on wellbore stability

Abstract

A great number of variables (such as rock properties, in-situ stresses, drilling fluid, inclination etc.) are effective on wellbore stability and the combined effect of those variables determines the stability condition of wellbore. The main aim of this study is to analyse the combined effect of drilling fluid, stress state and inclination angle considering that there is no comprehensive study conducted on the topic. In this study, three stress conditions (normal faulting stress condition, reverse faulting stress condition and strike slip faulting stress condition), three drilling fluids (bentonite based, KCl based and polymer based), three inclination angles (0, 45 and 90°) and two depths (2800 and 5000 ft) were considered to evaluate the effect of those parameters on wellbore stability. The results of analyses showed that the number of instabilitity zones around the wellbore increases as the inclination angle increases under normal fault stress state However, number of instability zones are higher for low inclination angle values under reverse and strike slip fault stress states. While the best stability condition was obtained with polymer drilling fluids due to limited decrement on strength properties, the worst stability condition was satisfied with bentonite drilling fluids due to considerable decrement on strength properties as a result of interaction with drilling fluid.