Evaluation Of Effect Of Drilling Fluids On Wellbore Stability

Wellbore stability evaluation is one of the important applications of rock mechanics and should be regarded to prevent instabilities that may develop in the wells for various reasons (such as complex field conditions, interaction between rock and drilling fluid, directional drilling etc.). The saturation of rocks by drilling fluids causes significant changes on mechanical properties of rocks. However the level of these variations may be changed due to drilling fluid compositions. In this study, the effect of these variations on wellbore stability was examined for different well profiles. In order to assess the effect of saturation on stability of wellbores, three different drilling fluid compositions (bentonite, KCl and polymer based muds) were considered. The results of assessment suggested that the saturation by bentonite and KCl based drilling fluids create adverse effect on critical fracture and collapse pressures. However the pressure values determined as a result of saturation by polymer based drilling fluid are close to those obtained for dry condition. It should be also stated that the range of mud pressure ensuring the stability of wellbore is also significantly affected by variation of collapse and fracture pressures calculated under drilling fluid saturation.