Effects of drilling fluids on the strength properties of clay-bearing rocks

Certain drilling problems are frequently observed during different types of drilling (e.g. geothermal, oil, geotechnical). In order to maintain the stability of boreholes, analysis of the essential parameters affecting their stability, including induced post-drilling stresses, the geomechanical properties of the rocks encountered during drilling and the type of drilling fluid used, should be considered. Well pressure, which is a function of the density of the drilling fluid, should be kept at a level that will not create shear- or tensile-type failure around the borehole walls; however, the mechanical properties of rocks are not constant and can be varied due to the interaction between drilling fluid and rock materials especially for clay-bearing rocks. Despite being aware of these factors, to date, no comprehensive study has been conducted on these issues. In order to overcome these shortcomings, the variations in mechanical properties of two clay-bearing sediments, which were saturated with different types of drilling fluids that are widely used in drilling practices, were investigated. By considering the obtained geomechanical properties, the effect of drilling fluids was compared and evaluated in terms of degradation of mechanical properties. As the occurrence of excess pore pressures that can develop under undrained conditions around a borehole is essential to borehole stability, the distribution of excess pore pressures and plastic deformations was also investigated by a finite element-based borehole simulation under drained and undrained conditions. By considering the results of experimental and numerical studies, some recommendations were presented for understanding wellbore stability.