Utilization of mud weights in excess of the least principal stress to stabilize wellbores: Theory and practical examples

In this paper, we address the theoretical possibility of drilling with mud weights in excess of the least principal stress for cases of particularly high pore pressure or severe wellbore instability. Because lost circulation caused by hydraulic fracturing is to be avoided, we consider three critical wellbore pressures, P_frac, P_link, and P_grow. Tensile fractures initiate at the wellbore wall at Pfrac, link up to form large axial fractures that are subparallel to the wellbore axis at P_link, and propagate away from the wellbore at P_grow. It is obvious that lost circulation cannot occur if the wellbore pressure during drilling is below P_frac. However, even if P_frac is exceeded and tensile fractures are initiated at the wellbore wall, fracture propagation (and, hence, lost circulation) will be limited as long as the wellbore pressure is below P_link. Finally, if the wellbore pressure is greater than P_link, the fractures will not grow away from the wellbore (and significant lost circulation will not occur) if the wellbore pressure is below P_grow, which must exceed (if only slightly) the least principal stress. In general, our modeling shows that P_frac and P_link can be maximized by drilling the wellbore in an optimally stable orientation, and P_grow can be maximized with noninvading drilling muds that prevent fluid pressure from reaching the fracture tip. We apply the model that uses in-situ stress data collected in real fields, such as the South Eugene Island field in the Gulf of Mexico and the Visund field in the northern North Sea.