Fracture initiation from oval boreholes

Growing evidence indicates that wellbore in hydrocarbon reservoirs exhibits an anisotropic shape. In this study, the dual strength and toughness criterion is used to study the fracture initiation characteristics from oval wellbores subject to a far-field stress field. First, a critical value of the ellipse shape factor, defined as the length ratio between the difference over the summation of the two semi-axes, is derived under the condition that the fracture nucleation site is random on the oval circumference and the corresponding initiation pressure is solely dependent on the remote mean stress and the tensile strength. If we specified the length of the semi-minor axis, the results show that the critical ellipse shape factor at a given stress field can be used to divide the borehole shapes into two classes based on the fracture nucleation site. If the elliptical wellbore is enclosed inside the critical ellipse, the fracture is initiated along with the maximum principal stress; otherwise, the fracture is initiated along the wellbore elongation direction perpendicular to the maximum principal stress. Moreover, the initiation pressure and fracture nucleation size show completely different varying trends between these two cases. Next, the fracture initiation for an ellipse-like wellbore shape, modified by an exponent from the ellipse, is considered. Of interest, for the ellipse-like wellbore outside its counterpart ellipse, the fracture nucleates in an inclined direction within one quadrant circumference and the four nucleated fractures exhibit an X-shape. As the semi-axis lengths are fixed, with increasing the exponent, the nucleation site renders an asymptotic angle determined by two semi-axis lengths. With an increase of the remote stress ratio, when the ellipse-like wellbore is enclosed inside an ellipse, the fracture nucleation site tends to align with the principal stresses, and otherwise, it shifts clock-wisely within a narrow range.