Classification Of Drilling-induced Fractures And Their Relationship To In-situ Stress Directions

Natural and drilling-induced fractures that can be identified on borehole walls are classified in terms of the failure initiation mechanisms that generate them and the principal stress orientations with respect to well trajectory. Fracture initiation mechanisms include tensile failure in vertical holes, tensile failure in inclined holes, tensile failure in elliptical boreholes, tensile failure resulting from plastic deformation, shear failure in vertical boreholes subject to low fluid pressures, shear failure in vertical boreholes subject to high fluid pressures, shear failure in inclined boreholes subject to low fluid pressures, shear failure in inclined boreholes subject to high fluid pressures, and failure in boreholes subject to horizontal stresses of equal magnitudes. Analyzing these situations assuming linear elasticity shows that the geometry of fracture traces on borehole walls depends on the in-situ stress tensor, the relative orientations of the stress tensor and the borehole, the fluid pressures in the borehole and in the surrounding rock, the rock''s cohesive strength, and its friction angle for shear. Furthermore, hydraulically induced tensile fractures are governed by the least principal stress, but shear fractures are governed by the intermediate and the least principal stresses, contrary to previous conclusions. A classification based on the fracture initiation analysis is presented and is illustrated by examples of different types of borehole wall fractures recorded by image logging tools run in wells in western Canada.