Significance of Coring-Induced Fractures in Mesaverde Core, Northwestern Colorado (1)

Abstract

Several types of mechanically induced fractures occur in Mesaverde core from the U.S. Department of Energy's Multiwell Experiment wells. Two of these types of fractures have significance for the determination of in-situ stress orientations. Scribe-line fractures originate at orientation grooves on the surface of the core and are initiated by scribing knives in the core barrel minutes after the core is cut free from the surrounding rock. In sandstone core, these fractures strike parallel to the maximum in-situ horizontal stress direction that has been determined in these wells by anelastic strain recovery measurements. Coring-induced petal and petal-centerline fractures form just below the advancing core bit. Their strike is also controlled by the in-situ stress field. Loc lly, however, strike may be secondarily influenced by shear stress created by bit rotation, reorienting petal fracture strikes in the direction of rotation. Nevertheless, in many situations the orientations of these two types of fractures may be used to determine the orientation of the horizontal in-situ stress field if the core is oriented. Even in unoriented core, the relative orientation between in-situ stress and natural fractures may be determined.