Significance of Drilling and Coring-Induced Fractures in Mesaverde Core, Northwestern Colorado

Sharon J. Finley, John C. Lorenz

Several types of mechanically induced fractures occur in Mesaverde core from the U.S. Department of Energy's Multiwell Experiment wells. Two types of fractures have significance to the determination of in-situ stresses. Scribeline fractures originate at orientation grooves on the surface of the core and were caused by scribing knives in the core barrel minutes after the core was cut free from the surrounding rock. Their strike reflects the maximum in-situ horizontal stress direction that has been determined in these wells by anelastic strain recovery measurements. Drilling-induced petal and petal-centerline fractures from the same intervals of core formed just below the advancing core bit. They have a strike that is controlled by a combination of the in-situ stress and th shear stress created by the rotation of the bit. Greater shear stress produced more rotation of the fracture strike clockwise from the known maximum in-situ principal horizontal stress orientation. The contrast in the strikes of the two types of fractures in the same intervals of core is a function of whether the core was free-standing in the core barrel or still part of the rock mass below the rotating bit at the time of fracturing. In simple situations, orientations of these types of fractures may be used to determine the relative orientation between the in-situ stress and natural fractures, and ultimately whether a stimulation fracture will parallel or intersect natural fracture permeability systems in a reservoir.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.