Analysis of factors influencing fracture initiation and orientation in oil reservoir sandstone : Komar, CA Frohne, KH 11F. US Bur. Mines, RI 7813, 1973, 22P

Abstract

Hydraulic fracturing has developed into a widely applied technique for increasing production from petroleum and natural gas reservoirs. Although more than a million wells have been fractured, the degree of understanding of the mechanics of hydraulic fracturing is still incomplete. Laboratory-scale hydraulic fracture tests were conducted on directionally oriented sandstone material, along with measurements of directional rock properties such as tensile strength, permeability, ultrasonic velocity, and dynamic elastic constants. Fracture orientation was determined as a function of various combinations of stress, rock saturation, rate of borehole pressurization, and direction of minimum tensile strength relative to minimum horizontal compressive stress. The objective was to investigate whether horizontal stresses always controlled the orientation of induced vertical hydraulic fractures, or if under conditions of small differences in stress and/or slow borehole pressurization rates, the induced fracture direction was influenced by directional properties of the rock matrix. This study shows that the expected influence of horizontal earth stress on the orientation of an induced hydraulic fracture becomes negligible whenever the magnitude of the difference between the compressive stresses is 200 psi or less. When this occurs, fracture orientation is controlled by the directional properties of the rock--primarily permeability and tensile strength. (auth)