Factors that Predict Fracture Orientation In a Gas Storage Reservoir

Abstract

The data presented here indicate that surface joints and lineaments as revealed by aerial photographs are related to the maximum horizontal stress in a region and to the orientation of induced vertical wellbore fractures. This information could help engineers to make the optimum choice of wells for fracturing, thus enhancing reservoir performance. Introduction The natural state of earth stresses, in which the vertical stress generally exceeds the horizontal stress, is a condition favoring the formation of vertical fractures. Petroleum-producing reservoirs have vertical fractures Petroleum-producing reservoirs have vertical fractures along a preferred azimuthal direction. The orientation of vertical fractures in areas of flat or gently dipping rocks may be ascertained both quickly and accurately from regional surface features. In certain areas where no apparent relationship exists between jointing and present folding or stress orientation, the jointing may be due to failure planes or directions inherited from deep-seated basement rocks. In relating surface joint trends to hydraulically induced wellbore fractures, we hypothesized that one direction of jointing in surface rocks could be related to the orientation of the maximum horizontal stress in that locality. This hypothesis, in conjunction with the presently accepted theory that hydraulically induced vertical fractures will be oriented parallel to the maximum horizontal stress and nominal to the minimum horizontal stress direction, allows us to infer the orientation of the stress field by comparing surface-orientation data with subsurface-orientation data. In some tectonic provinces, the stress-field orientation can be inferred from the structural geometry; however, stress history must be examined or great errors arise from this method. In a reservoir study at microscopic scale, investigators have detected permeability to be directional in samples from various oilfields and have indicated that the idea is applicable in primary- and secondary-recovery development. Relative primary- and secondary-recovery development. Relative to the preferred direction of fracturing, higher permeabilities were found parallel to the wellbore permeabilities were found parallel to the wellbore fracture in one Appalachian oilfield, indicating a relationship between directional permeability and fracture orientation. In a noteworthy laboratory investigation on the relationship between directional characteristics and the induced fracture orientation in sedimentary rocks, it was shown that fracture propagation was parallel to the direction defined by greatest strain-energy relief in relaxation during overcoring of strain gauges mounted in boreholes. In an effort to determine the preferred direction of fracturing in an established gas-storage reservoir, the Petroleum Div. of the Morgantown Energy Research Center initiated a program to relate microscopic rock properties with observed macroscopic reservoir fractures. properties with observed macroscopic reservoir fractures. We believe that an interdisciplinary study involving both laboratory and field experiments is the proper approach to understanding the factors influencing fracture orientation. Therefore, we have directed our research toward relating basic rock properties to present and past geologic stresses. present and past geologic stresses. JPT P. 546