Delineating a Subsurface Fracture System in A Petroleum Reservoir-An Experiment
Authors: Komar C.A. , Shuck L.Z. , Overbey W.K. , Anderson T.O. 
Published: December 2007

The use of a multisensor array has shown that hydraulic pressurization can be useful in delineating subsurface fracture systems in petroleum reservoirs. It can help in establishing effective patterns of injection and production wells to avoid premature waterfront break-through, and thus increase sweep efficiency and oil recovery in flooding operations.

Introduction

Well stimulation by hydraulic fracturing is often a prerequisite to effective oil recovery by prerequisite to effective oil recovery by waterflooding, but acceptance of this idea has been inhibited by fears that fractures would permit channeling of the displacing fluid and bypassing of oil. Donahue et al. conducted model studies of a five-spot pilot flood and found that different orientations of the pattern significantly affected the sweep efficiencies at pattern significantly affected the sweep efficiencies at breakthrough (Fig. 1). Others report that consideration must be given to the type of fracture to be formed and to the presence of natural fluid conduits in the subsurface formation of the reservoir. Hubbert and Willis, Heck, Kehle, and Dunlap report that induced fractures will be vertical and will extend in a compass direction usually dictated by the earth stresses. Their studies show that the most probable direction that will be assumed by induced fractures can be estimated with some accuracy from a study of the principal horizontal tectonic stresses. However, communication between wells or between zones and the direction of magnitude of natural fracture trends must also be delineated . The pattern of injection and production wells can then be established to make maximum economic use of the subsurface natural and induced fracture system. For the Appalachian area, considerable data have been obtained from surface studies of joints and lineaments on the direction preferred by induced hydraulic fractures and on orientation predictions. This seems to confirm the work of Heck who concluded earlier that reservoir rocks and surface rocks are jointed similarly, except that the subsurface joints are relatively closed and exist as planes of weakness. We shall describe here a field experiment designed to establish a technique for delineating the subsurface fracture system in a petroleum reservoir. Essentially, the procedure employed is an interference test that was modified by the addition of a multisensor array of pressure transducers to measure responses simultaneously and at considerable distances from the test well. The test site is adjacent to leases where joint and lineament strikes, directional permeability, and the orientations of induced hydraulic fractures were measured in the manner described by Anderson and Stahl. We shall describe the instrumentation and tests that were employed, and interpret the fracture trends from this field experiment.

The Bradford Test Site Description and Location

A 36-acre tract about 10 miles east of Bradford, Pa., was made available for this research by the Minard Run Oil Co. (See Fig. 2.) Open-hole completion practices were used and considerable subsurface data were practices were used and considerable subsurface data were available in the form of cores, electric logs, and wellbore impression-packer surveys. The orientation of induced hydraulic fractures was compared with surface joint measurements.