The Geometry of a Large-Scale Nitrogen Gas Hydraulic Fracture Formed in Devonian Shale: An Example of Fracture Mapping With Tiltmeters

Ground deformations occurring around a well undergoing hydraulic fracturing stimulation are analyzed to obtain a description of the geometry and the development of the resulting fracture. Significant upward and lateral growth of a near-vertical fracture aligned with the proposed direction of maximum tectonic compression is inferred to take place initially. However, after six-tenths a the nitrogen gas fracturing fluid had been injected, fracture growth in a horizontal plane abruptly began, and vertical growth ceased. No indication of this breakout is evident in the wellhead pressure or flow rate records. The state of stress in the vicinity of the well is estimated and implications of the inferred fracture development behavior are discussed. Introduction Production enhancement of gas from wells draining low-permeability formations such as the Devonian shale series of the eastern U.S. commonly is attempted by use of hydraulic fracturing. The problem of designing such fractures to optimize the resulting yield has underlined the need for a more thorough understanding of the fracture process. To this end, several wells penetrating Devonian shale have been selected for intensive study during the fracturing treatments. The studies involve application of a wide variety of diagnostic technologies capable of supplying information pertinent to establishing the behavior of the fracture during the treatment and the principal factors that influence this behavior. This paper presents the results of applying the tiltmeter technique of fracture mapping to determine geometrical characteristics of the fracture resulting from injection of nitrogen gas into the 335 m (1,100 ft) deep Black No. 1 well, which penetrates the Devonian shale in Knox County, OH (Fig. 1). During treatment, which lasted from 16:47 to 17:15 hours (local time) on Oct. 22, 1979, a total of 2.59 × 104 std m3 (9.68 × 105 scf) of nitrogen gas at a wellhead temperature of 46 deg. C (115 deg. F) and pressure of 9.0 MPa (1.300 psi) was administered to the well. No proppant was used. The well was offered to the research program by its owners after production had declined from an initial 8.6 × 102 std m3/d (3.2 × 104 scf ft/D) after drilling in 1975 to essentially zero by the end of 1977. During this period of production, the well was openhole for the entire Devonian shale section (Fig. 2) and had suffered neither explosive nor hydraulic stimulation treatments. It is not known whether the bulk of production came from the shale matrix or from sets of high-angle natural fractures that intersected the well. Before the experiment, the well was recased as shown in Fig. 2. Other diagnostic technologies involved in the stimulation experiment included borehole seisviewer imagery, extensive logging and coring, lineament analysis, downhole television, nitrogen analysis, and a downhole three-component geophone systems which, like the tiltmeter array, was operated continuously throughout the treatment period. The results of these analyses are cited freely to support the interpretation presented. Theory The underlying principle of the tiltmeter technique is to determine the deformation field at the earth's surface that results from growth of the fracture structure at depth. SPEJ P. 755^