On the Development of Shallow Hydraulic Fractures as Viewed Through the Surface Deformation Field: Part 2-Case Histories
Authors: Evans Keith , Holzhausen Gary 
Published: December 2007

Abstract
Five case histories are presented wherein aspects of the broad-scale development of shallow in-situ hydraulic fractures are deduced using the surface tiltmeter technique. The results show that prolonged periods of predominantly in-plane extension of the fracture predominantly in-plane extension of the fracture structure are prone to interruption from sudden changes in orientation of the plane of fracture growth. Fracturing pressure records may or may not provide an indication pressure records may or may not provide an indication that breakout has occurred. Mean fracture widths during treatment are generally in the range of 0.6 to 4.0 mm (0.025 to 0. 16 in.) and reflect the viscosity of the fracturing fluid. On termination of injection, fractures formed with proppant-free fluid invariably showed evidence of self-propping. Closures were usually 85 to 95% complete. Fractures formed with proppant-laden fluid, however, exhibited closures of less than 50% and showed residual propped volumes many times the volume of proppant injected.

Introduction
The surface tiltmeter method of hydraulic fracture mapping has been available as a commercial service to the energy industry since 1978. The method has advantage over other techniques in that it is sensitive to the broad-scale geometry of fractures rather than being limited to near-wellbore fracture detection. It is particularly well-suited to the study of shallow is less than 457-m ( is less than 1,500-ft) fracture growth and shut-in consolidation characteristics where the high signal-to-noise ratios obtained enable the time history of the evolving surface deformation to be recovered from the data. From this it is possible to reconstruct, albeit to a limited degree, the time history of fracture development. Mean width of the fracture also can be estimated from the amplitude of the surface deformations. In this paper we review a number of case histories pertaining to shallow fracture growth and consolidation upon shut-in as viewed through the associated surface deformation. The resulting descriptions of fracture development behavior are compared with data derived from other fracture-mapping technologies (e.g., wellhead pressure and acoustic emission), where such supplemental data exist, to evaluate consistency. Many, although not all, of the data discussed have been published in the form of contractural reports, and readers published in the form of contractural reports, and readers interested in obtaining copies should contact Fracture Technology Inc. (previously M.D. Wood Inc.). A discussion of the models used in the interpretation of the data and the limitations of the technique are presented in Part 1 (see Page 406) and are not repeated here. Part 1 (see Page 406) and are not repeated here. Models
Three analytic models have been used, as appropriate, in deriving the results discussed. All three represent the fracture as a planar discontinuity in the displacement field within a homogeneous, isotopic linear-elastic half-space of known Poisson's ratio. The most versatile is based on the three-dimensional dislocation model of Davis in which the fracture is represented by an opening mode slit of arbitrary dip that is rectangular in both cross-sectional and plane views.
JPT
P. 411