Forms of Hydraulic Fractures in Shallow Fine-Grained Formations

Abstract

Hydraulic fractures have been created in fine-grained formations at depths of 2–10 m to improve the performance of environmental remediation projects at dozens of locations and in a wide range of geologic conditions. The effectiveness of a hydraulic fracture during remediation will depend primarily on its form; that is, its shape, thickness, orientation, length, width, and location with respect to the borehole. The forms of many hydraulic fractures have been determined by mapping exposures in excavations and by compiling split-spoon sampling data. These observations indicate that a typical hydraulic fracture at shallow depths is gently dipping, slightly elongate in plan, and slightly asymmetric with respect to the parent borehole. Shallow hydraulic fractures lift the ground surface to produce gentle domes, and the pattern of uplift reflects the location and thickness of the fracture at depth. The forms of hydraulic fractures created over a narrow range of depths at the same site are similar, but the forms can vary markedly between sites and at different depths at the same site. This indicates that the forms of hydraulic fractures will be relatively consistent when they are created under similar conditions, but changes in geologic conditions can markedly affect fracture form.