Form of Hydraulic Fractures

The effectiveness of hydraulic fractures created for remediation purposes is dependant on resulting fracture form. Form refers to the shape, thickness, orientation, length, width, and location of hydraulic fractures with respect to the borehole. In general, all induced fractures have the same sheet-like shape, with apertures (thicknesses) that are typically less than 1% of the maximum dimension. The final shape and orientation of the hydraulic fracture is dependent upon depth, presence of surface structures, formation geological parameters, and injection methodology. FRx has created and characterized the forms of hydraulic fractures in a wide range of conditions and geologic materials. The knowledge that we have gained is effectively exploited to best achieve the purposes of hydraulic fracturing services.

The Major Control: In situ state of stress within the formation is the dominant control on ultimate fracture form. Vertical in situ stress is typically the weight of the overburden, whereas lateral in situ stress results from tectonic activity and a variety of other geologic processes. The fracture will propagate in the plane that is perpendicular to the least principal in situ stress, i.e. it will overcome the least resistance. Essentially the fracture plane will demonstrate whether it is easier to jack-up the overburden (horizontal fracture plane) or compress the formation laterally (vertical fracture plane). Weight of overburden increases with depth, whereas lateral stress may not vary systematically. Accordingly, fractures created at shallower depths tend to follow a more horizontal plane and fractures created at greater depths tend follow a more vertical plane.

Typical Fracture Form: Research projects involving fracture excavation and/or characterization by soil coring have provided detailed information about hydraulic fracture form. The majority of fractures created in fine grained materials at depths appropriate for most environmental applications (1 to 30 ft) tend to display a characteristic fracture form. In plan view, the typical fracture form is an ellipse with an aspect ratio of 1.2:1 and a centroid that is shifted some distance from the point of initiation. The plane of the fracture tends to gently dip (10 to 30°) towards the point of initiation. The resulting proppant-filled aperture of a hydraulic fracture is typically thickest near the point of injection, however, the maximum can also occur away from the point of injection as shown in the example figure.

Figures showing the 3 dimensional shape (above) and aperture distribution (right) of an excavated hydraulic fracture. From Richardson, 2003.

Fracture Aperture