Understanding and modeling near-surface hydraulic fracture growth is of interest because fracturing is being used to induce caving near mine openings and for remediation work at shallow environmental waste or spill sites. Thus motivated, near-surface hydraulic fracturing experiments were performed in Polymethylmethacrylate (PMMA). The fractures were driven under conditions such that the internal fluid pressure may be assumed uniform, that is, propagation was in the so-called toughness-dominated regime. As the fractures extend they interact with the free surface and grow towards it, producing a bowl-shaped fracture that eventually daylights at the surface. Injection pressure, fluid injection rate, fracture radius, and surface displacement were monitored during each test. Additionally, an experimental technique that is based on the Beer-Lambert law of light absorption was developed which enables measurement of the full-field opening of hydraulic fractures in transparent materials. The experimental results for opening and radius compare within 10% of published numerical results - which ignore the fracture curving effect - for a toughness-dominated hydraulic fracture. There is greater than 30% difference between the data and published model results for the injection pressure, and reasons for this discrepancy are discussed. Tests were performed with between zero and 12 MPa of radially-directed confining stress. Examination of the shape of the resulting fractures suggests that the radius of the fracture when it daylights is three times the initial depth when radial confinement is zero. The fracture shape data together with scaling considerations suggest a simple empirical relationship whereby the daylighting fracture radius increases with the magnitude of the radial confinement. The findings of this study, in addition to providing guidelines applicable to field design of near-surface fractures, also clearly expose the successes and areas for improvement associated with recent modeling efforts. The paper concludes with a discussion of ongoing research directed at developing a more complete mechanistic model of near-surface hydraulic fracture growth.
1. INTRODUCTION
Hydraulic fracturing in near-surface conditions, that is, where the fracture radius is on the same order as the depth of the fracture, has found a number of industrial and scientific applications. Nearly one century ago, hydraulic fractures were used to induce horizontal sheeting in granite quarrying operations [1,2]. More recently, hydraulic fractures have been used in mining operations to induce caving [3] and/or precondition rock masses for caving [4]. Hydraulic fractures have also been used to stimulate contaminant recovery and to form barriers to contaminant transport in environmental remediation projects [5,6,7]. Furthermore, it is thought that hydraulic fracture is an important mechanism in a number of near-surface geophysical processes [1,8].
In all of these cases, the behavior of the fracture is heavily influenced by the nearby free surface. Hence, these fractures exhibit some unique characteristics. Among these features is the tendency of the fracture to curve, thus forming a bowl shape (Fig. 1a). Another feature is the increase in fracture compliance as the radius increases with respect to the depth. Quantitative understanding of these sorts of features is imperative to near-surface hydraulic fracture analysis and design. This has motivated some recent contributions.
