Experimental Study of Hydraulic Fracturing in Unconsolidated Materials
Authors: Germanovich L. N. , Hurt R. S. , Ayoub J. A. , Siebrits E. , Norman W. D. , Ispas I. , Montgomery C. 
Published: June 2014

In this work, we argue that resistance (apparent toughness) to fracture propagation is an inherent characteristic of cohesionless particulate materials. We developed experimental techniques to quantify the initiation and propagation of fluid-driven fractures in saturated particulate materials. The fracturing liquid is injected into particulate materials, where the fluid flow is localized in thin, self-propagating, crack-like conduits. By analogy, we call these conduits ‘cracks’ or ‘hydraulic fractures.’ The experiments were performed on three particulate materials – (1) fine sand, (2) silica flour, and (3) their mixtures. Based on the laboratory observations and scale (i.e., dimensional) analysis, this work offers physical concepts to explain the observed phenomena. The goal of this study is to determine the controlling parameters of fracture behavior and to quantify their effects.

When a fracture propagates in a solid, new surfaces are created by breaking material bonds. Consequently, the material is in tension at the fracture tip. In contrast, all parts of the cohesionless particulate material (including the tip zone of the hydraulic fracture) are likely to be in compression. In solid materials, the fluid front lags behind the front of the propagating fracture. However, for fluid-driven fractures in cohesionless materials the lag zone is absent. The compressive stress state and the absence of the fluid lag are important characteristics of hydraulic fracturing in particulate materials with low, or negligible, cohesion. At present, two kinematic mechanisms of fracture initiation and propagation, consistent with both the compressive stress regime and the absence of the fluid lag, can be offered. The first mechanism is based on shear bands propagating ahead of the tip of an open fracture. The second is based on the reduction of the effective stresses and material fluidization within the leakoff zone at the fracture tip.

Our experimental results show that the primary factor affecting peak (initiation) pressure and fracture aperture is the magnitude of the confining stresses. The morphology of the fracture and fluid leakoff zone, however, changes significantly not only with stresses, but also with other parameters such as flow rate, fluid rheology, and permeability. Typical features of the observed fractures are multiple offshoots (i.e., small branches, often seen only on one side of the fracture) and the bluntness of the fracture tip. This suggests the importance of inelastic deformation in the process of fracture propagation in cohesionless materials. Similar to solid materials, fractures propagate perpendicular to the least compressive stress.

Scaling indicates that in the experiments performed in the regime of limited leakoff (i.e., the thickness of the leakoff zone is much smaller than the fracture length); there is a high-pressure gradient in the leakoff zone in the direction normal to the fracture. Fluid pressure does not decrease considerably along the fracture, however, due to the relatively wide fracture aperture. This suggests that hydraulic fractures in unconsolidated materials propagate within the toughness-dominated regime. This is the main conclusion of our work. In addition, the theoretical model of toughness-dominated hydraulic fracturing can be matched to the experimental pressure-time dependences with only one fitting parameter. Scale analysis shows that large apertures at the fracture tip correspond to relatively large 'effective' fracture (surface) energy, which can be orders of magnitude greater than typical for hard rocks.

In this work, we present a comprehensive experimental development focusing on four main parameters: (1) confining stresses, (2) fluid rheology, (3) injection rate, and (4) permeability. Another important conclusion is that the primary parameter in determining the peak injection pressure is that of confining stresses.