HR: 0800h
AN: H41B-0416    [Abstracts]
TI: On Mechanisms of Hydraulic Fracturing in Cohesionless Materials
AU: * Hurt, R S
EM: gtg108p@mail.gatech.edu
AF: Georgia Institute of Technology, School of Civil and Envromental Engineering, Atlanta, GA 30332-0355 United States
AU: Wu, R
EM: gte755q@mail.gatech.edu
AF: Georgia Institute of Technology, School of Civil and Envromental Engineering, Atlanta, GA 30332-0355 United States
AU: Germanovich, L
EM: leonid@ce.gatech.edu
AF: Georgia Institute of Technology, School of Civil and Envromental Engineering, Atlanta, GA 30332-0355 United States
AU: Chang, H
EM: hongchang@gmail.com
AF: Georgia Institute of Technology, School of Civil and Envromental Engineering, Atlanta, GA 30332-0355 United States
AU: Dyke, P V
EM: Peter-Erik@Van-Dyke.Net
AF: Pinnacle Technologies, Inc., Park North Technology Center 219 Airtex Boulevard, Houston, TX 77090-6627 United States
AB: Based on the developed experimental techniques, hydraulic fracturing in particulate materials has been directly observed in the laboratory. We have conducted an experimental series by varying such controlling parameters as the properties of particulate materials and fracturing fluids, boundary conditions, initial stress states, and injection volumes and rates. As a result, we suggested some (hopefully, fundamental) mechanisms of hydraulic fracturing in particulate materials and determined some (hopefully, relevant) scaling relationships (e.g., the interplay between elastic and plastic processes). While the ongoing work is likely to change at least some conclusions, it is important that the results reported in this paper appear to form the framework for modeling and, perhaps, even for (qualitative) interpretation of some field data. The main conclusion of our work is that hydraulic fracturing in particulate materials is not only possible, but even probable if the fluid leakoff is minimized (e.g., high flow rate, high viscosity, or low permeability). The observed fracture geometry and the measured pressure injection curves suggest that hydraulic fracturing occurs in soft sediments in the following sequence: (i) cavity expansion before the injection pressure reaches its peak; (ii) fracture front initiation from the expanding cavity near the pressure peak; and (iii) propagation of the developed fracture after the peak. Another important conclusion of our work is that all parts of the particulate material (including the tip zone of hydraulic fracture) are likely to be in compression. The compressive stress state is an important characteristic of hydraulic fracturing in particulate materials with low, or no, cohesion (such as were used in our experiments). For the fracture initiation at the peak pressure (i.e., following the initial cavity expansion), there exists a threshold value of cohesion that results in compressive stresses everywhere in the particulate material. For less cohesion, the cohesive materials can be considered to be effectively cohesionless. Three main types of fracture fronts were observed in our laboratory experiments: round, beveled and fingered. Accordingly, three physical mechanisms of fracture propagation corresponding to the three observed front types were suggested. These are ``pile driving'' or cavity expansion, shear banding, and induced cohesion, which appear to be consistent with round, beveled, and fingered fracture fronts, respectively. It is important to emphasize the importance of leakoff effect on hydraulic fractures in particulate materials. In our experiments, fractures appear rather different compared to those in the no-leakoff case. The fluid leakoff region manifests itself as a (bubbly) layer around the fracture, which thins towards the fracture tip, which is similar to brittle fractures. Furthermore, even the tip details of these fractures remarkably resemble cracks in brittle materials. Therefore, currently available experimental observations suggest that even small leakoff may change the fracture pattern rather dramatically. For example, while at the stage of fracture initiation, the cavity expansion mechanism may still be important, the mechanism of induced cohesion may prevail at the developed stage of fracture growth.
DE: 1899 General or miscellaneous
SC: Hydrology [H]
MN: Fall Meeting 2005