HR: 0800h
AN: H11B-0492    [Abstracts]
TI: HYDRAULIC FRACTURING IN SATURATED COHESIONLESS MATERIALS
AU: Germanovich, L N
EM: leonid@ce.gatech.edu
AF: Georgia Institute of Technology, School of Civil and Environmental Engineering 790 Atlantic Drive, Atlanta, GA 30332-0355, United States
AU: * Hurt, R S
EM: r.hurt@gatech.edu
AF: Georgia Institute of Technology, School of Civil and Environmental Engineering 790 Atlantic Drive, Atlanta, GA 30332-0355, United States
AU: huang, h
EM: haiying.huang@ce.gatech.edu
AF: Georgia Institute of Technology, School of Civil and Environmental Engineering 790 Atlantic Drive, Atlanta, GA 30332-0355, United States
AB: Based on the developed experimental techniques, hydraulic fracturing in particulate materials has been directly observed in the laboratory. As a result, we suggested several mechanisms of hydraulic fracturing in particulate materials and determined 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 work appear to form the framework for modeling and, perhaps, even for (qualitative) interpretation of field data. 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, (ii) fracture front initiation, and (iii) propagation of the developed fracture. Our experiments show that liquid can indeed propagate as a crack-like feature when injected into cohesionless saturated materials. Laboratory observations suggest that at the initial stage, the cavity expansion process ends with fracture initiation. Sometimes, the growing fracture resembles penetration of one movable material into another less movable material, which may be a manifestation of the Taylor-like instability. An important conclusion of our work is that all parts of the cohesionless 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). At present, two kinematic mechanisms of fracture propagation, consistent with the compressive stress regime, 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 tensile strain ahead of the fracture tip and reduction of the effective stresses to zero within the leak-off zone. Additionally, an important characteristic feature of fractures in our experiments is the bluntness of the fracture tip, which suggests that plastic deformation at the fracture tip is important. Scaling indicates that fluid pressure does not decrease considerably along the fracture, due to the wide fracture aperture. However, there is a high pressure gradient in the leak-off zone in the direction normal to the fracture. Scaling also suggests the importance of fluid leaf-off in the cavity expansion and, possibly, in the fracture propagation process. First estimates show that large openings at the fracture tip correspond to large fracture energy, an order or two greater than for typical rocks. Unfortunately, it is not currently clear what defines the characteristic dimension at the process (tip) zone, which does not allow devising a comprehensive theoretical model. Without a model, it is not clear how to estimate an in-situ value of the fracture energy (or the corresponding value of the effective fracture toughness), that is, how to "extract" the fracture energy from available data of hydraulic fracturing tests or observations on natural hydraulic fractures (e.g., sand dikes propagated through unconsolidated sediments). However, it may still be possible that the fracture in field conditions is similar to that in conventional cohesive materials. Since what have been observed so far does not contradict to the condition of autonomity at the fracture tip (front), a tip-scale (local) fracture criterion still may be feasible to develop.
DE: 1699 General or miscellaneous
SC: Hydrology [H]
MN: 2007 Fall Meeting