Development of the polyaxial hydraulic fracturing test under realistic in-situ conditions is a part of an ongoing joint research program conducted at Shell EPT /UIC/CSM. This testing is important for understanding the effects of material and treatment parameters upon fracture mechanisms and crack growth. However, the fractographic analysis of the hydraulically fractured specimens indicates that the progression of damage during these tests cannot be fully reconstructed from the post-mortal analysis. In addition, it is a time-consuming and expensive program. This consideration motivates the development of a small-scale uniaxial and bi-axial pilot test which could supply more adequate information regarding the hydraulic fracture mechanisms and can be used for selection of polyaxial test conditions. A new specimen geometry, i.e., Edge Cleavage Specimen (ECS), was developed for small-scale testing of rock. A special loading device used with the specimen enables a stable crack propagation which is critical for studies of micromechanisms of fracture growth pertinent to hydraulic fracture. The ECS test geometry permits video recording of the crack growth process via a long-range microscope in addition to measurements of the fracture length and Crack Opening Displacements (in two locations). Therefore, it allows for a complete quantitative fracture characterization. The ECS geometry can also accommodate a bi-axial loading applied to the specimen boundaries. This paper presents the results of ECS testing of porous rocks under various loading conditions (ramp/fatigue, uniaxial/biaxial loading) and bedding planes orientation with respect to the loading direction(s). The observed micromechanisms of fracture and critical parameters such as critical crack depth and fracture opening are discussed.
The development of laboratory polyaxial hydraulic fracturing tests under realistic in-situ conditions is part of an ongoing joint research project between Shell, UIC, and CSM. This research is focused on identifying various fracture mechanisms that may affect and control the growth of large-scale hydraulic fractures in the field. This objective, which clearly separates this project from many other laboratory investigations of hydraulic fracturing, requires micromechanical characterization of hydraulically fractured blocks. Originally, the development of micromechanical fracture characterization techniques was the main task to be performed in the Fracture Mechanics Laboratory at UIC. However, initial studies of several hydraulically fractured blocks from polyaxial tests and cylindrical conventional triaxial compression test specimens showed that the progression of damage during these tests could not be determined from post-mortal analysis. In addition, material failure in triaxial tests occurs under stress conditions quite different from those in hydraulic fracturing. It is a very difficult task to monitor the fracture progression in the polyaxial loading frame. Monitoring techniques are presently being developed (Hand and Glaser, 1996) and when ready will still require some calibration, i.e., independent means of observation. The same limitations are present in observations of failed triaxial compression specimens. This consideration motivates the development of small-scale uniaxial and bi-axial pilot tests which could supply more adequate information regarding fracture tip mechanisms and are better suited for defining polyaxial test conditions. A second motivating factor concerns the proper material selection for polyaxial experiments.
