Longitudinal and transverse fractures were observed in two laboratory hydraulic fracturing experiments under similar loading conditions. In-depth data analyseshowed that the difference might be due to the secondary stress field induced by the packers. A three-dimensional, non-linear module from a comn?rcial numerical simulator (ABAQUS) was used to model the stress field in the tested blocks. It is found that when the packer is working properly, it transfers tensile stress concentration from the packer edge to the central section of the sealed borehole and forms a longitudinal stress concentration band. This band induces a longitudinal fracture initiated from the wall of the sealed section. On the other hand, if the packer malfunctions, high tensile stress concentrations will be induced at its edges. As a result, circular tensile stress concentration bands form which eventually initiate transverse fractures.
Packers are widely used in both field hydraulic fracturing (I-IF) operations (Brown et al. 2000) and laboratory experiments (Guo et al. 1993, Morita et al. 1996, Wilson et al. 1999). The primary function of packers is to seal the pressurized section from the rest of the borehole.
There have been many investigations on the influence of packers. While most of them were concemed with field applications, some involved topics related to laboratory work. yon Schoenfeldt & Fairhurst (1969) were the first to mention that the stress field in the borehole would be influenced by the type of packer, though no quantitative results were given in their paper. Using a 2-D FEM simulator, Roegiers et al. (1973) investigated the distribution of longitudinal and circumferencial stresses in the borehole near a packer. Both influences of the packer rigidity and the steel mandrel length were studied. Ong (1994) investigated the function of different packers on laboratory tests of inclined boreholes and developed an epoxy to backup the packers located in the borehole, limiting their inherent deformation.
In a recent experimental study, longitudinal and transverse fractures were observed in two different laboratory tests, which were run under the same conditions except for the packer length. The purpose of those HF experiments was to investigate the fracture initiation and propagation in asymmetrical stress field situations. The one with a longer packer showed normal fracturing behavior, which means the fracture initiated on the borehole wall in the sealed section and propagated in the direction of higher stresses (Scott et al. 2000). On the other hand, the one with a shorter packer showed transverse fracturing behavior, that is, the fracture initiated at the end of the packer and propagated in the direction perpendicular to the borehole.
Due to the complexity of the applied stress, the previously mentioned 2-D FEM modeling could not fully explain the results. In order to understand these phenomena, 3-D FEM numerical modeling using ABAQUS was carried out.
