Hydraulic fracturing of uncemented sand was investigated with theoretical calculations and physical laboratory experiments to investigate crack tip mechanics for soft rock applications. The mechanics of soft rock fracturing is an important topic for the better understanding of injection operations in weakly or unconsolidated sands. Fracpack treatments, waste injection, and steam injection for heavy oil recovery are all applications for which the application of standard elastic hydraulic fracturing models may be inadequate due to expected nonelastic deformation components.

Analytical calculations indicate that shear associated with hydraulic fracturing is likely in unconsolidated sands at depth, but cases of high overpressure promote tensile mode fracturing. An expression is derived to predict the rock strength required in terms of UCS to achieve tensile fracture. Preliminary physical experiments were performed to ascertain failure mode in uncemented sand at low confining stress and to investigate shear failure under high viscosity injection. Physical specimens were created using sand ranging from 100 to 300 micron in diameter, compacted with wet vibration. Preliminary fracture opening experiments performed using solid wedging were imaged with a CT-scanner and demonstrated near-tip shear failure propagating diagonally away from the crack tip, but not along the in-plane propagation direction. Injection experiments indicated complex fracture propagation geometries involved fracture plane segmentation and curving. Preliminary results indicate fracture propagation mode is predominantly tensile and not shear, although deformation in the vicinity of the hydraulic fracture showed evidence of shear. Higher stress anisotropy is anticipated to increase the likelihood of shear failure over tensile failure.

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