REACTIVATION OF A HYDRAULIC FRACTURE IN PERMEABLE ROCK
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The problem of massive fluid injection into a pre-existing fracture has many applications in petroleum industry including underground liquid waste disposal (e.g., hydraulic fracturing wastewater, supercritical CO2) and waterflooding to increase the hydrocarbon recovery from a reservoir. Understanding the conditions leading to the reactivation of pre-existing fractures and ensuing propagation is critical for a successful injection project design, and it may also help to mitigate potential environmental hazards, such as contamination of underground aquifers and induced seismicity. Extensive analytical and numerical studies are carried out to quantify the transient pressurization and breakdown of a pre-existing fracture due to a fluid injection in the context of unconventional hydraulic fracturing when the fluid diffusion can vary over wide range of scales from 1-D to 2-D or 3-D. We establish the range of the problem parameters and injection time when the fluid pressure is approximately uniform along the crack. The pressure uniformity assumption allows for a simpler and more accurate solution method (based on the Green’s function approach). As the fracture is pressurized, the condition for the onset of propagation (the breakdown condition) is eventually reached. We quantify how the breakdown depends upon the problem parameters, such as fluid and rock properties, the in-situ stress, and the fluid injection rate. The poroelastic effects on the transient pressurization and initiation of the fracture in a critically over-pressured reservoir (i.e., when the initial reservoir pore pressure p_0 is approximately equal to the minimum confining stress sigma_0) are also investigated. We show that the poroelastic effects will substantially delay the breakdown time compared to the non-poroelastic case when the fracture breakdown occurs at later stages of injection characterized by large-scale (2D) pore pressure perturbation in the reservoir. Finally, we extend the analysis to the transient pressurization of multiple fractures. The results of this study are transportable to the production well test analysis of a fractured well. The history of the transient pressurization prior to breakdown can also be used to provide the initial conditions for the fracture propagation problem.