Potential for hydraulically induced fracturing of the Floridan Aquifer System (FAS) and the overlying Hawthorn Group deposit exists due to operation of seven potential aquifer storage and recovery facilities planned to be developed in south-central Florida to enhance Everglades restoration. The purpose of this study was to determine critical threshold water pressures at which hydraulically induced fracturing of the FAS rock matrix may occur. Several FAS rock matrix samples were collected, tested, and evaluated to define representative mechanical properties, which were then used in relation with in situ stresses to determine critical threshold water pressures. Three hydraulically induced fracturing failure mode evaluation methods based on shear, tensile, and microfracture development were utilized. Microfracture development requires the lowest critical threshold water pressure to induce fracturing, followed by tensile and then shear failure modes. Predictive critical threshold water pressures for tensile and microfracture development failure modes can potentially be achieved during full-scale operation of the planned aquifer storage and recovery facilities; therefore, appropriate design considerations and operational precautions should be taken to minimize water pressures that exceed this operational constraint. If hydraulically induced fractures are developed in the FAS, their propagation into the Hawthorn Group deposit would likely be arrested by or re-directed along the discontinuity zone at the contact of these two deposits. Additionally, the Hawthorn Group deposit exhibits a significantly lower modulus of elasticity than the FAS, which would tend to effectively arrest hydraulically induced fracture propagation.

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