The growth of a hydraulic fracture increases the period of free oscillations in a well. Simultaneously, the decay rate of free oscillations decreases. The properties of forced oscillations in a well also change during fracture growth. All of these effects result from the changing impedance of the hydraulic fracture that intersects the well. Fracture impedance can be determined directly by measuring the ratio of downhole pressure and flow oscillations, or determined indirectly from wellhead measurements using impedance transfer functions. Because impedance is a function of fracture dimensions and the elasticity of the surrounding rock, impedance analysis offers a promising new approach for evaluating fracture geometry. Because oscillatory flow conditions occur continuously a hydraulic-fracturing treatment, data collection is simple and economical, adding to the attractiveness of this technique.

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