Monitoring the width of hydraulic fractures with ultrasonic measurements J.Groenenboom A.J.W.Duijndam J.T.Fokkema

Introduction During hydraulic fracturing experiments in the laboratory the opening of hydraulic fractures is monitored with ultrasonic transducers. The experiment closely resembles seismic monitoring surveys in the field [MEADOWS AND WINTERSTEIN 1994, WILLS ET AL. 1992]. The extraction of information out of these experiments is critically dependent on the understanding of the elastodynamic behaviour of the thin fluid filled fractures. The laboratory experiments provide useful information on what determines the seismic visibility of these fractures, both for compressional and shear waves. The role of the fracture thickness or width on the elastodynamic response and a new method for monitoring fracture opening is investigated. Most theoretical approaches postulate the use of the classical boundary conditions. The void boundary condition assumes a stress free surface. The “fluid-filled” fracture boundary condition assumes vanishing shearing stresses at an infinitely thin fracture interface. Considering perpendicular incidence, in the first case all wave energy is reflected. In the second case all compressional energy is transmitted whereas all shear energy is reflected. Data examples from laboratory experiments clearly show that (for a perpendicular incident beam) the effect of the intervening fracture is an attenuation and dispersion of the transmitted wave, both for transmitted compressional (Fig.1) and shear waves (Fig.2). No pure P-wave transmission or shear-wave shadowing is seen. Although these data can be explained by modified boundary conditions which allow for a displacement discontinuity [PYRAK-NOLTE ET AL. 1990] no physical interpretation exist for the fracture compliance or stiffness that arises in this theory. Method As an idealised model the acoustic response of a flat fluid layer embedded in a rock is taken. The transmission response of a compressional wave for perpendicular incidence is a classical problem, already known to Lord Rayleigh: