A method is decribed where microseismic mapping of hydraulic fractures is improved by using crosswell data to calibrate and/or verify dipole-sonic velocity data. The microseismic technique uses an array of advanced tri-axial receivers to detect microearthquakes induced by a fracture and, thus, provides a real-time image of the fracture and the way it propagates. To apply this technique, the orientation of the receivers must be determined by detecting perforations or other energetic sources in the treatment well, and the velocity structure of the intervening rock interval must be adequately known. Currently, dipole-sonic logs provide high-resolution velocity data, but significant errors may occur if the logs are old (e.g., formation has depleted), anisotropy is large (e.g., vertical velocity different from horizontal velocity), or several other conditions exist. In addition, faults or lithology changes may separate the wells and alter the velocity structure. Accurate location of the microseisms, and thus the fracture image, is strongly dependent on accurate information about the velocity structure.

In the perforation-timing procedure, crosswell-velocity data are obtained by monitoring the firing pulse from the receiver-orientation perforations (or string shots) and recording the timing pulse along with the arrival data. From these results, a simple one-dimensional model of velocities can be extracted and used to validate, refine, or correct the detailed dipole-sonic data or provide a warning of discrepancies. The ultimate goal is improved accuracy in microseismic mapping, but the results are also useful for assessing the applicability of dipole-sonic logs. Perforation-timing measurements for velocity structure have now been performed in 10 separate projects that have been used to analyze approximately 120 fracture treatments.

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