Active seismic monitoring of hydraulic fractures in laboratory experiments

In scaled laboratory tests, we perform acoustic measurements in a time-lapse sequence to separate the fracture response from the background signal. Using both compressional and shear waves (that are very sensitive to fluid filled fractures) we can, not only detect the hydraulic fracture, but also characterize its shape and geometry during its growth. We show the application of the technique to propagation and reopening of hydraulic fractures. During fracture growth the acoustic waves excite diffractions at the tip of the fracture. Depending on the acquisition geometry, we detect many events related to surface waves propagating along the fracture. Also, we observe that shear waves detect the migration of the fluid front during reopening of a pre-existing hydraulic fracture. in contrast with the compressional waves which are insensitive to the fluid front during reopening. (C) 2001 Elsevier Science Ltd. All rights reserved.