"Watching Fractures Open and Close Using VSP P and S Wave Diffractions"
by M. E. Willis, D. Pei, A. Cheng from Halliburton and X. Feng and X. Shang from MIT
Microseismic monitoring of hydraulic fracturing treatments is currently the primary method for accessing the lateral and vertical extent of each fracture stage. However, this only reveals the location where rocks are breaking and/or where existing faults have been reactivated. The previously published Lost Hills Field hydraulic fracture experiment demonstrated that scattered (diffracted) shear waves of fluid-filled hydraulic fractures can be almost as large in amplitude as the incident wave field, but diminish as the fluid pressure drops and the fracture stiffens. Based upon that study, as well as our modelling results, we propose a hydraulic fracture monitoring program consisting of quickly repeating a series of shear wave VSPs at a small collection of shot points immediately after each fracture stage. A series of migrated images is created after each fracture stage, where the output image plane is defined by a fit through the microseismic events. Thus, the use of the scattered PP, PS, SP and SS wave energy can define the dimension of the fluid-filled fracture and be visualized on these images.