The development of unconventional reservoirs is dependent on horizontal and vertical well spacing, completion strategy, and timing among other factors. Various methods have been used to determine the stimulated volume created by hydraulic fracturing. In conventional reservoirs, 4D seismic is often used to monitor changes in gas or fluid volumes and to determine differences between drained and undrained areas. In unconventional reservoirs, hydraulic fracturing is expected to change the elastic properties of the reservoir and thus alter seismic amplitude and sonic traveltime responses and may show a time-lapse signature.
3D vertical seismic profiles (VSPs) were acquired in a vertical well equipped with an external distributed acoustic sensing (DAS) optical fiber, both before and immediately after the stimulation of two offset horizontal wells Fiber optic cable makes the acquisition of 3D VSPs fairly inexpensive compared to traditional geophone sensors. In addition to the DAS VSP, a conventional VSP with geophones was acquired along with microseismic data, tiltmeters, external pressure gauges, radioactive tracers, and distributed thermal sensing (DTS) in the vertical well to monitor the hydraulic fracture treatments. After processing the shot records to remove downgoing energy and enhance upgoing energy, prestack depth migration was used to image the seismic volumes from the DAS fiber both before and after the stimulation treatments. Amplitude changes and traveltime differences were observed that changed azimuthally between the two volumes. The height of the stimulation inferred from these differences was similar to the height estimates obtained from analysis of the other methods employed.
The Midland basin in West Texas has been a significant source of hydrocarbons for over 80 years. With the advent of horizontal drilling in conjunction with hydraulic fracturing this basin continues to be a prolific source of now unconventional shale oil. To effectively develop these shale resources it is necessary to understand the stimulated rock volume created during the hydraulic fracturing process to help establish appropriate well spacing and horizontal well stacking for field development.