Fiber-optic distributed acoustic sensing (DAS) offers advantages in time-lapse VSP seismic monitoring of an unconventional reservoir. Petrophysical changes to the reservoir due to hydraulic fracturing of the rocks change the character of seismic waves. Repeatable DAS VSP measurements within the stimulated zone can reveal the areas affected by the fracturing. The goal of this study is to assess how these changes affect DAS seismic data acquired before and after stimulation. One of the main advantages of DAS VSP seismic is that receivers cover the entire well including the deviated and horizontal sections. This provides not only velocity/image control in the overburden and target, but also high-resolution images within the frac zone from the horizontal receivers. Another advantage of DAS VSP, if the fiber is permanently installed behind casing, is that the receiver locations are fixed, allowing for high repeatability between surveys.
A fiber-optic cable was installed in a treatment well in the Meramec Shale covering the entire length of the well from surface to target depth, resulting in approximately 1000 recorded channels. The large number of channels, combined with the wide aperture, allowed us to record and locate seismic events from both vertical and horizontal portions of the well. Seismic processing consisted of time-lapse cross-equalization (XEQ) of data using receivers within the vertical portion of the well, where no changes are expected, as proxies to assess the validity of responses observed in the horizontal portion of the well. The XEQ data was then imaged with respect to the monitoring well in order to assess the changes to the reservoir. Complex arrivals within the deviated well were modeled in order to calibrate the wavefield separation prior to prestack Kirchhoff depth migration (PSKDM). The resulting amplitude anomalies in the vicinity of the fibered well have been analyzed in tandem with traditional DAS diagnostic measurements such as crosswell strain and microseismic.
The analysis of this DAS data set demonstrates that current fiber-optic technology can provide enough sensitivity to map seismic anomalies which we can integrate with temperature and strain data for an improved reservoir description. It further demonstrates the value of having DAS receivers within the stimulated zone as they provide in-situ information about the subsurface changes. The importance of the DAS measurements is that they reduce acquisition costs while providing additional monitoring tools from the same hardware.