Hydraulic fracturing operations in unconventional reservoirs are typically monitored using geophones located either at the surface or in adjacent wellbores. A new approach to record hydraulic stimulations utilizes fiber-optic Distributed Acoustic Sensing (DAS). A fiber-optic cable was installed in a treatment well in the Meramec Shale. A variety of physical effects, such as temperature, strain and microseismicity are measured and correlated with the treatment program during hydraulic fracturing of the well containing the fiber as well as an adjacent well. The analysis of this DAS data set demonstrates that current fiber-optic technology can provide enough sensitivity to detect a significant number of microseismic events and that these events can be integrated with temperature and strain measurements for an improved reservoir description.
Monitoring a reservoir's behavior before, during and after hydraulic fracturing is key to efficient and safe operations (Molenaar et al 2012; Webster et al, 2013). A variety of well-known reservoir surveillance techniques, such as surface seismic imaging, vertical seismic profiling, or microseismic analysis, are used in these scenarios. All these techniques have advantages and disadvantages, but in general they augment the understanding of treatment processes.
While Distributed Temperature Sensing (DTS) has been available for many years, Distributed Acoustic Sensing (DAS) has only recently found increased usage during hydraulic fracture monitoring (Webster et al. 2013, 2016; Cole et al., 2016), from allowing relative flow measurements through perforations during the hydraulic injection process, to suggesting and validating diversions, to detecting fluid leaks through packers or behind casing, or to measuring seismic wave fields (Molenaar, 2012; Panhuis et al, 2014).
In contrast to traditional point sensors, such as accelerometers, geophones, and seismometers, DAS uses the fiber itself to detect minute changes in strain. The sensing principle relies on Rayleigh scattering of light within the optical fiber (Lumens, 2014). A laser interrogator generates sequences of light pulses, detects the back-scattered light and translates it into individual strain measurement values at sensing points with whatever sampling interval is desired. A fiber that is permanently installed (cemented) behind casing is preferred for optimum signal to noise ratio. Since the fiber extends all the way to the surface, DAS measurements of the overburden as well as the reservoir can be obtained.
