Distributed Acoustic Sensing (DAS) optical fiber for downhole geophysical measurements is a fairly new technology the industry is utilizing to better characterize hydraulic stimulations. Data were acquired with a vertical observation well that was instrumented externally with dual and single mode fiber optics for strain, acoustics (DAS), temperature (DTS), and external pressure gauges as well as internally instrumented with conventional tiltmeters and geophones. We used this instrumented well multiple times to record a number of nearby offset horizontal hydraulic stimulations as well as for a 3D 4D vertical seismic profile (VSP).
By using several tools, we can more accurately determine the height and length of the hydraulically stimulated zone to calibrate fracture models and determine where to place future horizontal wells during our field development.
While numerous papers can be written about the data collected, this paper focuses mainly on the microseismic data acquired with fiber. We have made the following observations using these microseismic data:
We can observe fiber based microseismic (MS) events down to a certain magnitude level but conventional geophones can still record weaker events.
MS events appear to often occur along distinct depths that could be considered bedding planes.
In some cases, we have noted very weak energy precursors to MS events in the injection zone with no distinct radiation pattern on the fiber data.
MS events here are noted to not appear to occur throughout the stimulated zone. For a number of the stages here imaged, MS events do not extend as far downward as compared to other downhole measurements, possibly due to ductile zone interactions.