Introduction
Fiber optic cables deployed within or outside wellbore casing provide opportunities to record orders of magnitude more borehole seismic data than has been economically feasible with traditional 3-component borehole seismic geophones. The recording method, known as DAS (Distributed Acoustic Sensing) provides seismic information at a “receiver” spacing of typically 3 to 15 ft over the entire length of the fiber optic cable. A DAS cable deployed in a well that has a depth of 10,000 ft and a 10,000 ft horizontal section will provide thousands of separate seismic data channels, all recorded simultaneously, and with a receiver spacing that is typically reserved for very shallow near-surface seismic surveys (3-15 ft).
In spite of the viability of DAS seismic recording being a relatively recent event, the technology has been successfully used in multiple seismic disciplines. Zero-offset, 2D and 3D VSP surveys have been successfully recorded, microseismic data is commonly recorded on DAS cables, and active surface seismic sources recorded on the horizontal part of the DAS cable all provide high quality and useful P-wave and S-wave data.
The purpose of this paper is to present results from a DAS microseismic seismic survey and from a time-lapse seismic reflection imaging survey, both recorded on the same two DAS cables in separate wells. The seismic reflection imaging part of the paper is presented first to introduce a new method of seismic reflection imaging using DAS seismic data. The methodology that leads to successful processing of the active seismic source data is then extended to using microseismic events as seismic sources, thus yielding S-wave reflection images with 40-ft vertical resolution. Finally, microseismic data showing reflections from transient fractures that open and close during hydraulic fracture stimulation of an adjacent well provides highly accurate fracture locations that intersect the horizontal wellbore containing the DAS cable.
Fundamentals of DAS Recording
Figure 1 is a sketch that shows a well that turns horizontal at some depth and which we assume has a DAS cable temporarily deployed within the casing or permanently deployed on the outside of casing. A surface seismic source is represented by a photo of a Vibroseis truck though any seismic source can be used for DAS recording including dynamite or other impulsive source. The figure indicates that laser light is shone down one of the fibers. Impurities in the fiber tend to generate reflections that travel back up the fiber in the opposite direction of the “downgoing” laser light. Since the distance between the impurities is fixed, the changing interference patterns between the reflected light is used an electronic system at the surface called an Interrogator to determine the strain rate along the long axis of the DAS fiber. The interrogator divides the DAS cable into segments of between typically 3 and 15 ft for measurement of strain rate and outputs those strain rate measurements as seismic data.
