Taking downhole measurements during a hydraulic fracturing operation has many challenges, mainly the instrument survivability inside the hostile, high pressure, abrasive environment inside the casing during injection. Therefore, instrumentation must be attached on the outside of the casing, which has its own challenges in running-in-hole, cementing, pressure, etc. A fiber-optic cable with protective jackets can be permanently installed on the outside of the production casing and used to measure acoustics and temperature across the entire length of the borehole, without well intervention, for all operations from completions to production to abandonment. Fiber-optic distributed acoustic sensing (DAS) systems work by pulsing light into a fiber and measuring the backscattered light along the fiber length. An acoustic pressure wave that contacts the cable will create a small strain in the fiber and change the backscatter profile. The strain can be measured at surface, depth-matched using the speed of light in the fiber, and converted back into an acoustic signal. This paper will describe some of the acoustic events and signatures related to a typical plug-and-perf hydraulic fracturing operation including: wireline gun tracking, perforating, bridge plug setting, ball drops, and injection (axial flow and flow through perforations). Although this paper will only describe a typical signature of the listed activities, it will be evident that such continuous full wellbore measurements can be useful in redesigning and optimizing future operations, troubleshooting well problems, and making real-time operational decisions to avoid or mitigate non-productive time.
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