Passive Time-Lapse Distributed Acoustic Sensing (DAS) Data Recorded in an Open-Hole, Production Well
Distributed acoustic sensing (DAS) is a method that utilizes the Rayleigh backscattering of laser light pulses generated at numerous randomly spaced scattering points along a fiber optics cable, and then recorded by an “interrogator box”, where it’s measured by a computer known as an Optical Time Domain Reflectometer. Using the principles of two-way travel time, we can locate where backscatter originates along the fiber and any subsequent changes in arrival time, phase, or frequency of input laser light, which act as a measurement of the strain rate acting on the fiber cable. DAS technology can be useful in petroleum industry applications such as vertical seismic profiling (VSP), microseismic detection and imaging, and hydraulic fracture monitoring. In this study, we analyze a unique DAS dataset recorded from a producing horizontal oil well with an open-hole completion design, meaning the lateral section is not hydraulically stimulated. The DAS fiber rod is coupled directly to the reservoir under its own weight, directly on top of the reservoir rock. The well’s production alternated on and off multiple times over a 60-hour window, while DAS data recorded continuously. Signal analysis was performed on the data to identify and differentiate signal events from noise, and determine an optimal workflow for imaging the data, suitable for the large volumes (e.g., 7 Terabytes) of data that DAS creates. We have shown it is possible to use an extremely large dataset reduced by more than 99%, while retaining clear acoustic signal in the unenhanced data. This study positively identifies and interprets fluid-flow signal events visible on the DAS data during both phases of production and models the interpreted DAS signal to physical phenomena occurring in the wellbore. Understanding the acoustic fluid-flow signals in DAS data in a passive, time-lapse environment has potential to impact oil field and reservoir development including optimizing production, fluid injection, and knowledge of fluid-flow behavior during changing phases of production. This study provides a basis for future work on the affects that different temporal resolutions and frequency bins will have on imaging fluid-flow and elastic wave phenomena.