Seismic Applications of Downhole DAS

Abstract

Distributed Acoustic Sensing (DAS) is gaining vast popularity in the industrial and academic sectors for a variety of studies. Its spatial and temporal resolution is ever helpful, but one of the primary benefits of DAS is the ability to install fibers in boreholes and record seismic signals in depth. With minimal operational disruption, a continuous sampling along the trajectory of the borehole is made possible. Such resolution is highly challenging to obtain with conventional downhole tools. This review article summarizes different seismic uses, passive and active, of downhole DAS. We emphasize current DAS limitations and potential ways to overcome them.

Keywords: DAS; distributed; downhole; exploration; seismic; seismology.

Figure 1

Different types of DAS installation. Elements of the well are marked, and the fiber is in yellow. Figure from Naldrett et al. (2020), First Break.

Figure 2

Colocated fiber and downhole geophone installation in early experimental deployment. (a) Image of flatpack deployment including the fiber and (b) its schematic representation. (c) Flat and borehole geophone clamped on the tubing. Figure and caption from Daley et al. (2013), The Leading Edge.

Figure 3

One of the first examples of DAS-VSP records, with data acquired at Pinedale in 2011. Despite a large number of stacked sources, DAS data are noisy. However, downgoing and upgoing P-wave energy is visible, demonstrating the potential of DAS for VSP studies. For shallow DAS locations, the direct S-wave is also visible. DAS measurements are in good agreement, kinematically, with downhole geophones deployed in the same well. Figure from Mateeva et al. (2013), The Leading Edge.

Figure 4

DAS data quality acquired with a modern interrogator, using regular and engineered fiber. For a regular fiber (left), a stack of 38 shots is required to obtain the SNR of a single shot recorded with an engineered signal (right). Recorded amplitude as a function of frequency, using both fibers, is at the bottom. Figure from Naldrett et al. (2020), First Break.

Figure 5

DAS-VSP conducted in flowing wells, with image quality as a function of acquisition efforts (a) Image obtained with a small seismic source recorded by a single DAS well. In (b), a larger source is deployed, improving the illumination of targets deep below the well. When the same source is recorded by three different DAS wells (c), the image extent and continuity are much improved. (d) a comparison to the costly alternative—an OBN survey with a very large seismic source. Figure from Kiyaschenko et al. (2020), The Leading Edge.

Figure 6

Legacy and DAS-VSP salt imaging, taken along two perpendicular lines. In both lines, the legacy salt interpretation (black) agrees with the DAS-based image (red) for the top of the salt. However, there are significant differences in the salt flank. Figure from Duan et al. (2020), SEG Annual Meeting Expanded Abstracts.

Figure 7

Time-lapse DAS-VSP imaging tracking a CO₂ plume. As the volume of injected CO₂ increases, the amplitude change relative to the baseline image increases. Figure from While et al. (2020), SEG Annual Meeting Expanded Abstracts.

Figure 8

Seismic velocities estimated in a DAS-VSP well. The result of a zero-offset VSP survey with geophones (dashed black line) agrees very well with velocities derived using a recorded earthquake (solid blue and red lines). Earthquake analysis also yields a shear-wave velocity profile (dotted blue and red) which cannot be obtained from the VSP survey. Ambient seismic signals can be used to recover P-wave velocity (green) with low-frequency resolution. Figure from Lellouch et al. (2019), Journal of Geophysical Research: Solid Earth.

Figure 9

A microseismic event recorded with DAS in a deviated well. Direct P and S arrivals are visible and marked, but there are also reflections and coda events. Recording in the vertical part of the well has much lower SNR, as can also be seen in the later microseismic event that is only visible in the horizontal section. Figure from Karrenbach et al. (2017), The Leading Edge.

Figure 10

Comparison between event locations derived from two deviated DAS wells (light gray) and surface geophones (black) in map (a) and cross-section (b) views. DAS locations are generally more clustered around the stimulated well (pink line in a). In (b), red triangles denote the locations of the wells. Figure from Verdon et al. (2020), Geophysics.

Figure 11

Earthquake recorded by vertical DAS. It shows clear P (annotated in blue) and S arrivals (green), as well as trailing coda events (orange). First arrivals have a refraction-like behavior, as the energy dissipates quickly below a depth of ~840m. High levels of noise (red) in the near-surface are visible, illustrating the benefits of downhole recording. Adapted from Lellouch et al. (2021), Journal of Geophysical Research: Solid Earth.

Figure 12

Low-frequency DAS recording of fractures and pressure fronts interacting with a fiber in an offset well. Events of interest are labeled. Figure from Jin and Roy (2017), The Leading Edge.

Figure 13

Guided waves and their interaction with fractures. Two sets of arrivals, both dispersive, can be observed. A clear imprint (yellow arrow) from open fractures can be seen on the slower guided S-waves. Figure adapted from Lellouch et al. (2020), Geophysics.