3D vertical seismic profile acquired with distributed acoustic sensing on tubing installation: A case study from the CO2CRC Otway Project

Abstract

Distributed acoustic sensing (DAS) can revolutionize the seismic industry by using fiber-optic cables installed permanently to acquire on-demand vertical seismic profile (VSP) data at fine spatial sampling. With this, DAS can solve some of the issues associated with conventional seismic sensors. Studies have successfully demonstrated the use of DAS on cemented fibers for monitoring applications; however, such applications on tubing-deployed fibers are relatively uncommon. Application of tubing-deployed fibers is especially useful for preexisting wells, where there is no opportunity to install a fiber behind the casing. In the CO2CRC Otway Project, we acquired a 3D DAS VSP using a standard fiber-optic cable installed on the production tubing of the injector well. We aim to analyze the quality of the 3D DAS VSP on tubing, as well as discuss lessons learned from the current DAS deployment. We find the limitations associated with the DAS on tubing, as well as ways to improve the quality of the data sets for future surveys at Otway. Due to the reduced coupling and the long fiber length (approximately 20 km), the raw DAS records indicate a high level of noise relative to the signal. Despite the limitations, the migrated 3D DAS VSP data recorded by cable installed on tubing are able to image interfaces beyond the injection depth. Furthermore, we determine that the signal-to-noise ratio might be improved by reducing the fiber length.

Seismic Applications of Downhole DAS

“…The quality of recorded DAS data allows for active seismic imaging in the vicinity of the reservoir. There have been many examples, in- and off-shore, of successful imaging [ 37 , 77 , 78 , 79 , 80 ]. Figure 5 shows imaging results obtained with different DAS configurations compared to an ocean bottom node (OBN) survey.…”

“…This type of deployment suffers from strong noise in the form of tubing ringing, which can be dealt with during data processing stages [ 36 ]. Nonetheless, coupling to the surrounding formations is weaker, and signal quality is lower [ 37 ]. Deployment inside the tubing is fully retrievable and less prone to failure, but noise levels are very high due to direct interaction with flowing fluid, among other factors [ 38 , 39 ].…”

Distributed Acoustic Sensing as a Distributed Hydraulic Sensor in Fractured Bedrock

“…When considering the use of DAS in downhole applications, it is important to keep in mind that the fiber must be mechanically coupled to the formation to measure displacement. This can be accomplished using an overpressured liner, as was done in these experiments, or by grouting/cementing the cable into a borehole or outside a well casing as is done for seismic applications (Correa et al, 2019;Daley et al, 2013Daley et al, , 2016. Simply hanging a fiber in a well will not reliably produce any measure of formation displacement and, therefore, hydraulic response.…”

“…For vertical boreholes, the fiber must be either grouted into the borehole or pressed against the borehole wall. Grouting or cementing the fiber outside of the casing provides the most robust coupling (Correa et al, 2019), although clamping systems have also been designed (Daley et al, 2015). For shallow applications, a pressured flexible liner may be used to temporarily couple the fiber against the borehole wall (Becker, Ciervo, et al, 2017; Munn et al, 2017).…”

The initial appraisal of buried DAS system in CO2CRC Otway Project: the comparison of buried standard fibre-optic and helically wound cables using 2D imaging

“…Concurrently with this test, Daley et al (2013) tested DAS acquisition and obtained promising results. These tests were followed in 2015 by the installation of a permanent 3D receiver array, a permanent 3D DAS array, and a retrievable borehole geophone array in the monitoring well, as well as a DAS array in the injection well (Correa et al, 2018). The purpose of these installations was to perform numerous experiments to ascertain an optimal TL seismic approach for monitoring the emplaced CO2 plume.…”