Distributed Acoustic Sensing (DAS) is a rapidly maturing fiber-optic technology with numerous applications in geophysical and in-well monitoring, and is being developed in a partnership between Shell and Optasense. DAS transforms nearly any fiber-optic cable into a distributed array of acoustic sensors. Recording data requires a special "Interrogator Unit", which is connected to the fiber optic infrastructure. Fiber optic cables may be installed in any kind of borehole, and routinely multiple fibers are included in a single cable, which can allow DAS measurements in conjunction with other fiber optic techniques such as Distributed Temperature Sensing (DTS). Previously the cost, inconvenience, and risk of well intervention associated with conventional downhole geophones made in-well geophysical monitoring expensive and often unfeasible. Permanently installed down-hole fiber-optic cables are ideal for low-cost nonintrusive geophysical monitoring. Once the cable is installed, no further well intervention is required. In order to test this DAS technology, Shell/OptaSense have executed field trials in several locations around the world. One of these locations is the Quest Carbon Capture and Storage (CCS) project in Alberta, Canada.
The Quest CCS project is intended to capture up to 1.08 million tonnes of CO2 annually from the Scotford Upgrader and deliver it via a pipeline to an underground storage site at 2km depth. To demonstrate conformance and containment of the CO2, during the first years of injection, time-lapse monitoring using Vertical Seismic Profiles (VSP) is included in the Quest Measurement, Monitoring and Verification (MMV) plan. In this study the feasibility of recording VSP data using Distributed Acoustic Sensing is tested.
The Quest field trials were executed in September 2010 and October 2011. In these field trials zero-offset and walk-away VSP data were acquired to assess feasibility of, monitoring CO2 containment and tracking the CO2 injection plume. The trials show that, for these applications DAS data can be substituted for geophone data: the zero-offset VSP using DAS is capable of deriving good quality checkshot velocity profiles and the walk-away VSP DAS image is similar to the corresponding geophone image. After imaging, the DAS walk-away VSP images are quite repeatable (NRMS=15%), a requirement for a successful time-lapse survey. In summary, this work demonstrates that it is feasible to use VSPs data recorded with DAS for geophysical monitoring.
Barbara Cox is geophysicist working for Shell's Innovation, Research & Development department and is based in Calgary. She holds an MSc in geophysics from the Utrecht University (The Netherlands), and a PhD in physics from the Delft University of Technology (The Netherlands). She joined Shell International in The Netherlands in 2004, where she worked on new velocity model building and migration methods, and the development of reservoir monitoring technologies. In 2008 she moved to Canada where she is a geophysicist for the Areal Monitoring research team, working on geophysical monitoring technologies for Heavy Oil, CO2 storage, and Tight Gas projects.