VSP and surface seismic

VSP and surface seismic2018-06-01T19:21:14+00:00

CO2 storage & seismic

Many of the in-well iDAS™ , Carina® and DTS applications targeted at the upstream oil and gas industry are also a natural fit for CO2 well monitoring.

Silixa has played an active part in the installation of fibre optic enabled monitoring systems for carbon capture and storage projects and continue to acquire data on several of these projects for a variety of reasons.

With a permanent fibre optic cable installed in a COdisposal or observation well the user has access to on-demand, and time lapse ArraySeis™ borehole seismic data which can help describe the growth of the CO2 plume within the reservoir. Concerns over the integrity of the cap rock, or indeed the wellbore, can be addressed by passively monitoring for potential micro-seismic events, an area that is developing quickly as the iDAS sensitivity improves through system development and signal processing.

The installation at a well site in Spain

VSP and surface seismic

iDAS has been used in many seismic acquisitions, encompassing vertical seismic profiling in CO2 storage wells and surface seismic.

Distributed Fibre Optic Sensing is a viable alternative to geophone arrays for the acquisition of borehole seismic data. The ability to deploy optical fibres into a well, either as a cable-based intervention or as part of a completion string, allows for the entire wellbore to be surveyed with every source activation. This can dramatically reduce the operating time required to complete a normal survey as well as offering the opportunity to achieve much higher spatial coverage than is typical of traditional technologies.

The ability to acquire ArraySeis™ borehole seismic data in a CO2 injection well without the need to disrupt the injection process or drill expensive observation wells also offers significant benefits to the operator.

Benefits

  • Seismic surveys on temporary and permanent installations
  • cost-effective wide coverage
  • 3D seismic for the fraction of the time required for traditional surveys
  • data quality identical to geophone data

Applications of iDAS at the surface and shallow subsurface of horizontal soil transects have provided results comparable to geophone arrays. iDAS provides the unique advantage of having a high density of measurement points. Recently, iDAS data have allowed the 3D characterization of the mechanical properties of highly heterogeneous systems of water-soil-rocks, based on an accurate distributed detection of seismic signals propagating at different velocities through the different media.

Such results open the possibility to characterizing the subsurface and combined with 3D VSP they enable data acquisition from the storage complex with unprecedented precision.

Case Study: High-resolution, far-offset VSP survey with Carina® Sensing System for permanent CO2 storage monitoring in Otway, Australia

Challenge

The benefits of iDAS as a seismic acquisition tool are proven and well-suited to this application. High-resolution measurements with full wellbore coverage for every shot provide unique levels of detail. However, with most VSP type measurements, the imaging aperture is limited, meaning only a small area of the reservoir around the wellbore is imaged. Using far-offset source locations can widen this aperture but often the signal-to-noise ratio is not sufficiently high to extend the seismic image, even when using conventional wireline geophone sensors.

Solution

The advanced Carina Sensing System, developed by Silixa, provides a further 20dB (100 times) improvement in the signal-to-noise performance of DAS measurements, and offers capabilities beyond conventional geophones in many seismic applications. This step-change improvement in performance was confirmed during a baseline VSP acquisition at the Otway research site.

Results

The figures below show a comparison between shot gather data for both a conventional wireline geophone tool and the Carina Sensing System at two different shot locations. Figure 1 shows data from a 700m offset, Figure 2 shows data from a 1800m offset. The source was a single 26,000lbs vibroseis truck using a 6-150Hz, 24sec sweep.

Download the case study.