Active and passive sesmic monitoring of CO2 storage facilities
Many of the in-well and surface iDAS™ , Carina® and DTS applications targeted at the upstream oil and gas industry are also a natural fit for CO2 storage site characterisation.
Distributed fibre optic sensing is a viable alterative to geophone arrays for the acquisition of seismic data. 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 CO2 disposal or observation well the user has access to on-demand, and time lapse ArraySeis™ or Carina® Seismic VSP data which can help describe the growth of the CO2 plume within the reservoir.
Optical fibres can be deployed the entire length of a well, either as a cable-based intervention or as part of a completion string, allowing the entire depth of the wellbore to be surveyed with every shot. 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. Data collected at the CO2CRC Otway test site shows how Carina Seismic offers much greater detail in each shot record compared to geophones.
The ability to acquire ArraySeis™ or Carina Seismic borehole seismic data in a CO2 injection well without the need to disrupt the injection process or drill expensive additional observation wells also offers significant benefits to the operator.
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.
Additionally, concerns over seismic hazard and the integrity of the caprock, 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
- 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
Case Study: High-resolution, far-offset VSP survey with Carina® Sensing System for permanent CO2 storage monitoring in Otway, Australia
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.
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.
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.