Optical fiber availability for data acquisition

The Ketzin CO2 pilot capture site is coordinated by the German Research Centre for Geosciences (Deutsches GeoForschungsZentrum, or GFZ) to advance scientific understanding of how to store CO2 geologically and to research the processes of underground CO2 injection and migration. The site has one injector well and three monitor wells. For monitoring purposes, the wells have single-mode optical fiber cemented behind the casing, and one of the monitor wells also has cemented multimode optical fiber.

Efficient seismic acquisition using cemented fibers

The Optiq Seismic solution brings new efficiency to borehole seismic operations. The solution’s optical interrogator unit at surface is connected to any optical-fiber cable deployed in a well, from hybrid wireline logging cable to production tubing with fiber installed or optical fiber permanently cemented behind casing. The interrogator records seismic signals, employing the downhole fiber as a vibration sensing device along the full length of the wellbore.

Because the Optiq Seismic solution makes every logging run or fiber installation an opportunity for seismic data acquisition, it significantly improves the efficiency of borehole seismic operations while lowering the cost. No longer is time required for rigging up and down and deployment of conventional borehole seismic tools because the solution is simply connected to the fiber at surface and records seismic data in conjunction with other stationary logging services or on its own. A full well profile is recorded for each shot.

Good-quality VSP from either single- or multimode fiber

A truck-mounted accelerated weight-drop source was used as the seismic source, delivering approximately 9,500 J per shot. The data was median stacked using 50 shots, which is equivalent to about 10 minutes of acquisition.

Good-quality VSP data was recorded by the Optiq Seismic solution in the bottom 70% of the monitoring well with both single- and multimode well fibers cemented behind casing. A strong seismic reflector is present, along with direct shear. Casing arrival is present in the shallow section of the well, which has uncemented double casing, so the optical fibers are not mechanically coupled to the formation.

Single-mode fiber is typically the preferred deployment for VSP data acquisition. However, because multimode fiber can be the only option in such permanent installations, the Optiq Seismic solution can still use it to obtain usable seismic data.

For single-mode fiber acquisition, VSP data was simultaneously acquired at all four wells at the site by daisy-chaining the single-mode fiber across the wells to the Optiq Seismic solution interrogator. In this configuration, although the cemented fiber installation in the farthest well was more than 5 km away from the optical interrogator, better-quality data was returned compared with acquisition by the multimode fiber at less than 1 km from the laser source.