Fiber-optic borehole seismic saves 15-h rig time, 40 t of CO₂e

Location

Norway, Europe, Offshore

Summary

A North Sea operator acquired a fiber-optic vertical incidence walkaway vertical seismic profile (VI-WA VSP) in an offshore exploration well by deploying Optiq TuffLine™ torque-balanced fiber-optic wireline conveyance. The innovative borehole seismic survey eliminated a dedicated geophone logging run, saving 15 h of rig time and 40 metric tons of CO₂e.

Objective

One of the most active and successful oil and gas explorers in Norway drilled an S-shaped offshore exploration well to 3.8-km TD using a semisub rig. As part of its formation evaluation, the operator needed accurate vertical transit times, velocities, and seismic response to provide a robust well-to-seismic tie and calibrate sonic log and seismic velocity models.

A conventional VI VSP using geophones would take approximately 16 hours to complete. However, the North Sea’s challenging environment often adds unplanned weather downtime. To improve offshore safety, operational efficiency, and environmental performance, SLB proposed using the Optiq TuffLine torque-balanced fiber-optic wireline conveyance system, which provides industry-leading safe working load, to collect VSP data in minutes using distributed acoustic sensing (DAS) technology.

SLB used the Optiq TuffLine conveyance to run the Sonic Scanner™ acoustic scanning platform and Quanta Geo™ photorealistic reservoir geology service in tandem. Together with the Optiq™ DAS interrogator, they provided high-quality DAS data, accurately measuring seismic velocities and the reflectivity response in both the open and cased sections of the well. The VI-WA VSP took just 1 h to record, using a supply vessel with a VSP source to shoot 160 shotpoints every 6.25 m.

Results

After reviewing the real-time monitoring and quicklook data processing, SLB recommended resurveying the final openhole section with the boat positioned over the well TD. This was done during the third logging descent, when magnetic resonance and Litho Scanner™ high-definition spectroscopy service tools were run, taking only 15 min to complete.

The high-density DAS data, with VSP traces every 5 m, achieved a bandwidth of 4 to 60 Hz at the target level at >3-km TVD. A triple G-gun source was deployed at 5 m depth from a supply vessel crane and autotuned using the TRISOR™ acoustic source control system to ensure perfect synchronization within 0.5 ms. The source was positioned using the SWINGS™ seismic navigation and positioning system and its coordinates saved with DAS data in real time.

As a result of the successful and efficient data acquisition using DAS technology, the operator is estimated to have reduced greenhouse gas (GHG) emissions by 40 metric tons. By eliminating the need for a dedicated geophone descent, the Optiq TuffLine conveyance helped the operator save 15 hours of deepwater rig time and avoid potential bad weather delays.

Figure 1. Wellbore sketch, trajectory, and vertical incidence walkaway VSP geometry.

Vertical incidence shots extracted from the DAS walkaway survey.

Figure 2. DAS VI VSP shots (left) extracted from the VI-WA survey common receiver gathers (CRG) recorded every 5 m with 160 shots each (right panel shows six CRGs).

Figure 3. DAS VI VSP interval velocities overlaid on the sonic velocities versus true vertical depth below mean sea level (TVDBMSL). Figure 4. The DAS VI VSP image matches the surface seismic and uses only the vertical incidence subset of the walkaway data. However, the full VI-WA fold is much higher and further imaging improvements and elastic full-waveform inversion (eFWI) should be possible.