Walkaway VSP Using Multimode Optical Fibres in a Hybrid Wireline

A Walkaway and a Walkaround Vertical Seismic Profile (VSP) data were acquired with an iDAS system. The survey took place in a CNPC formerly producing well in northeastern China, near Tangshan, Hebei. A standard VSP data pre-processing workflow was applied, followed by prestack Kirchhoff time migration. The DAS pre-processing step was faced with additional and special challenges: strong coherent noise due to cable slapping and ringing along the borehole casing. The final pre-processed DAS data rendered a vertical aperture the height of the well that resulted in a much wider lateral image than customary from a single well. This single well DAS Walkaway and Walkaround VSP image provided a good result with higher vertical and lateral resolution than the surface seismic in the objective area.

The vertical well environment without the ability to effectively clamp the sensor to the borehole casing wall creates a unique set of challenges. This paper reports on lessons learned in the handling of the wireline cable and subsequent special DAS data processing steps developed to remediate some of the practical wireline deployment issues. Optical wireline cable as a conveyance method for VSP in vertical wells will open the use of the DAS system to much wider applications.