Drillbit SWD Reverse Walkaway Using Downhole Reference Signal

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDevelopment drilling occurs sometimes in areas where the surface seismic quality is poor. Drilling in such areas is risky enough to justify the application of drillbit SWD for seismic illumination ahead of the bit. A multidisciplinary project partially supported by the EC was undertaken combining the following techniques:• Drillbit SWD, Seismic While Drilling survey using the drillbit as seismic source and many lines of surface receivers. The Bottom Hole Assembly (BHA) includes a roller cone bit, a shock absorber and a downhole reference sensor in between. • EMWD, as Electro-Magnetic While Drilling wireless transmission surface to downhole. GEOSERVICES built a new Downhole Recorder Unit (DRU) integrated in the BHA in order to record the random axial drillbit vibrations in synchronization with a Surface Command Unit (SCU). Most of the commercial surface seismic recorders can be interfaced with the SCU. Large amount of uncorrelated seismic data are memorized both on surface and downhole while drilling, as the drillbit signal is available only when the drillstring is pulled out of the well.The objective of the method is to produce a seismic image of the geological structure around the first well in order to confirm the target structure, or to indicate the drilling targets of the following directional wells. The field operation is limited to one or two drilling runs around mid-target depth, due to operational and cost constraints.A full scale SWD reverse walkaway was recorded in this fashion around an ENEL well in a " no seismic result " area, near Larderello, using the OGS SEISBIT surface seismic recorder with 170 channels deployed over 6 radial lines.The whole chain of field equipment behaved successfully and good seismic reflections have been obtained. The present SWD reverse walkaway technique does not disturb the drilling process significantly. The time delay for processing needs to be reduced in order to render the method even more attractive.

Seismic While Drilling (Swd) Techniques with Downhole Measurements, Introduced by Ifp and Its Partners in 1990-2000

“…Additionally, strong interfering S-P converted reflected waves (downgoing S-wave generated by the rock bit, followed by upgoing P-wave reflection) were observed during the processing stage, as illustrated by the propagation sketch Figure 38, and required appropriate velocity filter rejection (details in [16]). The efficiency of the S-P reflected wave cancellation can be appreciated on the final binstack images Figures 39a and 39b: the S-P reflected events appear on the top binstack Figure 39a Sketch of propagation.…”

“…In the well vicinity, the surface seismic image is totally blurred. Full technical details on this experiment can be found in [16].…”