MWD VSP and Checkshot Surveys Using the Drill Bit as a Downhole Energy Source

Abstract

Vertical Seismic Profiling (VSP) has been used over the past decade to provide seismic images near a wellbore and to make time-to-depth calibrations of surface seismic data. A new wellbore seismic technique, the TOMEX® Survey, uses the vibrations produced by a drill bit while drilling as a downhole seismic energy source to produce VSP data. No downhole instrumentation is required to acquire the data, and the data recording does not interfere with the drilling process. Hence, there is no loss of rig time and MWD (measurement-while-drilling) borehole seismic data is acquired. Rig time savings are particularly important on offshore wells where the daily cost of rig time can be very high. The continuous signals generated by the drill bit during drilling are monitored with a reference sensor attached to the top of the drillstring. The reference sensor signals are processed in combination with the signals from receivers placed at or near the earth's surface to produce VSP data. In a marine environment, sensors are deployed on the ocean bottom. Results from a land test well demonstrate that the processed drill-bit source data are virtually identical to conventional VSP data. Offshore tests demonstrate that drill-bit VSP data quality offshore is superior to that on land. INTRODUCTION Vertical Seismic Profiling (VSP) consists of recording seismic data using a surface energy source and a receiver lowered by a wireline into a borehole. VSP surveys are used like checkshot surveys to convert surface seismic sections to depth. They are also used to estimate formation interval velocities. The reflected energy recorded in VSP surveys are used to image formations near the wellbore and to predict formations yet to be encountered by the drill bit. One disadvantage of VSP is that the drilling activity must cease while the data are being recorded. The TOMEX Survey, or drill-bit VSP, uses the natural vibrations created by the drill bit during the drilling of a well as a downhole energy source. No downhole instrumentation is required, so there is no lost rig time and, complications of open-hole wireline operations are avoided. Offshore, the savings in rig time achieved by a drill-bit VSP versus conventional VSP is often greater than the cost of the entire survey. Ideally, an explorationist wants the information obtained with a VSP to be available while the well is being drilled, because the ability to locate the seismic target with respect to the current drill-bit position could enormously reduce drilling expenditures. However, prior to acquiring the VSP, the driller must trip out of the hole, and in many cases, a wiper run must be performed to condition the borehole walls. This drilling inactivity period can be rather long. Hence, VSPs are usually performed during the final logging run, after it is too late to affect the drilling plan. By contrast, drill-bit VSP data are acquired and processed as drilling proceeds.