A major operator needed better quality seismic data than that obtained from surface seismic on a high-pressure high-temperature (HPHT) well in the central North Sea. The Schlumberger SlimXtreme slimhole, HPHT well logging platform acquired more accurate velocities from within and below a chalk layer in the well to be able to reprocess surface seismic data and obtain a much clearer image of the reservoir. This operator is now also using these accurate velocities to reprocess existing surface seismic data to reduce risk in future drilling.

 

The HPHT 3 3/8-in. [8.6-cm] seismic acquisition tool was run in association with the logging platform to acquire full vertical seismic profile (VSP) data in the deviated well with a total depth below 15,000 ft [4,600 m]. The unobscured VSP data enabled the well and surface seismic data to be correlated, building a more accurate picture of the reservoir.

 

Despite an operating temperature approaching 400 degF [204 degC] and a well trajectory that deviated above the chalk and sidetracked out of the plane of deviation, the logging operation progressed smoothly, with data being acquired at 50-ft [15 m] stations within the reservoir and into the chalk. The deepest of the 73 stations recorded a temperature of 380F [193 degC].

 

In addition to overcoming the velocity uncertainties and generated noise, use of the seismic tool and platform resolved differences in velocity models based on the surface seismic and VSP data. This indicated that the VSP detected higher velocities in the chalk layer and lower velocities below the chalk than the surface seismic model. The more accurate velocities obtained from the survey continue to aid the operator in reprocessing data to create a highly defined image of the reservoir.