Pseudo-Sticking and Pull – A New Old Artifact on Image Logs Acquired in Ocean Wireline Log Operations

Abstract

Abstract The "pseudo-sticking and pull", also nicknamed "blocky effect" (because of its general aspect), is a common acquisition artifact that reduces considerably the quality of wireline acoustic and microresistive image logs, even though it is not well known among image log interpreters, well-site geologists and engineers. This artifact is generated by heave motion influence when the wave movement is not minimized enough by the compensation systems during offshore wireline operations. In fact, it is well known that the linear or rotary compensator systems reduce the heave variance between 52-80% and, sometimes, other factors such as string weight and fluid density can affect, significantly, wireline logs quality by the heave variance not well compensated. In this case, mainly the wireline stationary tools and the ones that need lower acquisition speed, present depth uncertainties that may cause relevant problems. The image logs are largely used in pre-salt reservoirs for geomechanics, petrophysics, structural, sedimentological and stratigraphic studies due to their great contrast responses on carbonates. However, because acoustic image logs run at speeds around 425 ft/h (against 3600 ft/h from basic logs) and acquire between 180 and 250 samples data each 0,2 inches depth, this log becomes more affected by this artifact than the microresistive one (over 1500 ft/h). The residual heave affects directly the image quality by creating "pseudo-sticking and pull" at each wave period, distorting the image log significantly. Besides the artifact that immediately appears on the image during the logging, the residual heave can also be confirmed by observing when cable tension and vector Z accelerometer variation have the same frequency as the period of ocean waves, monitored by the meteo-oceanographic daily reports. Even though this artifact can be minimized with processing (better than true sticking and pull corrections), the loss of quality should be considered. Petrobras experience has also shown that interventions directly on the source by adjusting compensators and changing steering towards can be effective to handle this problem. In order to help the interested community in acquiring better data with minimum costs, this work shows some examples of this artifact on acoustic image logs from pre-salt wells. It can also show the results with processing, the differences reached after adjusting compensators and demands for future works.