When drilling through shales, the borehole wall is fully exposed to the actions of the rotating drillstring and stabilizers, which can affect the wellbore stability. In the present work, we report a Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) study of how the shale borehole wall is affected by prolonged exposure to a rotating drillstring/stabilizer and circulating mud during drilling. A for-purpose built pressurized cell was used to expose the samples to drilling conditions, and drilling-induced damages were preserved by fluorescent epoxy after the experiment. The samples were thereafter dried, and the zone closest to the hole was investigated by SEM and EDX. It was found that the strongly rotating drillstring/stabilizer caused shale "smearing" in the near-hole region, meaning that loosened shale fragments were re-deposited onto the borehole wall. This smear effect may have a stabilizing effect for a certain timeframe, and it is important to investigate the stability regarding further downhole operation in a well. This material may slow down migration of mud into the borehole wall, and thus contribute to stabilization of the borehole.

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