Cuttings transport is one of the most important aspects to control during drilling operations, but the effect of wellbore geometry on hole cleaning is not fully understood. This paper presents results from experimental laboratory tests where hydraulics and hole cleaning have been investigated for two different wellbore geometries; circular and a non-circular, where spiral grooves have been deliberately added to the wellbore wall in order to improve cuttings transport. Improving hole cleaning will improve drilling efficiency in general, and will in particular enable longer reach for ERD wells.
The experiments have been conducted as part of a research project where friction and hydraulics in non-circular wellbores for more efficient drilling and well construction is the aim. The experiments have been performed under realistic conditions. The flow loop includes a 12 meter long test section with 2″ diameter freely rotating drillstring inside a 4″ diameter wellbore made of concrete. Sand particles were injected while circulating the drilling fluid through the test section in horizontal and 30° inclined positions. The test results show that borehole hydraulics and cuttings transport can be significantly improved in a non-circular wellbore relative to a circular wellbore.
Investigating the cutting transport in non-circular wellbores with available models is even more complex than for circular wellbores. Most drilling models assume circular wellbores, but in reality the situation is often different. Also, it may be possible to create non-circular wellbores on purpose, as in the present study. Such a comparative, experimental study of hole cleaning in different wellbore geometries has to our knowledge previously never been performed, and the results were obtained in a custom-made and unique experimental flow loop. The results and the experimental approach could therefore be of value for any one working with drilling.