Torque & drag and hole cleaning are important parameters to control during drilling operations, but the effect of wellbore geometry on these parameters are not fully explored. This paper presents results from an experimental study of torque and cutting transport in two different wellbore geometries; one circular and one non-circular, where spiral grooves have been deliberately added to the wellbore wall in order to improve cuttings transport. Improving borehole hydraulic and mechanics will improve drilling efficiency in general, and will in particular enable longer reach for extended reach drilling (ERD) wells.
The experiments have been conducted as part of an R&D project where the goal was to obtain reduced friction and improved borehole hydraulics and hole cleaning for more efficient drilling and well construction. The experiments have been performed under realistic conditions using a flow loop which includes a 12 meter long test section with a 2" OD freely rotating drillstring inside a 4" ID wellbore made of concrete. Solid particles were injected while circulating the drilling fluid through the test section at two inclinations; horizontal and 30° inclined from horizontal. The test results show that drillstring torque is reduced and cutting transport is improved in the non-circular wellbore relative to the circular wellbore. The experiments show up to 40% reduction in dynamic pressure drop for similar hole cleaning conditions of a non-circular geometry. In addition, the results show that the torque decreases by up to 15% in the non circular geometry relative to the circular geometry. The difference in torque is attributed to the reduced contact area between drillstring and wellbore for the non-circular geometry.
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. The results show that a non-circular wellbore with spiral grooves can improve borehole hydraulics and mechanics.