• Structure controlled, where the discontinuities around the wellbore control stability e.g. planar, wedge failure.

  • Stress-controlled, causing localised plastic behaviour which leads to a failure around the wellbore.

  • Chemical reaction, where the fluid content in rock mass changes the rock properties causing instabilities such as swelling of shale rock.

  • Fluid flow/rock interaction in which the fluid flow through the borehole or discontinuities exerts drag or lift forces on the formation and causes erosion.

One of the challenges of drilling petroleum and mineral exploration wells is borehole instability posing various risks such as poor drilling efficiency, drill string sticking/stuck, hole cleaning and fluid loss. One of the mechanisms of open borehole instability is a failure caused by the eroding action of drilling fluid in the annulus, called borehole erosion. The shear stress and lifting force mobilized on the formation and the impact of cuttings at the rock grains lead to detachment of formation grains and enlargement of the borehole. This failure is more critical when a turbulent regime is imposed in the annulus in a small gap between the drill string and the formation, which is usually the case during coiled tube drilling. While bore hole erosion, further cutting contamination can also occur from the borehole erosion, which is particularly important for the new drilling system being developed by the Deep Exploration Technologies Cooperative Research Centre (DET CRC) for mineral exploration use. This paper aims to review existing experiments of soil or rock erosion and to introduce new experimental setup for borehole erosion study.

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