Annular Velocity vs
Limpieza de Pozo

Annular Velocity Calculator (AV) is one of the “Three Pillars” of effective hole cleaning in high-angle wellbores, with the other two being pipe rotation and the low-end rheology of the drilling fluid.

Figure 2

In general, it is recommended to achieve a minimum AV of 150 ft/min (46 m/min) in order to provide for effective cuttings transportation in high-angle holes. While hole cleaning efficiency will generally always increase with increasing AV, ECD will also increase and so 200 ft/min (61 m/min) is often considered as an optimal upper value. The relative importance of AV, along with the practicalities achieving the above range, is dependent on the specific hole-size / pipe-size combination. Therefore, Merlin's recommended flow ranges for each hole size (see Figure 3) don't always correspond with the above AV range. However, the AV value should still always be calculated as an indicator as to whether the flowrate could be a weak link in the overall hole cleaning system.

Figure 3

Please note that the above AV calculations and associated discussion is only valid where the hole retains the assumed gauge. Over-gauge hole (e.g. due to either washouts or borehole instability), can be one of the main enemies of good hole cleaning in high-angle wellbores, as the over-gauge sections can act as cuttings traps, presenting an increased pack-off risk during trips. Where the over-gauge section is due to wellbore instability, then there will likely be cavings to deal with as well, which present an even greater hole cleaning challenge (more material to remove and material that is harder to remove).