What is Annular Velocity and Why Does It Matter?
Annular Velocity Formula (AV) is one of the three key elements of effective hole cleaning in high-angle wellbores (“The Three Pillars”, with the other two being pipe rotation and low-end rheology of the drilling class fluid). AV in a high-angle wellbore varies around the annulus due to the drilling tubulars lying on the low side of the hole (see Figure 1), which is why pipe rotation and rheology play such important roles in enabling solids to be removed from the wellbore (see Figure 2). However, it is the average value of AV, as calculated from the volume flowrate and the Annular Cross Section area, which is used as one of the three key hole cleaning effectiveness parameters.
In general, it is recommended to acheive 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 and borne in mind as an indicator as to whether or not AV could be a weak link in the overall hole cleaning system.
Please note that the above AV calculations and associated discussuion is only valid where the hole retains the assumed gauge. Over-gauge hole (e.g. due to either washouts borehole instability), can be one of the main enemies of good Annular Velocity for Hole Cleaning in high-angle wellbores, as the overgauge 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).
