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What is Annular Velocity and Why Does It Matter?

Annular Velocity (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 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-sectional 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 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).

Figure 2

Figure 1

Figure 3

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Good AV

For large hole sizes greater than 16in (406mm), it is often not practical to achieve flow rates high enough such that the AV is in the 150-200 ft/min (46-61 m/min) range. For example, in order to achieve an AV of 150 ft/min in a 17 1/2″ hole with 5.5in drill pipe, a minimum flow rate of 1684 gpm (6375 lpm) would be required. Therefore a more practical range of 900 – 1200 gpm is recommended by Merlin for this hole size range. However, because the AV will be below 150 ft/min this flow range, it is even more important to maximise the RPM and 6rpm rheology, within system limitations and Merlin recommendations.

For example, with reference to the above hole / pipe combination and Merlin’s recommended parameters table, below, whatever flow rate in the 900 – 1200 gpm range is selected, it is strongly recommended to have a 6 rpm rheology in low twenties and rotate at above 150 rpm. Another option would be to consider drilling a 16″ hole, rather than 17 1/2″.

For this hole size, the Merlin recommended range of 800 – 1000 gpm most closely matches the 150 – 200 ft/min range for most typical pipe sizes. In general, any AVs below 150 ft/min are likely to be compromising the effectiveness of the hole cleaning system, while AVs greater than 200 ft/min should be providing for good hole cleaning (provided the RPM and low-end rheology is within the recommended range), with the main limiters being stand pipe pressure and ECD vs facture gradient.

For hole sizes less than 12 1/4″, achieving 150 ft/min, or even 200 ft/min is much less of a challenge and the predominant challenge is typically going to shift more towards ECD limitations. That two important elements need to be kept in mind when drilling these hole sizes:

1. Maintaining rpm and low-end rheology within the recommended ranges is still essential for effective hole cleaning.

2. Be aware of liners: always model the AV (with the relevant drill pipe) above the top of the liner where this is in a high-angle section of the hole, as this zone will often represent the weak point of the whole system. In these cases, it is worth considering the use of a circulation sub to improve the AV above the liner, if required. Note that a split flow circulation sub will be required as rotating the BHA without flow can lead to rapid over-heating.

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