¿Qué es la Velocidad Anular y por qué es importante?

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.

Se recomienda lograr una VA mínima de 150 ft/min (46 m/min) para asegurar un transporte efectivo de recortes en pozos de alto ángulo. Aunque la eficiencia de limpieza del pozo generalmente aumenta con una mayor VA, también lo hace la Densidad Equivalente en Circulación (ECD), por lo que 200 ft/min (61 m/min) suele considerarse como un valor superior óptimo. La importancia relativa de la VA, junto con la viabilidad práctica de alcanzar ese rango, depende de la combinación específica de tamaño del pozo y tamaño de la tubería. Por lo tanto, los rangos de caudal recomendados por Merlin para cada tamaño de pozo (ver Figura 3) no siempre coinciden con el rango de VA mencionado. Sin embargo, el valor de VA siempre debe calcularse y tenerse en cuenta como un indicador de si la VA podría ser un punto débil en el sistema general de limpieza del pozo.

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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Velocidad Anular vs Limpieza del Pozo