Annular Velocity and Its Importance to Drilling Hydraulics

Annular velocity is the speed of fluid moving up the annulus and it must be high enough to transport cuttings generated while drilling from the wellbore. However, if the annular velocity is too high, it can create hole wash out and excessive equivalent circulating density.

When the drilling mud is circulated through a system, the moving speed is lower at location where the cross section area is bigger. Conversely, when the fluid flows through the small cross section area, the annular velocity at that point is higher.

The cross section area around drill collar and BHA has the smallest area so the annular velocity is the highest.


(Annular velocity around drill collar and BHA)

On other hand, the area around drill pipe has the biggest cross sectional area, hence, the speed of fluid around the drill pipe area is smallest.


(Annular velocity around drill pipe)

The annular velocity around drill pipe must be used to determine if it is good enough for hole cleaning because it is the lowest velocity in the wellbore. If the annular velocity around the drill pipe is good enough for hole cleaning purpose, it will definitely be sufficient for hole cleaning around drill collars, BHA and tool joints.

If you have some formations which can be easily washed out, you need to look at the annular velocity around drill collars, BHA and tool joints. The size of drill collars and BHA should be reduced if the flow rate can cause excessive wellbore erosion.

To adequately transport the cuttings from the wellbore, the annular velocity is affected by mud properties, rate of penetration, mud types, formation types, hole angle, size of cuttings, etc.

A formula for the annular velocity in an oilfield unit is shown below:


Va = annular velocity, ft/min

Q = flow rate, gallon per minute

Dh = hole diameter, inch

Dp = outside diameter of drill pipe, inch

Note: You can see the annular velocity calculation from this post => Annular Velocity Calculation

Reference:  Drilling Hydraulic Books

Tagged , . Bookmark the permalink.

About DrillingFormulas.Com

Working in the oil field and loving to share knowledge.

2 Responses to Annular Velocity and Its Importance to Drilling Hydraulics

  1. leon robinson says:

    ANNULAR VELOCITY DOES not “WASH A HOLE OUT” – have you ever run a real test to prove that was the only variable which changed when you decreased AV and the hole came back to gauge? No engineering data is available in the literature that proves this myth to be true. Lots of random observations but none where only the AV was changed to show washout. see SPE PAPER 30497

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.