So use the following formula to calculate annular pressure loss. This formula will give you idea about how much annular pressure loss you will encounter.
P= [(1.4327 x 10-7) x MW x Lx V2] ÷ (Dh – Dp)
P = annular pressure losses, psi
MW = mud weight in ppg
L = length of annular in ft
V = annular velocity in ft/min
Dh = hole or casing ID in inch
Dp = drill pipe or drill collar OD in inch
Mud weight = 13.0 ppg
Length = 8000 ft
Circulation rate = 320 gpm
Hole size = 6.5 in.
Drill pipe OD = 4.0 in.
Determine annular velocity, ft/mm: v = (24.5 x 320) ÷ (6.52 – 4.02)
v = 299 ft/min
Determine annular pressure losses, psi: P = [(1.4327 x 10-7) x 13.0 x 8000 x 2992] ÷ (6.5 – 4.0)
P = 531.65 psi
Please find the Excel sheet to calculate annular pressure loss
PS, if you want to get the more accurate, you may need to contact your drilling fluid company to do it for you because they have the actual database and more complex formula that may help you get the good number.
Ref books: Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.
Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.
Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.