When you get differentially stuck, you may consider spotting light weight fluid to reduce force created by differential pressure between mud in wellbore and formation pressure. However, you really need to understand and be able to calculate how much light fluid you can put in hole that it will not create well control situation by accidentally underbalance formation pressure. This post demonstrates you how to determine maximum height of light weight spot pill that can be in the annulus.
Firstly, we really need to know how much under balance with current mud weigh is. You may get this information by calculating current hydrostatic pressure minus expected pressure based on pressure profile. Secondly, you must know how much your light weight pill is. Then please follow the steps below to determine how height of light weight spot pill in the annulus should be.
a) Determine the difference in pressure gradient in psi/ft between the mud weight and light weight spot fluid:
Difference pressure in pressure gradient in psi/ft = (current mud wt in ppg – light weight spot pill in ppg) x 0.052
b) Determine height in ft of light weight spot fluid that will balance formation pressure in the annulus:
Height ft in vertical = overbalance pressure with current mud weigh in psi ÷ difference in pressure gradient in psi/ft
This height is the maximum allowable height of light weight spot pill in the annulus. If you have higher length of light weight spot pill in the annulus, it may create well control situation.
Example: Use the following data to determine the height in ft of light weight spot pill that will balance formation pressure in the annulus:
Mud weight = 13.0 ppg
Light weight spot pill = 8.3 ppg
Amount of overbalance = 300 psi
a) Difference in pressure gradient in psi/ft:
Difference pressure in pressure gradient in psi/ft = (13 ppg – 8.3 ppg) x 0.052
Difference pressure in pressure gradient in psi/ft = 0.2444
b) Determine the height in ft of light weight spot liquid that will balance formation pressure in the annulus:
Height = 300 psi ÷ 0.244 psi/ft
Height = 1227 ft
You must ensure than height of light weight pill in the annulus must less than 1227 ft in order to prevent wellcontrol situation.
Please find the Excel sheet to calculate maximum height in annulus of light weight spot pill.
Ref book: Formulas and Calculations for drill, production and workover by Norton J. Lapeyrouse
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