# Leak Off Test (Procedures and Calcuation)

Leak Off Test is conducted in order to find the fracture gradient of certain formation at casing shoe depth. The results of the leak off test also dictate the maximum equivalent mud weight that should be applied to the well during drilling operations.

Leak Off Test (LOT) guide line procedures are as follows  (note: this is just only guide line.

You may need to follow your standard procedure in order to perform leak off test):

1.Drill out new formation few feet, circulate bottom up and collect sample to confirm that new formation is drilled to and then pull string into the casing.

2.Close annular preventer or pipe rams, line up a pump, normally a cement pump, and circulate through an open choke line to ensure that surface line is fully filled with drilling fluid.

3.Stop the pump and close a choke valve.

4.Gradually pump small amount of drilling fluid into well with constant pump stroke. Record total pump strokes, drill pipe pressure and casing pressure. Drill pipe pressure and casing pressure will be increased continually while pumping mud in hole. When plot a graph between strokes pumped and pressure, if formation is not broken, a graph will demonstrate straight line relationship. When pressure exceeds formation strength, formation will be broken and let drilling fluid permeate into formation, therefore a trend of drill pipe/casing  pressure will deviate from straight line that mean formation is broken and is injected by drilling fluid. We may call pressure when deviated from straight line as leak off test pressure.

5.Bleed off pressure and open up the well. Then proceed drilling operation.

Note:  the way people call leak off test pressure depends on each company standard practices.

Leak off test pressure can be calculated into equivalent mud weight in ppg as formula below:

Leak off test in equivalent mud weight (ppg) = (Leak off test pressure in psi) ÷ 0.052 ÷ (Casing Shoe TVD in ft) + (current mud weight in ppg)

Pressure gradient can be determined from Leak Off Test pressure by the following formula.

Pressure gradient in psi/ft = (Leak off test pressure in psi) ÷ (Casing Shoe TVD in ft)

Example:

Leak off test pressure = 1600 psi
Casing shoe TVD = 4000 ft
Mud weight = 9.2 ppg

Leak off test in equivalent mud weight (ppg) = 1600 psi ÷ 0.052 ÷ 4000 ft + 9.2ppg ppg = 16.9

Pressure gradient = 1600 ÷ 4000 = 0.4 psi/ft

Please find the Excel sheet to calculate leak off test

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.

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### 17 Responses to Leak Off Test (Procedures and Calcuation)

1. Rufor says:

Hello,
http://www.drillingformulas.com – da best. Keep it going!

Thanks
Rufor

2. kumaran says:

thanks for wonderful explanation

3. Janusz Danda says:

Many thanks!

4. Danny boy says:

hi there ! in your example you do have 1600 psi given for leak off test pressure.
If you don’t have the given 1600 psi., How will you able to calculate the leak off test and get the result for maiximum allowable mud density? I’am confused because Leak off test pressure is one component for getting maximum allowable mud weight..I hope you understand my query or question.Your answer will help me a lot..thank you so much !!!

• Hi Danny,
The 1600 psi is just for example number calculation. For the actual leak off number, you need to get it when you attempt to do the leak off at shoe.

5. Loyai alshafte says:

I study at Tripoli university and I have to make a presentation about leak off test and i got a clear information on your website thank you so much

6. jim says:

Is there any way to calculate LOT if you have FIT ? or any other way to find an estimate of LOT (value) ?

• The calculation concept of FIT and LOT is the same. If you get FIT, your equivalent mud weight is equal to your planned FIT value.

7. s.alba says:

Hi there,

Is this pressure (1600psi) obtained at the shoe or at surface? I am having problems with that. If I have a leak off pressure at surface of 1600 psi calculated using a 10.5ppg mud, in order to calculate the break down pressure at casing shoe (i.e 7000 ft) should I use the following formula?

Break down pressure = LOP at surface + (0.052x7000x10.5) = 1600 + (0.052x7000x10.5)

Thanks in advance and congrats for this website!!

• This is correct. 1600 psi is surface pressure. Like what you show, you add surface pressure and hydrostatic pressure which will be equal to break down pressure at shoe in psi.

8. prince says:

when calculating total break down pressure,do i use total depth of well or depth of casing shoe