Leak Off Test (Procedures and Calcuation)

Leak Off Test is conducted in order to find the fracture gradient of certain formation. 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 increase 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.

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 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

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

Please find the Excel sheet tocalculate leak off test

Ref book: Drilling Formula Book Formulas and Calculations for Drilling, Production and Workover, Second Edition

Convert specific gravity to mud weight (ppg and lb/ft3) and pressure gradient (psi/ft)

Referring to the previous post, Calculate Specific Gravity (SG) in oilfield unit , you get specific gravity (SG) from mud weight (ppg and lb/ft3) and pressure gradient (psi/ft) but you may need to reverse calculation from specific gravity (SG) to mud weight or pressure gradient (psi/ft). So you will learn how to how to convert specific gravity to mud weight (ppg and lb/ft3) and pressure gradient (psi/ft) by using simple formulas as follows:

1. Convert specific gravity to mud weight in pounds per gallon (ppg)
Convert specific gravity fomula: mud weight in ppg = specific gravity (SG) x 8.33
Example: specific gravity = 1.50
mud wt, ppg = 1.50 x 8.33
mud wt = 12.5 ppg

2. Convert specific gravity to mud weight in lb/ft3
Convert specific gravity fomula: Mud weight in lb/ft3 = specific gravity x 62.4
Example: specific gravity = 1.50
Mud weight in lb/ft3 = 1.50 x 62.4 lb/ft3
Mud weight in lb/ft3 = 93.6 lb/ft3

3. Convert specific gravity to pressure gradient in psi/ft
Convert specific gravity fomula: Pressure gradient in psi/ft = specific gravity (SG) x 0.433
Example: specific gravity = 1.5
Pressure gradient in psi/ft  = 1.5 x 0.433
Pressure gradient in psi/ft = 0.650 psi/ft

Please also find the Excel sheet regarding how to convert specific gravity (SG)

Ref book: Drilling Formula Book Formulas and Calculations for Drilling, Production and Workover, Second Edition

Calculate Specific Gravity (SG) in oilfield unit

You may need to calculate specific gravity from different density or pressure gradient units. This post demonstrates you about specific gravity calculation and specific gravity formulas frequently used in oilfield business.

1. Calculate specific gravity using mud weight in PPG

 

Specific gravity formula : Specific Gravity (SG) = mud weight in ppg ÷ 8.33

Example: 
Fluid = 13.0 ppg
Specific Gravity (SG) = 13.0 ppg ÷ 8.33
Specific Gravity (SG)= 1.56

2. Calculate specific gravity using pressure gradient in psi/ft

 

Specific gravity formula : Specific Gravity (SG) = pressure gradient in psi/ft ÷ 0.433

Example: pressure gradient = 0.50 psi/ft
Specific Gravity (SG) = 0.50 psi/ft ÷ 0.433
Specific Gravity (SG) = 1.15

3. Calculate specific gravity using mud weight in lb/ft3

 

Specific gravity formula : Specific Gravity (SG) = mud weight in lb/ft3 ÷ 62.4

Example: Mud weight = 90 lb/ft3
Specific Gravity (SG)= 90 lb/ft3÷ 62.4
Specific Gravity (SG) = 1.44

Please also find the Excel Sheet: Specific Gravity Calculation Sheet

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.

Convert Pressure into Equivalent Mud Weight

Learn how to convert pressure into equivalent mud weight in PPG as per following formulas.

1. Convert pressure in psi  unit into equivalent mud weight in ppg by using feet as the unit of measurement

Equivalent Mud Weight, ppg = pressure in psi ÷ 0.052 ÷ True Vertical Depth (TVD) in ft
Example:
Pressure = 5,000 psi
True vertical depth = 8,000 ft
Equivalent mud weight, ppg = 5,000 psi ÷ 0.052 ÷ 8,000 ft
Equivalent mud weight, ppg = 12.0 ppg

2. Convert pressure in psi  unit into equivalent mud weight in ppg by using meters as the unit of measurement

Equivalent Mud Weight in ppg = Pressure in psi ÷ 0.052 ÷ True Vertical Depth (TVD) in meters ÷  3.281

Example: pressure = 5,000 psi
True Vertical Depth = 2,500 meters
Equivalent mud weight, ppg = 5,000 psi ÷ 0.052 ÷ 2,500 m ÷ 3.281
Equivalent mud weight, ppg = 11.7 ppg

Please find the Excel sheet for converting pressure into equivalent mud weight.

Ref book: 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.

Hydrostatic Pressure Calculation

Hydrostatic pressure is created by fluid column. Two factors affecting hydrostatic pressure are mud weight and True Vertical Depth. So this post will demonstrate how to calculate hydrostatic pressure in different oilfield units.

1. Calculate hydrostatic pressure in psi by using mud weight  in PPG and feet as the units of
True Vertical Depth.

Hydrostatic pressure equation:
HP = mud weight in ppg x 0.052 x True Vertical Depth (TVD) in ft

Example: mud weight = 12.0 ppg
True Vertical Depth = 10,000 ft
HP = 12.0 ppg x 0.052 x 10,000 ft
HP = 6,240 psi

2. Calculate hydrostatic pressure in psi by using pressure gradient in psi/ft and feet as the units of True Vertical Depth.

Hydrostatic pressure equation:
HP = Pressure gradient in psi/ft x True Vertical Depth (TVD) in ft

Example: Pressure Ggradient = 0.5 psi/ft
True Vertical Depth = 10,000 ft
HP = 0.5 psi/ft x 10,000 ft
HP = 5,000 psi

3 Calculate hydrostatic pressure in psi by using mud weight in lb/ft3 and feet as the units of True Vertical Depth.


Hydrostatic pressure equation:
HP = 0.006944 x mud weight, lb/ft3 x TVD, ft

Example: mud weight = 80 lb/ft3 true vertical depth = 10,000 ft
HP = 0.006944 x 80 lb/ft3 x 10,000 ft
HP = 5,555 psi

4. Calculate hydrostatic pressure in psi by using mud weight in PPG and meters as unit of True Vertical Depth.

Hydrostatic pressure equation:
HP = mud weight, ppg x 0.052 x TVD in meters x 3.281

Example: Mud weight = 12.0 ppg true vertical depth = 5000 meters
HP = 12.0 ppg x 0.052 x 5000 x 3.281
HP = 10,237 psi

Download the Excel sheet for calculating hydrostatic pressure

Ref book: Drilling Formula Book Formulas and Calculations for Drilling, Production and Workover, Second Edition