Leak Off Test (Procedures and Calculation)

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.

Oilfield Unit

Leak Off  Test pressure in mud density

Leak off test in equivalent mud weight = (Leak Off Test pressure ÷ 0.052 ÷ Casing Shoe TVD ) + (Current Mud Weight)

Where;

Leak off test in equivalent mud weight in ppg

Leak Off Test pressure in psi

Casing Shoe TVD in ft

Current Mud Weight in ppg

Note: Always round down for LOT calculation

Example:

Leak off test pressure = 1,600 psi
Casing shoe TVD = 4,000 ft
Mud weight = 9.2 ppg
Leak off test in equivalent mud weight (ppg) = (1,600 psi ÷ 0.052 ÷ 4,000 ft )+ 9.2ppg  = 16.8 ppg

Metric Unit

Leak Off  Test pressure in mud density

Leak off test in equivalent mud weight  = (Leak Off Test pressure ÷ 0.00981 ÷ Casing Shoe TVD ) + (Current Mud Weight)

Where;

Leak off test in equivalent mud weight in kg/m3

Leak Off Test pressure in KPa

Casing Shoe TVD in m

Current Mud Weight in kg/m3

Note: Always round down for LOT calculation

Example:

Leak off test pressure = 1,740 KPa
Casing shoe TVD = 1,300 m
Mud weight = 1,100 Kg/m3
Leak off test in equivalent mud weight (Kg/m3) = (1,740 KPa ÷ 0.00981 ÷ 1,300 m)+ 1,100 Kg/m3 = 1,236 Kg/m3

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.

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.

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

Referring to the previous article, 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). In this article, you will learn how to how to convert specific gravity to mud weight  and pressure gradient in both oilfield and metric unit by using simple formulas as follows:

Convert Specific Gravity to Mud Weight In Oilfield Unit

1. Convert specific gravity to mud weight in pounds per gallon (ppg)

Mud weight in ppg = specific gravity (SG) × 8.33



Example: specific gravity = 1.50
mud wt, ppg = 1.50 × 8.33
mud wt = 12.5 ppg

2. Convert specific gravity to mud weight in lb/ft3

Mud weight in lb/ft3 = specific gravity × 62.4


Example: specific gravity = 1.50
Mud weight in lb/ft3 = 1.50 × 62.4 lb/ft3
Mud weight in lb/ft3 = 93.6 lb/ft3

3. Convert specific gravity to pressure gradient in psi/ft

Pressure gradient in psi/ft = specific gravity (SG) × 0.433

Example: specific gravity = 1.5
Pressure gradient in psi/ft  = 1.5 × 0.433
Pressure gradient in psi/ft = 0.650 psi/ft

Convert Specific Gravity to Mud Weight In Metric Unit

4. Convert specific gravity to mud weight in kg/m3

Mud weight in kg/m3 = specific gravity × 1000

Example: specific gravity = 1.5

Mud weight in kg/m3 = 1.5 × 1,000

Mud weight in kg/m3 = 1,500

5. Convert specific gravity to mud weight in g/cc

Mud weight in g/cc = specific gravity × 1.0

Example: specific gravity = 1.5

Mud weight in  g/cc = 1.5 × 1.0

Mud weight in g/cc  = 1.5

6. Convert specific gravity to pressure gradient in kPa/m

Pressure gradient in kPa/m = specific gravity (SG) × 9.81

Example: specific gravity = 1.5
Pressure gradient in kPa/m  = 1.5 × 9.81
Pressure gradient in kPa/m = 14.7

Please also find the Excel sheet  for  this topic.

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.

Calculate Specific Gravity (SG) in oilfield unit

This article demonstrates how to calculate Specific Gravity (SG) from different density or pressure gradient  in both oilfield and metric unit.

Calculate Specific Gravity from Mud Weight In Oilfield Unit

1. Calculate specific gravity using mud weight in PPG

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

Example: 
Mud weight = 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 (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 (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

Calculate Specific Gravity from Mud Weight In Metric Unit

4. Calculate specific gravity using mud weight in kg/m3

 Specific Gravity (SG) = mud weight in kg/m3÷ 1000

Example: 
Mud density = 1200 kg/m3
Specific Gravity (SG) = 1200 kg/m3÷ 1000
Specific Gravity (SG)= 1.2

5. Calculate specific gravity using mud weight in g/cc

 Specific Gravity (SG) = mud weight in g/cc÷ 1.0

Example: 
Mud density = 1.2 g/cc
Specific Gravity (SG) = 1.2 g/cc÷ 1000
Specific Gravity (SG)= 1.2

6. Calculate specific gravity using pressure gradient in kPa/m

 Specific Gravity (SG) = pressure gradient in kPa/m ÷ 9.81

Example: 
Pressure gradient = 10 kPa/m
Specific Gravity (SG) = 10 kPa/m÷ 9.81
Specific Gravity (SG)= 1.02

Please also find the Excel sheet  for  this topic – 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 into equivalent mud weight as per following formulas.

Oilfield Unit

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

Equivalent Mud Weight in 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

Metric Unit

Convert pressure in KPa unit into equivalent mud weight in Kg/m3 by using meters as the unit of measurement

Equivalent Mud Weight in Kg/m3 = Pressure in KPa × 1000  ÷ True Vertical Depth (TVD) in meters ÷  9.81

Example:

Pressure = 35,000 KPa
True Vertical Depth = 2,500 meters
Equivalent mud weight, kg/m3= 35,000 × 1,000 ÷ 2,500 ÷ 9.81
Equivalent mud weight, kg/m3= 1,427  kg/m3

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 pressure created by fluid column and two factors affecting hydrostatic pressure are mud weight and True Vertical Depth.

You can read full details about hydrostatic pressure from this article, Understand Hydrostatic Pressure.

In this article, it will demonstrate how to calculate hydrostatic pressure in different oilfield and metric unit.

1. Calculate hydrostatic pressure in psi by using mud weight  in pound per gallon (ppg) and feet as the units of True Vertical Depth.

Hydrostatic pressure equation:

Hydrostatic Pressure (HP) = 0.052 × Mud Weight × True Vertical Depth (TVD) 

Where;

Mud Weight in ppg

True Vertical Depth in ft

Example;

Mud Weight = 12.0 ppg
True Vertical Depth (TVD) = 10,000 ft
Hydrostatic Pressure (HP) = 0.052 × 12.0 × 10,000
Hydrostatic Pressure (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:

Hydrostatic Pressure (HP) = Pressure gradient in psi/ft × True Vertical Depth (TVD)

Where;

Pressure gradient in psi/ft

True Vertical Depth in ft

Example;

Pressure Gradient = 0.5 psi/ft
True Vertical Depth (TVD) = 10,000 ft
Hydrostatic Pressure (HP) = 0.5 psi/ft × 10,000 ft
Hydrostatic Pressure (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:

Hydrostatic Pressure (HP) = 0.006944 × Mud Weight × True Vertical Depth (TVD)

Where;

Mud Weight in lb/ft3

True Vertical Depth in ft

Example;

Mud Weight = 80 lb/ft3
True Vertical Depth (TVD) = 10,000 ft
Hydrostatic Pressure (HP) = 0.006944 × 80 lb/ft3 × 10,000 ft
Hydrostatic Pressure (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:

Hydrostatic Pressure (HP) = Mud Weight  × 0.052 × TVD in meters x 3.281

Where;

Mud Weight in ppg

True Vertical Depth in meter

Example:

Mud Weight = 12.0 ppg
True Vertical Depth (TVD) = 5000 meters
Hydrostatic Pressure (HP) = 12.0 × 0.052 × 5000 × 3.281
Hydrostatic Pressure (HP) = 10,237 psi

5. Calculate hydrostatic pressure in Pascal by using mud weight  in kg per cu-meter (kg/m3) and meter (m) as the units of True Vertical Depth.

Hydrostatic Pressure (HP) = 9.81 × Mud Weight × True Vertical Depth (TVD) 

Where;

Mud Weight in kg/m3

True Vertical Depth in meter

Example;

Mud Weight = 1,000 kg/m3
True Vertical Depth (TVD) = 100 m
Hydrostatic Pressure (HP) = 9.81 × 1,000 × 100
Hydrostatic Pressure (HP) = 981,000 pascal

6. Calculate hydrostatic pressure in bar by using mud weight  in kg per cu-meter (kg/m3) and meter (m) as the units of True Vertical Depth.

Hydrostatic Pressure (HP) = 9.81 × Mud Weight × True Vertical Depth (TVD) ÷ 100,000

Where;

Mud Weight in kg/m3

True Vertical Depth in meter

Example;

Mud Weight = 1,000 kg/m3
True Vertical Depth (TVD) = 100 m
Hydrostatic Pressure (HP) = 9.81 × 1,000 × 100 ÷ 100,000
Hydrostatic Pressure (HP) = 9.81 bar

7ุ. Calculate hydrostatic pressure in atm by using mud weight  in kg per cu-meter (kg/m3) and meter (m) as the units of True Vertical Depth.

Hydrostatic Pressure (HP) = 9.81 × Mud Weight × True Vertical Depth (TVD)  × 9.8692 × 10-6

Where;

Mud Weight in kg/m3

True Vertical Depth in meter

Example;

Mud Weight = 1,000 kg/m3
True Vertical Depth (TVD) = 100 m
Hydrostatic Pressure (HP) = 9.81 × 1,000 × 100 × 9.8692 × 10-6
Hydrostatic Pressure (HP) = 9.6817 atm

Download Free Excel Spread Sheet

Download the Excel sheet for calculating hydrostatic pressure – This is 2020 update with Metric Unit

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.