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Leak Off Test Drilling Procedures & Calculations Sheet

Before starting leak off test drilling procedures, gauges should be checked for accuracy. Also the upper pressure limit should be determined. It is so simple to run leak off test drilling but if you exceeded certain pressure, you may break the formation and a loss of circulation may occur.

Leak Off Test Pressure Equations

Field Units

LOT in equivalent mud weight = (LOT pressure ÷ 0.052 ÷ Depth of Casing Shoe TVD ) + (Working 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

Important Hints For A Successful Leak Off Test

  1. The casing should be tested prior to drilling out the shoe.
  2. Drill out the shoe and cement, exposing 5 – 10 ft of new formation.
  3. Circulate and condition the mud, check mud density in and out before running Leak off Test.
  4. Pull the bit inside the casing. Line up cement pump and flush all lines to be used for the test.
  5. Close Blowout Preventer.
  6. With the well closed in, the cement pump is used to pump a small volume at a time into the well typically a 1/4  or 1/2  bbl per min. Monitor the pressure build up and accurately record the volume of mud pumped. Plot pressure versus volume of mud pumped.
  7. Stop the pump when any deviation from linearity is noticed between pump pressure and volume pumped.
  8. Bleed off the pressure and establish the amounts of mud, if any, lost to the formation.

Examples Of Leak Off Test Plot Interpretation

In non-consolidated or highly permeable formations fluid can be lost at very low pressures. In this case the pressure will fall once the pump has been stopped and a plot such as that shown in Fig 1a will be obtained. Figs 1b and 1c show typical plots for consolidated permeable and consolidated impermeable formations respectively.

Leak Off Test Drilling Curves
Fig 1

Working example of leak off test drilling procedure (floating rigs)

Operational Drilling Procedures for Floating Rigs is designed to determine the equivalent mud weight at which the formation will accept fluid. This test is not designed to break down or fracture the formation. This test is normally performed at each casing shoe.   Prior to the formation leak-off, have a handy piece of graph paper (see graph 1 ), pencil and straight edge (ruler). Utilizing the high pressure cement pumping unit, perform leak-off as follows:  

  1. Upon drilling float equipment, clean out rat hole and drill 15 ft of new hole. Circulate and condition hole clean. Be assured mud weight in and mud weight out balance for most accurate results.  
  2. Pull bit up to just above casing shoe. Install circulating head on DP.  
  3. Rig up cement unit and fill lines with mud. Test lines to 2500 psi. Break circulation with cementing unit, be assured bit nozzles are clear. Stop pumping when circulation established.  
  4. Close pipe rams. Position and set motion compensator, overpull drill pipe (+/- 10,000 lbs), close choke/kill  valves.  
  5. At a slow rate (i.e. 1/4  or 1/2  BPM), pump mud down DP as following
    1. Pump 1/4  bbl – record/plot  pressure on graph paper.
    2. Pump 1/4  bbl – record/plot  pressure on graph paper.
    3. Pump 1/4  bbl – record/plot  pressure on graph paper.
    4. Pump 1/4  bbl – record/plot  pressure on graph paper.
    5. Pump 1/4  bbl – record/plot  pressure on graph paper.
    6. Continue this slow pumping. Record pressure at 1/4  bbl increments until two points past leak-off. (See examples, Graph 1, 2 & 3.)
  6. Upon two points above leak-off, stop pumping. Allow pressure to stabilize. Record this stabilized standing pressure (normally will stabilize after 15 mins or so).
  7. Bleed back pressure into cement unit tanks. Record volume of bleed back.
  8. Set and position motion compensator, open rams.
  9. Rig down and cement unit lines. Proceed with drilling operations.
  10. Leak-off can be repeated after step 6 if data confirmation is required, otherwise leak off test is complete.  

NOTE: For 20″ and 13 3/8″ csg leak-off tests, plot pressure every 1/2 bbl. Results will be the same. It should be noted that in order to obtain the proper leak off and pumping rate plot, it will be necessary to establish a continuous pump rate at a slow rate in order to allow time to read the pressure and plot the point on the graph. (Barrels pumped vs. pressure – psi), normally 1/2  BPM is sufficient time.   A pressure gauge of 0-2000 psi with 20 or 25 increments is recommended.  

NOTE: In the event Standing Pressure is lower than leak off point. Use standing pressure to calculate equivalent mud weight. Always note volume of mud bled back into tanks.  

Leak Off test graph
leak off test drilling graph
graph

there are a lot of videos explaining how to perform a leak off test calculations and procedures. I have choose one of them which I found that it was simple and clear.    

Download Leak Off Test Calculation Sheet Report