The leak-off test establishes a practical value for the input into fracture pressure predictions and indicates the limit of the amount of pressure that can be applied to the wellbore over the next section of hole drilled. It provides the basic data needed for further fracture calculations and it also tests the effectiveness of the cement job. The test is performed by applying an incremental pressure from the surface to the closed wellbore/casing system until it can be seen that fluid is being injected into the formation. Leak-off tests should normally be taken to this leak-off pressure unless it exceeds the pressure to which the casing was tested. In some instances as when drilling development wells this might not be necessary and a formation competency test, where the pressure is only increased to a predetermined limit, might be all that is required.
Leak-Off Test Procedure:
Before starting, gauges should be checked for accuracy. The upper pressure limit should be determined.
- The casing should be tested prior to drilling out the shoe.
- Drill out the shoe and cement, exposing 5 – 10 ft of new formation.
- Circulate and condition the mud, check mud density in and out.
- Pull the bit inside the casing. Line up cement pump and flush all lines to be used for the test.
- Close BOPs.
- 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.
- Stop the pump when any deviation from linearity is noticed between pump pressure and volume pumped.
- 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. This graph bellow show leak-off test process with graph P Vs Barrel mud Pumping
Working example of leak-off test 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 “handy” a piece of graph paper (see graph 1 ), pencil and straight edge (ruler). Utilising the high pressure cement pumping unit, perform leak-off as follows:
- 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.
- Pull bit up to just above casing shoe. Install circulating head on DP.
- 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.
- Close pipe rams. Position and set motion compensator, overpull drillpipe (+/- 10,000 lbs), close choke/kill valves.
- At a slow rate (i.e. 1/4 or 1/2 BPM), pump mud down DP.
- Pump 1/4 bbl – record/plot pressure on graph paper.
- b. Pump 1/4 bbl – record/plot pressure on graph paper.
- c. Pump 1/4 bbl – record/plot pressure on graph paper.
- d. Pump 1/4 bbl – record/plot pressure on graph paper.
- e. Pump 1/4 bbl – record/plot pressure on graph paper.
- f. Continue this slow pumping. Record pressure at 1/4 bbl increments until two points past leak-off test.
- g. 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).
- h. Bleed back pressure into cement unit tanks. Record volume of bleed back.
- i. Set and position motion compensator, open rams.
- j. Rig down and cement unit lines. Proceed with drilling operations.
- k. 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. Resultswill 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.
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