The Leak-off Test

[dropcap]T[/dropcap]he leakoff test is the ultimate method for the positive determination of the maximum mud weight permitted in the open hole section of the well. The crew performs the test in the first few feet of a new hole drilled after a new casing point, which is the likely weakest point of the open hole section if no highly permeable formations exist further down hole. The test result when converted to equivalent mud density determines the maximum mud weight that the section can withstand without loss of circulation.

Leakoff tests should be run usually of a few wells in a new block. The test
consists of closure of the hole at surface, then application of pressure until mud just begins to inject into the formation. A Leakoff Test is usually as follows:

1. After cementing casing, run in hole with bit and drillstring.
2. Pressure test casing , then drill out casing shoe and a further minimum of
10feet of new formation.
3. Pull bit up to casing shoe.
4. With bit at shoe depth, shut off pumps, wait for flow to cease then close the kelly cock and blow out preventer.(mainly the annular preventer)
5. Then use cementing unit to pump drilling mud slowly through the choke line into the hole annulus. While pumping, always monitor the pressure build up and volume pumped.
6. The pressure build-up should be more or less linear until mud begins to bleed into the formation. The pressure at which the build-up curve departs from linearity is the Leakoff pressure (PLOT)
7. As pumping continues, the build-up curve flattens out until pressure no longer increases. At this point, the pump is injecting mud into the formation pores and fractures. The pressure of the mud at this point is the INJECTION
PRESSURE.
8. At injectivity point, pump should be shut off and the choke line closed.
9. Monitor the pressure. Normally at this point the shut-in pressure will fall until it reaches an equilibrium point that is slightly above the leakoff Pressure. The equilibrium point is the Bleedoff point

10.Hold bleed off pressure for several minutes to confirm that no breakdown has taken place. If bleedoff pressure remains steady, open the choke valve to vent the rest of the pressure

Bottom Hole Pressure at Leakoff

The leakoff pressure determines the Bottom Hole Pressure at leakoff. The
maximum mud weight or ECD permitted can then be calculated.
The equation for the BHP is as follows

BHP(at leakoff) = 0.052rmD + PLOT
PLOT = Leakoff pressure(psi)
rm = Mud Density, lb/gal
D = True Vertical Depth of well, ft.
Maximum Mud Weight Permitted is computed to be
rmax = BHP/(O.052*D)