Page 24 - Well Control for Completions and Interventions

18 Well Control for Completions and Interventions

with depth, a rough approximation of fracture pressure can be estimated
if it is assumed that average density of the overlying formation and the
associated liquids is roughly equivalent to a gradient of 1 psi/ft (22.6 kPa/m).
For most completion and intervention activities, fracture pressure will
have been determined during the drilling of the well by performing a
leak-off test (LOT).

1.1.5.1 Formation leak-off tests
A formation LOT is performed to confirm the integrity of the cement
bond, and the formation directly below the casing seat. Normally, the
zone directly beneath the casing seat is assumed to be the weakest point
during the drilling of the next hole section. Since it is the shallowest part
of the next section of formation to be drilled, it will have the lowest
overburden pressure.
LOTs are normally carried out at each casing point. After setting,
cementing, and testing the new casing string, the shoe track and casing
shoe are drilled out, and a few feet of new formation drilled. Normally,
this is about 15 ft, to ensure enough formation is exposed. A formation
LOT is then performed. A routine formation LOT is typically performed
as follows:
• Check pressure gauges are working and have been recently calibrated.
• Condition (circulate) the mud to ensure weight is consistent through-
out the system and confirm mud density.
• Ensure the bit is back inside the casing shoe, then close the well (close
the BOP annular preventer or pipe rams).
• Start to slowly increase pressure by pumping a small volume at a steady
1
1
rate (/4 /2 bbl/min). Measure and record the pressure increase against
volume pumped.
• Note: Slightly different techniques are used by some operators.
Some will increase pressure incrementally, stopping between incre-
ments. Others do not like to pump into a closed system, and will
circulate whilst increasing back pressure by gradually closing the
choke.
• As the formation fractures, mud will start to leak into the formation,
and the rate of pressure increase will fall off. Pump rate should be
reduced.
• When no further increase in pressure is observed, or pressure begins
to fall off, stop pumping.