lot test
DESCRIPTION
lot testTRANSCRIPT
Fracture pressure is the pressure in the wellbore at which a
formation will crack
The stress within a rock can be resolved into three principal stresses.
A formation will fracture when the pressure in the borehole exceeds
the least of the stresses within the rock structure. Normally, these
fractures will propagate in a direction perpendicular to the least
principal stress.
Fracture Formation Pressure Definition and Mechanism
At sufficient depths (usually below 1000 m or 3000 ft) the minimum
principal stress is horizontal; therefore, the fracture faces will be
vertical. For shallow formations, where the minimum principal stress
is vertical, horizontal (pancake) fractures will be created.
Fracture Formation Pressure Definition and Mechanism
The pressure at which formations will fracture when exposed to
borehole pressure is determined by conducting one of the following
tests:
• Leak-off test
• Limit Test
• Formation Breakdown Test
Fracture Formation Pressure The Leak-off Test – Limit Test - Formation Breakdown Test
The procedure used to conduct these tests is basically the same in all
cases. The test is conducted immediately after a casing has been set
and cemented. The only difference between the tests is the point at
which the test is stopped. The procedure is as follows:
1. Run and cement the casing string
2. Run in the drillstring and drillbit for the next hole section and drill
out of the casing shoe
3. Drill 5 - 10 ft of new formation below the casing shoe
4. Pull the drillbit back into the casing shoe (to avoid the possibility of
becoming stuck in the openhole)
Fracture Formation Pressure The Leak-off Test – Limit Test - Formation Breakdown Test
5. Close the BOPs (generally the pipe ram) at surface
6. Apply pressure to the well by pumping a small amount of mud
(generally 1/2 bbl) into the well at surface. Stop pumping and record
the pressure in the well. Pump a second, equal amount of mud into
the well and record the pressure at surface. Continue this operation,
stopping after each increment in volume and recording the
corresponding pressure at surface. Plot the volume of mud pumped
and the corresponding pressure at each increment in volume.
7. When the test is complete, bleed off the pressure at surface, open the BOP rams and drill ahead
It is assumed in these tests that the weakest part of the wellbore is the formations which are exposed just below the casing shoe. It can be seen in the next slide that when these tests are conducted, the pressure at surface, and throughout the wellbore, initially increases linearly with respect to pressure. At some pressure the exposed formations start to fracture and the pressure no longer increases linearly for each increment in the volume of mud pumped into the well. If the test is conducted until the formations fracture completely, the pressure at surface will often drop dramatically.
5. Close the BOPs (generally the pipe ram) at surface
6. Apply pressure to the well by pumping a small amount of mud
(generally 1/2 bbl) into the well at surface. Stop pumping and record
the pressure in the well. Pump a second, equal amount of mud into
the well and record the pressure at surface. Continue this operation,
stopping after each increment in volume and recording the
corresponding pressure at surface. Plot the volume of mud pumped
and the corresponding pressure at each increment in volume.
The “Leak-off test” is
used to determine the
pressure at which the
rock in the open hole
section of the well just
starts to break down (or
“leak off”). In this type of
test the operation is
terminated when the
pressure no longer
continues to increase
linearly as the mud is
pumped into the well. The “Limit Test” is used to
determine whether the
rock in the open hole
section of the well will
withstand a specific,
predetermined pressure.
This pressure represents
the maximum pressure that
the formation will be
exposed to while drilling
the next wellbore section.
While performing a leak off test, the surface pressure at leak off was
940 psi. The casing shoe was at a true vertical depth of 5010 ft and a
mud weight of 10.2 ppg was used to conduct the test. Calculate the
maximum allowable mud weight.
The Maximum bottom hole pressure during the leakoff test can be
calculated from: hydrostatic pressure of column of mud + leak off
pressure at surface
= (0.052 x 10.2 x 5010) + 940 = 3597 psi
The maximum allowable mud weight at this depth is therefore
= 3597 psi / 5010 ft = 0.718 psi/ft = 13.8 ppg
Allowing a safety factor of 0.5 ppg,
The maximum allowable mud weight = 13.8 - 0.5 = 13.3 ppg.
Fracture Formation Pressure Example