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