The pressure at which formations will fracture when exposed to borehole pressure is determined by conducting one of the following tests:
1. Leak-off test
2. Limit Test
3. Formation Breakdown Test
The basic principle of these tests is to conduct a pressure test of the entire system in the wellbore (See Figure 21 ) and to determine the strength of the weakest part of this system on the assumption that this formation will be the weakest formation in the subsequent open hole. The wellbore is comprised of (from bottom to top): the exposed formations in the open hole section of the well (generally only 5-10ft of formation is exposed when these tests are conducted); the casing (and connections);
the wellhead; and the BOP stack. 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)
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. (Figure 22).
(Note: the graph shown in Figure 21 represents the pressure all along the wellbore at each increment. This shows that the pressure at the formation at leak off is the sum of the pressure at surface plus the hydrostatic pressure of the mud).
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 Figure 21, 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 (see point A in Figure 22). If the test is conducted until the formations fracture completely (see point B in Figure 22) the pressure at surface will often dop dramatically, in a similar manner to that shown in Figure 22.
The precise relationship between pressure and volume in these tests will depend on the type of rock that is exposed below the shoe. If the rock is ductile the behaviour will be as shown in Figure 22 and if it is brittle it will behave as shown in Figure 23.
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 (See Figure 24). In practice the pressure and volume pumped is plotted in real time, as the fluid is pumped into the well.
When it is seen that the pressure no longer increases linearly with an increase in volume pumped (Point C) it is assumed that the formation is starting to breakdown.
When this happens a second, smaller amount of mud (generally 1/4 bbl) is pumped into the well just to check that the deviation from the line is not simply an error (Point D). If it is confirmed that the formation has started to “leak off” then the test is stopped and the calculations below are carried out.
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 whilst drilling the next wellbore section. The pressure to volume relationship during this test is shown in Figure 25. This test is effectively a limited version of the leak-off test.
The “Formation Breakdown Test” is used to determine the pressure at which the rock in the open hole section of the well completely breaks down. If fluid is continued to be pumped into the well after leak off and breakdown occurs the pressure in the wellbore will behave as shown in Figure 26.
Institute of Petroleum Engineering, Heriot-Watt University