Rig Site Tips: Formation Pressure Tests

The Leak Off Test and the formation integrity test are pressure tests which are carried for many reasons:
- To investigate the strength of the cement around the casing shoe and to ensure that there is no communication with higher layers of formation.
- To estimate the fracture gradient in order to determine the upper limit of the mud weight which is the primary well control barrier.
- To collect information to use them for the optimization of the well design for future projects
- The information gathered from this test can be used to select the casing seats
- It can be used also for planning the hydraulic fracturing.

There are three types of formation pressure tests:

Shoe bond test: The objective of this test is verifying the integrity of cement bond around the casing shoe.
Formation Integrity Test (FIT): This test is carried out to investigate the formation strength at any depth. This test is performed for a pre-determined surface pressure.
Leak Off Test (LOT): It is similar to the FIT. The main difference between Leak Off Test and Formation Integrity Test is that LOT is that the formation is pressurized till getting to the injection level. The leak off test can lead to get the fracture gradient not like the formation integrity test which can give just an idea about the resistance of the formation at a certain level of pressure.
During these tests, the surface pressure can be converted to equivalent mud weight using the formulas below:
EMW= (P/(0.052 x TVD)) + MW

P: Surface Pressure or Test Pressure (Psi)
TVD: True vertical depth (ft)
MW: Actual Mud Weight (ppg)

Pressure Test Procedure

All the formation pressure tests are performed similarly, the difference is the level of surface pressure reached at each test. Cementing unit is used to carry out these tests due to the precision of the gauges and the accurate pumping rates. The mud pumps are not recommended to be used in such tests. these are some hints to perform the test safely and accurately.

- The cementing unit has to be checked and tested prior to the test. 
- To get accurate and precise data from the tests, the bore hole has to be cleaned. The well has to be circulated for at least one bottom-up volume. The mud should be cleaned and conditioned
- The drilling bit have to be pulled inside the casing above the casing shoe.
- Pumping circuit: the test can be performed using the inner string or through the choke line. When using the choke line to test the formation, it is recommended to disconnect the top drive and use a safety valve to isolate the drill string even with the use of the float valve.
- The testing circuit should be flashed and tested. A break circulation should be established through opened BOP till getting return into the flow line. A valve should be at the upstream of the pressure gauges when getting to pressure stabilization to monitor the trend of the pressure.
- After checking and testing the lines, close the upper rams or the annular BOP.
- Pump slowly till noticing pressure build up.
- Depending on the company policy, the pumping procedure is preformed continuously or by steps till getting the desired testing pressure.
- The pumping rate has to be the lowest possible rate. Flow rate should be at the range of 0.25 bbl/min to 0.50 bbl/min.
- The pumping rate should be kept constant and steady during the test.
- If the test is performed by volume increments, a time steps of 2 to 3 minutes are used to wait for pressure stabilization. 
- The pumping is continued depending on the objective of the test. It is ceased when getting to the predetermined pressure or when noticing the deviation from straight trend of pressure. The point of deviation is called the Leak Off Point.
- The pressure is monitored for at least 15 minutes till getting the stabilization. 
- Bleed off the pressure slowly and smoothly then record the fluid volume returned to the cementing unit and compare it with the volume used during the test. 
- Open the BOP, then come back to the bottom and start the operations normally.
The surface pressure reached during the test should be converted to equivalent mud weight scale and reported to the office.

Fig1 : Qualitative graph of the formation integrity test

Further Reading

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