FST Procedure

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This article presents Formation Strength Tests Equipment and Procedure.

Equipment Requirements

A formation strength test requires:

  • a Cement Pump with mud tank calibrated in ¼ bbl
  • calibrated gauges covering anticipated pressure range mounted on a dedicated manifold
  • a chart recorder
  • a recording form.

Test Pressure Limitations

The test pressure shall be limited to a maximum pressure that does not exceed the lowest:

  • leak-off pressure.
  • limited pressure advised in the Drilling Programme (Limit Test) of the casing burst pressure
  • wellhead test pressure
  • BOP test pressure.

Procedure

The ‘hesitation’ method shall be used for all FST’s.  The following procedure should be adhered to:

1. DRILL OUT of the casing shoe to a minimum of 10ft of new formation below the shoe, unless advised differently in the Well Programme.

2. CIRCULATE and CONDITION the new mud until densities in and out are the same (no foam).

Note: At least one full circulation shall be made. The mud density out shall be checked using a pressure mud balance. All leak-off  tests shall be carried out with the mud gradient necessary to  over balance the anticipated reservoir pressures at the shoe by  200 psi.

3. PULL the bit back into the casing shoe.

4. ENSURE that the hole is filled up then CLOSE the BOP around the drillpipe.

5. PUMP mud slowly using the cement unit until the pressure builds up.

Note: Rig pumps shall not be used.

6. PUMP fixed volumes in pre-determined steps by using the calibrated displacement tank.

7. RECORD the pressure as soon as pumping ends and the gauge needle stabilises. WAIT two minutes then RECORD the static pressure.

8. REPEAT Steps 6 and 7 plotting versus cumulative volume pumped until the maximum pressure determined in accordance with company policies has been achieved or until formation breakdown occurs.

Note: Experience proves that graph should be plotted as the test is ongoing for optimum results. Formation breakdown is indicated when the final pump pressure deviates from the final     static pressure after shut in time. This point should coincide with the point where the pressure-volume plot departs from the normal straight line relationship.

9. MAINTAIN the well closed in for five minutes to verify that a constant pressure has indeed been obtained.

10. BLEED OFF pressure into the calibrated tank and CALCULATE and RECORD the volume of mud lost to the formation.

Note: If leak-off occurs before the minimum limit specified in the Drilling Programme is reached the Drilling Engineer shall be notified immediately to determine if remedial action or changes to the Drilling Programme are required.  The leak-off test results shall be reported in the morning reports.

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