Leak Off Test Procedure
A Leak-Off Test (LOT) will be performed On Wild-Cat wells at each casing shoe after setting the surface casing. LOTs are also recommended to be carried out on both appraisal and development wells.
Leak Off Test and Formation Integrity Test (FIT), also termed the Limit Test, are for formation strength pressure tests made just below the casing seat prior to drilling ahead.
These tests are carried out to:
- Investigate the cement seal around the casing shoe which should be at least as high as the predicted fracture pressure for the area.
- Investigate the wellbore capability to withstand pressures below the casing shoe in order to allow proper well planning with regard to the setting depth of the next casing, mud weights and alternatives during well control operations.
- Collect regional information on formation strengths and stress magnitude for different applications including optimisation of future well planning, hole stability analysis and modelling, reservoir application.
Prior to a test, a decision should be made to either increase the pressure until leak off occurs (as in the LOT) or to stop at a predetermined pressure for a (FIT). It should be noted that:
- FIT does not obtain information on stress magnitude.
- A LOT is designed and should be performed to determine, in a better way, the desired data without breaking down the formation.
When a Formation Integrity Test is required, the maximum pumping pressure is often expressed in terms of 'Equivalent Mud Weight' (EMW):
Imperial units EMW =-+ MW
P = Pumping Pressure (psi) TVD = True Vertical Depth (ft) MW = Actual Mud Weight (PPG)
Pumping Pressure (kg/cm ) True Vertical Depth (m) Actual Mud Weight (kg/l)
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