Detailed explanation of formation fracture pressure test!

May 06, 2022

1. Formation fracture pressure and formation leakage pressure
Formation fracture pressure refers to the pressure that the formation can withstand when fractures and fractures occur at a certain depth. When the fracture pressure of the formation is reached, the original fractures in the formation expand and extend or the formation without fractures produces fractures. Under normal circumstances (following the compaction law), the formation fracture pressure increases with the increase of the well depth.
During drilling, the lower limit of the drilling fluid column pressure should be kept in balance with the formation pressure to achieve pressure control. The upper limit cannot exceed the fracturing pressure of the formation to avoid lost circulation caused by fracturing the formation.
Formation loss pressure refers to the pressure at which drilling fluid is lost in the formation at a certain depth. For high-permeability sandstones, fractured formations and fault fracture zones with normal pressure, the formation leakage pressure is often much smaller than the fracture pressure, and it is very harmful to the safe operation of drilling. It is customary to use the formation leakage pressure as the basis for determining the shut-in pressure for well control operations. This tends to be more secure.

2. Method for determining formation fracture (loss) pressure

2.1. Prediction method - using empirical formulas to predict formation fracture pressure as a basis for drilling design.
2.2. Verification method - After the casing is cemented, a leak test must be carried out to verify the predicted rupture pressure.

It is worth noting that the data obtained from hydraulic tests in the same formation in vertical wells and directional wells cannot be used interchangeably.
When the first layer below the casing shoe is a brittle rock layer, only the ultimate pressure test is performed on it, and the limb fracture pressure test is not performed. Because the brittle rock layer is subjected to the fracture pressure test, the deformation amount is very small before it is cracked, and once it is fractured, the Pressure capacity will decrease. The ultimate test pressure should be determined according to the maximum mud density that will be used in the drilling of the lower stratum and the requirements for the pressure bearing capacity of the stratum when the well is shut in and killed after overflow occurs. The test method is the same as the burst pressure test, but only to the ultimate pressure.

3.matters needing attention
3.1. The experimental pressure should not exceed the pressure-bearing capacity of ground equipment (especially pipe fittings such as movable elbows and high-pressure straight pipes) and casing.
3.2. When the hydraulic test is carried out after a few days of drilling, the test pressure may be very high due to the clogging of the rock pores by the cuttings. This is an illusion and should be paid attention to.
3.3. The hydraulic test is only applicable to areas dominated by sand and shale, and the hydraulic test of limestone, dolomite and other strata has yet to be solved.

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