Formation rupture and loss pressure
Formation fracture pressure refers to the pressure that a certain depth stratum can bear when it is broken and fractured.
When the formation fracture pressure is reaching, the original fracture of the formation expands and extends or the formation without fracture produces fracture.
Under the general condition (following the compaction law), the formation fracture pressure increases with the increase of well depth.
When 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 should not exceed the fracture pressure of the formation to avoid fracturing the formation and causing leakage.
Formation loss pressure is the pressure at which the formation at a certain depth produces loss of drilling fluid.
For high permeability sandstone, fractured formation and fault fracture zone with normal pressure, the formation loss pressure is often much smaller than the fracture pressure, and it is very harmful to the safety of drilling.
Formation loss pressure is traditionally using as the shut-in pressure basis for well control operations. It’s much safer.
To determine the formation fracture (leakage) pressure method
- 1. Prediction method — Using empirical formulas to predict formation fracture pressure as a basis for drilling design.
- 2. Verification method — After running casing cementing, a leak test must be performing to verify the predicted rupture pressure.
- 1. Lift the drill bit above the casing shoe, fill the well with drilling fluid, and close the well.
- 2. The drilling fluid is pumping into the well at a selecting small rate from either the drill hole or the annulus.
- 3, every interval of 20~50L (select a fixed amount) pump volume or every interval of 10~ 20S (pump speed constant, select a fixed time interval) record the corresponding pump pressure and injection volume or time.
- 4, when the pump pressure begins to drop, stop the pump. If it is not requiring to calculate the minimum horizontal principal ground stress of the stratum and the tensile strength of the leakage test layer, the test is finishing.
- 5. Stop the pump for 1~2min, and record the pump pressure every 10~20s.
- 6. After the pump pressure is relatively stable, restart the pump for 1-2 minutes, and record the retension pressure every 10 to 20s.
- 7. Make a typical leakage test curve as shown in the following figure. In the figure, the pressure PL at the point deviating from the straight line is the leakage pressure.
The rupture pressure equivalent density (υ F) is: υ F = υ M +100Pl/H
υ M: Mud density, g/cm3;
Pl: leakage pressure, MPa;
H: midpoint depth of open hole, M.
The fracture pressure gradient (Gf) of the formation is:
Gf = 0.01 υ M +Pl/H Unit: MPa/ M
It is important to note that data obtained from hydraulic tests performed on the same formation in vertical and directional Wells are not interchangeable.
When the first layer below the casing shoe is brittle rock, only the ultimate pressure test is carriing out on it, but not the limb fracture pressure test.
Because the brittle rock has a small deformation before cracking during the fracture pressure test, the bearing capacity will decrease once it is fracturing.
The ultimate test pressure is determined based on the maximum mud density to be used in drilling the lower formation and the bearing capacity of the formation when shutting in and killing the well after overflow occurs.
The test method is the same as the rupture pressure test, but only up to the ultimate pressure.
Typical leakage test curve analysis
- Leakage pressure (PL)
As you can see from the graph, the vertical pressure changes almost in a straight line with the injection volume, indicating that there is no downhole loss.
A turning point occurred from L. The test curve deviated from the straight line to show a curve change, but the pressure continued to rise.
It indicates that the skeleton particles of the formation begin to separate, but no fracture is forming, and the drilling fluid starts to lose (but the leakage rate is less than the injection amount).
The point where the test curve deviates from the straight line is the point where the formation starts to lose, and the pressure on the formation is calling the formation loss pressure.
- 2. Rupture pressure (PF)
As can be seen from the figure, after the turning point from L, the pressure changes in a curve, but the pressure continues to rise and drops after reaching the maximum peak point F.
At this time, the formation breaks and cracks are forming. The drilling fluid leaks into the cracks (the leakage rate is greater than the injection amount), and then the pessure drops.
At this point, the pump is stopping and the pressure becomes stable. The highest point of the test curve. It reflects that the pressure in the well overcomes the strength of the formation and causes it to break, forming a fracture, and the drilling fluid is lost to the fracture, and then the pressure will fall.
- 3. Extension pressure (PPRO)
The point at which pressure flattens out. It causes the crack to spread far and wide.
Matters needing attention
- 1. The experimental pressure should not exceed the bearing capacity of ground equipment and casing.
- 2. When the hydraulic test is carriied out a few days after drilling, it may be due to the rock debris blocking the rock pore, resulting in high experimental pressure. It is an illusion and should be paid attention to.
- 3. Hydraulic tests are only suitable for sand and shale areas. The hydraulic tests for limestone,dolomite and other strata are still to be solving.