1. Formation Fracture Pressure

2. What is a Fracture pressure?
Fracture Pressure can be defined as the maximum pressure that a formation can sustain before it’s tensile strength is exceeded and it fails.
Estimating formation fracture pressure made before setting the casing can be calculated using correlation methods.
Leak Off Tests

3. What is a Fracture pressure?
Leak Off Tests

4. Fracture Pressure Correlations
The following are the most common used correlations :
1. Hubbert and Willis.
2. Mathews and Kelly.
3. Benebaker
4. Eaton
Leak Off Tests

5. Fracture Pressure Correlations
The following are the most common used correlations :
1. Hubbert and Willis.
2. Mathews and Kelly.
3. Benebaker
4. Eaton
Leak Off Tests

6. Fracture Pressure Correlations
Hubbert and Willis. States That the minimum wellbore pressure required to extend and existing fracture is the pressure needed to over come the minimum horizontal Stress
When sx & sy are equal, the local horizontal stress at
welbore is twice the minimum horizontal stress
Later it was concluded that the minimum matrix stress in
shallow sediments is 1/3 of the vertical stress, thus the
fracture extension pressure is represented by :
Leak Off Tests

7. Fracture Pressure Correlations
The following are the most common used correlations :
1. Hubbert and Willis.
2. Mathews and Kelly.
3. Benebaker
4. Eaton
Leak Off Tests

8. Fracture Pressure Correlations
Mathews and and Kelly. Replaced the assumption
that the minimum matrix stress was one third of the
overburden stress by, Matrix Stress coefficient (Fs)
To use these curves for abnormally pressured formation the
equivalent depth is used instead of the depth when
Determining Matrix Stress coefficient (Fs)
Benebaker used the same correlation using the
same depth instead of the equivalent depth and called
Effective Stress ration (Fs)
Leak Off Tests

9. Fracture Pressure Correlations
Leak Off Tests

10. Fracture Pressure Correlations
Leak Off Tests

11. Fracture Pressure Correlations
Eaton : Poison’s Ratio is needed to predict the observed fracture gradient :
FPG = [m / (1 - m )]sv + FPPG
sv = S - FPPG
FPG = [m / (1 - m )](S - FPPG) + FPPG
Poisson’s ratio is 0.25 for clean sands, sandstone and carbonate rocks down to medium depth
n = 0.28 for sands with shale, sandstone and carbonate rocks at great depth. 2

12. Fracture Pressure Correlations
Evaluated overburden load and Poison’s ratio as a function of vertical depth.
Overburden load and Poisson’s ratio is easily evaluated and presented in graphical form for specific applications
FPG = [m / (1 - m )](S - FPPG) + FPPG 2