Formation Fracture Pressure

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

What is a Fracture pressure? Leak Off Tests

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

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

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

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

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

Fracture Pressure Correlations Leak Off Tests

Fracture Pressure Correlations Leak Off Tests

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

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