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Fracture Gradients 1.11- 2 Prediction of Fracture Gradients 4 Well Planning 4 Theoretical Fracture Gradient Determination 4 Hubbert & Willis 4 Matthews & Kelly 4 Ben Eaton 4 Comparison of Results 4 Experimental Frac. Grad. Determination 4 Leak-off Tests
Fracture Gradients 1.11- 3 Well Planning 4 Safe drilling practices require that the following be considered when planning a well: 4 Pore pressure determination 4 Fracture gradient determination 4 Casing setting depth selection 4 Casing design

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Fracture Gradients 1.11- 4 Formation Pressure and Matrix Stress Given: Well depth is 14,000 ft. Formation pore pressure expressed in equivalent mud weight is 9.2 lb/gal. Overburden stress is 1.00 psi/ft . Calculate: 1. Pore pressure, psi/ft , at 14,000 ft 2. Pore pressure, psi, at 14,000 ft 3. Matrix stress, psi/ft 4. Matrix stress, psi
Fracture Gradients 1.11- 5 Formation Pressure and Matrix Stress σ + = P S overburden pore matrix stress = pressure + stress (psi) (psi) (psi) S = P + σ

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Fracture Gradients 1.11- 6 Formation Pressure and Matrix Stress Calculations: 1. Pore pressure gradient = 0.433 psi/ft * 9.2/8.33 = 0.052 * 9.2 = 0.478 psi/ft 2. Pore pressure at 14,000 ft = 0.478 psi/ft * 14,000 ft = 6,692 psig Depth = 14,000 ft . Pore Pressure = 9.2 lb/gal equivalent Overburden stress = 1.00 psi/ft .
Fracture Gradients 1.11- 7 Formation Pressure and Matrix Stress Calculations: 3. Matrix stress gradient, psi psi/ft σ / D = 0.522 psi/ft σ + = P S D D P D S or σ + = ( 29 ft / psi 478 . 0 000 . 1 D P D S D ., e . i - = - = σ

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Fracture Gradients 1.11- 8 Formation Pressure and Matrix Stress Calculations: 4. Matrix stress at 14,000 ft = 0.522 psi/ft * 14,000 ft σ = 7,308 psi
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