Petroleum Engineering 405 Drilling Engineering * Well Drilling Engineering Prediction of Fracture Gradients Dr DO QUANG KHANH * Prediction of Fracture Gradients Well Planning Theoretical Fracture Grad[.]
Well Drilling Engineering Prediction of Fracture Gradients Dr DO QUANG KHANH Prediction of Fracture Gradients Well Planning Theoretical Fracture Gradient Determination Hubbert & Willis Matthews & Kelly Ben Eaton Comparison of Results Experimental Frac Grad Determination Leak-off Tests Lost Circulation Read: Applied Drilling Engineering, Ch HW # Well Planning Safe drilling practices require that the following be considered when planning a well: Pore pressure determination Fracture gradient determination Casing setting depth selection Casing design Mud Design, H2S considerations Contingency planning Fig 7.21 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: Pore pressure, psi/ft , at 14,000 ft Pore pressure, psi, at 14,000 ft Matrix stress, psi/ft Matrix stress, psi Formation Pressure and Matrix Stress S =S PP + pore overburden stress (psi) = pressure (psi) + matrix stress (psi) Fracture Gradient Determination In order to avoid lost circulation while drilling it is important to know the variation of fracture gradient with depth Leak-off tests represent an experimental approach to fracture gradient determination Below are listed and discussed four approaches to calculating the fracture gradient Fracture Gradient Determination Hubbert & Willis: where Fmin 1 2P 1 3 D Fmax P 1 D F = fracture gradient, psi/ft P = pore pressure gradient, psi/ft D 10 19 Example Ben Eaton: P S P F * D D 1 S ? D ? 20