# 11. Fracture Gradient Determination - PETE 625 Well Control...

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PETE 625 Well Control Lesson 11 Fracture Gradient Determination

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2 Fracture Gradient Determination Hubbert and Willis Matthews and Kelly Ben Eaton Christman Prentice Leak-Off Test (experimental)
Assignments HW # 6: Ch 3, Problems 1- 10 due Wednesday, June 23 HW # 7: Ch 3, Problems 11- 20 due Monday, June 28 Read: Chapter 3

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4 Well Planning Safe drilling practices require that the following be considered when planning a well: Pore pressure determination Fracture gradient determination Casing setting depth Casing design H 2 S considerations Contingency planning
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7 Provides the basis of fracture theory and prediction used today. Assumes elastic behavior. Assumes the maximum effective stress exceeds the minimum by a factor of 3. min _ e max _ e * 3 σ = σ

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8 Cohesion, c = 0 Angle of Internal Friction, ϖ = 30 deg.
9 The Hubbert & Willis Equation If the overburden is the maximum stress, the assumed horizontal stress is: σ H = 1/3( σ ob - p p ) + p p Equating fracture propagation pressure to minimum stress gives p fp = 1/3( σ ob - p p ) + p p min _ e max _ e * 3 σ = σ obe min _ e * 3 1 σ = σ ( 29 ( 29 p ob p min p * 3 1 p - σ = - σ

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10 The Hubbert & Willis Equation p fp = 1/3( σ ob - p p ) + p p p fp = 1/3( σ ob + 2p p ) (minimum) p fp = 1/2( σ ob + p p ) (maximum)
11 Matthews and Kelly Developed the concept of variable ratio between the effective horizontal and vertical stresses, not a constant 1/3 Stress ratios increase according to the degree of compaction σ He = K MK σ ve

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12 Matthews and Kelly σ eH = K MK σ ev K MK = matrix stress coefficient Including pore pressure, σ H = K MK ( σ ob - p p ) + p p
13 Matthews and Kelly Equating fracture initiation pressure to the minimum in situ horizontal stress gives p fi = K MK ( σ ob - p p ) + p p and g fi = K MK (g ob - g p ) + g p

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14 Example 3.8 Given: Table 3.4 (Offshore LA) Estimate fracture initiation gradients at 8,110’ and 15,050’ using Matthews and Kelly correlation
15 TABLE 3.4 psi/ft ft

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11. Fracture Gradient Determination - PETE 625 Well Control...

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