Natural fracture reactivation or fault slip potential after fluid injection

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Propensity of existing, closed natural fractures to get reactivated after increased pore pressure due to fluid injection depends mostly on the relative orientation of the fracture plane with respect to the orientation of maximum stress and the coefficient of friction. Fractures whose orientations are closer to the angle of friction are more likely to get reactivated (critically stressed) with increased pore pressure and therefore, more likely to flow. Further increases in pore pressure may reactivate other orientations effectively creating a "network" of reactivated fractures. 

 

Under the assumption of vertical fractures, this spreadsheet helps determine which natural fractures from a given set are critically stressed under a certain stress state. If they are not, the calculator determines the additional pore pressure needed to reactivate them. Reactivated fractures will determine the preferential orientations of flow. For more details see Enderlin, M.B. (2010) ("A method for evaluating the effects of stress and rock strength on fluid along the surfaces of mechanical discontinuities in low permeability rocks"). For a more recent reference, see Zoback, M.D. and Lund Snee, J-E. (2018) ("Predicted and observed shear on pre-existing faults during hydraulic fracture stimulation").

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Interested in generalizing this concept using 3D seismic data to estimate recovery? Take a look at this presentation. 

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Change the pore pressure to see to see which natural fractures get reactivated for the given stress state.

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Input:

• Minimum horizontal stress gradient (in psi/ft)

• Maximum horizontal stress gradient (in psi/ft)

• Orientation of maximum horizontal stress Shmax in degrees (measured from North)

• Initial pore pressure gradient (psi/ft)

• Coefficient of friction Mu

• Depth (ft)

• Fracture strikes in degrees (measured from North)

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Assumptions: 

Natural fractures are vertical.

 

Output:

• Minimum horizontal stress at given depth (in psi)

• Maximum horizontal stress at given depth (in psi)

• Pore pressure at given depth (in psi)

• Indicator of whether each input fracture orientation is critically stressed (Yes or No)

• Additional pore pressure required to reactivate fractures that are not critically stressed

• Plot of extra pore pressure needed to reactivate fractures vs fracture strike (zero means "reactivated"). If fractures are cannot be reactivated under the current stress state they are indicated as "Impossible".

• Plot of orientations of reactivated fractures and Shmax strike.

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This calculator is not designed to work on mobile devices

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