Natural Fractures Reactivation Calculator
Estimation of pressure required for natural fracture reactivation
Propensity of existing, closed natural fractures to get reactivated after increased pore pressure due to fluid injection in a low permeability reservoir depends mostly on the relative orientation of the fracture plane with respect to the orientation of maximum stress. 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 preferencial orientations of flow. The application of this spreadsheet can be generalized a larger seismic volumes to estimate spatial variations in fracture reactivation. 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, JE. (2018) ("Predicted and observed shear on preexisting faults during hydraulic fracture stimulation").
Change the pore pressure to see to see which natural fractures get reactivated for the given stress state.
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)
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")

Plot of orientations of reactivated fractures and Shmax strike
This calculator is not designed to work on mobile devices