Geomechanics and FracturingIn-Situ Stress and Rock Mechanics

Pore Pressure Increase Due to Fluid Activity Mody and Hale Formula

\delta P=E_m\left(\frac{RT}{V}\right)\ln\left(\frac{A_p}{A_m}\right)

Pore Pressure Increase Due to Fluid Activity Mody and Hale calculates pore pressure increase for in-situ stress and rock mechanics workflows in geomechanics and fracturing.

Calculate

How engineers use this formula

Use this formula when the listed inputs (E_m, R, T, V, A_p, A_m) are known and the assumptions behind the cited in-situ stress and rock mechanics relationship match the engineering case being checked.

Assumptions

Input values are representative for the well, reservoir, fluid, or equipment case being evaluated.
The declared units match the field-unit constants used in the formula.
The cited formula applies to the selected petroleum engineering workflow.

Limitations

The calculation does not replace a full engineering model or operating procedure.
Accuracy depends on the source correlation, assumptions, input quality, and unit consistency.

Common mistakes

Mixing unit systems without converting the inputs.
Using default example values as field recommendations.
Applying the formula outside the source assumptions.

Default example

Using the default inputs, dP equals 813.825087 psi.

E_mdimensionless

0.8

Rpsi*ft^3/(mol*K)

10.73

520

Vft^3/mol

A_pdimensionless

0.9

A_mdimensionless

0.75

Inputs

E_m

dimensionless

Membrane Efficiency

R

psi*ft^3/(mol*K)

Gas Constant

T

Temperature

V

ft^3/mol

Volume

A_p

dimensionless

Pore Fluid Activity

A_m

dimensionless

Mud Activity

Outputs

dP

psi

Pore Pressure Increase

E_m

dimensionless

Membrane Efficiency

R

psi*ft^3/(mol*K)

Gas Constant

T

Temperature

V

ft^3/mol

Volume

A_p

dimensionless

Pore Fluid Activity

A_m

dimensionless

Mud Activity

Source and review

reviewed

Reservoir Geomechanics, Zoback, M.D. (2007)

Zoback, M.D. 2007. Reservoir Geomechanics. Cambridge University Press, Page 321.

Source