Pore Pressure: Eaton Pressure Volume
The Eaton equation is a technique used to estimate pore pressure in 3D. This process creates new volumes of pressure (in psi) and pressure gradient (in ppg) using Eaton's equation.
To fully read and understand the fundamental theory behind pore pressure analysis and Eaton's equation, see the Pore Pressure Prediction chapter.
The Eaton equation
The Eaton equation is defined as:
Pore Pressure Gradient (PPG) = OBG - (OBG - Png) * (NCTL / Dt) ^ Exp
OBG = Overburden Gradient
Png = Hydrostatic Pore Pressure Gradient
NCTL = Normal Compaction Trend Line
Dt = Compressional Slowness (as 1 / Vp, from the velocity volume)
Exp = Exponent
- In the Control Panel, open the Process tab.
- At the tab header, click the Add icon and select New Process.
- Double-click PP: Eaton Pressure Volume.
- Type a new name for the process and click OK.
Velocity Volume — A 3D volume of interval velocities in depth. If you have a velocity model of another type, use the Velocity Conversion process to transform it to Interval Velocity (Depth) (after which we recommend that you QC it, to verify that the interval velocities are sensible).
Note: Velocity volumes should start from the seismic reference datum. If the depth the velocity starts is not consistent with the project's reference datum, see Configure a Project to change your project's seismic reference datum.
- Density Volume — A TVD volume of densities, generally created using the PP: Density Volume process. These densities are integrated to calculate Overburden Gradient (OBG).
- Exponent — The exponent (Exp) in the Eaton equation.
- Hydrostatic Gradient — The pressure gradient resulting from a fluid (water) column at equilibrium due to the force of gravity. Hydrostatic pressure increases in proportion to depth measured from the surface because of the increasing weight of fluid exerting downward force from above.
- NCTL Formula — A function of depth below sea level (tvdss_m or tvdss_ft) and/or depth below the mud-line/waterbottom (tvdbml_m or tvdbml_ft). The default is:
NCTL = ΔTm + (ΔTml - ΔTm)*e^(-c * tvdbml_m)
ΔTm is the compressional transit time in the zero porosity shale matrix (default = 50).
ΔTml is the mudline transit time (default = 185).
c is a constant (default = 0.0006).
tvdbml_m is the depth below mudline in metres.
- Waterbottom — A constant depth or a horizon. This is required if the NCTL function is defined in terms of tvdbml. If a horizon is chosen, a constant value can be used to offset the horizon.
As a result of this process, the pore pressure and pore pressure gradient will be added in the Volume tab.
Zhang, Jincai, 2011, Pore pressure prediction from well logs: Methods, modifications, and new approaches. Earth-Science Reviews, 108, 51-62.