Calibration of Predicted Pore Pressure: Perceptions and Pitfalls

Transforming petrophysical properties into pore pressure is a very challenging process to which unscientific calibration templates are often applied.

The assumption that measured pore pressure in permeable beds (sand) is equal to predicted pore pressure in low permeable beds (shale, mud) does not hold true in most cases. If it was true, drilling problems and hydrocarbon trapping risk would be negligible.

Seismic velocity of a proposed wildcat location and data derived from offset wells, such as sonic and resistivity, are utilized to predict the subsurface geopressure profile. A pre-drilling geopressure model is built based on the variables of velocity picks, normal compaction trends slope and effective stress exponents.

During and post-drilling, a calibration fitting process should be applied to the modeled values of shale beds. However, some of these calibrating methods are based on illogical assumptions, such as:

• Predicting pore pressure in sand using the same effective stress model designed for shale,

• Assuming measured pore pressure in sand is equal to predicted pore pressure in shale,

• Applying universal effective stress exponent,

• Breaking the normal compaction trend into different segments for the purpose of fitting the pore pressure profile,

• Using pressure-depth plots in PPG-MWE (pound per gallon of mud weight equivalent) for calibrating the predicted values to the measured ones.

The correct calibrated data and model should incorporate:

During drilling

• Shut in pressure (SIP)

• Mud Weight vs. loss and gain

• Cuttings volume vs. drilled bore hole size

• Mud log gas gauge

Post drilling

• Formation testers (RFTs, MDTs, etc.)

• Bore hole profile (calipers logs)

Subsurface sedimentation and structure are the building blocks in determining the geopressure transgression/regression events of geopressure compartmentalization and, consequently, dictate trap seal effectiveness. Applying the correct calibration methods avoids the pitfalls of inaccurately assessing predicted pore pressure.