Uncertainty – Shale Pore Pressure from Borehole Measurements

Martin D. Matthews
Knowledge Systems Inc, Stafford, TX

The degree of certainty in pore pressure estimates from resistivity and sonic data is examined through a case study interpreted by eight trained analysts. The relevant data included resistivity, sonic travel time, gamma ray, density, and mud weight as a function of depth. A shallow kick pressure and several deeper MDTs were provided for calibration.

While the models and tools are quite simple, the shale pressures are derived from several measurements and decisions. The results each analysts interpretation were compiled and combined with some theoretical analysis to estimate the uncertainty attached to components used to estimate shale pore pressure. Two sources of variation dominate the uncertainty in estimating pore pressure from resistivity and sonic logs:

1.trust that sand pressure calibration points are representative of shale pressures, and 2.choice of the normal compaction trend.

Uncertainties in shale pore pressure ranged as low as plus or minus 0.25 ppg to as much as plus and minus 0.75 ppg depending on these decisions. Other factors accounted for approximately less than 0.1 ppg in regions of good data. An approach for minimizing and quantifying the impact they have on an analysis is discussed.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005