Standard seismic/ acoustic log Pp prediction techniques developed for young sediments in offshore basins are not very effective in unconventional reservoirs. The age and lithification of shale reservoirs, the variability in lithology, and different overpressure generation mechanisms and basin histories all lead to poor quality predictions using standard Eaton or Bowers methods. But Pp prediction remains important in unconventional reservoirs due to the correlation between overpressured areas and productivity, and the correlations between thermal maturity and pore pressure.
We have developed a method that extends the theoretical basis of the Eaton and Bowers methods to the geologic and basin history conditions of unconventional reservoirs. The method has been developed using standard log suite along with dipole acoustic logs. Key components of the method are:
use of a normal pressure trend section of wells and extension of that trend to deeper, overpressured zones,
correction of sonic velocities for lithologic and porosity variations through rock physics models and lithology logs,
use of lith logs to determine Biot alphas and thus accurate effective stresses and thus pore pressures,
calibration of the predicted pressure profile with measured pore pressures from DFIT, DST, or shut-in pressures.
A test of the method in a series of 32 wells in Wyoming's Powder River Basin have shown it to be very effective for a range of productive formations across the basin. We find little measured pressure data is needed to calibrate the results and that most of the measured pressure data serves to validate the method and its ability to predict pressures in multiple formations. But the effectiveness of the method over a range of wells and formations gives credence to the theoretical underpinnings of the method. Integration of results from multiple wells and with geochemistry, thermal maturity, and basin modeling gives insight into overpressure generation and expulsion mechanisms.
The prediction of pore pressure, and in particular overpressured intervals, has been a key part of geomechanical/geophysical/petrophysical analysis in offshore basins since Eaton first developed his acoustic/seismic/resistivity method in 1975. Prediction of overpressure in offshore basins is especially critical for avoiding the hazards of drilling into overpressured formations unaware. But the prediction of pore pressure has become important more widely for understanding potential sweet spots (from a thermal maturity, basin modeling, and productivity perspective), drilling hazards due to low effective stress and wellbore strength, and basic hydrocarbon system understanding. However, the basic Eaton (1975), Athy (1930) and Bowers (1995) methods tend to have issues with older, more lithified, more lithologically varied formations than the thick, young, turbidite sequences they were originally developed for.