An Estimation of Pore Pressure in Shales from Sonic Velocities

Abstract

Pore pressure in shaly formations is difficult to measure due to their extremely low permeability. For this reason, indirect methods of pore pressure estimation in shales are crucially important. Overpressured zones in shales are currently identified by deviation of sonic log velocities from the velocities obtained from generic or regional compaction trends. As such trends can be extremely variable, such comparison can only give an approximate estimation of the pore pressure. Here we compare sonic log velocities measured in overpressured shale with the velocities obtained from a rock physics model, that has been shown to predict shale velocities quite accurately for regions with normal compaction trends. The model is based on the parameters measurable in the same well and, thus, provides more detailed information than generic/regional velocity trends. Here we show that for a vertical well with abnormally high pore pressure, the modeled velocities strongly overestimate sonic log velocities. The difference between the measured sonic velocities and the modeled ones is used to estimate pore pressure. To this end, results of laboratory measurements of elastic properties at different effective stresses are used.