Techniques for estimating formation pore pressures from relative changes in log derived shale properties have been in the industry and accepted for years. The basic premise, of course, is that, at depth, shale porosity is a function of the pore fluid pressure and the log derived shale properties are a function of the shale porosity. There are difficulties, however, in that there are other factors which can influence the log properties of shales than just porosity alone. For this reason, the determination of formation pore pressures from log properties can be difficult and inaccurate.
Recalibration techniques and considerations have been developed for the various log derived properties typically used in the industry, however this paper will focus on those affecting shale resistivity, or conductivity if you prefer. In particular, the effects of changes in formation salinities and the effects of multiple log runs are the significant factors which have been addressed and dealt with. As increasing pore pressures are encountered, shale resistivities decrease as an indication of increasing pressure. However, if the formation water salinity increases, the shale resistivity will also drop, complicating the analysis. Also, as we set each string of casing, we log each subsequently smaller hole with a different log tool, probably a different logging engineer and in many cases with an entirely different logging unit. These changes which occur at each log run also result in difficulty in pore pressure determinations across these intervals.
This method addresses the effect of changing salinities and log runs and enables one to accurately determine formation pore pressures from shale resistivity and conductivity trends. The technique has been utilized on hundreds of wells throughout the Gulf of Mexico with great success. Typically pore pressure determinations are within a few tenths of a pound per gallon of measured pore pressures in the adjacent virgin sands.