One of the aims of a pre-drill pore pressure prediction is to create a mud weight and casing program to allow the well to be drilled safely and with as much forewarning of potentially overpressured zones as possible. Knowledge and understanding of overpressured zones can be derived from many sources such as formation pressure tests, daily drilling and end of well reports. In addition, a common practice is the interpretation of well-based (sonic) velocity from offset wells and increasingly, seismic interval velocities as a means of predicting pore pressure in shales. However, as exploration moves into frontier areas where no wells have been drilled or to deeper targets in an area that has only been explored at shallower depths the reliance on seismic interval velocities for pore pressure prediction becomes increasingly important. Concerns arise in that the workflows used to derive these seismic velocities can differ depending on whether improving the image is the requirement, i.e. optimizing the stacking process, or whether velocities are to be used for pressure prediction. There are also significant challenges in terms of matching seismic and well velocity due to resolution and frequency differences. Using seismic data therefore presents some unique challenges. Both well-based and seismic interval velocities (regardless of how they are processed) are affected by changes in rock properties such as increasing temperatures with increasing depth which may cause diagenetic changes in shales resulting in additional overpressure which is no longer related to effective stress and porosity. In addition such changes can also result in the precipitation of silica cement, which produces artificially fast shales, where velocity, porosity and therefore effective stress are no longer related in a predictable manner. The use of velocity based pressure prediction in both of these scenarios will result in the under-prediction of the pore pressure thereby invalidating its use for pressure prediction in these environments. Other factors to consider include; the total organic content (TOC) of the shales, the effect of carbonate cements on compaction and porosity and the stress history of the basin. In addition, diagenetic changes can create a more laminated rock which may produce velocity anisotropy whereby the rock has a different velocity depending on the direction of measurement. In such cases, the horizontally-measured seismic velocities increase relative to the vertically-measured well sonic velocities requiring some form of calibration to match the well-based and seismic velocities. If only seismic velocities are available and therefore no calibration is possible, the tendency is that pressure is under-predicted.
1Ikon Science, Durham, DH1 5TS, UK