An estimate of formation pore pressure prior to drilling is a key requirement for successful exploration and drilling. During the exploration phase, an estimate of pore pressure can be used to develop fluid-migration models, to study the effectiveness of seals, and to rank prospects. In drilling, a predrill pore-pressure estimate allows selection of the appropriate mud weight and optimization of the casing program, thus enabling safe and economic drilling. In this chapter, the basic concepts used to predict pore pressure are defined, and the methods for estimating pore pressure by using elastic wave velocity measurements are explained. An estimate of pore pressure can be obtained from seismic velocities by using a velocity-to-pore-pressure transform calibrated with offset-well data. The rock-physics basis underlying such transforms is discussed, and their application for estimating pore pressure is illustrated by several examples from the Gulf of Mexico.
Pore pressure can be estimated from elastic wave velocities by using a velocity-to-pore-pressure transform. Early examples include the work of Hottman and Johnson (1965) using sonic velocities and that of Pennebaker (1968) using seismic interval velocities obtained from stacking velocities. Most velocity-to-pore-pressure transforms in use today are based on the effective-stress principle, which first was formulated by Terzaghi (1943) and was mentioned in Chapter 2 of this book. The effective-stress principle states that all measurable effects of a change in stress — such as compaction and variation in elastic wave velocities — are functions only of the effective stress.