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.
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Geophysics Under Stress: Geomechanical Applications of Seismic and Borehole Acoustic Waves
This is a recording of the 2010 Distinguished Instructor Short Course (DISC) by Colin Sayers. The state of stress within the earth has a profound effect on the propagation of seismic and borehole acoustic waves, this leads to many important applications of elastic waves for solving problems in petroleum geomechanics. This course provides an overview of the sensitivity of elastic waves in the earth to the insitu stress, pore pressure, and anisotropy of the rock fabric resulting from the depositional and stress history of the rock, and introduces some of the applications of this sensitivity. The course will provide the basis for applying geophysics and rock physics solutions to geomechanical challenges in exploration, drilling, and production. See catalog #233A for the accompanying DISC book.