Undercompacted shales generally have a lower acoustic impedance (product of density and velocity) than those that follow a normal compaction trend. Departure from the normal compaction trend may indicate potential drilling hazards due to overpressure. Techniques that can monitor acoustic impedance can be used to indicate the existence of such potential hazards, and thereby, help in designing the casing and mud program.

Prediction of pressure ahead of the bit starts with the best predrill model. In frontier wells, commonly seismic data are the only data available. Seismic velocities from analysis of stacking velocities and impedances from reflection sequence analyses, in conjunction with a predrill rock model, can be used to develop a predrill pressure vs. depth profile. This has been used with considerable success in deep-water wells. The limitations, however, are the lack of resolution in the reflection seismic data and uncalibrated velocity models. Thus, a strategy is developed that can update this so-called static model in real time using borehole data.

Conventional wire-line vertical seismic profile (VSP) measurements are commonly used to provide high-quality reflection data within and below the bottom of the well. Inversion of VSP data for acoustic impedance has been demonstrated to be a reliable way to accurately predict acoustic impedance below the bit, with more resolution than the conventional velocity data from stacking-velocity analyses. This has been found to yield pressure vs. depth profiles, at the bit level, with more resolution. Downtime on the rig is required to acquire the wire-line data.

Vertical seismic profile inversion allows the location of the overpressured zone to be accurately determined in two-way traveltime. This time estimate can be converted to depth if the formation acoustic velocity ahead of the bit is known. The drill bit seismic technique, which uses a working drill bit as the seismic source, provides continuous time to depth information. These data can be used to estimate the formation acoustic velocity continuously, in real time, and to calibrate seismic velocity at the bit

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level and hence allow an accurate, continuously updated, prediction of the depth to the overpressure hazard in real time at the well site.

In this chapter we present a methodology to quantify and predict overpressure hazards ahead of the bit using surface seismic, VSP, and drill bit seismic.