Page 194 - Origin and Prediction of Abnormal Formation Pressures

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Chapter 7

SEISMIC METHODS OF PRESSURE PREDICTION

E AMINZADEH, G.V. CHILINGAR and J.O. ROBERTSON JR.

INTRODUCTION

Geophysical methods, in conjunction with other tools, can provide the means to
predict reservoir pressure in many cases. Overpressured shales can act as good reservoir
seals, but can also cause drilling difficulties, particularly in maintaining an adequate
safety margin for the drilling mud weight. Geophysical techniques are based on the
impact of reservoir pressure on the seismic velocities (primarily compressional waves).
Many studies have demonstrated the effectiveness of geophysical methods for pore
pressure prediction. One of the first of such studies was reported by Pennebaker (1968).
Subsequently, the Society of Exploration Geophysicists published Geopressure (Dutta,
1987) that included major geophysics-related methods for overpressure prediction. With
the advent of 3-D seismic and, more recently, four components and 4-D seismic, it
has become possible to make pressure predictions that are more reliable and create
three-dimensional pressure profiles.
In general, the seismic reflections are functions of acoustic impedance (velocity
times density) and are influenced by reservoir pressure. On the other hand, the type of
reservoir fluid impacts sonic velocities. Shear waves and compressional waves respond
differently to various reservoir fluids (and lithology) as well as reservoir pressure. These
phenomena offer the following two practical applications: (1) prediction of abnormal
pressure from seismic velocities before drilling; (2) mapping reservoir fluid movement
and dynamic changes of reservoir pressure using time lapse (4-D seismic).

PREDICTION OF ABNORMAL PRESSURE FROM GEOPHYSICAL DATA

Most of the methods of predicting reservoir overpressures utilize the following
phenomena: (1) lower bulk densities (thus lower seismic velocity); (2) higher porosity;
(3) lower stress; (4) higher reservoir temperature.
Table 7-1 shows how specific types of measurement at different stages of well
development are employed to predict reservoir pressure using geophysical data. There
are two major categories of approaches for predicting pore pressure and effective
stress. They are based either on empirical relationships derived from statistical data
and case histories or on laboratory measurements and rock physics models. In general,
most methods use the seismically derived velocities as a basis for prediction. Some
of the earlier work on the subject has been reported by Dutta (1987) and Fertl et
al. (1994). The sonic velocities are calibrated against velocities derived from sonic