The Equation for Geopressure Prediction from Well Logs

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract This study has resulted in the development of four equations that may be used for the prediction of geopressure magnitudes from well prediction of geopressure magnitudes from well log and drilling parameter data. Equations are given for use with resistivity plots, conductivity plots, sonic travel-time plots, and corrected "d" exponent plots. All equations have the same theoretical basis. Introduction In 1965, Hottman and Johnson presented a method for predicting geopressure magnitudes by using resistivity and sonic log data. This technique has received wide acceptance even though the prediction charts were based only on data concerning Tertiary age sediments in the Gulf Coast area. It was specifically pointed out that these techniques were applicable only in areas where the generation of geopressures is primarily the result of compaction in response primarily the result of compaction in response to the stress of overburden. In 1972, this author presented a theory on the effect of overburden stress gradients n geopressure prediction techniques. Compaction caused by overburden stress was described classically in a soil mechanics book by Terzaghi and Peck in 1948. With a vessel containing a spring and a fluid, they simulated the compaction of clay that contained water. Overburden stress was simulated by a piston, as in Fig. 1. It was shown that the overburden stress, S, was supported by the stress in the spring, sigma, and the fluid pressure, p. Thus, the long-accepted equation of equilibrium was established. (1) S = + p If Fig. 1 and Eq. 1 are studied, it is obvious that if S is increased and the fluid is allowed to escape, sigma must increase while p remains as hydrostatic pressure. However, if the fluid cannot escape, p must also increase as S is increased. Hubbert and Rubey published a comprehensive treatment of this theory as related to sedimentary rock compaction. They showed that, as the overburden stress is increased as a result of burial, the porosity of a given rock is decreased. Therefore, some fluid that was once in the pores of a given formation was later squeezed out by compaction. In many such cases, there is no escape route for the fluid, and thus the fluid becomes overpressured according to Eq. 1. This happens in many areas, and such generated overpressured zones are often called "abnormal" pressure zones or "geopressure" zones.