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.

Compaction of diagenetically altered mudstones – Part 2: Implications for pore pressure estimation

“…The most widely applied method of pore pressure estimation in mudstones over the past 40 years has probably been the empirical method of Eaton (1975) using the sonic log. We start by applying it to the mudstones of the Cretaceous Kvitnos and Lange formations at Haltenbanken to demonstrate where some more sophisticated approach is needed.…”

“…This plot helps us to visualize why the vertical effective stress is not uniquely defined by density and sonic transit time in diagenetically altered mudstones, even if their depositional lithology is known and they have been uniaxially consolidated. We then compare pore pressure estimates from wireline logs in the Cretaceous mudstones at Haltenbanken obtained by the widely used method of Eaton (1975) and the Fudge approach proposed by Sargent et al (2015). We argue that the Budge-Fudge approach is preferable because it requires explicit accounting for both disequilibrium compaction and unloading mechanisms of overpressure generation, although it does require geological input to help estimate the overpressure contribution from disequilibrium compaction.…”

“…7b. We emphasize that the sonic reference trend is not the hydrostatic normal compaction trend, as used, for example, in the method of Eaton (1975), because it is not, in general, the same sonic-depth trend that the sonic log would follow in the case of hydrostatic pore pressure. The sonic reference trend is the expected trend of the sonic log, given the density log, which would be followed by illite-rich mudstones if they were presently at the maximum vertical effective stress they had previously experienced.…”

Pre-drill pore pressure prediction using seismic velocities for prospect areas at Beni Suef Oil Field, Western Desert, Egypt

“…There are several methods in literature dealing with the pre-drill prediction of abnormal pore pressure from seismic survey data (see Eaton Eaton 1975;Bowers 1995;ENI 1999;Kan et al 1999;Dvorkin et al 1999;Carcione and Helle 2002;Sayers et al 2002;Chopra and Huffman 2006;Sundaram and Jain 2008;Lu et al 2009;Babu and Sircar 2011;Brahma et al 2013). Comparing of the measured physical properties of subsurface formations in the abnormal pressure region with the normally pressurized formation properties is the general approach in all the overpressured prediction methods.…”

Effect of Rock Properties on ROP Modeling Using Statistical and Intelligent Methods: A Case Study of an Oil Well in Southwest of Iran

“…The most commonly used method to predict the pore pressure is the Eaton equation (Eaton, 1975). The equation is as follow:…”