The Effect of Overburden Stress on Geopressure Prediction from Well Logs

Abstract

Think of the money that could be put to better use if we could predict the depth below which commercial production will not be found. It has been suggested that the magic level in geopressured areas is where log resistivity ratios exceed 3.50. The theory offered here, with the hope that it will be carried further, is that the limiting ratio is a function also of overburden stress gradient. Introduction In 1965, Hottman and Johnson presented a method for predicting geopressure magnitudes by using resistivity 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 to the stress of overburden. 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, and the fluid pressure, p. Thus, the long-accepted equation of equilibrium was 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, or 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. Hottman and Johnson recognized the main significance of the preceding theory and developed a very useful relationship between electrical log properties and geopressures. They reasoned that since rocks are more resistive to electrical current than is formation water, a well compacted shale containing less water (because the water has escaped) is more resistive than a less compacted shale containing more water (one in which the water has not escaped to the same degree). Also, they reasoned that a sequence of normally compacted sediments (in which water is free to escape) should have a normally increasing resistivity trend. They substantiated this when they plotted resistivities from actual well logs. Any resistivity decrease from the well established normal trend indicates the, presence of abnormally high-pressured zones. Empirical data from well tests and logs were used to develop a correlation of the pore pressure gradient as a function of the resistivity departure ratio (see Fig. 2). JPT P. 929