Abstract
This paper showing that using rock-density measurement from well-logs affect the calculation of overburden stress because "environmental effects" caused by borehole conditions, overpressures, gas content, high mud filtrates, formation fluid saturation and rock mineralogy. Those effects influence the rock-density measurements and consequently both the overburden stress and geopressures computations; furthermore, the lack of rock-density well-logs in the first drilling stages also affect the overburden stress determinations. However, we consider that the most important bias-effect in the overburden stress calculation is that the overburden must be determined exclusively using the rock-density due to normal compaction of sediments. Hence, we are proposing a new method and model to compute the overburden stress that utilize a power law approach to reproduce rock density values instead to use log-density measurements. The new method is based on the theory of consolidation of clays described by Terzagui and Peck in 1948 which considers that rock-porosity reduction because of compaction when sediments burial increases. This rock-porosity reduction or rock-density increment can be modeled by a power law approach and use its results to calculate overburden instead rock-density measurements from well-logs. Here, we describe how establish the normal compaction trend for rock-density and its integration into overburden equation. This modified overburden model is showed and some examples are presented to permit readers to use the method with their own data. The examples compare the overburden stress calculated with new method against those using rock-density measurements from well-logs. The results show better overburden estimations using the power law model and how the use of rock-density readings from well-logs impact in the geopressures results because mislead of the overburden stress. The results allow to conclude that the overburden stress must be calculated using rock-density due to normal compaction of sediments and avoid the use of density-log readings that are influenced by "borehole environmental effects".
