Identification of Zones of Abnormal Pressures and Determination of the Mechanical Properties of the Rock Through Pseudo-Sonic and Pseudo-Density Logs in Conventional and Unconventional Reservoirs

Abstract

The objective of the work is to identify zones of abnormal pressures and determine the dynamic mechanical properties of the rock in wells without information of sonic and density logs. In the area of study, geomechanical problems have been detected in intermediate hole sections that make it difficult for drilling operations, thus generating non-productive times. Density log (ρb), compressional sonic log (Δtc) and shear sonic log (Δts) are essential to attack this problem and provide possible solutions.

To determine the pseudo-sonics logs, it was necessary to modify the correlation of L.Y. Faust (1951), introducing a third variable, the clay volume, it was called Faust Modified Correlation. The pseudo-density log was obtained from the G.H.F. Gardner adjusted correlation (1974). The zones of abnormal pressures were identified by comparing the normal compaction train of the sonic log (Δtcn) with the compressional sonic log (Δtc). And finally the dynamic mechanical properties of the rock were determined such as Poisson's ratio (n), Shear modulus (μ), Bulk modulus (K), Young's modulus (E) and Bulk compressibility (C).

The Faust modified correlation showed excellent results of compresional sonic logs, obtaining a correlation coefficient of 93%. The Gardner adjusted correlation as a function of the P wave velocity obtained good results of density logs, with a correlation coefficient of 94%. The zones of abnormal pressures were identified towards the Miocene base with an average pore pressure of 9.17 ppg. In the Pliocene and Miocene high Poisson's ratio was determined that varies between 0.28 and 0.36, and low Young's modulus between 0.85 and 5 Mpsi, this indicates that the rocks are deformed more easily. In the Eocene and Cretaceous, low Poisson's ratio was determined between 0.21 and 0.27, and high Young's modulus between 6.1 and 10 Mpsi, this indicates that the rocks do not easily deform.

In addition, the velocity models of the P wave and S wave (VP and VS) were simplified through graphical methods, where VP is a function of the Bulk modulus (K) and Shear modulus (μ), while VS is a function of the Shear modulus (μ). From these models, cubes of Lamé's parameters (λ, μ), elastic properties and S wave velocity were determined using the velocity cube of the RMS compressional wave of seismic as input data to generate cubes of clay volume and fluid saturation with the purpose of looking for opportunities in exploration areas.