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CONSIDERATION ON APPRAISAL OF THE RESERVOIR PRESSURE BY GEOPHYSICAL METHODS

C. Jugastreanu, L. Ionescu, T. Cristescu, D. Frunzescu, V. Vasiliu
Wednesday 19 December 2018 by Libadmin2018

ABSTRACT

Establishing of pore fluids pressure is one of the critical challenges encountered in the formations opening in drilling. Pores fluids pressure evaluation through geophysical methods involves live recordings of the collector parameters using LWD (Logging While Drilling)/MWD (Measurements While Drilling) techniques. Empirical relationships of the dependence of pore pressure and the measured parameters use exponents whose values are accepted as constant. These exponents were determined on the basis of experimental data on certain oil fields or structures. These exponents can be considered constant for that area or structure, but cannot be extended to all basins, especially because in the evolution of normal clays, large differences occur from one basin to another due to both the very different clay composition (lower or higher content of quartz, silt, mica, calcite, organic matter, different clay minerals) as well as different geological and tectonic evolutions. Studies conducted by various researchers on different types of clay show that compaction of clays cannot follow a general law, but it is admitted, however, in the first approximation, that the analysis of geophysical parameters with depth allows obtaining of pressure data valid over an important depth range or geological age. In this study will be analyzed the variation of the exponent used in B. Eaton’s relation for calculating the formation pressure from the electrical resistivity data. Electrical devices for conductivity measuring do not give the same values in the clay formations due to the anisotropy of this type of rocks (different vertical and horizontal resistivities). Beside the above mentioned causes of variation for α exponent, can be highlighted also the following additional factors which affect the resistivity values measurements: (1) the temperature that increases with the depth, causing a decrease in resistivity for waters with the same salinity; (2) the presence of hydrocarbons in the porous clay space has the effect of increasing its resistivity considerably; (3) the presence of organic matter in significant proportions in the clays leads to the increase of the resistivity and the underestimation of the pressure in the pores. The following study is focused on the validation of the reservoir pressure based on the processing of electrical diagrams, valid for clay of a certain type of lithology and a certain age in a sedimentary basin. To get the value of the exponent α from B. Eaton equation will be used the equivalent depth method – estimation of pore pressures. Consequently, the scope will be the analysis of the B. Eaton’s equation exponent α variation for the accurate estimation of the formation pressure from the electrical resistivity data. The authors suggest a new approach for pore fluid pressure evaluation (in order to establish a different value for α exponent). This is necessary in the case of abnormal pore pressure (abnormal clay compaction) and influenced by geological age and lithology of the formation.

Keywords: reservoir pressure, electrical resistivity, equivalent depth method, B. Eaton equation, exponent α


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