Petrophysical Characterization of Carbonates Heterogeneity

Abstract

The heterogeneity of carbonate rocks results from their origin and their sensitivity to physical, chemical and biological alterations, such as dissolution, cementation and recrystallization of their minerals. These phenomena lead to pores with different shapes, origins and sizes, and, consequently, makes it difficult to predict the porosity and permeability distributions of these rocks. In addition, the high heterogeneity of carbonates affects the flow of fluids inside the reservoir and, therefore, the final oil recovery, evidencing the importance of an exhaustive petrophysical characterization of these rocks. In the present paper carbonate rocks from the same formation were investigated, grouped into two sets: Low Permeability set (LP), with a permeability range between 5 and 15 mD; and High Permeability set (HP), with a permeability range between 90 and 150 mD. Besides basic petrophysical properties (porosity and permeability), other properties, such as pore size and pore throat size distributions, surface area, capillary pressure and relative permeability curves of each set were studied in order to correlate their properties with rock heterogeneity. Nuclear Magnetic Resonance (NMR) was used to evaluate pore size distributions and the Mercury Intrusion Porosimetry (MICP) was used to determinate pore throat size distributions and capillary pressure curves. It was observed that HP set presents predominantly macropores and macro-throats (about 90% of the porous structure), whereas the LP set presents about 80% of macropores, but approximately the same distribution of mesoand macrothroats (from 40 to 50 %). The lack of macro-throats strangles fluids flow, and it may be one of the factors responsible for lower permeability of LP set samples. Furthermore, when the pore size and pore throat size distributions are similar, it is possible to convert the T2 distribution data to pore throat size with an acceptable correlation. Capillary pressure curves obtained by MICP were normalized with saturation data obtained by High Speed Centrifuge (HSC) tests. Initial water saturation was higher in LP set, as expected. Capillary pressure curves were compared with the data obtained by the two methods, presenting very comparable results. In the same way, the relative permeability curves also presented similar results by the two methods. In this study we could observe the association that different permeabilities of rocks from the same formation have with petrophysical properties, such as pore size and pore throats distributions, capillary pressure and relative permeability curves.