Some Petrophysical Characteristics of a Brazilian Pre-Salt Carbonate
Authors: Sampaio R. V. , Righetto G. L. , Gomes R. G. S. , Fernandes L. F. L. , Fontoura S. A. B. , Guimarães V. L. O. , Bueno J. F. , Honório B. C. Z. , Lima C. , Naumann M. , Zhou C. 
Published: September 2023

ABSTRACT
Brazilian pre-salt carbonate reservoirs are known by their complex and varied pore shapes with multimodal pore-size distribution. The complexity in these lacustrine carbonates can be related to sensitiveness of salinity, pH, water level changes, topography, and sediment supply variations, forming deposits that can be very challenging for petrophysical characterization. This paper presents a geological-petrophysical experimental program based on thin sections, x-ray powder diffraction, gas permeameter-porosimeter, mercury intrusion porosimetry, well logs, nuclear magnetic resonance, and digital rock analyses on samples recovered from a well in a reservoir at Santos Basin, aiming to capture the multi-scale petrophysical properties associated with these rocks. The samples contained calcite (∼82 %), ankerite (∼12 %), and both quartz and clay minerals (∼3 %) composing only one facies that has calcite shrubs and spherulites as the main textural components. The average porosity (13.3 %), obtained in gas permeameter-porosimeter was similar to the one obtained from thin sections, well logs, nuclear magnetic resonance, and digital rock analyses. The carbonates’ heterogeneity was manifested in the permeability values (9.7 to 137.9 mD), which agrees with the diversity of pore types and the multi-modal pore size distributions obtained. The results of digital rock analysis enabled an internal view of the rock complementing the laboratory data and supporting its heterogeneous nature. In conclusion, the designed experimental program provides a rich collection of geological-petrophysical data towards a better understanding of the rock.

INTRODUCTION
Approximately 50-60 % of the world's hydrocarbons reserves and the largest oil and gas fields are composed of carbonate rocks (Roehl & Choquette, 1985; Ramakrishnan et al., 2001; Bust et al., 2011; Burchette, 2012; Garland et al., 2012). Initially, there was a common perception that carbonate reservoirs were mostly associated with a marine, especially organic, origin and were characterized by superficially similar mineralogical and diagenetic imprints (‘limestones and dolomites’), as addressed by Burchette (2012). Since the 1970s, however, the joint efforts between industry and academia provided major advances in the understanding of carbonate rocks. Some of the research-driver issues related to carbonate reservoirs include their complex and varied pore shapes with multimodal pore-size distribution, permeability ranges within orders of magnitude for a given porosity, and non-linear porosity/permeability relationships. Besides that, properties like porosity are very helpful to estimate mechanical parameters, through the use of empirical correlations, when there is no available sample for laboratory tests. Thus, operators working in Brazil are very much motivated to increase their knowledge about the petrophysics of pre-salt carbonates, which is the focus of this paper.