Strength Characterization and Empirical Correlations for Carbonate Rocks from the Brazilian Pre-Salt Reservoirs

The pre-salt regions in Brazil, especially the Campos and Santos Basins, are responsible for most of the country's hydrocarbon production. Although there has been much discussion about the geological and petrophysical aspects of the carbonate reservoirs found in these areas, information about their mechanical properties is still limited. This study aims to help fill this gap by characterizing the strength of well plugs from four different wells located in the Santos Basin and developing empirical correlations. The samples are lacustrine carbonates of varying lithologies from the Barra Velha Formation, with porosity ranging between 0.7% and 20.5%, and permeability ranging from near-zero to 22.1 mD. The results showed cohesion (c') varying from 11.1 to 43.1 MPa, internal friction angle (φ') between 12.7° and 39.2°, and UCS between 29.2 and 171.7 MPa. The average Poisson's ratio (ν50) was 0.26 ± 0.1, and the Young's modulus (E50) results were between 8.7 and 72.4 GPa. The established empirical correlations between porosity and E50, UCS, c', and φ' were exponential and power equations that provided suitable fitting of the data and could be applied to other carbonates from the Barra Velha Formation. 1. INTRODUCTION The worldwide exploration and production of carbonate reservoirs for oil and gas have been a significant driving force for the study of these rocks. In the offshore basins of Southeastern Brazil, the lacustrine carbonates are a major challenge for the industry. These rocks are known for being complex and heterogeneous, requiring a wide understanding of the interplay of their geological, petrophysical, and mechanical aspects. Even though most of the country's production comes from pre-salt reservoirs, obtaining well samples for mechanical characterization is expensive and, therefore, rare. Thus, there is a lack of information about the mechanical behavior of these lacustrine carbonates. Considering this scenario, many authors have been addressing their efforts to develop empirical correlations to be used with parameters such as porosity, sonic wave's transit time, and density, which are easily obtained from well logs. Some of these correlations available in the literature were gathered by Santos and Ferreira (2010). However, due to the complexity of these carbonate rocks, it is important to verify the suitability of the correlations. In this paper, some well plugs from Santos Basin were used in triaxial tests to obtain the Mohr-Coulomb failure envelopes and elastic parameters. In addition, empirical correlations were developed based on the tests' results and the porosity of the specimens.