Petrophysical evaluation of coquinas by means of X-ray microtomography: an example of Morro do Chaves formation, Sergipe-Alagoas Basin

Evaluate the pore system of carbonate rocks is an extremely complicated task. The heterogeneity makes the correct characterization even more difficult and, consequently, the extraction of petrophysical parameters, which could help us improve our understanding regarding the capacity of the rock to act as a reservoir. For this reason, it is important that new methods be implemented in order to improve the visualization and quantification of petrophysical properties. Among the methods that have been proposed to help petrophysical evaluation, X-ray microtomography (μ-CT) has been highlighted as one the most promising techniques. Its excellent investigative capacity, related to its higher spatial resolution, allied with the possibility to generate a 3D model of the pore space of rocks has allowed the emergence of a new petrophysical branch, digital rock petrophysics. Nevertheless, nowadays this technique is just considered an auxiliary method, and for this reason, should be integrated to other conventional characterization and evaluation methods, like optical microscopy and experimental petrophysics, and, above all, it should also be correlated to geological information, so it would be possible to institute an integrated context. In this work the objective was to calculate the Total Porosity (PT), porosity profile, Pore and Pore-throat Size Distribution (DTP and DTG), Coordination Number (CN), pore/pore-throat ratio and absolute permeability of different coquinas taphofacies from Morro do Chaves Formation (Sergipe-Alagoas Basin). Thus, outcrop samples were selected and plugged, firstly, in 1-inch diameter, in which experimental petrophysical analysis and Nuclear Magnetic Resonance (NMR) were carried out. Additionally, thin-section of the same samples were produced and, later on, 8mm core plugs were extracted from the first ones. Both core plug sizes yielded μ-CT images of approximately 28 μm (R1) and 7 μm (R2). These images were pre-processed and segmented, allowing the extraction of petrophysical parameters directly from the binarized images or by its post-processing. The results of this work is divided in two original articles. The first one deals with the petrophysical parameters obtained using the 28 μm images resolution, while the other is about the multiscale characterization considering the integration of both resolutions (R1 and R2). Based on this, it was possible to conclude, firstly, that there is basically two taphofacies of coquinas that present petrophysical parameters which are better than the others. In this case, μ-CT has allowed this distinction mainly based in parameters inaccessible by other techniques. Hence, this method has proved to be useful to detected petrophysical differences that other methods were not able to perceive. However, due to the limitation imposed by spatial resolution, in the second article a multiscale integration of μ-CT images was implemented. By doing this, it was possible to note that the petrophysical parameters showed to be nearer to those values calculated by experimental petrophysics and NMR, but still lower. This demonstrates that an expressive part of the porosity of coquinas is associated, most likely, to the micropores present in micrite and/or clay minerals. To compute this pores, we suggest the utilization of nanomicrotomography (nCT), or even SEM with 3D reconstruction, to a better evaluation of the pore space of coquinas. (author)