Acidificação matricial em rochas areníticas e seus efeitos sobre a porosidade e a permeabilidade

Abstract

The matrix acidification operation can be applied to both sandstone and carbonate rocks, and different types of acids provide different results for each type of formation mineralogy. In addition to this factor, which is relevant to the possibility of precipitate formation, other factors can interfere with the acidification process, such as formation heterogeneity, types of minerals and clay minerals that make up the rock, acid reactivity with the constituent minerals, among others. This work seeks to study the permoporous alterations resulting from the imbibition, on a bench scale, of sandstone rocks from the Cabeças Formation, located in the Parnaíba Basin, in acidic solutions of hydrochloric acid (HCl) and nitric acid (HNO3), separately, and also in an acid mixture composed of these two components. A comparative analysis was carried out, before and after the acidification experiment, in order to verify the acid composition that obtained the highest success rate in relation to the increment of the permoporous properties. An integrated analysis of the results was also carried out, mainly in relation to the alterations suffered by the constituent minerals, the consequent change in the morphological structure of porosity and permeability, and the diagenetic processes (recrystallization, precipitation and dissolution of minerals), in order to understand how they contributed to the alteration of the original characteristics of the sandstones and their influence on their quality as reservoirs. It was observed that, among the analyzed concentrations, the most significant results in relation to the increase in porosity were obtained in the sample acidified with 5% HCl, which generated an increase of approximately 3% in total porosity, since the sample presented porosity of 11% before acidification and, after the technique, a porosity of 14% was recorded. It is noteworthy that in this sample considerable percentages of potassium feldspars were recorded, represented by orthoclases and microclines, as well as the presence of plagioclases. Despite the affinity of HCl with carbonate minerals, it had a positive influence on the generation of secondary porosity, acting on the dissolution of more unstable minerals, such as potassium feldspars, plagioclase and clay mineral fractions. However, when subjected to the acid mixture of HCl and HNO3, the sample showed an increase in the percentages of alterations, substitutions and precipitations for clay minerals, as observed by optical microscopy. It is noteworthy that the samples that showed low porosity and permeability, influenced by the great performance of diagenetic processes, such as mechanical and chemical compaction and silicification, were not effective using the imbibition methodology.