Impactos da dissolução cárstica na permo-porosidade de rochas carbonáticas: implicações para o desenvolvimento de redes de fraturas alargadas e camadas super-k

Abstract

We evaluated the impact of karst dissolution on the permo-porous system of carbonate rocks, using field data obtained from outcrops of the Jandaíra Formation (Potiguar Basin). The karst system of this formation has excellent exposures, which allow the study of karst features both in vertical and horizontal planes. The integrated and multidisciplinary approach of this thesis was carried out in two stages. The first stage was predominantly two-dimensional (2D), in which we used scanlines and aerial images of Unmanned Aerial Vehicles to characterize networks of karstified subvertical fractures. In this stage, we show that the process of fractures enlargement by karstic dissolution modifies the statistical relationships between fracture length and aperture. For early stages of karst dissolution, the length and aperture values follow approximately the classical power law distribution typical of mechanical fractures. However, as karstification progresses, fracture aperture tends to remain constant, regardless of its length. Thus, by adapting the aperture-length curve, according to the known stage of karst dissolution, it is possible to build discrete fracture models that include the effects of karstic dissolution. We call the resulting model DFKN (Discrete Fracture and Karst Network). We also show that fracture enlargement can increase the transmissivity of a karstified fracture network by up to five orders of magnitude, without changing its connectivity. The second stage of the research consists of a three-dimensional (3D) analysis of fracture networks, lithostratigraphic interfaces and karst features, based on the joint interpretation of Ground Penetrating Radar (GPR) cubes, core logs, porosity, permeability and total radiation profiles, images of borehole walls and thin sections. In this stage we verified that GPR signal attenuation zones are related to secondary alterations of carbonate rocks, which are caused by fracturing, fracture and pore enlargement, as well as stylolite dissolution. Such secondary alterations significantly increase the porosity and permeability of rocks. Fractures are the main pathways for fluid percolation, contributing to the evolution of karstification processes, especially in layers with higher primary porosity/permeability. On the other hand, layers with low primary porosity/permeability, but with a high concentration of stylolites, can also develop zones of high permeability due to the interconnection of dissolved stylolites. The attenuation/dissolution zones can compose connected volumes of high permeability and generally have tabular geometry, because they are limited by subvertical fractures and subhorizontal stratigraphic interfaces. At the reservoir scale, these tabular bodies may constitute super-k layers. In a unified view, our results contribute to a better understanding of the three-dimensional permo-porous system of fractured and karstified carbonate reservoirs.