Criação de zonas de alta permeabilidade em fraturas: modelagem numérica do Lajedo de Soledade, Formação Jandaíra, Bacia Potiguar

The fracturing in carbonate rocks has been attracting increasingly attention due to new oil discoveries in carbonate reservoirs. This study investigates how the fractures (faults and joints) behave when subjected to different stress fields and how their behavior may be associated with the generation of karst and consequently to increased secondary porosity in these rocks. In this study I used satellite imagery and unmanned aerial vehicle UAV images and field data to identify and map faults and joints in a carbonate outcrop, which I consider a good analogue of carbonate reservoir. The outcrop comprises rocks of the Jandaíra Formation, Potiguar Basin. Field data were modeled using the TECTOS software, which uses finite element analysis for 2D fracture modeling. I identified three sets of fractures were identified: NS, EW and NW-SE. They correspond to faults that reactivate joint sets. The Ratio of Failure by Stress (RFS) represents stress concentration and how close the rock is to failure and reach the Mohr-Coulomb envelopment. The results indicate that the tectonic stresses are concentrated in preferred structural zones, which are ideal places for carbonate dissolution. Dissolution was observed along sedimentary bedding and fractures throughout the outcrop. However, I observed that the highest values of RFS occur in fracture intersections and terminations. These are site of karst concentration. I finally suggest that there is a relationship between stress concentration and location of karst dissolution in carbonate rocks.

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O fraturamento em rochas carbonáticas vem atraíndo atenção devido as novas descobertas de petróleo em reservatórios carbonáticos. Este estudo investiga como fraturas (falhas e juntas) submetidas a diferentes campos de tensões se comportam e como seu comportamento pode estar associado à geração de carste e, consequentemente, ao aumento da porosidade secundária em carbonatos. Neste estudo usei imagens de satélite e imagens de veículos aéreos não tripulado, além de dados de campo para identificar e mapear falhas e juntas no lajedo de rochas carbonáticas, que considero ser um bom análogo para reservatório carbonatíco. O lajedo está localizado na Formação Jandaíra, Bacia Potiguar. Os dados de campo foram modelados utilizando o software TECTOS, que usa análise de elementos finitos para fraturas 2D. Formam identificados três sets de juntas NS, EW e NW-SE, que foram reativadas como falhas. Os maiores valores de Ratio of Failure by Stress (RFS) obtido na modelagem representam o quão perto a rocha está de tocar a envoltória Mohr-Coulomb, que indica a concentração da tensão. Os resultados indicam que as tensões tectônicas estão concentradas em zonas estruturais preferenciais, que são locais ideais para a geração de dissolução nos carbonatos. Observei dissolução ao longo de fraturas e planos de estratificação em todo o lajedo. No entanto, concluo que os maiores valores de RFS ocorrem no cruzamento e terminações das fraturas. Estes são lugares de concentração do carste. Finalmente sugiro que existe uma relação direta entre a concentração da tensão e a localização da dissolução em rochas carbonáticas. ABSTRACT The fracturing in carbonate rocks has been attracting increasing attention. This study investigates how the fractures (faults and joints) behaves subjected to the stress field and how their behavior may be associated with the generation of karst and consequently to increased secondary porosity in these rocks. We use satellite imagery and VANT image and field data to identify and map the faults and joints, analogous outcrop carbonate rock reservoir in the Jandaíra Formation, Potiguar Basin. Then, we modeled our data using the TECTOS software, which uses finite element analysis for 2D fractures. The results indicate that the tectonic stresses are concentrated in preferred structural zones, which are ideal places for the generation of dissolution in carbonates. The highest values of Ratio of Failure by Stress (RFS) obtained in modeling represents how close the rock is touching the envelopment Morh-Coulomb. We conclude that the highest values of RFS occur at the intersection of fractures and termination thereof. These sites are places of concentration of karst. From the data collected in the field there were three sets of fractures with NS, EW and NW-SE directions. In all cases, the regular pattern of fractures indicates that the system of lineaments 80 º and 340 º direction are sets of joints that were reactivated as faults. The joints and faults are shown consistent with the extracted lineaments in the GoogleEarth image. However, it is observed that, in marioria cases, both sets are reactivated as strike-slip faults. Tectonic stylolites occur mainly found in Lajedo showing shortening parallel to the joints (stylolites NS direction), formed in the same tectonic event. In every part of the flagstone we observed dissolution along fractures and bedding planes of the rock. Combining field data with numerical modeling we suggest that there is a direct relationship between stress concentration and location of dissolution.