Geometry and Stability Analysis of Caves in Bahia, Brazil

Abstract

More than 30% of the world’s hydrocarbon reserves are located in carbonate reservoirs, and this percentage is likely to increase, as a result of discoveries of new giant oil fields in carbonate rocks, generically named “Pre-salt layers”. However, there are still some problems in understanding karst systems that still unresolved. The karst caves are one of the suitable analogs for karstic reservoirs that also spread all over the world. In this study, the mobile LIDAR (Light Detection and Ranging) data is used to characterize the geometries and to analyze the stability of tunnels under several depths from 5 caves in Bahia, Brazil. The studied caves are representing both of the karstification mechanism (Epigene & Hypogene). In general, there are two tunnel shapes among the caves: horizontal ellipse & Vertical ellipse shapes. Several factors could be controlled the shape origin of the tunnels, but from this study mainly caused by lithology or the geology structure factor. By comparing with the structural data, the conduit orientation generally shows the same trend. Therefore, these conditions suggest a geometrical correlation between the fractures and the caves, and that the observed fractures almost certainly acted as conduits for fluid flow. The stability analysis showed that the vertical ellipse tunnels are more stable compared to the horizontal ellipse. However, all of the tunnels are already unstable in shallow depth (on average less than 2 km depth). The sensitivity tests show several parameters that would affect the stability: Rock Properties (Rock Mass strength, Rock mass elastic, density), number of tunnels in the system and the distances between multiple tunnels.