Linking natural fractures to karst cave development: a case study combining drone imagery, a natural cave network and numerical modelling

Abstract

In carbonate rocks, channelized fluid flow through fracture conduits can result in the development of large and connected karst networks. These cavity systems have been found in multiple hydrocarbon and geothermal reservoirs, and are often associated with high-permeability zones, but also pose significant challenges in drilling and reservoir management. Here, we expand on the observed interplay between fractures, fluid flow and large cave systems, using outcrop analysis, drone imagery and fluid-flow modelling. The studied carbonate rocks are heavily fractured and are part of the Salitre Formation (750–650 Ma), located in central Bahia (NE Brazil). Firstly, the fracture and cave network data show a similar geometry, and both systems depict three main orientations, namely; NNE–SSW, NW–SE and ESE–WNW. Moreover, the two datasets are dominated by the longer NNE–SSW features. These observed similarities suggest that the fractures and caves are related. The presented numerical results further acknowledge this observed correlation. These results show that open fractures act as the main fluid-flow conduits, with the aperture model defining the fracture-controlled flow contribution. Furthermore, the performed modelling highlights that geometrical features such as length, orientation and connectivity play an important role in the preferred flow orientations. Thematic collection: This article is part of the Naturally Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/naturally-fractured-reservoirs

An automated fracture trace detection technique using the complex shearlet transform

“…The complex shearlet measure first introduced by Reisenhofer (2014) and improved by King et al (2015) was used for applications like coastline detection (King et al, 2015), flame front detection , and feature extraction from terrestrial lidar inside tunnels (Bolkas et al, 2018). Karbalaali et al (2018) used the complex shearlet transform for channel edge detection from synthetic and real seismic slices.…”

“…Reisenhofer et al (2016) presented a comprehensive comparison of complex shearlet-based feature detection compared with conventional edge detectors such as Canny (Canny, 1986), Sobel (Sobel and Feldman, 1973), phase congruency (Kovesi, 1999), and another shearlet-based edge detector (Yi et al, 2009). Bolkas et al (2018) also made specific comparisons between the performance of Canny, Sobel, and Prewitt (Prewitt, 1970) edge detection methods versus space-frequency transform methods such as wavelets, contourlets, and shearlets. A detailed overview of the complex shearlet transform is provided in Appendix A for the interested reader.…”

An automated fracture trace detection technique using the complex shearlet transform

“…The Neoproterozoic cover was affected by the Brasiliano Orogeny (750-540 Ma) in two separate folding events resulting in fold belts around edges of the São Francisco Craton (Ennes-Silva et al, 2016). The Brejões pavement UAV imagery that we used for our analysis was acquired by Boersma et al (2019). The orthomosaic covers an area of 0.81 km 2 and consists of fractured, black oolitic limestones that correspond to Unit A1 of the Salitre stratigraphy (Guimarães et al, 2011).…”

“…The automatically extracted features from the Brejões image data was compared with manual interpretations performed by and obtained from Boersma et al (2019) at seven stations. The automatic interpretations were trimmed to the peripheries of the manual interpretations for a fair comparison between both the vectorizations.…”

“…Comparison between automatic and manual interpretation on seven stations within the Brejões outcrop. The manual interpretations were obtained fromBoersma et al (2019) https://doi.org/10.5194/se-2019-104 Preprint. Discussion started: June 2019 c Author(s) 2019.…”