Linking natural fractures to karst cave development: a case study combining drone imagery, a natural cave network and numerical modelling by Quinten Boersma, Rahul Prabhakaran, Francisco Hilario Bezerra, and Giovanni Bertotti Petroleum Geoscience Volume ():petgeo2018-151 April 10, 2019 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

(a) Regional geological map of the study area (RPMB, Riacho do Pontal Mobile Belt; SFC, Sao Francisco Craton) (figure modified and simplified from Bizzi et al. 2003). (a) Regional geological map of the study area (RPMB, Riacho do Pontal Mobile Belt; SFC, Sao Francisco Craton) (figure modified and simplified from Bizzi et al. 2003). (b) Simplified chart of the main tectonic events, deposition and stress regimes during the Mesoproterozoic and Neoproterozoic. (c) Simplified stratigraphic column of the Salitre Formation. Figure modified from Guimaraes et al. (2011). Coordinate system: WGS 84. Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

The study area highlighted in Figure 1a. The study area highlighted in Figure 1a. (a) Satellite image of the Brejoes Field area, including the locations of different sampling points, the entrance of the cave and the Brejoes fractured pavement. (b) Drone image acquired from the pavement. (c) Entrance to the Brejoes cave system. Coordinate system: WGS 84. Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

Data-driven workflow to obtain dynamic fluid-flow model and structural data from acquired drone images. Data-driven workflow to obtain dynamic fluid-flow model and structural data from acquired drone images. (a) (Step 1.1) Drone imagery, photogrammetry and (Step 1.2) GIS-based fracture interpretation (the example shown has a 1:1000 scale interpretation). (b) (Step 2.1) Simplification and meshing of the interpreted fracture network, (Step 2.2) local stress modelling and (Step 2.3) fracture-aperture modelling using the simplified mesh. Node duplication and model boundary conditions workflow are after Bisdom et al. (2017a). (c) (Step 3.1) Fluid-flow modelling and (Step 3.2) effective permeability calculations using AD-GPRS (Karimi-Fard & Firoozabadi 2001; Karimi-Fard et al. 2004; Voskov et al. 2009). Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

Cave map digitization and interpretation workflow. Cave map digitization and interpretation workflow. (a) Original map of the Brejoes cave system. The cave map is acquired by, and courtesy of, Grupo Bambuí de Pesquisas Espeleológicas (https://blogdobambui.wordpress.com/2013/04/07/expedicao-brejoes/). (b) GIS-based georeferencing and digitization of the cave map. (c) GIS-based interpretation using the polyline tool. Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

Detailed structural field data. Detailed structural field data. (a) Pavement outcrop depicting the structural features observed on the Brejoes fractured pavement. (b) Interpretation of the pavement outcrop. (c) Vertical outcrop next to the Brejoes fractured pavement. (d) Fractures and vertical stylolites on top of the Brejoes pavement. (e) Stereonet of the measured features (taken from different sampling points: Fig. 2). (f) & (g) Inversion analysis of the measured data using the right dihedron method. Results show two horizontal compression phases, namely: NNW–SSE compression and ENE–WSW compression. Inversion calculations were performed using Win-Tensor® (Delvaux & Sperner 2003). All data are shown in strike/dip. Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

(a) Cliff outcrop showing reactivated fractures acting as fluid-flow conduits. (a) Cliff outcrop showing reactivated fractures acting as fluid-flow conduits. (b) Reactivated stylolite, which acted as a fluid-flow conduit. Both images were taken at the Brejoes cave entrance. Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

Structural interpretation of the Brejoes fractured pavement and cave system. Structural interpretation of the Brejoes fractured pavement and cave system. (a) Satellite and drone images of the field area with the interpreted structural features (fractures and cave tunnels) highlighted as polylines. (b). Drone images (stations) of the detailed fracture interpretations. The location of each station is highlighted on the fractured pavement (Fig. 8a). (c) Rose diagrams acquired from small- and large-scale fracture interpretations. (d) Orientation distribution from the cave tunnel interpretation. Coordinate system: WGS 84/Pseudo-Mercator. Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

(a) Length data from both caves and fractured pavement. (a) Length data from both caves and fractured pavement. The presented data have been discriminated for each fracture and cave orientation. (b) Normalized cumulative length distribution and power-law fit of the Brejoes fractured pavement and cave system. Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

Stress and aperture modelling results. Stress and aperture modelling results. List of mechanical parameters and applied stresses: E = 30 GPa, υ = 0.25, σH = 30 MPa, σh = 10 MPa and σH (orient)  = 70°. (a) The fracture network implemented into the model. (b) Nodal fracture contact pressures [MPa] (subsection: Meshing and stress modelling using FEM). (c) Modelled nodal fracture slip [m] (subsection: Meshing and stress modelling using FEM). (d) Calculated mechanical aperture [mm], using the Barton–Bandis equation (5). (e) Calculated hydraulic aperture [mm] (equation 10). (f) Hydraulic conductive fractures (fracture aperture >1.0 × 10−5 m). Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

Fluid-flow modelling results. Fluid-flow modelling results. (a) Barton–Bandis aperture results. (b) Fluid pressure field for both modelled flow directions. (c) Modelled velocity field for both flow directions. (d)–(f) The modelled fracture apertures, fluid pressures and flow velocities for the constant aperture scenario. (g)–(i) Modelled apertures, fluid pressures and flow velocities for the power-law aperture scenario. For these results, the modelled matrix permeability is 1.0 mD. Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved

(a) Equivalent permeability over the matrix permeability for each modelling scenario (keq/kM). (a) Equivalent permeability over the matrix permeability for each modelling scenario (keq/kM). (b) Ratio between the two modelled equivalent permeability directions (kNS/kEW). Quinten Boersma et al. Petroleum Geoscience 2019;petgeo2018-151 © 2019 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved