Multiscale Geological Modelling for Fluid Flow Evaluation on Deformed Carbonates

In this Ph.D. thesis, the effect of both lithological and structural heterogeneities on fluid flow was investigated, in both porous and tight carbonates, by means of multiscale geological models and fluid flow simulations. The petrophysical properties (i.e., porosity, permeability) of the analyzed multiscale fault zones have been investigated by the implementation of 3D models based on different stochastic and deterministic approaches such as the Discrete Fracture Network modelling (DFN), Structure from Motion photogrammetry (SfM), X-ray computed microtomography (micro-CT) and Lattice-Boltzmann Method (LBM). Furthermore, a 2D elastic-petrophysical model of a seismic scale fault zone in tight carbonates was investigated through the seismic modelling pre-stack depth migration (PSDM) technique, performing a sensitivity analysis of different geophysical and geological conditions to test the seismic signature of a seismic scale fault zone internal architecture.The bulk of this doctoral thesis consists of four scientific papers: Chapter 1. From fracture analysis to flow simulations in fractured carbonates: the case study of the Roman Valley quarry (Majella Mountain, Italy), published in Marine and Petroleum Geology 100 (2019) 95–110. Chapter 2. Analysis of fracture roughness control on permeability using SfM and fluid flow simulations: implications for carbonate reservoir characterization, published in Geofluids, Volume 2019, Article ID 4132386. Chapter 3. Pore-scale dual-porosity and dual-permeability modeling in an exposed multi-facies porous carbonate reservoir, published in Marine and Petroleum Geology 128 (2021) 105004. Chapter 4. Outcrop-scale fracture analysis and seismic modelling of a basin-bounding normal fault in platform carbonates, central Italy, submitted in Journal of Structural Geology. The studies related to the first three papers have been carried out within the same study area, the inactive Roman Valley quarry (Majella Mountain, central Italy), well-known for its historical bitumen extraction. This site facilitates the study of a well-exposed analogue of a porous deformed carbonate reservoir and allows gaining information about matrix, fracture and fault characteristics that influenced hydrocarbon migration. Furthermore, the bitumen shows distribution within the quarry helps to further discuss and validate the obtained results. In the first chapter, the main objective was to assess the impact of both stratigraphic and structural heterogeneities on fluid flow at the outcrop scale. This was possible by creating a large-scale DP/P model of the study area, which includes the petrophysical properties (i.e., porosity and permeability) of the matrix and fracture pore systems associated with the different studied lithofacies and fault zones. The studied rocks consist of ramp carbonates belonging to the lower member of the Bolognano Formation (Oligocene-Miocene in age) composed of grainstones, packstones and wackestones. These rocks are crosscut by two high-angle oblique-slip faults WNW- ESE oriented with up to 40 m of throw. The petrophysical properties of matrix and fractures were derived from laboratory measurements and field-based Discrete Fracture Network (DFN) modelling, respectively. Finally, the DP/P model was used to run fluid flow simulation, testing different scenarios of well locations. The fluid distribution in the matrix, resulting from these flow simulations, is consistent with field observations wherebitumen localizes within the most pervious lithofacies (grainstones). In the fault zones, the fracture network gains a relevant fluid flow anisotropy, enhancing the fluid flow along the faults, whereas the across fault fluid flow is controlled by type and lateral continuity of fault rocks, where fault breccias represent conduits and cataclasites localized barriers. Although the use of DFN models is an acceptable representation of the macroscopy heterogeneities associated with sub-seismic resolution faults in a reservoir characterization, at the pore-scale the fluid flow is controlled by the matrix and fracture pore morphology. Therefore, the scale of investigation was changed in the second and third chapters focusing on the effect of pore scale heterogeneities on permeability. Specifically, the second chapter focuses on the analysis of the so-called fracture hydraulic aperture, which differs from the mechanical aperture due to a friction factor related to the roughness of the fracture walls. Samples of fracture surface have been collected from the different lithofacies outcropping within the Roman Valley quarry and digitalized using SfM photogrammetry in a highly controlled laboratory setting, applying a fracture surface micro-topography. This study incorporates fluid flow simulations, using the Lattice-Boltzmann method, and the use of synthetic computer- generated fractures for estimation of the fracture roughness. The quantitative analysis of fault surface roughness was achieved by implementing the power spectral density (PSD), which provides an objective description of the roughness, based on the frequency distribution of the surface asperities in the Fourier domain. This work evaluates the respective controls on permeability exerted by the fracture displacement (perpendicular and parallel to the fracture walls), surface roughness, and surface pair mismatch. The results may contribute to defining a more accurate equation of hydraulic aperture and permeability of single fractures. The third chapter aims to investigate the interaction between the fracture and matrix pore systems at the microscale. To do so, microscale DP/P models were generated by incorporating two different methods of 3D imaging such as, high resolution synchrotron X-ray microtomography (micro-CT) and SfM photogrammetry. Quantitative analyses of pristine rock and DP/P models were performed to evaluate the contribution of macrofracture segments to the porosity and connectivity of the pore network. These results were integrated with fluid flow simulations by applying a sensitivity analysis to evaluate the control exerted by fracture roughness parameters (i.e., asperity height distribution and fractal dimension) on porosity and permeability in various lithofacies. The results of this study demonstrate the utility of obtainingmicroscale DP/P models as complementary approach to explain the geofluids distribution in fractured multi-facies porous carbonates. Finally, the fourth chapter focuses on an integrated outcrop-based characterization and seismic modelling of the internal architecture of a seismic scale fault zone hosted in tight carbonates. This was possible in a key outcrop represented by an active quarry, located at the southeastern boundary of the Fucino Basin (Abruzzo region, central Italy). Here, the footwall damage zone of a seismic scale fault (throw ≈ 300 m), known as Venere Fault (VF), is well-exposed in a 3D view and crosscut by many subsidiary structures. This study presents a workflow to investigate the across strike distribution of petrophysical properties within the VF damage zone, through quantitative fracture analysis and in-situ permeability measurements. A large-scale 2D petrophysical-elastic base model of the VF zone was constrained incorporating results from field analyses and digital outcrop models (DOM) from SfM photogrammetry technique. This base model was tested in ray-based seismic modelling (PSDM; Lecomte et al., 2008), performing a sensitivity analysis of geological and geophysical parameters to investigate the seismic signature of the VF zone. The present contribution hence highlights the great importance of high- resolution structural analysis of fault damage zones for seismic modelling, and subsurface fault characterization.