3D MULTISCALAR GEOLOGICAL MODELS AND FLUID FLOW SIMULATIONS IN DEFORMED CARBONATES

Faults are key features to consider during reservoir evaluation due to their strong impact on the containing and the migration of fluids. Faults occurs at different scale and could be classified in terms of the detectability in seismic profiles as Seismic Resolution Faults (SRF) and Sub-Seismic Resolution Faults (SSRF). The threshold between the two groups is related to the maximum vertical resolution of seismic, which is currently in the order of 10-20 m. The impact of SRF on fluid flow is strictly related to the hydraulic behaviour of Fault Damage Zone (FDZ) and fault core composing the fault zone. The porosity and the permeability within the FDZ depend on the geometry, density, and connectivity of the fracture network. Similarly, the geometrical and hydraulic properties of array of SSRF are key parameters to be considered in fractured and faulted reservoir modelling. In this Ph.D. thesis, we investigated the effect of both SRF and SSRF on fluid flow, in both tight and porous carbonates, by means of multiscale geological models and fluid flow simulations based on different stochastic and determinist approaches such as the Discrete Fracture Network (DFN), X-ray computed microtomography (micro-CT) and Lattice-Boltzmann Method (LBM). A methodology to assess the effects of structural heterogeneities below seismic resolution in porous carbonate grainstones on reservoir performance during production is developed by integrating structural analysis, power law distributions, up-scaling, and numerical techniques. The novelty of the methodology consists in accounting for the buffering effects on permeability caused by compactive and cataclastic deformation bands. By using this methodology, a 3D deterministic field analogue and a 3D stochastic Discrete Fracture Network (DFN) representations of the reservoir/aquifer were built first, and then single-phase, steady state fluid flow numerical experiments in an equivalent porous medium framework were performed. The studied outcrop is located at the San Vito Lo Capo Peninsula (western Sicily, Italy), where deformation is mainly localized and represented by a network of SSRF composed by deformation bands (DBs). The permeability of these features is up to three orders of magnitude less than the surrounding porous carbonate rocks. The fluid flow numerical experiments show that results are similar for both deterministic and stochastic approaches except for wells located in areas of intense strain localization. Therefore, in structurally complex areas, SSRF might represent a drilling risk for water production and enhanced oil recovery (EOR) activities. Considering the applicability of the 3D stochastic DFN approach, this methodology was used for deriving the hydraulic properties of a highly fractured and faulted tight carbonates cropping out in Murge region (Italy). Here, individual attributes (i.e., orientation, length, height, aspect ratio, intensity, and aperture) of fault-related fractures were investigated with respect to their effect on porosity and permeability in fault damage zones. The achieved results indicate that the overall fault permeability is 3-to-4 orders of magnitude higher than the host rock permeability. The fault damage zones form the main fluid conduits, with the highest permeability values computed for fault-parallel fluid flow. Such a pronounced permeability anisotropy obtained for the fault damage zone is mainly related to the fracture dimension, both lengths and heights, and their aperture values. Even though the use of DFN models is an acceptable representation of the macroscopy heterogeneities induced by sub-seismic resolution faults in a reservoir/aquifer, at the pore-scale the fluid flow is controlled by the pore morphology. Therefore, the scale of investigation was changed focusing on the effect of pore scale heterogeneities on permeability. To do that, the X-ray micro-CT technique was applied for imaging the porous network of deformed porous carbonates grainstones. The studied rocks are highly affected by DBs cropping out in Sicily (Favignana Island and San Vito Lo Capo Peninsula) and Abruzzo (Maiella Mt), Italy. The obtained virtual rock samples have been used for characterizing the porous network architecture (i.e. porosity, connectivity) and textural properties (i.e. shape, surface area, tortuosity) within DBs and the surrounding host rock. In addition, fluid flow simulations were carried out by means of the LBM to evaluate their effect on the permeability. In general, the control exerted by both SRF and SSRF on fluid flow is determined by the morphological, geometrical and petrophysical properties at the pore-scale scale. In tight rocks, where the matrix porosity and permeability are negligible, the permeability is controlled by both aperture and roughness of fractures. In porous rocks, the permeability is controlled by the pore network properties, thus highly affected by strain and diagenesis localization. In common, both SRF and SSRF may buffer the fluid flow across them causing a high permeability anisotropy of the reservoirs.