Large-scale dynamic hydrofracturing, healing and fracture network characterization

Permeability of a rock is a dynamic property that varies spatially and temporally. Fractures provide the most efficient channels for fluid flow and thus directly contribute to the permeability of the system. Fractures usually form as a result of a combination of tectonic stresses, gravity (i.e. lithostatic pressure) and fluid pressures. High pressure gradients alone can cause fracturing, the process which is termed as hydrofracturing that can determine caprock (seal) stability or reservoir integrity. Fluids also transport mass and heat, and are responsible for the formation of veins by precipitating minerals within open fractures. Veining (healing) thus directly influences the rock’s permeability. Upon deformation these closed factures (veins) can refracture and the cycle starts again. This fracturing-healing-refacturing cycle is a fundamental part in studying the deformation dynamics and permeability evolution of rock systems. This is generally accompanied by fracture network characterization focusing on network topology that determines network connectivity. Fracture characterization allows to acquire quantitative and qualitative data on fractures and forms an important part of reservoir modeling. This thesis highlights the importance of fracture-healing and veins’ mechanical properties on the deformation dynamics. It shows that permeability varies spatially and temporally, and that healed systems (veined rocks) should not be treated as fractured systems (rocks without veins). Field observations also demonstrate the influence of contrasting mechanical properties, in addition to the complexities of vein microstructures that can form in low-porosity and permeability layered sequences. The thesis also presents graph theory as a characterization method to obtain statistical measures on evolving network connectivity. It also proposes what measures a good reservoir should have to exhibit potentially large permeability and robustness against healing. The results presented in the thesis can have applications for hydrocarbon and geothermal reservoir exploration, mining industry, underground waste disposal, CO2 injection or groundwater modeling

Structural and sedimentological controls on the evolution of carbonate platforms on equatorial margins

Carbonate platforms are common features on Cenozoic Equatorial Margins. The growth and development of carbonate platforms and their associated depositional settings depend on a series of controlling factors. This thesis analyses the structural and sedimentological factors controlling four different study areas with carbonate platforms, utilising a variety of datasets. Study areas include the Vulcan Sub-Basin, Bonaparte Basin (Northwest Shelf of Australia), the Cariatiz carbonate platform in the Sorbas Basin (SE Spain), the Pernambuco Basin (Eastern Brazil), and the Pará- Maranhão Basin (Equatorial Brazil). Datasets include 2D and 3D seismic data, wellbore data, airborne LiDAR maps, outcrop maps and multispectral satellite imagery, spanning multiple scales of observation. This thesis aims to improve the current understanding of shallow- and deep-water carbonate depositional and structural settings, aiding industry and academia in prospect identification and reservoir characterisation. A comprehensive analysis of fault evolution and its relationship with the distribution of isolated carbonate platforms is investigated in the Vulcan Sub-Basin, Northwest Australia, using 3D seismic and borehole data. Detailed fault-throw measurements along arrays of normal faults were completed to generate throw- depth (T-Z) and throw-distance (T-D) profiles, as well as fault-throw maps. The results obtained were useful to determine the fault styles and timing(s) of fault initiation in the Vulcan Sub-Basin, and data were compared to the growth rates of isolated carbonate platforms (ICPs). Three types of ICPs were defined: one in which fault-throw is larger than carbonate productivity (type 1), a second type in which fault-throw is equal or lower than carbonate productivity (type 2), and ICPs where fault-throw postdates the growth of carbonate platforms (type 3). An integrated method to characterise fracture networks and their scale relationships is proposed using multi-scale datasets from the Cariatiz and Pernambuco carbonate platforms. Small fractures are obtained via detailed outcrop mapping, while intermediate-scale fractures are mapped from airborne LiDAR imagery. Large-scale fractures are measured from 3D seismic data. Geometrical and topological data are acquired to demonstrate that fracture properties behave differently depending on their size, and that particular fracture types correlate to specific scales of observation. The key result in this Chapter is that small-scale fractures strike in all directions, and are highly connected in the two study areas. However, intermediate- and large-scale fractures strike predominantly parallel to the platform margin and have lower connectivity rates than small-scale fractures. Understanding sub-seismic fracture networks is theefore critical to quantify fluid flow and permeability in carbonate reservoirs. Toward the end of this thesis, deep-water depositional settings from the Pará- Maranhão Basin, Equatorial Brazil, are studied utilising 2D and 3D seismic, borehole and multispectral satellite data to better understand platform-to-basin sedimentary processes. Neogene calciclastic submarine fans and channel-levee systems are analysed, and a comprehensive geomorphologic analysis is undertaken with the ultimate aim of finding similarities (or major contrasts) with their siliciclastic counterparts. Mixed calciclastic and siliciclastic sediment was transported from shallow waters into deep and ultra-deep waters by turbidity flows. Of importance is the confirmation that the pre-existing palaeotopography - such as terraces and gullies - was key to funnel sediment and create distinct types of channel-levee systems in Equatorial Brazil. Three types of channels are recognised: channels related to calciclastic submarine fans (type 1), low-sinuosity, aggradational channels (type 2), and high-sinuosity channels (type 3)