Fourth Naturally Fractured Reservoir Workshop

An integrated fault characterization study was performed as part of a comprehensive mature field redevelopment project in the Austrian Flysch reservoirs. Structural interpretation was challenging due to the complexity of the faulting and the poor quality of the available seismic data. Structural complexity is often a reason to degrade the seismic image. The seismic was reprocessed with CRS for noise reduction and CBM as an imaging method. In a close cooperation between geophysicists and interpreters seismic fault attributes were derived. The heavily faulted structure was interpreted in an inductive manner based on numerous fault crossing in the wells and the observed faults on the seismic. An structural was developed honoring all available data leading to an improvement of the geological concept of regional tectonics. Further a number of prospective yet undrilled compartments could be identified in this mature field.

A thorough drill cuttings study was conducted for a fractured basement reservoir, focusing on fracture analysis. The aim of this study was to provide input to a conceptual fractured basement reservoir model, progress the understanding of lithological controls on fracturing, and to investigate how the connected hydrocarbon volume from fractures and/or potential matrix are distributed at the well scale. A thorough macroscopic cuttings screening combined with insights from all available log, production and seismic data led to a selection of 45 cuttings samples, which were thoroughly analyzed with thin section microscopy, X-ray diffraction (XRD) and X-ray fluorescence (XRF). Our study shows that cuttings can provide a valuable source of subsurface information at a low cost – also with regard to fracture description. Outcomes show that a stereomicroscopic cuttings screening proved to be most suitable for fracture frequency detection. Thin section analysis combined with geochemical measurements were crucial to understand cement generations, related fracture conductivity and porosity types. Furthermore the study highlighted the linkage between lithology/metamorphic facies/hydrothermal alteration and microfracturing along with methodology limitations such as scaling, cavings and mud additives.

Multiscale karst systems, typically below seismic resolution, are important contributors to permeability but also provide significant challenges in drilling operations and in flow simulations. A substantial portion of subsurface karsts is of hypogenic origin, that is, is associated with upward movement of chemically aggressive fluids. The Morro Vermelho Karst System and Cave developed within Proterozioc shallow water carbonates and is an inspiring analog for buried hypogenic karsts. MVKS formed in association with a regional strike-slip fault. In an initial stage, deformation in the “softer” carbonates overlying more rigid quartzites and basement was accommodated by a 100s of m wide semi-ductile shear zone. Wholesale dolomitization took place and numerous dolomite-filled veins formed. With progressing strike-slip, the fault propagated upward creating an anticline parallel to the fault zone, causing the development of m-long extensional fractures, the arrival of SiO2 rich fluids in the karts and, eventually, the formation of the cave. Vertical fracture corridors and/or faults capturing deep, generally layer parallel flow are present in different settings, from foredeeps to rifted margins and are primary candidates for widespread hypogenic karst development.

The result of these works are detailed mechanical earth models for wells within the area of investigation and description of behaviour of interpreted CSF under variable stress state as well as understanding of the connectivity of natural fractures within zone subjected to fracturing treatment.

Seismic diffraction imaging (DI) is applied to the Barents Sea Wisting field, using the velocity and reflection dip fields obtained from a legacy PSDM. The objective is to image steeply dipping faults. The fault definition obtained from the DI is compared to the legacy PSDM and also to a high resolution P-cable survey. The improved resolution and detectability of the fault systems by DI offer a significant added value for interpretation of the legacy PSDM as well as the P-cable.

We apply the mechanically driven approach to fracture modelling of Welch et al. (2019) to two fractured outcrops, Nash Point in South Wales and Robin Hood’s Bay in NE England. We show that the local stress field developed around larger faults is a key control on the geometry of the fractures, along with layer thickness, fracture propagation rate and layer stiffness. We can use this information to predict fracture anisotropy and connectivity.

This study focuses on the multi-scale analysis of lineaments on the pan-African base of the Arabo-Nubian shield. Several satellite images of different resolutions were used (DTM, Landsat, RapidEye, GeoEye). The lineaments measured highlight structures such as Najd-Faults (large shear zones), a dyke network and a primary fracture system. The length distribution was also analyzed and shows a power law applicable to most of the scales studied demonstrating control of large and small lineaments on the fracture network.

Karstification generated by the circulation of fluids along fracture zones leads to the formation of upward erosive features, which result in dolines. In this work, we investigate collapse structures and their relationship with flow corridors in Neoproterozoic carbonate units in the Irecê Basin, Brazil. This area has a high concentration of dolines that connect surface drains through a cryptorheic system. We used high-resolution unnamed aerial vehicle (UAV) imagery, field mapping of dolines, structural lineaments, drainage, and field measurements of bedding strikes and dips. We conclude that collapse structures along anticline fold hinges form flow corridors. These findings have implications for fractured reservoirs.

In recent years, the evolution of Unmanned Aerial Vehicles (UAV), together with advances in photogrammetry and Structure from Motion techniques (SfM) have enhanced the role of Digital Outcrop Models (DOM) in fracture studies, due also to the possibility to obtain accurate 3D measurements from large and inaccessible areas. In this work, we applied the Digital Outcrop Model techniques, developed by UAV Photogrammetry, to the study of fracturation of a strongly prograding carbonate buildup in the Dolomites. About 250 fault and fractures where measured from the Digital Model, and more than 400 fracture trends were collected in the field , both on the platform-top surface and in the overlying Heiligkreutz formation. Opening mode joints and neptunian dykes, orthogonal to the direction of progradation (i.e. margin parallel) were detected at seismic scale and in the field, and interpreted as early-stage fractures. Moreover, the carbonate body records two main tectonic events, which contribute to form the present-day fracture network: the Jurassic extension, forming N-S trending joints and normal faults, and the Alpine compressive phase. Syndepositional fractures are preferentially reactivated in strike-slip, showing that their orientation with respect to the stress field can influence the subsequent evolution of the fracture pattern.

Production from the mid-Cretaceous Sabiriyah Mauddud Formation results to a large extent from waterflood, yet field data indicate that reservoir performance is impacted by large permeability contrasts in the reservoir. High permeability (high-K) elements in the reservoir are likely the product of sedimentological and diagenetic processes as well as the structural evolution of the field. In this abstract, we concentrate on the fault and fracture components of an integrated study with included carbonate and structural geology, seismic interpretation diagenesis, geomechanics and reservoir engineering. One of the key outcomes of the study is an improved understanding of the complex High-K pathways summarized in a so-called plumbing diagram. The two first order elements for the High-K plumbing system are 1) cemented low porosity cycle tops that are sequence stratigraphically controlled and where dissolution enlarged open fractures and vugs make up flow paths and 2) fault damage zones with increased fracture density in cemented layers. While it has been observed that, statically, the reservoir is characterized by the presence of faults and fractures contributing to flow paths, from a dynamic-response point of view, the reservoir behaves overall as an unfractured reservoir.