Abstract An elastic–plastic material model, with strain-hardening or -softening, and volumetric strains, implemented within a general-purpose finite-element system (SAVFEM TM ), is shown to reproduce the stress–strain relationships and localized to de-localized (brittle to ductile) changes in strain response that have long been observed in typical laboratory experiments on common porous rocks. Based on that validation of the implementation, SAVFEM TM is then used to create numerical simulations that reproduce the patterns of localized shear zones, and their growth history, that occur in experimental (physical) models of fold–fault systems in layered rocks. These simulations involve a progressive evolution of the mechanical state, illustrating a geometrically dominated type of localization behaviour. Part of the deformation simulated here represents a crestal graben system. Analysis of the evolving mechanical state in the system of simulated faults poses challenges to some longstanding ideas concerning the way that faults operate, suggesting the need for a new fault-process paradigm.
Thin layers of less permeable materials can occur in cross‐bedded rocks and may function as flow baffles, thus influencing the bulk fluid flow. However, it can be challenging to model their flow impact using standard grid‐based techniques that can only capture the topology and geometry of the layers accurately using a large number of small cells. In this paper, a numerical method, recently developed by the authors for modeling fluid flow in fault damage zones that contain thin low‐permeability fault strands, is demonstrated as being applicable to modeling the flow impact of thin baffle laminae in cross beds. This method does not require thin baffles to be discretized explicitly. For a range of permeability contrasts between the thin baffle layers and the rest of the matrix, upscaled permeability values are derived for models that have the same volumetric fraction of baffle material. The results show that when the baffles are completely connected, the upscaled permeability is less than for cases where the baffles do not form a continuous impediment for all levels of permeability contrast and declines more steeply with the increase of the permeability contrast. The flow effect of the layer configuration becomes more apparent for the disconnected situation when the permeability contrast is high. The method is shown to be accurate and efficient for this type of work. These results highlight the importance of capturing the topology and volume of the thin baffle layers in flow modeling of cross beds and the necessity of using appropriate numerical techniques.
Abstract Understanding and predicting fracture propagation and subsequent fluid flow characteristics is critical to geoenergy technologies that engineer and/or utilize favorable geological conditions to store or extract fluids from the subsurface. Fracture permeability decreases nonlinearly with increasing normal stress, but the relationship between shear displacement and fracture permeability is less well understood. We utilize the new G eo‐ R eservoir E xperimental A nalogue T echnology (GREAT cell), which can apply polyaxial stress states and realistic reservoir temperatures and pressures to cylindrical samples and has the unique capability to alter both the magnitude and orientation of the radial stress field by increments of 11.25° during an experiment. We load synthetic analogue materials and real rock samples to stress conditions representative of 500–1,000 m depth, investigate the hydraulic stimulation process, and then conduct flow experiments while changing the fluid pressure and the orientation of the intermediate and minimum principal stresses. High‐resolution circumferential strain measurements combined with fluid pressure data indicate fracture propagation can be both stable (no fluid pressure drop) and unstable (fluid pressure drop). The induced fractures exhibit both opening and shear displacements during their creation and/or during fluid flow with changing radial stress states. Flow tests during radial stress field rotation reveal that fracture normal effective stress has first‐order control on fracture permeability but increasing fracture offset can lead to elevated permeabilities at maximum shear stress. The results have implications for our conceptual understanding of fracture propagation as well as fluid flow and deformation around fractures.
Sharp pressure transition zones, and alterations of stresses within overpressure compartments, are features that are predictable and expected consequences arising from the geomechanical behaviours of typical mud-rich successions. A qualitative analysis based on poro-plastic material responses explains how seals form, how they fail, and how the stress state evolves within compartments. The analysis predicts that rocks within overpressure compartments may be very weak due to dilational deformations. These conditions pose major challenges to drilling and production operations.
Temperature and lithological data from forty-six wells distributed along a SW-NE section line (from the Mid-North Sea High, across the Central Graben, to the Norwegian-Danish Basin) define the modern temperature field and the geological setting of a part of the North Sea Basin.
Abstract High‐speed neutron tomographies (1‐min acquisition) have been acquired during water invasion into air‐filled samples of both intact and deformed (ex situ) Vosges sandstone. Three‐dimensional volume images have been processed to detect and track the evolution of the waterfront and to calculate full‐field measurement of its speed of advance. The flow process correlates well with known rock properties and is especially sensitive to the distribution of the altered properties associated with observed localized deformation, which is independently characterized by Digital Volume Correlation of X‐ray tomographies acquired before and after the mechanical test. The successful results presented herein open the possibility of in situ analysis of the local evolution of hydraulic properties of rocks due to mechanical deformation.
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