Maria A. Nikolinakou

Geomechanical modeling of stresses adjacent to salt bodies: Part 1—Uncoupled models

We compare four approaches to geomechanical modeling of stresses adjacent to salt bodies. These approaches are distinguished by their use of elastic or elastoplastic constitutive laws for sediments surrounding the salt, as well as their treatment of fluid pressures in modeling. We simulate total stress in an elastic medium and then subtract an assumed pore pressure after calculations are complete; simulate effective stress in an elastic medium and use assumed pore pressure during calculations; simulate total stress in an elastoplastic medium, either ignoring pore pressure or approximating its effects by decreasing the internal friction angle; and simulate effective stress in an elastoplastic medium and use assumed pore pressure during calculations. To evaluate these approaches, we compare stresses generated by viscoelastic stress relaxation of a salt sphere. In all cases, relaxation causes the salt sphere to shorten vertically and expand laterally, producing extensional strains above and below the sphere and shortening against the sphere flanks. Deviatoric stresses are highest when sediments are assumed to be elastic, whereas plastic yielding in elastoplastic models places an upper limit on deviatoric stresses that the rocks can support, so stress perturbations are smaller. These comparisons provide insights into stresses around salt bodies and give geoscientists a basis for evaluating and comparing stress predictions.

Geomechanical modeling of stresses adjacent to salt bodies: Part 2—Poroelastoplasticity and coupled overpressures

We use a fully coupled poroelastoplastic geomechanical model to study how stresses and pore pressures evolve in sediments bounding a spherical salt body. Drained analyses (pore pressures remain hydrostatic) demonstrate that sediments yield in response to loading by the salt, which leads to a redistribution of stresses and to deformations larger than predicted by poroelastic or solid Coulomb-plastic models. Undrained analyses (overpressures develop while no dissipation occurs) illustrate that salt loading induces pore pressures that extend kilometers away from the salt body. We also model the flow and consequent dissipation that occur in the sediments because of this undrained salt loading. We show that with time, the pressure field dissipates and expands. The dissipation process takes millions of years, which suggests that pore-pressure perturbations caused by salt loading should still be present in mudstones near many salt bodies. Under drained conditions, stress perturbations generate low minimum principal stresses above and below the salt, resulting in convergence of pore pressure and minimum principal stress at these locations. Such conditions are challenging to drill through. In undrained systems, sharp drops in pore pressure may occur above and below the salt, whereas both the pore pressure and the minimum principal stress rise next to the salt. In contrast to previous models that do not couple changes in stress to changes in pore pressure, the coupled approach presented here has the potential to predict in-situ stresses and pore pressures more accurately in a wide variety of geologic settings.

Prediction and interpretation of the performance of a deep excavation in Berlin sand

This paper describes the application of a generalized effective stress soil model, MIT-S1, within a commercial finite-element program, for simulating the performance of the support system for the 20-m-deep excavation of the M1 pit adjacent to the primary station “Hauptbahnhof” in Berlin. The M1 pit was excavated underwater and supported by a perimeter diaphragm wall with a single row of prestressed anchors. Parameters for the soil model were derived from an extensive program of laboratory tests on the local Berlin sands. This calibration process highlights the practical difficulties both in the measurements of critical state soil properties and in the selection of model parameters. The predictions for excavation performance are strongly affected by the vertical profiles of two key state parameters: the initial earth pressure ratio, K0; and the in situ void ratio, e0. These parameters were estimated from field dynamic penetration test data and geological history. The results showed good agreement between c...

The role of pore fluid overpressure in the substrates of advancing salt sheets, ice glaciers, and critical‐state wedges

Critical-state wedges, ice glaciers, and salt sheets have many geometric and mechanical similarities. Each has a tapering geometry and moves along a basal detachment. Their motions result from the combined effects of internal deformation and basal sliding. Wedge deformation and geometry, basal conditions, and overpressure (pore fluid pressure less hydrostatic pore fluid pressure) development within the substrate interact with each other in this mechanically coupled system. However, the nature of this interaction is poorly understood. In order to investigate this coupled system, we have developed two-dimensional poromechanical finite-element models with porous fluid flow in sediments. We have simulated the advance of a salt sheet wedge across poroelastic sediments in this study. We emphasize that our results have applications beyond salt wedges to both critical-state wedges and ice glaciers. Overpressure develops within the substrate over time during the advance of the wedge. The magnitude of the overpressure influences the wedge geometry and the wedge advance rate. Lower overpressure results in a thicker and steeper wedge geometry, and a slower advance rate, while higher overpressure favors a thinner, wider, and more flattened wedge geometry and a faster advance rate. This study provides key insights into the links between wedge geometry, basal shear stress, and overpressure in substrates.

Deformation, stress, and pore pressure in an evolving suprasalt basin

We develop a two-dimensional plane strain large-deformation coupled poroelastoplastic finite element model to simulate initiation and rise of a salt wall from a flat salt body during sedimentation. We run transient analyses with high-permeability and low-permeability sediments in the model to simulate drained conditions and overpressure (or pore fluid overpressure). We investigate deformation, stress, and overpressure in the evolving suprasalt basin. Model results show that horizontal stress increases even higher than vertical stress at the flank of the salt wall and in the minibasin due to horizontal pushing out of the rising salt wall and that orientations of principal stresses rotate in the minibasin relative to far-field stress state. Model predicts that compared with far-field overpressure, the overpressure near the salt wall and within the minibasin is largely perturbed by the salt body. We find that perturbations of pore pressure (or pore fluid pressure) near the salt wall and within the minibasin cannot be resolved by traditional pore pressure prediction methods such as normal compaction trend approach and mean stress model approach. In order to predict pore pressure more accurately, especially in the regions near salt bodies or where stresses are perturbed, we need to apply a method that includes both effects of mean effective stress and deviatoric stress on pore pressure and compaction, for example, the Modified Cam Clay model approach, to pore pressure prediction. Our results provide geoscientists insights into evolution of salt basins, near-salt deformation, stress, overpressure, and overpressure prediction methods and have implications for near-salt wellbore drilling programs.

Initiation and growth of salt diapirs in tectonically stable settings: Upbuilding and megaflaps

We use finite element modeling to show that upbuilding can be a significant component of salt diapir growth in tectonically stable systems when basin sediments are elastoplastic mudrocks. The ability of such sediments to deform plastically and the dependence of their strength on confining pressure enable structural thinning, which allows salt to pierce through a relatively thick roof. Once pierced, the originally continuous roof uplifts to form a megaflap. We show that the evolution to an upturned megaflap adjacent to a salt stock causes shortening of the bedding layers in the radial and vertical directions and extension in the hoop (circumferential) direction. These deformations lead to significant shear strains within the sediments; as a result, in some areas within the upturned megaflap, mudrocks have reached their maximum level of shear resistance and are failing. Thinning and shear failure of sediments are also significant near salt walls, despite the absence of out-of-plane deformation. We illustrate that cross-sectional area and bedding line lengths are not necessarily preserved. Based on our results, we re-evaluate traditional assumptions of kinematic restoration and show that established workflows may not properly restore salt systems that interact with shallow plastic sediments. Finally, we show that when wall rocks are deformable, salt diapir shapes are not necessarily a simple function of sedimentation and salt flux rates (qfx/A) and that the diapir hourglass shape might result from lateral deformation of the megaflap.

Structure and form of early Gothic flying buttresses

ABSTRACT This paper explores the structural function of early Gothic flying buttresses. Their effectiveness is evaluated under minimum thrust conditions using conventional limit analysis. The significance of various formal characteristics of the flying buttress (length, intrados curvature, thickness, inclination) as well as probable failure modes (sliding and support displacement), are investigated both parametrically and using a series of twenty French early Gothic flyers. The results permit us to address certain long-standing art-historical assumptions and demonstrate that the method of study proposed here holds promise for future exploration for all types of flying buttresses—not just those from the early Gothic period.

Modeling of Shales in Salt-Hydrocarbon Systems

We model the stress–strain response of shale wall rocks to large deformations associated with the emplacement of salt bodies. We further identify the implications of these stress changes for hydrocarbon exploration. We model the mudrocks as porous elastoplastic materials. We employ both static and evolutionary approach for the modeling of salt systems and show that while the static one can model actual geologic geometries, only the evolutionary approach can provide a detailed description of the stress changes associated with the emplacement of salt. Hence, the evolutionary approach can register the overall stress history of the shale wall rocks, which is essential for predicting the present-day state of stress, porosity, and pore pressure. More generally, the evolutionary approach can provide useful insights for understanding Earth processes related to salt-hydrocarbon systems.

Pore Pressure and Stress around Dipping Structures

We employ laws of poro-elastoplasticity (Modified Cam Clay) to predict the stresses and pore pressures around inclined hydrocarbon reservoirs (e.g., dipping structures). We show that at the crest, the localized flow field increases the horizontal and vertical stresses above the lithostatic value; therefore, very high least principal stresses, even greater than the overburden value, are possible around the shallower end of dipping structures. At the deeper end of such structures, the local flow field loads the surrounding sediments, leading to elastoplastic strains and notable increase in porosity. We show that both the vertical and the horizontal total stresses decrease locally near the base. Traditional basin modeling approaches assume a horizontal-to-vertical stress ratio and so they cannot fully capture the stress changes around dipping structures. We illustrate that poromechanical analyses are essential to predict changes in pore pressure and the associated stress changes, especially in the vertical direction. The poromechanical results can significantly increase confidence in the trap integrity of inclined hydrocarbon reservoirs and provide a more reliable evaluation of borehole stability.