Sequential vertical gas charge into multilayered sequences controlled by central conduits

Four sets of stacked amplitude anomalies are described from a three-dimensional seismic survey acquired in Block A of the Dutch North Sea. The amplitude anomalies (AAs) have a subcircular planform and within each set they are stacked vertically: they have a high degree of spatial overlap in the vertical succession. These sets of AAs are interpreted as vertical anomaly clusters (VACs) composed of five to seven major AAs each. Seismic interpretation of the VACs and quantitative analysis of the reservoir intervals reveal that the VACs are gas-bearing silt-rich reservoirs hosted in the upper section of the Upper North Sea Group, a mixed clastic succession of Pliocene to Pleistocene age. Novel analysis based on the geometry of the individual anomalies reveals that these are likely to be the result of a gas migration process characterized by sequential upward gas charge into reservoir units and that the flow across the seals separating these reservoirs is controlled by central regions of focused fluid flow. These regions function as seal–bypass systems and are most likely formed by hydraulic fracturing

A homogenised model for flow, transport and sorption in a heterogeneous porous medium

A major challenge in flow through porous media is to better understand the link between pore-scale microstructure and macroscale flow and transport. For idealised microstructures, the mathematical framework of homogenisation theory can be used for this purpose. Here, we consider a two-dimensional microstructure comprising an array of circular obstacles, the size and spacing of which can vary along the length of the porous medium.We use homogenisation via the method of multiple scale to systematically upscale a novel problem that involves cells of varying area to obtain effective continuum equations for macroscale flow and transport. The equations are characterized by the local porosity, an effective local anisotropic flow permeability, and an effective local anisotropic solute diffusivity. These macroscale properties depend non-trivially on both degrees of microstructural geometric freedom (obstacle size and spacing). We take advantage of this dependence to compare scenarios where the same porosity field is constructed with different combinations of obstacle size and spacing. For example, we consider scenarios where the porosity is spatially uniform but the permeability and diffusivity are not. Our results may be useful in the design of filters, or for studying the impact of deformation on transport in soft porous media

Dynamics of compressible displacement in a capillary tube

We study two-phase displacement via the steady compression of an air reservoir connected to an oil-filled capillary tube. Our experiments and modelling reveal complex displacement dynamics depending on compression rate and reservoir volume that, for large reservoirs, depend on a single dimensionless compressibility number. We identify two distinct displacement regimes, separated by a critical value of the compressibility number. While the subcritical regime exhibits quasi-steady displacement after an initial transient, the supercritical regime exhibits burst-like expulsion

Post-injection spreading and trapping of CO2 in saline aquifers: impact of the plume shape at the end of injection

We use an analytical model for the post-injection spreading of a plume of CO[subscript 2] in a saline aquifer under the action of buoyancy and capillary trapping to show that the spreading behavior is at all times strongly influenced by the shape of the plume at the end of the injection period. We solve the spreading equation numerically and confirm that, at late times, the volume of mobile CO[subscript 2] is given by existing asymptotic analytical solutions. The key parameters governing plume spreading are the mobility ratio, M, and the capillary trapping number, Gamma—the former sets the shape of the plume at the end of the injection period, and the latter sets the amount of trapping. As a quantitative measure of the dependence of the spreading behavior on the initial shape, we use a volume ratio. That is, we evolve the plume from a true end-of-injection initial shape and also from an idealized “step” initial shape, and we take the ratio of these mobile plume volumes in the asymptotic regime. We find that this volume ratio is a power-law in M, where the exponent is governed exclusively by Gamma. For conditions that are representative of geologic CO[subscript 2] sequestration, the ratio of mobile volumes between “true” and “step” initial plume shapes can be 50% or higher

Fluid-driven deformation of a soft granular material

Compressing a porous, fluid-filled material drives the interstitial fluid out of the pore space, as when squeezing water out of a kitchen sponge. Inversely, injecting fluid into a porous material can deform the solid structure, as when fracturing a shale for natural gas recovery. These poromechanical interactions play an important role in geological and biological systems across a wide range of scales, from the propagation of magma through Earth’s mantle to the transport of fluid through living cells and tissues. The theory of poroelasticity has been largely successful in modeling poromechanical behavior in relatively simple systems, but this continuum theory is fundamentally limited by our understanding of the pore-scale interactions between the fluid and the solid, and these problems are notoriously difficult to study in a laboratory setting. Here, we present a high-resolution measurement of injection-driven poromechanical deformation in a system with granular microsctructure: We inject fluid into a dense, confined monolayer of soft particles and use particle tracking to reveal the dynamics of the multiscale deformation field. We find that a continuum model based on poroelasticity theory captures certain macroscopic features of the deformation, but the particle-scale deformation field exhibits dramatic departures from smooth, continuum behavior. We observe particle-scale rearrangement and hysteresis, as well as petal-like mesoscale structures that are connected to material failure through spiral shear banding. </p

Large deformations of a soft porous material

Compressing a porous material will decrease the volume of the pore space, driving fluid out. Similarly, injecting fluid into a porous material can expand the pore space, distorting the solid skeleton. This poromechanical coupling has applications ranging from cell and tissue mechanics to geomechanics and hydrogeology. The classical theory of linear poroelasticity captures this coupling by combining Darcy’s law with Terzaghi’s effective stress and linear elasticity in a linearized kinematic framework. Linear poroelasticity is a good model for very small deformations, but it becomes increasingly inappropriate for moderate to large deformations, which are common in the context of phenomena such as swelling and damage, and for soft materials such as gels and tissues. The well-known theory of large-deformation poroelasticity combines Darcy’s law with Terzaghi’s effective stress and nonlinear elasticity in a rigorous kinematic framework. This theory has been used extensively in biomechanics to model large elastic deformations in soft tissues and in geomechanics to model large elastoplastic deformations in soils. Here, we first provide an overview and discussion of this theory with an emphasis on the physics of poromechanical coupling. We present the largedeformation theory in an Eulerian framework to minimize the mathematical complexity, and we show how this nonlinear theory simplifies to linear poroelasticity under the assumption of small strain. We then compare the predictions of linear poroelasticity with those of large-deformation poroelasticity in the context of two uniaxial model problems: fluid outflow driven by an applied mechanical load (the consolidation problem) and compression driven by a steady fluid throughflow. We explore the steady and dynamical errors associated ith the linear model in both situations, as well as the impact of introducing a deformation-dependent permeability. We show that the error in linear poroelasticity is due primarily to kinematic nonlinearity and that this error (i) plays a surprisingly important role in the dynamics of the deformation and (ii) is amplified by nonlinear constitutive behavior, such as deformation-dependent permeabilit

CO2 migration in saline aquifers. Part 1. Capillary trapping under slope and groundwater flow

Injection of carbon dioxide (CO2) into geological formations is widely regarded as a promising tool for reducing global atmospheric CO2 emissions. To evaluate injection scenarios, estimate reservoir capacity and assess leakage risks, an accurate understanding of the subsurface spreading and migration of the plume of mobile CO2 is essential. Here, we present a complete solution to a theoretical model for the subsurface migration of a plume of CO2 due to natural groundwater flow and aquifer slope, and subject to residual trapping. The results show that the interplay of these effects leads to non-trivial behaviour in terms of trapping efficiency. The analytical nature of the solution offers insight into the physics of CO2 migration, and allows for rapid, basin-specific capacity estimation. We use the solution to explore the parameter space via the storage efficiency, a macroscopic measure of plume migration. In a future study, we shall incorporate CO2 dissolution into the migration model and study the importance of dissolution relative to capillary trapping and the impact of dissolution on the storage efficiency.</jats:p

Fluid-fluid phase separation in a soft porous medium

Various biological and chemical processes lead to the nucleation and growth of non-wetting fluid bubbles within the pore space of a granular medium, such as the formation of gas bubbles in liquid-saturated lake-bed sediments. In sufficiently soft porous materials, the non-wetting nature of these bubbles can result in the formation of open cavities within the granular solid skeleton. Here, we consider this process through the lens of phase separation, where thermomechanics govern the separation of the non-wetting phase from a fluid-fluid-solid mixture. We construct a phase-field model informed by large-deformation poromechanics, in which two immiscible fluids interact with a poroelastic solid skeleton. Our model captures the competing effects of elasticity and fluid-fluid-solid interactions. We use a phase-field damage model to capture the mechanics of the granular solid. As a model problem, we consider an initial distribution of non-wetting fluid in the pore space that separates into multiple cavities. We use simulations and linear-stability analysis to identify the key parameters that control phase separation, the conditions that favour the formation of cavities, and the characteristic size of the resulting cavities

Evidence for massive emission of methane from a deep‐water gas field during the Pliocene

Geologic hydrocarbon seepage is a natural atmospheric source of methane, considered to prevail in terrestrial and shallow‐water areas; deep‐water methane seepage is expected to have no atmospheric impact, as gas is typically consumed throughout the water column. Here, we present evidence for a sudden expulsion of a reservoir‐sized quantity of methane from a deep‐water seep during the Pliocene, resulting from natural gas reservoir overpressure. Combining 3D seismic data, borehole data and fluid‐flow modelling, we estimate that 18–27 of the 23–31 Tg released at the seafloor could have reached the atmosphere over 39–241 days. This emission is ~10 % and ~28 % of present‐day, annual natural and petroleum‐industry methane emissions, respectively. While no such ultra‐seepage events have been documented in modern times and their frequency is unknown, seismic data suggest they were not rare in the past and potentially occur at present in critically pressurized reservoirs. This neglected phenomenon can influence decadal changes of atmospheric methane