Search

Gigatonne scale geological storage of carbon dioxide and energy (such as hydrogen) will be central aspects of a sustainable energy future, both for mitigating CO2 emissions and providing seasonal-based green energy provisions. In this Review, we evaluate the feasibility and challenges of expanding subsurface carbon dioxide storage into a global-scale business, and explore how this experience can be exploited to accelerate the development of underground hydrogen storage. Carbon storage is technically and commercially successful at the megatonne scale, with current projects mitigating approximately 30 Mt of CO2 per year. However, limiting anthropogenic warming to 1.5°C could require gigatonnes of storage per year by 2050, and a scaleup from 2025 approaching rates of deployment that would be historic for energy technology. Scale-up is not limited by geology or engineering. Advances in understanding storage complex geology, subsurface fluid dynamics, and seismic risk underpin new engineering strategies including the development of multi-site, basin scale, storage resource management. Instead economic and societal contraints pose barriers to project development. Underground hydrogen storage, still in development, will face similar issues. Overcoming these barriers with strengthened financial incentives, and programs to address concerns inhibiting public acceptance, will enable the storage of CO2 at climate relevant scales.

There is growing concern about seismicity triggered by human activities, whereby small increases in stress bring tectonically loaded faults to failure. Examples of such activities include mining, impoundment of water, stimulation of geothermal fields, extraction of hydrocarbons and water, and the injection of water, CO2 and methane into subsurface reservoirs1. In the absence of sufficient information to understand and control the processes that trigger earthquakes, authorities have set up empirical regulatory monitoring-based frameworks with varying degrees of success2,3. Field experiments in the early 1970s at the Rangely, Colorado (USA) oil field4 suggested that seismicity might be turned on or off by cycling subsurface fluid pressure above or below a threshold. Here we report the development, testing and implementation of a multidisciplinary methodology for managing triggered seismicity using comprehensive and detailed information about the subsurface to calibrate geomechanical and earthquake source physics models. We then validate these models by comparing their predictions to subsequent observations made after calibration. We use our approach in the Val d’Agri oil field in seismically active southern Italy, demonstrating the successful management of triggered seismicity using a process-based method applied to a producing hydrocarbon field. Applying our approach elsewhere could help to manage and mitigate triggered seismicity. A multidisciplinary method for managing triggered seismicity is developed using detailed subsurface information to calibrate geomechanical and earthquake source physics models, and is applied to the Val d’Agri oil field in seismically active southern Italy.

We conduct pore-scale simulations of two-phase flow using the 2D Rothman–Keller colour gradient lattice Boltzmann method to study the effect of wettability on saturation at breakthrough (sweep) when the injected fluid first passes through the right boundary of the model. We performed a suite of 189 simulations in which a “red” fluid is injected at the left side of a 2D porous model that is initially saturated with a “blue” fluid spanning viscosity ratios$$M = \nu _\mathrm{r}/\nu _\mathrm{b} \in [0.001,100]$$M=νr/νb∈[0.001,100]and wetting angles$$\theta _\mathrm{w} \in [ 0^\circ ,180^\circ ]$$θw∈[0∘,180∘]. As expected, at low-viscosity ratios$$M=\nu _\mathrm{r}/\nu _\mathrm{b} \ll 1$$M=νr/νb≪1we observe viscous fingering in which narrow tendrils of the red fluid span the model, and for high-viscosity ratios$$M \gg 1$$M≫1, we observe stable displacement. The viscous finger morphology is affected by the wetting angle with a tendency for more rounded fingers when the injected fluid is wetting. However, rather than the expected result of increased saturation with increasing wettability, we observe a complex saturation landscape at breakthrough as a function of viscosity ratio and wetting angle that contains hills and valleys with specific wetting angles at given viscosity ratios that maximize sweep. This unexpected result that sweep does not necessarily increase with wettability has major implications to enhanced oil recovery and suggests that the dynamics of multiphase flow in porous media has a complex relationship with the geometry of the medium and the hydrodynamical parameters.

Bacteria live in heterogeneous environments, so it is important to investigate their behaviour in porous media. Here the authors show that flow disorder enhances the effect of chemical gradients in micropockets in a porous medium, which then aid the transport of bacteria. Natural soils are host to a high density(1)and diversity(2)of microorganisms, and even deep-earth porous rocks provide a habitat for active microbial communities(3). In these environments, microbial transport by disordered flows is relevant for a broad range of natural and engineered processes, from biochemical cycling to remineralization and bioremediation(4-7). Yet, how bacteria are transported and distributed in the subsurface as a result of the disordered flow and the associated chemical gradients characteristic of porous media has remained poorly understood, in part because studies have so far focused on steady, macroscale chemical gradients(8-10). Here, we use a microfluidic model system that captures flow disorder and chemical gradients at the pore scale to quantify the transport and dispersion of the soil-dwelling bacteriumBacillus subtilisin porous media. We observe that chemotaxis strongly modulates the persistence of bacteria in low-flow regions of the pore space, resulting in a 100% increase in their dispersion coefficient. This effect stems directly from the strong pore-scale gradients created by flow disorder and demonstrates that the microscale interplay between bacterial behaviour and pore-scale disorder can impact the macroscale dynamics of biota in the subsurface. ISSN:1745-2473 ISSN:1745-2481

Viscous fingering is the occurrence of narrow fingers of an invading less viscous fluid such as water in a porous medium filled with a more viscous fluid such as oil, and its occurrence dramatically affects enhanced oil recovery by water flooding. We conduct 2D simulations using the lattice Boltzmann method for two-phase flow through a porous medium initially saturated with a fluid of a given viscosity in which a fluid of another viscosity is injected from the left side of the model. We conduct suites of simulations over viscosity ratios ( ∼ 1 / ( mobility ratio ) $\sim 1/(\mathrm {mobility\ ratio})$ ) from M = 0.01 through M = 100 and for wetting angles from non-wetting to fully wetting. We plot the phase space of saturation (= Recovery Factor) versus wetting angle and viscosity ratio. We remove the dominant viscosity ratio effect to study the effect of wetting angle and find that while there is some tendency for the saturation to be higher with increasing wettability, the saturation landscape is complex with hills and valleys in which optimal wetting angles exist that maximize saturation. Furthermore, the phase space landscape is found to depend on the porous matrix geometry. We also plot saturation post-breakthrough and find that the saturation continues to increase albeit at an ever decreasing rate. This research demonstrates the potential of the lattice Boltzmann method for two-phase flow to reveal unexpected behavior and phenomena with both scientific and practical significance such as optimization of recovery factors in enhanced oil recovery (EOR)., King Fahd University of Petroleum & Minerals

We present simulations of two-phase flow using the Rothman and Keller colour gradient Lattice Boltzmann method to study viscous fingering when a “red fluid” invades a porous model initially filled with a “blue” fluid with different viscosity. We conducted eleven suites of 81 numerical experiments totalling 891 simulations, where each suite had a different random realization of the porous model and spanned viscosity ratios in the range $$M\in [0.01,100]$$ M ∈ [ 0.01 , 100 ] and wetting angles in the range $$\theta _w\in [180^\circ ,0^\circ ]$$ θ w ∈ [ 180 ∘ , 0 ∘ ] to allow us to study the effect of these parameters on the fluid-displacement morphology and saturation at breakthrough (sweep). Although sweep often increased with wettability, this was not always so and the sweep phase space landscape, defined as the difference in saturation at a given wetting angle relative to saturation for the non-wetting case, had hills, ridges and valleys. At low viscosity ratios, flow at breakthrough is localized through narrow fingers that span the model. After breakthrough, the flow field continues to evolve and the saturation continues to increase albeit at a reduced rate, and eventually exceeds 90% for both non-wetting and wetting cases. The existence of a complicated sweep phase space at breakthrough, and continued post-breakthrough evolution suggests the hydrodynamics and sweep is a complicated function of wetting angle, viscosity ratio and time, which has major potential implications to Enhanced Oil Recovery by water flooding, and hence, on estimates of global oil reserves. Validation of these results via experiments is required to ensure they translate to field studies.

Understanding the physical mechanisms that underpin the link between fluid injection and seismicity is essential in efforts to mitigate the seismic risk associated with subsurface technologies. To that end, here we develop a poroelastic model of earthquake nucleation based on rate-and-state friction in the manner of spring–sliders, and analyze conditions for the emergence of stick-slip frictional instability—the mechanism for earthquakes—by carrying out a linear stability analysis and nonlinear simulations. We find that the likelihood of triggering earthquakes depends largely on the rate of increase in pore pressure rather than its magnitude. Consequently, fluid injection at constant rate acts in the direction of triggering seismic rupture at early times followed by aseismic creep at late times. Our model implies that, for the same cumulative volume of injected fluid, an abrupt high-rate injection protocol is likely to increase the seismic risk whereas a gradual step-up protocol is likely to decrease it.

We present a numerical analysis of the sensitivity of the density driven CO2 convection results in a vertical Hele-Shaw cell with respect to different modeling assumptions. The role of density driven convection phenomenon in CO2 geological storage capacity and safety has already been pointed out in several studies. We showed that in order to accurately simulate the phenomenon occurring in lab experiments, multi-phase transfer has to be considered and variations in the permeability field should also be taken into account. Taylor dispersion has been found to have no significant effect on the results. Experimental results of the convection fingering process development and of quantitative determination of the total mass of dissolved CO2 were used to validate the numerical simulation results. Understanding how accurate numerical models can simulate lab experiments is an important step in confirming their reliability to predict underground CO2 storage capacity.

Accurate streamline tracing and travel time computation are essential ingredients of streamline methods for groundwater transport and petroleum reservoir simulation. In this paper we present a unified formulation for the development of high-order accurate streamline tracing algorithms on unstructured triangular and quadrilateral grids. The main result of this paper is the identification of velocity spaces that are suitable for streamline tracing. The essential requirement is that the divergence-free part of the velocity must induce a stream function. We recognize several classes of velocity spaces satisfying this requirement from the theory of mixed finite element methods and, for each class, we obtain the precise functional form of the stream function. Not surprisingly, the most widely used tracing algorithm (Pollock's method) emanates in fact from the lowest-order admissible velocity approximation. Therefore, we provide a sound theoretical justification for the low-order algorithms currently in use, and we show how to achieve higher-order accuracy both in the streamline tracing and the travel time computation.

We present a variational multiscale mixed finite element method for the solution of Darcy flow in porous media, in which both the permeability field and the source term display a multiscale character. The formulation is based on a multiscale split of the solution into coarse and subgrid scales. This decomposition is invoked in a variational setting that leads to a rigorous definition of a (global) coarse problem and a set of (local) subgrid problems. One of the key issues for the success of the method is the proper definition of the boundary conditions for the localization of the subgrid problems. We identify a weak compatibility condition that allows for subgrid communication across element interfaces, a feature that turns out to be essential for obtaining high-quality solutions. We also remove the singularities due to concentrated sources from the coarse-scale problem by introducing additional multiscale basis functions, based on a decomposition of fine-scale source terms into coarse and deviatoric components. The method is locally conservative and employs a low-order approximation of pressure and velocity at both scales. We illustrate the performance of the method on several synthetic cases and conclude that the method is able to capture the global and local flow patterns accurately.

In this paper we complete the description and application of a computational framework for the numerical simulation of first-contact miscible gas injection processes. The method is based on the front-tracking algorithm, in which numerical solutions to one- dimensional problems are constructed in the form of traveling discontinuities. The efficiency of the front-tracking method relies on the availability of the analytical Riemann solver described in Part 1 and a strategy for simplifying the wave structure for Riemann problems of small amplitude. Several representative examples are used to illustrate the excellent behavior of the front-tracking method. The front-tracking method is extended to simulate higher-dimensional processes through the use of streamlines. The paper presents a validation exercise for a quarter five-spot homogeneous problem, and an application of this computational framework for the simulation of miscible flooding in three-dimensional, highly heterogeneous formations. In this case, we demonstrate that a miscible water-alternating-gas injection scheme is more effective than waterflooding or gas injection alone.

In this series of two papers, we present a front-tracking method for the numerical simulation of first-contact miscible gas injection processes. The method is developed for constructing very accurate (or even exact) solutions to one-dimensional initial-boundary-value problems in the form of a set of evolving discontinuities. The evolution of the discontinuities is given by analytical solutions to Riemann problems. In this paper, we present the mathematical model of the problem and the complete Riemann solver, that is, the analytical solution to the one-dimensional problem with piecewise constant initial data separated by a single discontinuity, for any left and right states. The Riemann solver presented here is the building block for the front-tracking/streamline method described and applied in the second paper.

In this paper, we analyze an empirical model of viscous fingering for three-component, two-phase, first-contact miscible flows. We present the complete range of analytical solutions to secondary and tertiary water-alternating-gas (WAG) floods. An important ingredient in the construction of analytical solutions is the presence of detached (nonlocal) branches of the Hugoniot locus, that is, curves in composition space that satisfy the Rankine–Hugoniot conditions but do not contain the reference state. We illustrate how, in water–solvent floods into a medium with mobile water and residual oil (immobile to water), the solvent front and the water Buckley–Leverett front may interact, resulting in a leading water/solvent shock that is stable to viscous fingering. The analytical solutions explain why in these miscible tertiary floods, oil and solvent often break through simultaneously. We discuss the implications of the new solutions in the design of miscible tertiary floods, such as the estimation of the optimum WAG ratio.

In a previous paper (Transp. Porous Media,55(1): 47–70), algorithms are given for computing the analytical solution to the three-phase Riemann problem. Application of those algorithms requires that the wave configuration is known. The purpose of this note is to provide a procedure to determine the wave structure for any initial and injected saturation states.

Under certain physically reasonable assumptions, three-phase flow of immiscible, incompressible fluids can be described by a 2×2 nongenuinely nonlinear, hyperbolic system. We combine analytical solutions to the corresponding Riemann problem with an efficient front-tracking method to study Cauchy and initial-boundary value problems. Unlike finite difference methods, the front-tracking method treats all waves as discontinuities by evolving shocks exactly and approximating rarefactions by small entropy-violating discontinuities. This way, the method can track individual waves and give very accurate (or even exact) resolution of discontinuities. We demonstrate the applicability of the method through several numerical examples, including a streamline simulation of a water-alternating-gas (WAG) injection process in a three-dimensional, heterogeneous, shallow-marine formation.

Traditional mathematical models of multiphase flow in porous media use a straightforward extension of Darcy’s equation. The key element of these models is the appropriate formulation of the relative permeability functions. It is well known that for one-dimensional flow of three immiscible incompressible fluids, when capillarity is neglected, most relative permeability models used today give rise to regions in the saturation space with elliptic behavior (the so-called elliptic regions). We believe that this behavior is not physical, but rather the result of an incomplete mathematical model. In this paper we identify necessary conditions that must be satisfied by the relative permeability functions, so that the system of equations describing three-phase flow is strictly hyperbolic everywhere in the saturation triangle. These conditions seem to be in good agreement with pore-scale physics and experimental data.