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Geologic carbon sequestration ( GCS) is being considered as a climate change mitigation option in many future energy scenarios. Mathematical modeling is routinely used to predict subsurface CO2 and resident brine migration for the design of injection operations, to demonstrate the permanence of CO2 storage, and to show that other subsurface resources will not be degraded. Many processes impact the migration of CO2 and brine, including multiphase flow dynamics, geochemistry, and geomechanics, along with the spatial distribution of parameters such as porosity and permeability. In this article, we review a set of multiphase modeling approaches with different levels of conceptual complexity that have been used to model GCS. Model complexity ranges from coupled multiprocess models to simplified vertical equilibrium (VE) models and macroscopic invasion percolation models. The goal of this article is to give a framework of conceptual model complexity, and to show the types of modeling approaches that have been used to address specific GCS questions. Application of the modeling approaches is shown using five ongoing or proposed CO2 injection sites. For the selected sites, the majority of GCS models follow a simplified multiphase approach, especially for questions related to injection and local-scale heterogeneity. Coupled multiprocess models are only applied in one case where geomechanics have a strong impact on the flow. Owing to their computational efficiency, VE models tend to be applied at large scales. A macroscopic invasion percolation approach was used to predict the CO2 migration at one site to examine details of CO2 migration under the caprock. [ABSTRACT FROM AUTHOR]

• Air-water flow through thin porous layers is studied using a state-of-the-art dynamic pore-network model. • Our pore-scale results provide insight into the continuum-scale modeling of thin porous layers. • We demonstrate the role of phase change in water removal from the cathode gas diffusion layer of a polymer electrolyte fuel cell. Thin porous layers, that have large aspect ratios, are seen in many applications such as hydrogen fuel cells and hygiene products, in which air-water immiscible flow is of great interest. Direct numerical simulations based on Navier-Stokes equation are computationally expensive, and even prohibitive for low capillary number flow such as water flooding in low-temperature polymer electrolyte fuel cells. Alternatively, the pore-network modeling needs much less computational resources, while still retaining essentials of the pore-structure information. In this work, a dynamic pore-network model of air-water flow with phase change has been developed. We focus on drainage processes through thin porous layers, in which liquid water is the nonwetting phase. Three test cases are conducted, namely, air-water flow through a thin porous layer, air-water flow through a bilayer of fine and coarse thin porous layers, and water flooding in the gas diffusion layer of a polymer electrolyte fuel cell with phase change between water and its vapor. Using these test cases, we aim to demonstrate the application of dynamic pore-network modeling in thin porous media studies. In particular, we discuss the challenge of modeling thin porous media at the average scale, and highlight the role of phase change in removing liquid water from the cathode gas diffusion layer. [ABSTRACT FROM AUTHOR]

Abstract Recent studies have reported methane (CH 4) emissions from abandoned and active oil and gas infrastructure across the United States, where measured emissions show regional variability. To investigate similar phenomena in West Virginia, we measure and characterize emissions from abandoned and active conventional oil and gas wells. In addition, we reconcile divergent regional CH 4 emissions estimates by comparing our West Virginia emissions estimates with those from other states in the United States. We find the CH 4 emission factors from 112 plugged and 147 unplugged wells in West Virginia are 0.1 g CH 4 h−1 and 3.2 g CH 4 h−1, respectively. The highest emitting unplugged abandoned wells in WV are those most recently abandoned, with the mean emission of wells abandoned between 1993 and 2015 of 16 g CH 4 h−1 compared to the mean of those abandoned before 1993 of 3 × 10−3 g CH 4 h−1. Using field observations at a historic mining area as a proxy for state-wide drilling activity in the late 19th/early 20th century, we estimate the number of abandoned wells in WV at between 60,000 and 760,000 wells. Methane emission factors from active conventional wells were estimated at 138 g CH 4 h−1. We did not find an emission pattern relating to age of wells or operator for active wells, however, the CH 4 emission factor for active conventional wells was 7.5 times larger than the emission factor used by the EPA for conventional oil and gas wells. Our results suggest that well emission factors for active and abandoned wells can vary within the same geologic formation and may be affected by differences in state regulations. Therefore, accounting for state-level variations is critical for accuracy in greenhouse gas emissions inventories, which are used to guide emissions reduction strategies. Graphical abstract Unlabelled Image Highlights • WV CH 4 emissions of plugged, unplugged and active wells were 0.1, 3.2 and 138 g CH 4 h−1. • Methane emissions were higher at active wells compared to plugged and unplugged abandoned wells. • We estimate the number of abandoned wells in WV at between 60,000 and 760,000. • Average emission from active wells was 7.5 times larger than the EPA emission factor. • Well emission can vary along geologic formation and be affected by state regulations. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing in shale gas formations involves the injection of large volumes of aqueous fluid deep underground. Only a small proportion of the injected water volume is typically recovered, raising concerns that the remaining water may migrate upward and potentially contaminate groundwater aquifers. We implement a numerical model of two-phase water and gas flow in a shale gas formation to test the hypothesis that the remaining water is imbibed into the shale rock by capillary forces and retained there indefinitely. The model includes the essential physics of the system and uses the simplest justifiable geometrical structure. We apply the model to simulate wells from a specific well pad in the Horn River Basin, British Columbia, where there is sufficient available data to build and test the model. Our simulations match the water and gas production data from the wells remarkably closely and show that all the injected water can be accounted for within the shale system, with most imbibed into the shale rock matrix and retained there for the long term. [ABSTRACT FROM AUTHOR]

We study the axisymmetric flows generated from fluid injection into a horizontal confined porous medium that is originally saturated with another fluid of different density and viscosity. Neglecting the effects of surface tension and fluid mixing, we use the lubrication approximation to obtain a nonlinear advection-diffusion equation that describes the time evolution of the sharp fluid-fluid interface. The flow behaviors are controlled by two dimensionless groups: M, the viscosity ratio of displaced fluid relative to injected fluid, and G, which measures the relative importance of buoyancy and fluid injection. For this axisymmetric geometry, the similarity solution involving R²/T (where R is the dimensionless radial coordinate and T is the dimensionless time) is an exact solution to the nonlinear governing equation for all times. Four analytical expressions are identified as asymptotic approximations (two of which are new solutions): (i) injection-driven flow with the injected fluid being more viscous than the displaced fluid (Γ ≪ 1 and M < 1) where we identify a self-similar solution that indicates a parabolic interface shape; (ii) injection-driven flow with injected and displaced fluids of equal viscosity (Γ ≪ 1 and M = 1), where we find a self-similar solution that predicts a distinct parabolic interface shape; (iii) injection-driven flow with a less viscous injected fluid (Γ ≪ 1 and M > 1) for which there is a rarefaction wave solution, assuming that the Saffman-Taylor instability does not occur at the reservoir scale; and (iv) buoyancy-driven flow (Γ ≫ 1) for which there is a well-known self-similar solution corresponding to gravity currents in an unconfined porous medium [S. Lyle et al. "Axisymmetric gravity currents in a porous medium," J. Fluid Mech. 543, 293-302 (2005)]. The various axisymmetric flows are summarized in a G-M regime diagram with five distinct dynamic behaviors including the four asymptotic regimes and an intermediate regime. The implications of the regime diagram are discussed using practical engineering projects of geological CO2 sequestration, enhanced oil recovery, and underground waste disposal. [ABSTRACT FROM AUTHOR]

Since 1850 the atmospheric mixing ratio of methane (CH 4), a potent greenhouse gas, has doubled. This increase is directly linked to an escalation in emissions from anthropogenic sources. An inexpensive means to identify and monitor CH 4 emission sources and evaluate the efficacy of mitigation strategies is essential. However, sourcing reliable, low-cost, easy-to-calibrate sensors that are fit for purpose is challenging. A recent study showed that CH 4 mixing ratio data from a low-power, low-cost CH 4 sensor (Figaro TGS2600) agreed well with CH 4 mixing ratios measured by a high precision sensor at mixing ratios between 1.85 ppm and 2 ppm. To investigate, as a proof of concept, if this low-cost sensor could be used to measure typical ambient CH 4 mixing ratios, we operated a TGS2600 in conjunction with a Los Gatos Ultra-portable Greenhouse Gas Analyzer (UGGA) in controlled laboratory conditions. We then explored the sensor's long-term reliability by deploying the TGS2600 near an onshore gas terminal to calculate emissions from May to July 2018. Our initial studies showed that previously published linear algorithms could not convert TGS2600 output to CH 4 mixing ratios measured by the UGGA. However, we derived a non-linear empirical relationship that could be used to reliably convert the output of a TGS2600 unit to CH 4 mixing ratios over a range of 1.85–5.85 ppm that agree to a high-precision instrument output to ±0.01 ppm. Our study showed that the TGS2600 could be used to continuously measure variability in CH 4 mixing ratios from 1.82 to 5.40 ppm for three months downwind of the gas terminal. Using a simplified Gaussian Plume approach, these mixing ratios correspond to an emission flux range of 0–238 g CH 4 s−1, with average emission of 9.6 g CH 4 s−1 from the currently active North Terminal and 1.6 g CH 4 s−1 from the decommissioned South Terminal. Our work here demonstrates the feasibility of utilizing a low-cost sensor to detect methane leakage at concentrations close to ambient background levels, as long as the device is routinely calibrated with an accurate reference instrument. Having a widely deployed network of such low-cost CH 4 sensors would allow improved identification, monitoring and mitigation of a variety of CH 4 emissions. • Low-cost, low-power methane sensor is investigated for use in a measurement network. • Calibration shows a non-linear response to methane change between 2 and 6 ppm. • Long-term deployment shows sensor is reliable and can be used to measure autonomously. • Sensor can detect methane close to background but requires routine calibration. • A network of low-cost sensors could help identification and mitigation of methane. [ABSTRACT FROM AUTHOR]

• We examine 458 power plants built since 2005 in 15 representative BRI countries. • Fossil fuel power generation accounts for 75% of capacity addition. • The number of solar PV and wind projects is increasing but capacity remains small. • Estimated committed CO 2 emissions are 56 (40–72) Gt by 2030. • This corresponds to 7–17% of the remaining carbon budget for 1.5 °C climate goal. The Belt and Road Initiative (BRI), initially proposed by China in Fall 2013, involves large-scale development of infrastructure, including energy infrastructure, in Asia, Europe, and Africa, and thus has the potential to affect global climate. In this work, we analyze publicly available information for 458 power plant development projects in 15 representative countries across the BRI regions from eight years prior to BRI (January 2005), to 6 years after BRI (June 2019) in which Chinese engineering contracting companies played a significant role. The data indicate that coal, gas, and solar photovoltaics (PV) activities increased to different degrees after BRI was introduced. We find that 75% of the new generation capacity after October 2013 has gone to fossil fuels, while the rest has gone to hydroelectricity (14%), solar PV (6%), wind (3%), and others (2%). Those numbers have important carbon emissions implications. Based on current trends, the total BRI-associated power development in the 15 countries could generate 37 (range 26–48) gigatonnes (Gt) of committed carbon dioxide (CO 2) emissions by the end of 2030, corresponding to 4–11% of the remaining carbon budget for the 1.5-degree climate goal. Extrapolation of these results to all BRI countries gives an estimate of 56 (range 40–72) Gt CO 2 for cumulative BRI-related committed emissions by 2030, corresponding to 7% to 17% of the remaining 1.5-degree carbon budget. If the projected growth of fossil fuel power generation in BRI slows down prior to 2030, there are commensurate reductions in emissions implications. To provide context for these numbers, we define a "greenness ratio," which can be used to measure the level of environmental sustainability of BRI in the power sector. [ABSTRACT FROM AUTHOR]

Various modeling approaches, including fully three-dimensional (3D) models and vertical-equilibrium (VE) models, have been used to study the large-scale storage of carbon dioxide (CO2) in deep saline aquifers. 3D models solve the governing flow equations in three spatial dimensions to simulate migration of CO2 and brine in the geological formation. VE models assume rapid and complete buoyant segregation of the two fluid phases, resulting in vertical pressure equilibrium and allowing closed-form integration of the governing equations in the vertical dimension. This reduction in dimensionality makes VE models computationally much more efficient, but the associated assumptions restrict the applicability of VE models to geological formations with moderate to high permeability. In the present work, we extend the VE models to simulate CO2 storage in fractured deep saline aquifers in the context of dual-continuum modeling, where fractures and rock matrix are treated as porous media continua with different permeability and porosity. The high permeability of fractures makes the VE model appropriate for the fracture domain, thereby leading to a VE dual-continuum model for the dual continua. The transfer of fluid mass between fractures and rock matrix is represented by a mass transfer function connecting the two continua, with a modified transfer function for the VE model based on vertical integration. Comparison of the new model with a 3D dual-continuum model shows that the new model provides comparable numerical results while being much more computationally efficient. [ABSTRACT FROM AUTHOR]

Recent measurements of methane emissions from abandoned oil/gas wells show that these wells can be a substantial source of methane to the atmosphere, particularly from a small proportion of highemitting wells. However, identifying high emitters remains a challenge. We couple 163 well measurements of methane flow rates; ethane, propane, and n-butane concentrations; isotopes of methane; and noble gas concentrations from 88 wells in Pennsylvania with synthesized data from historical documents, field investigations, and state databases. Using our databases, we (i) improve estimates of the number of abandoned wells in Pennsylvania; (ii) characterize key attributes that accompany high emitters, including depth, type, plugging status, and coal area designation; and (iii) estimate attribute- specific and overall methane emissions from abandoned wells. High emitters are best predicted as unplugged gas wells and plugged/vented gas wells in coal areas and appear to be unrelated to the presence of underground natural gas storage areas or unconventional oil/gas production. Repeat measurements over 2 years show that flow rates of high emitters are sustained through time. Our attribute-based methane emission data and our comprehensive estimate of 470,000-750,000 abandoned wells in Pennsylvania result in estimated state-wide emissions of 0.04-0.07 Mt (1012 g) CH4 per year. This estimate represents 5-8% of annual anthropogenic methane emissions in Pennsylvania. Our methodology combining new field measurements with data mining of previously unavailable well attributes and numbers of wells can be used to improve methane emission estimates and prioritize cost-effective mitigation strategies for Pennsylvania and beyond. [ABSTRACT FROM AUTHOR]

We develop a probabilistic collocation Eulerian–Lagrangian localized adjoint method on sparse grids for assessing CO 2 leakage through wells in randomly heterogeneous porous media, by utilizing the intrinsic mathematical, numerical, and physical properties of the mathematical model. We model the process in which CO 2 is injected into a deep aquifer, spreads within the aquifer and, upon reaching a leaky well, rises up to a shallower aquifer, to quantify the leakage rate, which depends on the pressure build-up in the aquifer due to injection and the buoyancy of CO 2 . The underlying Eulerian–Lagrangian framework has high potential to improve the efficiency and accuracy for the numerical simulation of complex flow and transport processes in CO 2 sequestration. The sparse grid probabilistic collocation framework adds computationally efficient uncertainty quantification functionality onto pre-existing Eulerian–Lagrangian methods in a nonintrusive manner. It also provides a scalable framework to consider uncertainty in a straightforward parallel manner. Preliminary numerical experiments show the feasibility and potential of the method. [ABSTRACT FROM AUTHOR]

Abandoned oil and gas (AOG) wells can provide pathways for subsurface fluid migration, which can lead to groundwater contamination and gas emissions to the atmosphere. Little is known about the millions of AOG wells in the U.S. and abroad. Recently, we acquired data on methane emissions from 42 plugged and unplugged AOG wells in five different counties across western Pennsylvania. We used historical documents to estimate well depths and used these depths with the emissions data to estimate the wells' effective permeabilities, which capture the combined effects of all leakage pathways within and around the wellbores. We find effective permeabilities to range from 10-6 to 10² millidarcies, which are within the range of previous estimates. The effective permeability data presented here provide perspective on older AOG wells and are valuable when considering the leakage potential of AOG wells in a wide range of applications, including geologic storage of carbon dioxide, natural gas storage, and oil and gas development. [ABSTRACT FROM AUTHOR]

Abandoned oil and gas wells provide a potential pathway for subsurface migration and emissions of methane and other fluids to the atmosphere. Little is known about methane fluxes from the millions of abandoned wells that exist in the United States. Here, we report direct measurements of methane fluxes from abandoned oil and gas wells in Pennsylvania, using static flux chambers. A total of 42 and 52 direct measurements were made at wells and at locations near the wells ("controls") in forested, wetland, grassland, and river areas in July, August, October 2013 and January 2014, respectively. The mean methane flow rates at these well locations were 0.27 kg/d/well, and the mean methane flow rate at the control locations was 4.5 × 10-6 kg/d/location. Three out of the 19 measured wells were high emitters that had methane flow rates that were three orders of magnitude larger than the median flow rate of 1.3 × -3 kg/d/well. Assuming the mean flow rate found here is representative of all abandoned wells in Pennsylvania, we scaled the methane emissions to be 4-7% of estimated total anthropogenic methane emissions in Pennsylvania. The presence of ethane, propane, and n-butane, along with the methane isotopic composition, indicate that the emitted methane is predominantly of thermogenic origin. These measurements show that methane emissions from abandoned oil and gas wells can be significant. The research required to quantify these emissions nationally should be undertaken so they can be accurately described and included in greenhouse gas emissions inventories. [ABSTRACT FROM AUTHOR]