CO2 Accumulations Research Articles

Origin of CO2 in Upper Devonian Duperow Formation and the Bakken Petroleum System at Kevin Dome, Northwest Montana

Carbon capture and storage (CCS) is a key mitigation strategy in achieving global net-zero emissions. It is therefore essential to identify and characterize potential subsurface storage repositories. Natural CO2 accumulations provide an opportunity to understand the behavior of CO2 in the subsurface. Here, we investigate the source(s), migration, and storage of CO2 in the Upper Devonian Duperow Formation at Kevin Dome, northwest Montana, USA within the Bakken Petroleum System. We report, major gas, stable and noble gas isotopic compositions in bulk gas samples (n = 19) produced from nearby hydrocarbon-bearing reservoirs at Kevin Dome and compare with CO2 bearing fluid inclusions (n = 24) from the Duperow Formation. Using the same methods, bulk gas samples (n = 9) from the adjacent Ferguson Field, (Exshaw/Bakken Petroleum System) in southern Alberta, Canada, and fluid inclusions from the Sweetgrass Hills igneous complex (n = 2) were analyzed to understand CO2 generation and the subsequent processes affecting CO2 regionally. We find that CO2 in the Upper Devonian Duperow Formation is magmatic in origin, likely related to the nearby Sweetgrass Hills igneous complex intrusion (∼52 Ma), whereas CH4 and N2 gases are generated predominantly by thermogenic processes associated with hydrocarbon generation during burial. Since emplacement, most CO2 in the hydrocarbon-bearing reservoirs of the Bakken petroleum system at Kevin Dome (∼98%) and the Ferguson Field (∼82%) has subsequently been dissolved into the groundwater. We employ a solubility model to calculate minimum gas/water ratios in Kevin Dome and the Ferguson Field, which are consistent with more groundwater interaction and more dissolution at Kevin Dome. Understanding the subsurface processes affecting CO2 is critical for future CO2 storage site selection.

Climate change mitigation under uncertainty and inequality: A classroom experiment

A novel game that captures the central dimensions of climate change mitigation as a social dilemma is presented. Students play the role of countries sharing a global atmosphere. In each round, carbon emissions are released and accumulated in the atmosphere, making climate change consequences more severe and difficult to mitigate over time. Without mitigation, CO2 accumulations will cause losses to all countries. During each round, countries decide on how to invest their endowments between economic development and mitigation to slow down carbon emissions causing climate change. Central to the game and the subsequent classroom discussions are implications emanating from introducing uncertainty and inequality in the game. The game is suitable for courses related to environmental economics and climate policy at undergraduate and graduate levels.

Effects of addition of leaf litter with different chemical properties on soil organic carbon mineralization and priming effect

Litter inputs can affect the mineralization of soil organic carbon (SOC). However, it is yet unknown how the input of leaf litter with different chemical properties drives SOC mineralization and priming effect. In this study, 13C-labeled leaf litter of six tree species were added to soil cores (10 cm depth) collected from a natural secondary forest in subtropical region. We examined the effects of different leaf litters on total soil CO2, litter-derived and soil-derived CO2 emission rates and accumulation and priming effect. We further examined the relationships between litter chemical properties and CO2 accumulation and priming effect. Our results showed that leaf litter addition increased total soil CO2 and soil-derived CO2 emission rates and accumulations, and that there were positive priming effects ranging from 68% to 128%. Soil organic carbon mineralization and priming effects varied among tree species. The Pearson correlation and stepwise multiple linear regression analysis showed that the litter-derived CO2 accumulation had negative correlation with leaf litter C, P and cellulose concentrations, whereas the soil-derived CO2 accumulation were positively correlated to litter C:N and lignin:N. The results implied that tree species could influence SOC mineralization and litter-induced priming effect. Thus it could mitigate soil C loss when we afforested plantation with high quality leaf litter in subtropical region.

Basin architecture controls on the chemical evolution and 4He distribution of groundwater in the Paradox Basin

Fluids such as 4He, H2, CO2 and hydrocarbons accumulate within Earth's crust. Crustal reservoirs also have potential to store anthropogenic waste (e.g., CO2, spent nuclear fuel). Understanding fluid migration and how this is impacted by basin stratigraphy and evolution is key to exploiting fluid accumulations and identifying viable storage sites. Noble gases are powerful tracers of fluid migration and chemical evolution, as they are inert and only fractionate by physical processes. The distribution of 4He, in particular, is an important tool for understanding diffusion within basins and for groundwater dating. Here, we report noble gas isotope and abundance data from 36 wells across the Paradox Basin, Colorado Plateau, USA, which has abundant hydrocarbon, 4He and CO2 accumulations. Both groundwater and hydrocarbon samples were collected from 7 stratigraphic units, including within, above and below the Paradox Formation (P.Fm) evaporites. Air-corrected helium isotope ratios (0.0046 - 0.127 RA) are consistent with radiogenic overprinting of predominantly groundwater-derived noble gases. The highest radiogenic noble gas concentrations are found in formations below the P.Fm. Atmosphere-derived noble gas signatures are consistent with meteoric recharge and multi-phase interactions both above and below the P.Fm, with greater groundwater-gas interactions in the shallower formations. Vertical diffusion models, used to reconstruct observed groundwater helium concentrations, show the P.Fm evaporite layer to be effectively impermeable to helium diffusion and a regional barrier for mobile elements but, similar to other basins, a basement 4He flux is required to accumulate the 4He concentrations observed beneath the P.Fm. The verification that evaporites are regionally impermeable to diffusion, of even the most diffusive elements, is important for sub-salt helium and hydrogen exploration and storage, and a critical parameter in determining 4He-derived mean groundwater ages. This is critical to understanding the role of basin stratigraphy and deformation on fluid flow and gas accumulation.

Reassessment of the Distribution of Mantle CO2 in the Bohai Sea, China: The Perspective from the Source and Pathway System

AbstractResearch on the distribution of mantle CO2 should involve comprehensive analysis from CO2 source to accumulation. The crust‐mantle pathway system is the key controlling factor of the distribution of mantle CO2, but has received little attention. The pathway system and controlling factors of CO2 distribution in the Bohai Sea are analyzed using data on fault styles and information on the mantle and lithosphere. The relation between volcanic rocks and the distribution of mantle CO2 is reassessed using age data for CO2 accumulations. The distribution of mantle CO2 is controlled by uplift of the asthenosphere and upper mantle, magma conduits in the mantle and fault systems in the crust. Uplifted regions of the asthenosphere are accumulation areas for CO2. The area with uplift of the Moho exhibits accumulation of mantle CO2 at depth. CO2 was mainly derived from vertical migration through the upper mantle and lower crust. The fault style in the upper crust controls the distance of horizontal migration and the locations of CO2 concentrations. The distribution of mantle CO2 and volcanic rocks are not the same, but both probably followed the same pathways sometimes. Mantle CO2 in the Bohai Sea is concentrated in the Bozhong sag and the surrounding area, particularly in a trap that formed before 5.1 Ma and is connected to crustal faults (the Bozhang faults) and lithospheric faults (the Tanlu faults).

Mineralogical and geochemical changes in conglomerate reservoir rocks induced by CO2 influx at Mih\xe1lyi-R\xe9pcelak natural analogue, NW-Hungary

A temporary solution to massive anthropogenic CO2 emissions can be the capture of industrial CO2 from flue gas and sequestering it in geological formations. For safe and effective storage of CO2, interaction processes in the rock-pore fluid–CO2 system should be known. Investigation of natural CO2 accumulations provides valuable examples to what physical and chemical effects could be expected during CO2 influx at future CO2 storage sites. One of the key controlling factors of the processes occurring in natural CO2 reservoirs is the lithology of the storage rocks, which is primarily determined by the formation conditions of these rocks. In this respect, the lithologies of individual CO2 accumulation areas influence the processes between the host rock, the pore fluid, and the CO2 in different ways. In the current study, we focus on a well-studied natural CO2 storage reservoir, namely the Mihályi-Répcelak area, NW Hungary. We provide insight into the so far unstudied conglomerate reservoirs that represent a stratigraphically deeper reservoir unit with significantly different lithology and pore water compositions compared to the sandstone reservoirs. Our results indicate that dawsonite /NaAlCO3(OH)2/ formation also affected the conglomerate reservoirs, which indicates that at least part of the CO2 could be trapped in mineral form. An important role of salinity in reducing the CO2 mineral trapping capacity of the storage system is also demonstrated. Furthermore, H isotope analysis of diagenetic kaolinite was applied to trace the origin of the pore water that was present during the rock formation. Based on the data, dawsonite formation was induced by the flux of meteoric water that infiltrated during a warm and humid period and mixed with ascending CO2.

Saline Water-Based Mineralization Pathway for Gigatonne-Scale CO2Management

This perspective proposes a potential pathway to diminish atmospheric CO2 accumulations which is distinct from traditional carbon capture and geological sequestration strategies and from existing negative emissions technologies (NETs). Unlike conventional sorbent- or solvent-based CO2 capture processes where substantial energy expenditures are associated with demixing and desorbing CO2, the single-step carbon sequestration and storage (sCS2) approach relies on electrolytic carbonate mineral precipitation using renewable energy within a simple and scalable process design. Although numerous approaches have implied electrolysis for carbon management, the sCS2 approach is unique in the following ways: (1) CO2 mineralization for promoting solid carbonate formation: The thermodynamic and kinetic barriers to carbonate precipitation are overcome by direct and in situ electrochemical forcing to stabilize dissolved inorganic carbon and divalent cations [Ca,Mg] to form carbonate minerals. (2) Flow-through membraneless electrolysis: A flowing electrolyte (seawater) is dissociated while in motion. The process utilizes cost-effective mesh electrodes while also decreasing the number of components and assembly steps and reducing the risk of device failure. (3) Integrated electrolytic reactor–rotary drum filter: An electroactive thin-film mesh cathode (eTFC) is suggested to be integrated within a rotary drum filter configuration, allowing for the filtration of dilute and polydispersed mineral precipitates at a low energy cost. These attributes render sCS2 as an approach worthy of more detailed evaluation, development, and scaling for global-scale carbon management.

A (not so) shallow controlled CO2 release experiment in a fault zone

The CSIRO In-Situ Laboratory Project (ISL) is located in Western Australia and has two main objectives related to monitoring leaks from a CO2 storage complex by controlled-release experiments: 1) improving the monitorability of gaseous CO2 accumulations at intermediate depth, and 2) assessing the impact of faults on CO2 migration. A first test at the In-situ Lab has evaluated the ability to monitor and detect unwanted leakage of CO2 from a storage complex in a major fault zone. The ISL consists of three instrumented wells up to 400 m deep: 1) Harvey-2 used primarily for gaseous CO2 injection, 2) ISL OB-1, a fibreglass geophysical monitoring well with behind-casing instrumentation, and 3) a shallow (27 m) groundwater well for fluid sampling. A controlled-release test injected 38 tonnes of CO2 between 336-342 m depth in February 2019, and the gas was monitored by a wide range of downhole and surface monitoring technologies. CO2 reached the ISL OB-1 monitoring well (7 m away) after approximately 1.5 days and an injection volume of 5 tonnes. Evidence of arrival was determined by distributed temperature sensing and the CO2 plume was detected also by borehole seismic after injection of as little as 7 tonnes. Observations suggest that the fault zone did not alter the CO2 migration along bedding at the scale and depth of the experiment. No vertical CO2 migration was detected beyond the perforated injection interval; no notable changes were observed in groundwater quality or soil gas chemistry during and post injection. The early detection of significantly less than 38 tonnes of CO2 injected into the shallow subsurface demonstrates rapid and sensitive monitorability of potential leaks in the overburden of a commercial-scale storage project, prior to reaching shallow groundwater, soil zones or the atmosphere. The ISL is a unique and enduring research facility at which monitoring technologies will be further developed and tested for increasing public and regulator confidence in the ability to detect potential CO2 leakage at shallow to intermediate depth.

A controlled CO2 release experiment in a fault zone at the In-Situ Laboratory in Western Australia

A controlled-release test at the In-Situ Laboratory Project in Western Australia injected 38 tonnes of gaseous CO2 between 336−342 m depth in a fault zone, and the gas was monitored by a wide range of downhole and surface monitoring technologies. Injection of CO2 at this depth fills the gap between shallow release (<25 m) and storage (>600 m) field trials. The main objectives of the controlled-release test were to assess the monitorability of shallow CO2 accumulations, and to investigate the impacts of a fault zone on CO2 migration.CO2 arrival was detected by distributed temperature sensing at the monitoring well (7 m away) after approximately 1.5 days and an injection volume of 5 tonnes. The CO2 plume was detected also by borehole seismic and electric resistivity imaging. The detection of significantly less than 38 tonnes of CO2 in the shallow subsurface demonstrates rapid and sensitive monitorability of potential leaks in the overburden of a commercial-scale storage project, prior to reaching shallow groundwater, soil zones or the atmosphere.Observations suggest that the fault zone did not alter the CO2 migration along bedding at the scale and depth of the test. Contrary to model predictions, no vertical CO2 migration was detected beyond the perforated injection interval. CO2 and formation water escaped to the surface through the monitoring well at the end of the experiment due to unexpected damage to the well’s fibreglass casing. The well was successfully remediated without impact to the environment and the site is ready for future experiments.

Effects of sedimentary processes and diagenesis on reservoir quality of submarine lobes of the Huangliu Formation in the Yinggehai Basin, China

Submarine lobe deposits of the upper Miocene Huangliu Formation in the Yinggehai Basin, characterised by high-temperature (132.4–141.2 °C), high overpressure (1.69–1.99), and inorganic CO2 accumulations, offer an excellent opportunity to investigate how reservoir quality is linked to sedimentary processes and diagenesis. This study examines the control on reservoir quality of submarine lobes using petrographic, geochemical, drilling test, and measured petrophysical data. Lobe axis deposits contain high-density turbidites (HDTs) that exhibit good reservoir quality, a higher degree of sandstone amalgamation, and larger grain sizes and pore-throat radii. The lobe off-axis deposits are composed of HDTs and low-density turbidites (LDTs) with moderate reservoir quality. In contrast, the lobe fringe deposits mainly comprising muddy debrites have the lowest reservoir quality. Sedimentary processes macroscopically controlled the initial grain size segregation, variation of textural properties, and reservoir quality throughout the submarine lobes. Flow transformation from turbidity currents in the lobe axis areas into muddy debris flows in the lobe fringe areas led to the high clay content of lobe fringe deposits, due to the incorporation of sediment eroded from the muddy seafloor. Mechanical compaction is the major diagenetic factor reducing reservoir properties of sandstones in the lobe axis and lobe off-axis positions. Calcite and ferrocalcite cements in the sandstones from the CO2-poor DF-B gas field were linked with thermal decarboxylation of organic matter in the adjacent mudstones. In contrast, the diagenetic processes in the CO2-rich DF-A gas field were more likely to be affected by inorganic CO2 accumulations. As a whole, the CO2-water-rock reactions in the CO2-rich DF-A area can not only lead to the precipitation of late dolomite and ankerite, and formation of authigenic clay minerals, but also cause intense dissolution of unstable compositions, resulting in an insignificant increase in porosity. In addition, the high-overpressure in both gas fields has effectively protected the primary porosity by reducing the effective stress exerted on the detrital grains. This study has implications for the assessment of reservoir potential in carbon sequestration and storage, and hydrocarbon exploration and production.

Effective stress constraints on vertical flow in fault zones: Learnings from natural CO2 reservoirs

Some fault zones leak vertically to the ground surface or seafloor, whereas most others remain naturally sealed. Understanding the factors that cause this leakage is essential for predicting and preventing such leakage for both conventional reservoir development and subsurface CO2 storage. This study, a comparison of leaking and nonleaking natural CO2 gas accumulations, provides such constraints. We compare and contrast trap configurations, fluid pressures, and stress states for several natural CO2 accumulations from the Colorado Plateau. Extensive surface geologic data are integrated with subsurface data from a large suite of groundwater and hydrocarbon wells. Leakage of CO2 is documented by geochemical surveys and the occurrence of extensive travertine deposits. The leakage occurs exclusively in fault fracture damage zones where the total fluid pressure reduces the minimum horizontal effective stress to approximately zero. These results are consistent with natural and accidentally induced fault seeps from some deep-water hydrocarbon reservoirs. These criteria can be used to evaluate the potential for fault zones to provide vertical leakage pathways and loss of fluid containment.

Geospatial Statistics Elucidate Competing Geological Controls on Natural CO2 Seeps in Italy

Site selection for the geological storage of CO2 for long timespans requires an understanding of the controls on containment, migration, and surface seepage of subsurface CO2 fluids. Evidence of natural CO2 migration from depth to the surface is documented at 270 sites from Italy, a prolific CO2 province. Previous studies indicate that CO2 delivery to and from buried structures that host CO2 accumulations is fault controlled but competing controls on the CO2 flow pathways affect the location and style of CO2 release. Here, we conduct a meta-analysis using a novel geospatial approach to statistically determine the relationship between the geological setting and structures and the CO2 seep spatial distribution and characteristics (morphological type, flux, and temperature) in Central Italy. We find that seep distribution differs on two spatial scales corresponding to the geological setting. On large scales (&gt;5 km), seeps are isotropically distributed and align with regional structures such as anticlines, decollements, and extensional faults. On local scales (&lt;5 km), seeps cluster and align with subsidiary geologic structures, including faults and lithological boundaries. The detailed location and flux of seeps within clusters are influenced by the regional structural domain: in the Tyrrhenian, seeps tend to be located along fault traces, whereas seeps are located as springs in the tip and ramp regions of fault scarps in the Apennines. Thus, our geospatial approach evidences, at a regional scale, how macrocrustal fluid flow is governed by deep extensional and compressional features but once CO2 reaches shallower structures, it evidences how smaller scale features and hydrogeological factors distribute the CO2 fluids in the near surface, dependent on the geological setting. This work not only demonstrates useful application of a novel geospatial approach to characterize competing crustal controls on CO2 flow at different scales but also informs the design of appropriate site characterization and surface monitoring programs at engineered carbon stores.

How to improve learners’ (mis) understanding of CO2 accumulations through the use of human-facilitated interactive learning environments?

Abstract On a global scale, the need for CO2 mitigation and adaptation strategies is both compelling and urgent. While there are several challenges including political, economic, and technological, IPPC has identified the lack of mitigation and adaptation strategies and the lack of “flows of knowledge and information relevant for adaptation decisions” as the fundamental barriers to adaptation. In fact, there exists much scientific evidence of how environmental emissions (e.g., CO2) are contributing to the global warming and climate change phenomena, yet subjects in numerous experimental studies show a poor understanding of, and engagement with, environmental accumulations. How can learners' understanding of environmental accumulations be improved? We argue that the adoption of the stock and flow perspective, which provides effective tools such as System Dynamics based Interactive Learning Environments (SDILEs) for education and engagement can address this challenge effectively. In our experiments, we assessed the impact of three interventions and found that education and training with human-facilitated SDILE improved learners’ understanding of, and ability to apply, the basic principle of accumulation much more than of those who were trained only with standalone SDILE or those who had a traditional lecture-based session. Such an improved understanding about CO2 accumulation is expected to support the design and implementation of mitigation and adaptation strategies needed for our sustainable future.

Carbon dioxide accumulation inside vehicles: The effect of ventilation and driving conditions

Limiting the air exchange of passenger vehicles by closing windows and recirculating cabin air (RC) restricts the influx of roadway pollutants and reduces in-vehicle particulate concentrations. However, the carbon dioxide (CO2) exhaled by the occupants can accumulate under these conditions to reach high concentrations. We characterized the factors (ventilation setting, vehicle age, speed, cabin volume, trip duration, and number of occupants) that allow CO2 accumulation to reach concentration thresholds found in other studies to produce cognitive or physiological effects of concern such as fatigue or difficulty concentrating. Ventilation setting was the primary determinant of CO2 accumulation; only the RC setting (and not outside-air intake) ever allows CO2 accumulations to exceed thresholds of concern. Longer trips with multiple occupants are a particular concern. Even so, under RC setting, a 2500ppm threshold—the threshold consistently linked to detrimental cognitive effects—would not be exceeded for most one- or even two-occupant average-duration commutes (twenty-six minutes in the U.S.). For multiple passenger commutes and/or longer trips, RC ventilation should be periodically interrupted or partially mixed with outside air to keep CO2 concentrations below 2500ppm.

Mantle and crustal gases of the Colorado Plateau: Geochemistry, sources, and migration pathways

The Colorado Plateau hosts several large accumulations of naturally occurring, non-hydrocarbon gases, including CO2, N2, and the noble gases, making it a good field location to study the fluxes of these gases within the crust and to the atmosphere. In this study, we present a compilation of 1252 published gas-composition measurements. The data reveal at least three natural gas associations in the field area, which are dominated by hydrocarbons, CO2, and N2+He+Ar, respectively. Most gas accumulations of the region exhibit compositions that are intermediate between the three end members. The first non-hydrocarbon gas association is characterized by very high-purity CO2, in excess of 75 mol% (hereafter, %). Many of these high-purity CO2 fields have recently been well described and interpreted as magmatic in origin. The second non-hydrocarbon gas association is less well described on the Colorado Plateau. It exhibits He concentrations on the order of 1–10%, and centered log ratio biplots show that He occurs proportionally to both N2 and Ar. Overall ratios of N2 to He to Ar are ≈100:10:1 and correlation in concentrations of these gases suggests that they have been sourced from the same reservoir and/or by a common process. To complement the analysis of the gas-composition data, stable isotope and noble-gas isotope measurements are compiled or newly reported from 11 representative fields (previously published data from 4 fields and new data from 7 fields). Gas sampled from the Harley Dome gas field in Utah contains nearly pure N2+He+Ar. The various compositional and stable and noble gas isotopic data for this gas indicate that noble gas molecule/isotope ratios are near crustal radiogenic production values and also suggest a crustal N2 source. Across the field area, most of the high-purity N2+He+Ar gas accumulations are associated with the mapped surface trace of structures or sutures in the Precambrian basement and are often accumulated in lower parts of the overlying Phanerozoic sedimentary cover. The high-purity gas association mostly occurs in areas interior to the plateau that are characterized by a narrow range of elevated, moderate heat flow values (53–74mW/m2) in the ancient (1.8–1.6Ga) basement terranes of the region. Collectively, the geochemical and geological data suggest that (1) the N2+He+Ar gas association is sourced from a crustal reservoir, (2) the gas association migrates preferentially along structures in the Precambrian basement, and (3) the sourcing process relates to heating of the crust. Prospecting for noble-gas accumulations may target areas with elevated Cenozoic heat flow, ancient crust, and deep crustal structures that focus gas migration. High-purity CO2 gas may also migrate through regional basement structures, however, there is not always a clear spatial association. Rather, CO2 accumulations are more clearly associated with zones of high heat flow (>63mW/m2) that sit above hot upper mantle and are proximal to Cenozoic volcanic rocks near the plateau margins. These observations are consistent with previous interpretations of a magmatic gas source, which were based on geochemical measurements.

Impact factors on reservoir quality of clastic Huangliu formation in overpressure diapir zone, Yinggehai Basin, China

A reservoir in the upper Miocene Huangliu formation from the Dongfang (DF) area of Yinggehai Basin is characterized by overpressure and hydrothermal fluids with CO2 accumulations. The shallow-marine gravity-flow deposits of the Huangliu formation provide an opportunity to study the impact factors on reservoir quality under overpressure and a CO2-rich environment. The current study examined the impact of sandbody types, diagenetic alteration, high-overpressure and CO2 accumulations on reservoir quality by comparing between the DF-A (CO2-rich) and DF-B (CO2-poor) areas. The channel sandbody, sheet sandbody and neritic sandbar are recognized from the Huangliu formation in the study area. The sandbody type is the most important factor for controlling reservoir quality. The bottom current reworked channel (BCRC) and reworked sheet sandstones of gas layer #I in the DF-B area have the best reservoir properties. This is due to winnowing away the original (very) fine component (such as mud) by bottom current, resulting in a slight increase of grain size and marked increase of pore-throat radius and permeability. Mechanical compaction and cementation are the main diagenetic factors for reducing primary porosity. High-overpressure can protect the primary porosity to some extent by resisting the compaction effect. The intrusion of late CO2-rich hydrothermal fluids affects mesodiagenesis in two ways. On the one hand, it can cause intense dissolution of detrital minerals and early calcite cements, resulting in a higher proportion of secondary porosity in the CO2-rich DF-A area than in the CO2-poor DF-B area. On the other hand, it can result in precipitation of cements, such as siderite and ankerite, through CO2-water-rock reactions. Overall, CO2 accumulations has an insignificant contribution on the increase of porosity. Intense dissolution by CO2 accumulations can increase the connectivity of pores and permeability, whereas precipitation of minerals leads to decreased permeability.

Experimental Study of Density-driven Convection in Porous Media by Using MRI

Density-driven convection is known as one of the physical phenomena for CO2 injection in saline aquifers, which accelerates the transfer of CO2 into the groundwater, increases the dissolution rate and shorten the dissolution time, thus it is favourable for long-term CO2 injection and the practical interest of storage security. Understanding how the occurrence and form of convective fingers is crucial to understanding the role of that Density-driven-convection behaviour. The purpose of this paper is to design a visual experimental set-up based on the MRI technique and present new analogue fluid pairs that reconstructed the convective process. The density-driven convection was formed by two miscible liquids at the interface demonstrated by fingers. We observed the development and growth of density-driven fingers of two-phase fluid i.e., water/ deuterium oxide, and brine/ deuterium oxide at different permeability porous media. Experiments indicated that the relationship of convection onset time to Rayleigh number changed with the density difference and permeability. We present an experimental study for the Rayleigh number in the range 583.279 to 2054.111. The results show that the higher permeability and density difference is conducive to shorten the convection onset time. The motivation behind this results is related to the implications for predicts the CO2 dissolution trapping time and accumulations in geological storage of carbon dioxide.

A model for CO2 storage and seismic monitoring combining multiphase fluid flow and wave propagation simulators. The Sleipner-field case

The main objective of this paper is to use a flow simulator to represent the CO2 storage and combine it with a wave propagation simulator in order to obtain synthetic seismograms qualitatively matching time-lapse real field data. The procedure is applied to the Utsira formation at Sleipner field. The field data at the site available to us is a collection of seismic sections (time-lapse seismics) used to monitor the CO2 storage. An estimate of the CO2 injection rate and the location of the injection point are known. Using these data, we build a geological model, including intramudstone layers with openings, whose coordinates are defined by performing a qualitative match of the field seismic data. The flow simulator parameters and the petrophysical properties are updated to obtain CO2 saturation maps, including CO2 plumes, so that the synthetic seismic images resemble the real data. The geological model is based on a porous-media constitutive equation. It considers a poroelastic description of the Utsira formation (a shaly sandstone), based on porosity and clay content, and takes into account the variation of the properties with pore pressure and fluid saturation. Moreover, the model considers the geometrical features of the formations, including the presence of shale seals and fractures. We also assume fractal variations of the petrophysical properties. The numerical simulation of the CO2-brine flow is based on the Black-Oil formulation, which uses the pressure-volume-temperature (PVT) behavior as a simplified thermodynamic model. The corresponding equations are solved using a finite difference IMPES formulation. Using the resulting saturation and pore-pressure maps, we determine an equivalent viscoelastic medium at the macroscale, formulated in the space-frequency domain. Wave attenuation and velocity dispersion, caused by heterogeneities formed of gas patches, are described with White’s mesoscopic model. The viscoelastic wave equation is solved in the space-frequency domain for a collection of frequencies of interest using a finite-element iterative domain decomposition algorithm. The space-time solution is recovered by a discrete inverse Fourier transform, allowing us to obtain our synthetic seismograms. In the numerical examples, we determine a set of flow and petrophysical parameters allowing us to obtain synthetic seismograms resembling actual field data. In particular, this approach yields CO2 accumulations below the mudstone layers and synthetic seismograms which successfully reproduce the typical pushdown effect.

Diagenesis and reservoir quality of overpressured deep-water sandstone following inorganic carbon dioxide accumulation: Upper Miocene Huangliu Formation, Yinggehai Basin, South China Sea

Turbidite deposits, overpressure, hydrothermal fluid activity, and CO2 accumulations are the basic geologic characteristics of the Upper Miocene Huangliu Formation sandstone reservoir in the Dongfang gas field, Yinggehai Basin. This study aims to unravel the timing and origin of dissolution in sandstone and evaluate the effects of inorganic CO2 accumulation on reservoir quality. A comparative study between area DFX-1 (CO2-rich) and area DFX-2 (CO2-poor) has illustrated the differences in diagenetic alteration and the effects of CO2 accumulation on the sandstone reservoir. Diagenetic alteration of sandstones in area DFX-1 was characterised by intense dissolution and medium cementation. Dissolution of silicates following inorganic CO2 charging in the DFX-1 area created a higher percentage of dissolution porosity (41.0%–53.9% of total visible porosity), compared with minor dissolution porosity in the DFX-2 area (16.7%–29.6% of total visible porosity). Small amounts of kaolinite, authigenic quartz, ankerite, and siderite were precipitated by CO2-water-rock reactions in area DFX-1; these minerals are almost absent in area DFX-2. The carbonate cement of the Huangliu Formation sandstone in area DFX-1 is characterised by light δ13C (−3.3‰ to −1.6‰, PDB), which is consistent with formation from inorganic carbon. The timing of dissolution by inorganic CO2 is suggested to be at about 0.4 Ma based on the previous simulation experiments. In the two areas studied, overpressure was caused by pressure transmission along deep open faults and was not related to mudstone undercompaction. Overpressure did a little to contribute to porosity preservation. CO2 accumulating at low partial pressures related to hydrothermal fluid flow cannot make a net increase in porosity in the Huangliu Formation sandstone of the Dongfang gas field.

CO2 outburst events in relation to seismicity: Constraints from microscale geochronology, geochemistry of late Quaternary vein carbonates, SW Turkey

Vein and breccia carbonates precipitated in a highly fractured/faulted carbonate bedrock in SW Turkey were investigated through high-resolution U-series geochronology, microstructural and geochemical studies including C–O–Sr isotope and rare-earth element and yttrium (REY) analyses. Petrographical observations and geochronological data are interpreted as evidence that the calcite veins formed through a crack-seal mechanism, mostly accompanied/initiated by intensive hydraulic fracturing of the host limestone in response to high-pressure fluids, which is manifested by multi-stage breccia deposits. Microscale U-series dates (272.6–20.5kyr) and geochemical compositions of the vein/breccia samples provide information on the timing and mechanism of the vein formation and identify the source of CO2-bearing fluids responsible for the carbonate precipitation. δ18OVPDB and δ13CVPDB values of the calcite veins range between −5.9 and −1.7‰, and −10.6 and −4.6‰, respectively. The isotopic compositions of the veins show highly fluctuating values as calcite grew successively perpendicular to vein walls, which, in combination with microstructural and geochronological constraints, are interpreted to reflect episodic CO2 degassing events associated with seismic and aseismic deformation. Oxygen and Sr isotope compositions (δ18OVPDB: −5.9 to −1.7‰; 87Sr/86Sr: 0.7082 to 0.7085) together with REY concentrations indicate deep infiltration of meteoric waters with various degrees of interactions mostly with the host limestone and siliciclastic parts of the basement rocks. Oxygen and carbon isotope compositions suggest CO2 degassing through intensive limestone dissolution. While majority of the veins display similar Post-Archaean Australian Shale (PAAS)-normalised REY variations, some of the veins show positive EuPAAS anomalies, which could be indicative of contributions from a deeply derived, heated, and reduced fluid component, giving rise to multiple fluid sources for the calcite veins. Vein calcite formed in fault-induced fractures offers insights into structural features, genetic characterisation of the parental fluids, and late Quaternary degassing of subsurface CO2 accumulations.