Sequestered CO2 Research Articles

First CO2-Enhanced-Oil-Recovery Demonstration Project in Saudi Arabia

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 181729, “Design and Implementation of the First CO2-Enhanced-Oil-Recovery Demonstration Project in Saudi Arabia,” by Sunil Kokal, SPE, Modiu Sanni, SPE, and Almohannad Alhashboul, SPE, Saudi Aramco, prepared for the 2016 SPE Annual Technical Conference and Exhibition, Dubai, 26–28 September. The paper has not been peer reviewed. An operator has designed a demonstration project for carbon dioxide (CO2) enhanced oil recovery (EOR) and has implemented it in one of its fields. The main objectives of the demonstration project are estimation of sequestered CO2, determination of incremental oil recovery, and evaluating the risks and uncertainties involved, including migration of CO2 within the reservoir and operational concerns. It is estimated that approximately 40% of the injected CO2 will be sequestered permanently in the reservoir. Project Design Conceptual Road Map and Screening Studies. Given the relatively light nature of crude oils and generally high reservoir pressures in Saudi Arabia, CO2 injection is a viable recovery method, especially in flooded reservoirs. An initial screening highlighted several good candidates for CO2 injection. A mature, waterflooded part of a large oil field with a carbonate reservoir was selected as a candidate for CO2 injection. Further studies were conducted for the candidate reservoir that included laboratory, feasibility, and detailed reservoir-simulation studies. This reservoir has been flooded for decades in a peripheral water-injection mode, and considerable reservoir and production data were available. Laboratory Studies. Two sets of experimental studies must be conducted for any given CO2-EOR prospect: fluid/fluid and fluid/rock interactions. The important laboratory experiments include the minimum miscibility pressure of the crude with CO2, swelling and fluid properties of CO2/oil mixtures, asphaltene precipitation onset and bulk asphaltene deposition, and oil-recovery potential by use of coreflooding studies. It must be emphasized that these experiments need to be conducted at reservoir conditions with live reservoir fluids and supercritical CO2; otherwise, the data have limited value at best. Simulation Studies. The reservoir selected for CO2 injection is a Jurassic carbonate reservoir, and the area selected is in a downflank, flooded part of the field. The selected area has been on peripheral water injection for more than 50 years and has been well-waterflooded because of its proximity to the peripheral injectors. Approximately 40 MMscf/D of relatively pure CO2 was available from a gas plant approximately 85 km from the pilot site. The slimtube data show that the minimum miscibility pressure is lower than the reservoir pressure, indicating that the CO2 will develop a miscible displacement in the reservoir at current reservoir pressures. The main objectives of the simulation study were as follows: Carry out screening and mechanistic studies and find areas suitable for a CO2-injection pilot. Assess the amount of CO2 sequestered over the period of the pilot testing. Assess incremental oil recoveries associated with different modes of CO2 injection. Optimize the pilot design within the reservoir and operational constraints.

Analytical Approach for Modeling of Multi Well CO2 Injection

Disposal of carbon dioxide (CO2) into underground geological formations is considered a viable strategy for the mitigation of global warming. It aims to reduce greenhouse gases emitted from point sources such as power plants. In order to select and evaluate a potential storage formation, many reservoir properties such as porosity, permeability, lateral and vertical extents, and a variety of residual fluid properties are considered. Injection design, which includes the placement of injectors and their flow rates, should be chosen to optimize injection capacity. One of the most important considerations to be addressed during design stage of sequestration is evaluation of pressure behaviour inside the reservoir during and after injection as the sequestered CO2 increases the pressure within the formation. In order to maintain the integrity of the reservoir the pressure needs to be maintained below the fracture pressure, typically at least 10% below. Thus evaluation of reservoir pressure is essential to ensure the reservoir remains under the maximum allowable pressure while sequestering the maximum amount of CO2 for long term storage in the reservoir. The optimal injection rates within a multi well injection site occur when the bottom-hole pressure at each injection site is at the maximum allowable pressure. In this study we present an analytical approach for modeling the pressure evolution during multi well CO2 injection into saline aquifers and using the model modify the injection rates to optimize injection capacity within a formation. We show that this optimization procedure significantly increases the capacity of formation as opposed to using the same injection rate at each wellbore.

CO2 sequestration in two mediterranean dune areas subjected to a different level of anthropogenic disturbance

Coastal sand dunes are among the most threatened habitats, especially in the Mediterranean Basin, where the high levels of human pressure impair the presence of plant species, putting at risk the maintenance of the ecosystem services, such as CO2 sequestration provided by these habitats. The aim of this study was to analyze how disturbance-induced changes in plant species abundance patterns account for variations in annual CO2 sequestration flow (CS) of Mediterranean sand dune areas. Two sites characterized by a high (site HAD) and a lower (site LAD) anthropogenic disturbance level were selected. At both sites, plant species number, cover, height and CS based on net photosynthesis measurements were sampled. At the plant species level, our results highlighted that Ammophila arenaria and Pancratium maritimum, had a key role in CS. Moreover, the results revealed a patchy species assemblage in both sites. In particular, HAD was characterized by a higher extension of the anthropogenic aphytoic zone (64% of the total transect length) than LAD. In spite of the observed differences in plant species composition, there were not significant differences between HAD and LAD in structural and functional traits, such as plant height and net photosynthesis. As a consequence, HAD and LAD had a similar CS (443 and 421 Mg CO2 ha−1 y−1, respectively). From a monetary point of view, our estimates based on the social costs of carbon revealed that the flow of sequestered CO2 valued on an average $ 3181 ± 114 ha−1 year−1 (mean value for the two sites). However, considering also the value of the CO2 negative flow related to loss of vegetated area, the annual net benefit arising from CO2 sequestration amounted to $ 1641 and $ 1772 for HAD and LAD, respectively. Overall, the results highlighted the importance to maximize the efforts to preserve dune habitats by applying an effective management policy, which could allow maintaining also a regulatory ecosystem service such as CO2 sequestration.

Isotopic evidence of passive mineral carbonation in mine wastes from the Dumont Nickel Project (Abitibi, Quebec)

Natural weathering of ultramafic rocks in mine tailings captures atmospheric CO2 through the formation of magnesium carbonates. The Dumont Nickel Project (DNP) is of particular interest as it will generate 1.7 Gt of ultramafic residues. A field experiment has been conducted at the DNP site in order to understand the process of natural CO2 sequestration. Two experimental cells were built using waste rock and mineral processing tailings and were instrumented with gas sampling ports and probes to monitor water saturation and suction. A decrease of the interstitial gas-phase CO2 concentration in both cells, from atmospheric values (∼390ppmv) near the surface to ∼100ppmv near the bottom, reflects active CO2 consumption by the residues. The total carbon content of the weathered DNP mine waste ranges from 0.2wt% to 6.5wt%C. Hydrotalcites supergroup minerals (pyroaurite-3R, brugnatellite, pyroaurite 2-H), aragonite, nesquehonite, dypingite and hydromagnesite were absent from the unweathered residues and precipitated in the cells during passive mineral carbonation. In situ measurements using Wavelength-Scanned Cavity Ring Down Spectroscopy reveal an increase of δ13C(air) from −8‰ near the surface of the cells to ∼2‰ near the bottom that is correlated with the decrease in CO2 concentration. This trend is explained by kinetic carbon isotope fractionation during dissolution of atmospheric CO2 in interstitial water (ΔDIC-CO2=−11.2‰). Secondary carbonates, precipitated from the interstitial water, are characterized by a moderately high δ18O and low δ13C. These isotopic compositions of the carbonates are consistent with precipitation in an evaporative environment where the kinetic carbon fractionation during atmospheric CO2 dissolution produces interstitial water depleted in 13C. Moreover, isotopic compositions of hydrotalcite supergroup minerals and other carbonate minerals are consistent with modern precipitation from the weathering of mining residue. These observations demonstrate the atmospheric source for the sequestered CO2 and help constrain a conceptual model of the carbonation reaction in the residues.

The biokarst system and its carbon sinks in response to pH changes: A simulation experiment with microalgae

AbstractThis study aims to explore the changes in a microalgal biokarst system as a potential carbon sink system in response to pH changes. The bidirectional isotope labeling method and mass balance calculation were adopted in a simulated biokarst environment with a series of set pH conditions and three microalgal species. Three key processes of the microalgal biokarst system, including calcite dissolution, CaCO3 reprecipitation, and inorganic carbon assimilation by microalgae, were completely quantitatively described. The combined effects of chemical dissolution and species‐specific biodissolution caused a decrease in overall dissolution rate when the pH increased from 7 to 9. CaCO3 reprecipitation and the utilization of dissolved inorganic carbon originating from calcite dissolution decreased when the pH increased from 7 to 9. The three processes exhibited different effects in changing the CO2 atmosphere. The amount of photosynthetic carbon sink was larger at high pH values than at low pH values. However, the CO2 sequestration related to the biokarst process (biokarst carbon sink) increased with decreasing pH. Overall, the total amount of sequestered CO2 produced by the biokarst system (CaCO3‐CO2‐microalgae) shows a minimum at a specific pH then increases with decreasing pH. Therefore, various processes and carbon sinks in the biokarst system are sensitive to pH changes, and biokarst processes play an important negative feedback role in the release of CO2 by acidification. The results also suggest that the carbon sink associated with carbonate weathering cannot be neglected when considering the global carbon cycle on the scale of thousands of years (<3 ka).

Assessing the carbon sequestration potential of magnesium oxychloride cement building materials

Magnesium oxychloride cement (MOC) boards have the potential to offset carbon emissions through carbon mineralization, a process whereby carbon dioxide (CO2) is converted to carbonate minerals. Boards (0–15 years old) contained MOC phase 5 (21–50 wt%), brucite, primary (e.g., magnesite) and secondary (hydromagnesite and chlorartinite) carbonate minerals. Quantitative mineralogy, electron microscopy and carbon abundance data demonstrate that secondary carbonates form through the reactions of MOC and brucite with CO2 within interfacial water layers after board manufacturing. Stable carbon isotopic data confirmed the source of sequestered CO2 as being from the atmosphere. Average carbonation rates were approximately 0.07 kg CO2/m2 board/year or 9.8 kg CO2/t board/year over 15 years, offsetting ∼20–40% of estimated carbon emissions. In experiments using 10% and 100% CO2 gas, carbonation was accelerated by approximately 400 and 1600 times in comparison to the passive rate. Integration of carbonation reactions into MOC board production could provide significant carbon offsets.

Comparing Low- and High-Temperature Electrochemical Process Pathways for Converting H2o and CO2 into Synthetic Natural Ga

The conversion of water and carbon dioxide into useful products such as synthetic natural gas using renewably supplied electrical energy can offer several advantages for stabilizing the electric grid and increasing the security of energy supply chains. In addition to providing increased grid operational flexibility for dealing with intermittent renewable electrical energy supplies, infrastructure changes such as hydrogen pipelines could conceivably be avoided in situations where natural gas pipelines and CO2 sources coincide. The natural gas distribution infrastructure is well developed in many countries, enabling the fuel to be transported long distances and easily delivered throughout cities. Using the existing pipeline to transport renewably generated synthetic natural gas (SNG) can further enhance the value of the product via offering grid services such as load leveling. While the price of natural gas is near record lows in the United States, many other countries are working to develop SNG as an alternative fuel for transportation and residential/commercial heating markets, especially in Europe and for island nations. This study presents pathway studies of low- and high-temperature electrolysis integrated with an SNG plant design and evaluates their performance for producing SNG by reacting renewably generated hydrogen with carbon dioxide. The carbon dioxide feedstock is assumed to be captured and scrubbed from either an existing coal fired power plant at the city-gate or from sequestered CO2, where the SNG plant is co-located. In the first pathway, water is split using either low-temperature alkaline or proton exchange membrane technology. In a second path, the co-electrolysis of steam and CO2 is done in-situ using high temperature (600°C) solid oxide cell technology. Historically, methanation has been a common practice for eliminating carbon monoxide and carbon dioxide in various chemical processes such as ammonia production and natural gas purification; for these processes, only small amounts (1-3% molar basis) of carbon oxides need to be converted to methane. A “bulk” methanation process is unique due to the high concentration of carbon oxides and hydrogen. In addition, the carbon dioxide is the only carbon source, and the reaction characteristics of carbon dioxide are much different than carbon monoxide. In process paths involving low-temperature electrolyzers, a methanation plant is required. In this study, thermodynamic and kinetic considerations of the methanation reaction are explored to model and simulate a system of reactors for the conversion of hydrogen and carbon dioxide to SNG. Multiple reactor stages are used to increase temperature control of the reactor and drain water to promote the forward direction of the methanation reaction. Heat recuperation and recovery using organic Rankine cycle units for electricity generation utilizes the heat produced from the methanation reaction. Bulk recycle is used to increase the overall reactant conversion while allowing a satisfactorily high methane content SNG product. A hydrogen membrane separates hydrogen for recycle to increase the Wobbe index of the product SNG by increasing the methane content to nearly 93% by volume. In both pathways, the product SNG has a minimum Wobbe index of 47.5 MJ/m3 which is acceptable for natural gas pipeline transport and end-use appliances in existing infrastructures. The overall plant efficiencies of both pathways are compared as well as the expected price of the product SNG using the H2A tool.

Multidomain-staggered coupling technique for Darcy–Navier Stokes multiphase flow: An application to CO2 geosequestration

This paper introduces a multidomain-staggered technique for coupling multiphase flow in a porous medium, dominated by the Darcy laminar flow, with multiphase flow in a wellbore, dominated by the Navier Stokes viscous, compressible flow. The Darcy flow in the porous medium is formulated using the averaging theory, and the Navier Stokes flow in the wellbore is formulated using the drift-flux model. The governing equations are discretized using a mixed discretization finite element scheme, in which the partition of unity finite element method, the level set method and the standard Galerkin finite element method are combined in an integrated numerical scheme. A multidomain technique is utilized to uncouple the physical system into two subdomains, coupled back by enforcing flow constraints at their interaction boundaries. The resulting system of equations is solved using an iterative staggered technique and a multiple time-stepping scheme. This combination between the multidomain technique and the staggered-multiple time-stepping technique enables the use of different mathematical and numerical formulations for the two subdomains, and facilitates the implementation of a standard finite element computer code. The proposed model is tailored to simulate sequestered CO2 leakage through heterogeneous geological formation layers and abandoned wellbores. A numerical example describing different leakage scenarios is given to demonstrate the computational capability of the model. The numerical results are compared to those obtained from a commercial simulator.

CO2 Energy Reactor – Integrated Mineral Carbonation: Perspectives on Lab-Scale Investigation and Products Valorization

To overcome the challenges of mineral CO2 sequestration, Innovation Concepts B.V. is developing a unique proprietary Gravity Pressure Vessel (GPV) reactor technology, and has focussed on generating reaction products of high economic value. The GPV provides intense process conditions through hydrostatic pressurization and heat exchange integration that harvests exothermic reaction energy, thereby reducing energy demand of conventional reactor designs, in addition to offering other benefits. In this paper, a perspective on the status of this technology and outlook for the future is provided. To date, laboratory-scale tests of the envisioned process have been performed in a tubular “rocking autoclave” reactor. The mineral of choice has been olivine (~Mg1.6Fe2+0.4(SiO4) + ppm Ni/Cr), although asbestos, steel slags and oil shale residues are also under investigation. The effect of several process parameters on reaction extent and product properties have been tested: CO2 pressure, temperature, residence time, additives (buffers, lixiviants, chelators, oxidizers), solids loading, and mixing rate. The products (carbonates, amorphous silica and chromite) have been physically separated (based on size, density and magnetic properties), characterized (for chemistry, mineralogy and morphology) and tested in intended applications (as pozzolanic carbon-negative building material). Economically, it is found that product value is the main driver for mineral carbonation, rather than, or in addition to, the sequestered CO2. The approach of using a GPV and focusing on valuable reaction products could thus make CO2 mineralization a feasible and sustainable industrial process.

Analysis of a time dependent injection strategy to accelerate the residual trapping of sequestered CO2 in the geologic subsurface

A time dependent injection strategy for greatly accelerating the immobilization of geologically sequestered CO2 is proposed and analyzed. The injection of high density CO2 into a brine aquifer is followed by brine flooding facilitating residual trapping and dissolution of the CO2 on time scales much shorter than those that would occur by natural processes. One-dimensional kinematic wave equations are derived for the two-phase flow of brine and CO2 and for the transport of dissolved CO2. A solution of these equations using the method of characteristics reveals that brine flooding is most effective when the kinematic wave speed of CO2 saturation is higher than the propagation velocity of a shock wave separating the two-phase flow from the native brine. Finite volume simulation using the reservoir simulator TOUGH2 with PetraSIM interface are generally in good agreement with the one-dimensional model, but show that gravitational overriding of the CO2 can become important if the duration of the injection process is too long. Both methods show that brine flooding is able to reduce the mass fraction of mobile CO2 to less than 10% using a volume ratio brine:CO2 of less than 2.75 on time scales comparable to that of the CO2 injection.

Impact indicators for caprock integrity and induced seismicity in CO2 geosequestration: insights from uncertainty analyses

The geological sequestration (geosequestration) of carbon dioxide (CO2) is a mitigation method for reducing greenhouse gas emission into the atmosphere. The security and safety of CO2 geosequestration are strongly dependent on the mechanical stability of the caprock overlying the reservoir. Underground injection of CO2 increases the pore pressure and thus decreases the effective stress. It may lead to caprock failure, as well as the subsequent leakage of sequestered CO2. In particular, geothermal exploitation and the underground disposal of hazardous liquid wastes have demonstrated a risk of induced seismicity. We performed an uncertainty analysis using a novel response surface methodology and a two-step statistical experimental design, evaluated the statistical significance of operator choices and subsurface uncertainties to caprock integrity, and quantified the moment magnitude of the induced seismicity. Furthermore, the optimal combination (i.e., the worst-case scenario) with the desired properties was forecast. A series of numerical experiments was well designed, and 130 combinations were statistically determined. Based on the results from the analysis of variance for the response surface quadratic model, the impact indicators were presented in histograms according to their significances to the Coulomb failure stress and moment magnitude of the induced seismicity. Lastly, the values of the selected independent impact indicators were predicted to obtain optimal compositions for object function of both Coulomb failure stress and moment magnitude, and the desired properties were being picked out. The optimal combinations had desirability values of 1.000, demonstrating the fitness of the selected statistical models in analyzing the experimental data.

Evaluating impacts of CO2 intrusion into an unconsolidated aquifer: I. Experimental data

Capture and deep subsurface sequestration of CO2 has been identified as a potential mitigation technique for rising atmospheric CO2 concentrations. Sequestered CO2 represents a potential risk to overlying aquifers if the CO2 leaks from the deep storage complex. Batch and column experiments combined with wet chemical extractions were conducted to evaluate these risks to groundwater quality and to understand effects of unintentional release of CO2 on groundwater chemistry and aquifer mineralogy. Sediments from the High Plains aquifer in Kansas, a largely unconsolidated aquifer, were used to study time-dependent release of major, minor and trace elements when exposed to CO2 gas. Results showed that Ca, Ba, Si, Mg, Sr, Na, and K increased either within the first 4h or followed nonlinear increasing trends with time, indicating that dissolution and/or desorption reactions controlled their release. In addition, other elements (e.g., Fe and Mn) and trace elements (e.g., As, Cu, Cr, Pb) were released during batch and column experiments, demonstrating the possibility for changes in mineralogy and groundwater quality degradation due to exposure to seepage of sequestered CO2. National drinking water regulations were exceeded for As and Mn in the batch experiments, and As, Se, Mn, Pb and Hg in the column experiments, despite low levels of these contaminants found in the sediments. In addition, the concentration of another potential contaminant, i.e., Mo, was consistently higher in the control batch experiments (i.e., absence of CO2) but was below detection in the presence of CO2 indicating a potential for removal of elements by CO2 gas exposure. Although results will be site specific for the High Plains aquifer and other mostly unconsolidated aquifers, these investigations will provide useful information to support site selection, risk assessment, and public education efforts associated with geological CO2 storage and sequestration.

Is there a potential of emission of sequestered CO2 from Illinois bituminous coal under shockwaves?

The mitigation of potential global warming due to the emission of greenhouse gases, especially CO2, would require large-scale geological sequestration. Though current emphasis has been on identifying and characterizing the potential geological sequestration reservoirs, there is also an urgent need to ensure that the sequestered CO2 will remain in place under reasonable perturbations, e.g., under seismic activity. One of the potential geological reservoirs identified is unmineable coal seams due to the advantage of recovering fuel gas like CH4 while sequestering CO2. Therefore, a closed experimental setup was constructed where Illinois bituminous coal cores could be subjected to compressive shockwaves while simultaneously monitoring the emission of gases from CO2 pressurized coal cores. The results from the pressurized coal cores were compared with the behavior manifested by un-pressurized coal cores as well as porous pumice stone cores, which were also subjected to shockwaves. As expected, the un-pressurized cores showed no significant emission of CO2 when subjected to shock; however, this was not the case for the cores which were pressurized with CO2. The results indicate that massive amounts of CO2 would be emitted if the cores were exposed to atmospheric pressure simulating a situation where caprock has been compromised during primary seismic activity. Irrespective of the belief that coal interacts strongly with CO2, both chemically and physically, compressive (0.374MPa) shockwaves forced almost all the CO2 to be ejected from the coal cores. Surprisingly, most, if not all, the sequestered CO2 would be emitted in less than 1h if the cores were subjected to reasonably moderate shocks. In actual seismic activity conditions, one expects conditions to be even more severe than in the experimental setup used in this study because of the presence of compressive and transverse stresses and shears. If such is the case, CO2 may be emitted even faster. It is reasonable to argue that Illinois bituminous coals may not be suitable hosts for sequestering CO2 because the region is prone to seismic activity.

Microbial growth under supercritical CO2.

Growth of microorganisms in environments containing CO2 above its critical point is unexpected due to a combination of deleterious effects, including cytoplasmic acidification and membrane destabilization. Thus, supercritical CO2 (scCO2) is generally regarded as a sterilizing agent. We report isolation of bacteria from three sites targeted for geologic carbon dioxide sequestration (GCS) that are capable of growth in pressurized bioreactors containing scCO2. Analysis of 16S rRNA genes from scCO2 enrichment cultures revealed microbial assemblages of varied complexity, including representatives of the genus Bacillus. Propagation of enrichment cultures under scCO2 headspace led to isolation of six strains corresponding to Bacillus cereus, Bacillus subterraneus, Bacillus amyloliquefaciens, Bacillus safensis, and Bacillus megaterium. Isolates are spore-forming, facultative anaerobes and capable of germination and growth under an scCO2 headspace. In addition to these isolates, several Bacillus type strains grew under scCO2, suggesting that this may be a shared feature of spore-forming Bacillus spp. Our results provide direct evidence of microbial activity at the interface between scCO2 and an aqueous phase. Since microbial activity can influence the key mechanisms for permanent storage of sequestered CO2 (i.e., structural, residual, solubility, and mineral trapping), our work suggests that during GCS microorganisms may grow and catalyze biological reactions that influence the fate and transport of CO2 in the deep subsurface.

Detection and monitoring of leaked CO2 through sediment, water column and atmosphere in a sub-seabed CCS experiment

Carbon capture and storage in sub-seabed geological formations (sub-seabed CCS) is currently being studied as a realistic option to mitigate the accumulation of anthropogenic CO2 in the atmosphere. In implementing sub-seabed CCS, detecting and monitoring the impact of the sequestered CO2 on the ocean environment is highly important. The first controlled CO2 release experiment, Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage (QICS), took place in Ardmucknish Bay, Oban, in May–September 2012.We applied the in situ pH/pCO2 sensor to the QICS experiment for detection and monitoring of leaked CO2, and carried out several observations. The cabled real-time sensor was deployed close to the CO2 leakage (bubbling) area, and the fluctuations of in situ pH and pCO2 above the seafloor were monitored in a land-based container. The long-term sensor was placed on seafloor in three different observation zones. The sediment pH sensor was inserted into the sediment at a depth of 50cm beneath the seafloor near the CO2 leakage area. Wide-area mapping surveys of pH and pCO2 in water column around the CO2 leakage area were carried out by using an autonomous underwater vehicle (AUV) installed with sensors. Atmospheric CO2 above the leakage area was observed by using a CO2 analyzer that was attached to the bow of ship of 50cm above the sea-surface.The behavior of the leaked CO2 is highly dependent on the tidal periodicity (low tide or high tide) during the CO2 gas release period. At low tide, the pH in sediment and overlying seawater decreased due to strong eruption of CO2 gas bubbles, and the CO2 ascended to sea-surface quickly with a little dissolution to seawater and dispersed into the atmosphere. On the other hand, the CO2 bubbles release was lower at high tide due to higher water pressure, and slight low pH seawater and high atmospheric CO2 were detected. After stopping CO2 gas injection, no remarkable variations of pH in sediment and overlying water column were observed for three months.

Application of tracers to measure, monitor and verify breakthrough of sequestered CO2 at the CO2CRC Otway Project, Victoria, Australia

At the Cooperative Research Centre for Greenhouse Gas Technology's (CO2CRC) field site in the Otway Basin of Victoria, Australia, investigations into the storage of CO2-rich gas in a depleted hydrocarbon gas field have been conducted in the Waarre C reservoir. The injected gas from the nearby Buttress field contained 75mol% CO2, 21mol% CH4 with the remaining balance being a mixture of wet hydrocarbons, condensate and nitrogen. Chemical tracers (sulphur hexafluoride, SF6; krypton, Kr; perdeuterated methane, CD4) were added on the basis of literature surveys and small volume trials at the Frio II Brine experiment in Texas. The aim of the project was to measure, monitor and verify the presence of injected CO2 in a depleted gas field and that the arrival of tracers was a major component of demonstrating breakthrough of CO2 at the monitoring well, Naylor-1. The paper focuses on methods developed for the injection, recovery and analysis of samples collected at the Naylor-1 well. Results of tracer analysis compare well with other data collected (including pH and density measurements) to demonstrate breakthrough.A slip-stream injection system was designed to deliver the tracers mixed with the CO2-rich gas into the subsurface at the CRC-1 well. The tracers were added to the gas stream 17days after the start of injection (CO2 injection commenced 18th March, 2008) into the depleted natural gas field at Naylor. A U-tube system was used to retrieve the samples from the Naylor-1 monitoring well. Collected gas and formation water samples were analysed in detail for gas composition, tracers, isotopes (13C CO2 mainly) and inorganic geochemistry for the broader project. The tracer results confirm that CO2 breakthrough at the monitoring well occurred within the predicted times. However the interval between samples taken from the U-tubes was too coarse to resolve detailed differences in arrival times between the CO2 and tracers.Of the three tracers used, SF6 provided the clearest evidence of breakthrough at U-tube 2. Kr, because of its abundance in air, and its potential to be present in the subsurface, was more prone to contamination and had higher background levels prior to breakthrough. CD4 was expected to provide some more unique data based on the presence of abundant CH4 in the reservoir interval. With hindsight, larger volumes should have been injected to facilitate comparisons with the other tracers and add value to the data set. The test of CD4 however acted as a suitable proof of concept that CD4 could be used in such a high background of CH4.Further work is ongoing to generate data for partition coefficients between supercritical CO2, CH4 and water under the injection conditions.

Deep chlorophyll maximum and primary productivity in Indian Ocean sector of the Southern Ocean: Case study in the Subtropical and Polar Front during austral summer 2011

To understand the implications of deep chlorophyll maximum (DCM) on primary productivity (PP), 3-hourly observations (over 72h) were carried out at the Polar Front (PF) and the Subtropical Front (STF) of the Indian Ocean sector of Southern Ocean during the austral summer (February) 2011. Pronounced vertical variations in hydrographic parameters (temperature, salinity, density, dissolved oxygen) were observed between the fronts, whereas temporal (diurnal) variations of these parameters within the fronts were minimal during the study period. At the PF, a well-defined temperature minimum layer (TML), which is the winter residue of Antarctic Surface Water (AASW), was observed between 50 and 320m. The DCM observed at the PF (~75m) was more prominent than that at the STF and coincided with the upper boundary of the TML. The elevated chlorophyll a (Chl a) concentration in DCM is believed to result from the proliferation of the low-light adapted phytoplankton (shade flora). Due to the presence of a strong DCM the average column-integrated Chl a was nearly two times higher at the PF than at the STF yielding ~1.4 times higher column integrated PP (IPP) at the PF (211mgCm−2d−1) compared to the STF (152mgCm−2d−1). Higher Chl a and productivity at the PF was attributed to the dominance of diatoms as indicated from diagnostic pigment analysis by high performance liquid chromatography (HPLC). Owing to their higher sinking rate, the diatoms with sequestered CO2, would generate substantial export production/flux, thereby making the PF region as a sink for atmospheric CO2.

Geochemical modeling of CO2–water–rock interactions for two different hydrochemical types of CO2-rich springs in Kangwon District, Korea

Naturally outflowing CO2-rich springs are a natural analogue of the seepage of sequestered CO2 in geological storage sites. In Kangwon district of South Korea, two hydrochemically different types of CO2-rich springs (i.e., Ca–HCO3-type and Na–HCO3-type) occur together in a granitic terrain. Hydrochemical and water-isotope data (i.e., δ18O–δD and tritium) show that Na–HCO3-type springs have experienced significant silicate weathering processes over a long residence time at depths, while Ca–HCO3-type springs were formed by the mixing of Na–HCO3-type springs with shallow groundwater during ascent. In this study, diverse geochemical models including mixing, ion exchange and reaction path were investigated to verify the geochemical processes accounting for the occurrence of two contrasting types of CO2-rich springs. The mixing and ion exchange models reveal that Ca–HCO3-type springs are well explained by reverse cation exchange occurring during the mixing of Na–HCO3-type springs with shallow groundwater. The Na–HCO3-type springs are well explained by the reaction path modeling including the dissolution of silicate minerals (plagioclase, K-feldspar and biotite) and the precipitation of secondary minerals (calcite, kaolinite, muscovite and Mg-beidellite), implying that dissolved carbon is sequestered by calcite precipitation (i.e., mineral trapping). However, the concentrations of K in our modeling results are far below those of K observed in Na–HCO3-type springs, because of the precipitation of muscovite considered in the model, suggesting the partial disequilibrium state of the aquifer during the hydrolysis of K-feldspar under high PCO2 conditions. This result implies that to better predict long-term CO2–water–rock interactions in a geological storage site with abundant K-feldspar, the secondary K-bearing minerals should be carefully predicted, because a target aquifer can be far from chemical equilibrium during the storage period. This study shows that geochemical modeling can be effectively used to predict the hydrochemical changes of groundwater during long-term CO2–water–rock interactions and subsequent leakage toward surface in K-feldspar rich aquifer, although it should be included in a fully coupled computational approach between fluid flow, heat transfer and reactive mass transport processes in the future research.

Experimental and Modelling Study of Storage of CO2 and Impurities in a Depleted Gas Field in Northeast Netherlands

Abstract In order to investigate the effects of impurities on subsurface storage of CO2, experiments were carried out on Permian Rotliegend reservoir and Zechstein caprock core samples at subsurface conditions of 300 bar and 100°C. The experiments were performed in the presence of brine and the following gas compositions: CO2, CO2+5000 ppm H2S, CO2+100 ppm H2S and CO2+100 ppm SO2 for a 30 day duration. Following CO2 injection, permeability of the reservoir and caprock samples increased by 10-30% and by a factor of 3-10 respectively. After co-injection of 5000 ppm H2S permeability of both reservoir and caprock samples reduced significantly. When the concentration of H2S was reduced to 100 ppm, minimal variation of permeability took place because the dissolution of minerals was balanced with the precipitation of secondary phases. In the case of co-injection of 100 ppm SO2 permeability of reservoir samples increased by a factor of 1.18 to 2.2. In the caprock samples permeability changed by a factor of 0.8 to 23. In addition, in order to determine long term (>100 years) interaction between CO2 and the reservoir mineralogy we need to rely on modelling programs. This requires accurate and fit for purpose input parameters, associated with the lithological composition of reservoirs and seals at the storage site. For this purpose, we specifically focused on the reactive surface area of minerals, which we measured by scanning electron microscopy. Using the range of measured reactive surface area of minerals as an input in the modelling software, we could obtain the distribution of the sequestered CO2 as a mineral (3-9 kg of CO2/m3).

Effect of marine environmental factors on the phase equilibrium of CO2 hydrate

We monitored the stability of CO2 hydrate by investigating its three-phase (LW-H-V) equilibrium at different marine environmental conditions in the range of 273.3–280K and 17–30bar. The phase equilibrium of CO2 hydrate in synthetic seawater (aqueous solution with 3.5% NaCl and 0.1M NaHCO3) was observed in higher pressure and lower temperature region of phase diagram (ΔP: 5.063±0.019bar, ΔT: −1.950±0.019K), compared to that in deionized water (DIW) control. Effect of soil minerals (illite and pyrite) and organic matters (glycine and humic acid) on CO2 hydrate phase equilibrium was not significant in both DIW and synthetic seawater. The phase equilibrium of CO2 hydrate in Ulleung basin (UB) sediment suspension was very similar to that of the synthetic seawater control (ΔP: 0.115±7.969×10−4bar, ΔT: −0.038±8.466×10−5K); however, in the absence of organic fraction in UB sample by baking the sediment, its phase equilibrium shifted to the unstable upper left region (ΔP: 0.360±0.043bar, ΔT: −0.215±8.442×10−4K). The results can provide fundamental knowledge to predict the stability of sequestered CO2 and estimate CO2 leakage from the sites in seabed sediment environments as a part of significant criteria to implement successful offshore CO2 storage.