CO2 Reservoir Research Articles

Complete Equation of State Thermal Formulation for Simulation of CO2 Storage

Summary Storage of carbon dioxide (CO2) in depleted gas reservoirs or large aquifers is one of the available solutions to reduce anthropogenic greenhouse gas emissions. Numerical modeling of these processes requires the use of large geological models with several orders of magnitude of variations in the porous media properties. Moreover, modeling the injection of highly concentrated and cold CO2 in large reservoirs with the correct physics introduces numerical challenges that conventional reservoir simulators cannot handle. We propose a thermal formulation based on a full equation of state (EoS) formalism to model pure CO2 and CO2 mixtures with the residual gas of depleted reservoirs. Most of the reservoir simulators model the phase equilibriums with a pressure-temperature-based formulation. With this usual framework, it is not possible to exhibit two phases with pure CO2 contents. Moreover, in this classical framework, the crossing of the phase envelope is associated with a large discontinuity in the enthalpy computation, which can prevent the convergence of the energy conservation equation. In this work, accurate and continuous phase properties are obtained, basing our formulation on enthalpy as a primary variable. We first implement a new phase-split algorithm with input variables as pressure and enthalpy instead of the usual pressure and temperature, and we validate it on several test cases. This algorithm can model situations in which the mixture can change rapidly from one phase to the other at constant pressure and temperature. Then, treating enthalpy instead of temperature as a primary variable in both the reservoir and the well modeling algorithms, our reservoir simulator can model situations with pure or near pure components, as well as crossing of the phase envelope that usual formulations implemented in reservoir simulators cannot handle. We first validate our new formulation against the usual formulation on a problem in which both formulations can correctly represent the physics. Then, we show situations in which the usual formulations fail to represent the correct physics and that are simulated well with our new formulation. Finally, we apply our new model for the simulation of pure and cold CO2 injection in a real depleted gas reservoir from the Netherlands.

Correction of gas adsorption capacity in quartz nanoslit and its application in recovering shale gas resources by CO2 injection: A molecular simulation

Accurate molecular simulations of the competitive adsorption behaviors of CH4 and CO2 in shale reservoirs are crucial for understanding the fundamental mechanisms of fluids storage and recovery under actual burial depth. In this work, the shale thickness of 79.82 nm (143 layers) at each side of quartz wall in theoretical model was advanced for gas adsorption capacity correction. In this case, the adsorption mechanism of shale gas and its recovery mechanism by CO2 injection were systematically pursued at varied burial depth, pore size, moisture content, and gas mole fraction by Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. Adsorbed and free capacities of CH4 gas were determined individually. Gas loss rate and recovery efficiency were put forward to evaluate the recovery of shale gas by H2O and CO2 molecules. The obtained results show that the enrichment region of CH4 is ∼2400 m, which is predicted to be the economically feasible mining depth of CH4. Shallow burial depth (600–800 m) should be favorable for the sequestration of CO2. The results obtained are expected to provide significant theoretical guidance for the reliable evaluation and economic exploitation of shale gas as well as the sequestration of CO2.

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.

Harvesting the Potential of CO2 before it is Injected into Geological Reservoirs

To store CO2 in geological reservoirs, expansion valves have been used to intentionally release supercritical CO2 from high-pressure containers at a source point to lower-pressure pipelines and transport to a selected injection site. Using expansion valves, however, has some shortcomings: (i) the fluid potential, in the form of kinetic energy and pressure which can produce mechanical work or electricity, is wasted, and (ii) due to the Joule-Thomson cooling effect, the reduction in the temperature of the released CO2 stream might be so dramatic that it can induce thermal contraction of the injection well causing fracture instability in the storage formation. To avoid these problems, it has been suggested that before injection, CO2, should be heated to a temperature slightly higher than that of the reservoir. However, heating could increase the cost of CO2 injection. This work explores the use of a Tesla Turbine, instead of an expansion valve, to harvest the potential of CO2, in the form of its pressure and kinetics, to generate mechanical work when it is released from a high-pressure container to a lower-pressure transport pipeline. The goal is to avoid throttling losses and to produce useful power because of the expansion process. In addition, due to the friction between the gas and the turbine disks, the expanded gas temperature reduction is not as dramatic as in the case when an expansion valve is used. Thus, as far as CO2 injection is concerned, the need for preheating can be minimized.

Quantum Dot-Sensitized Photoreduction of CO2 in Water with Turnover Number > 80,000.

Climate change and global energy demands motivate the search for sustainable transformations of carbon dioxide (CO2) to storable liquid fuels. Photocatalysis is a pathway for direct conversion of CO2 to CO, one step within light-powered reaction networks that could, if efficient enough, transform the solar energy conversion landscape. To date, the best performing photocatalytic CO2 reduction systems operate in nonaqueous solvents, but technologically viable solar fuels networks will likely operate in water. Here we demonstrate catalytic photoreduction of CO2 to CO in pure water at pH 6-7 with an unprecedented combination of performance parameters: turnover number (TON(CO)) = 72,484-84,101, quantum yield (QY) = 0.96-3.39%, and selectivity (SCO) > 99%, using CuInS2 colloidal quantum dots (QDs) as photosensitizers and a Co-porphyrin catalyst. At higher catalyst concentration, the system reaches QY = 3.53-5.23%. The performance of the QD-driven system greatly exceeds that of the benchmark aqueous system (926 turnovers with a quantum yield of 0.81% and selectivity of 82%), due primarily to (i) electrostatic attraction of the QD to the catalyst, which promotes fast multielectron delivery and colocalization of protons, CO2, and catalyst at the source of photoelectrons, and (ii) termination of the QD's ligand shell with free amines, which capture CO2 as carbamic acid that serves as a reservoir for CO2, effectively increasing its solubility in water, and lowers the onset potential for catalytic CO2 reduction by the Co-porphyrin. The breakthrough efficiency achieved in this work represents a nonincremental step in the realization of reaction networks for direct solar-to-fuel conversion.

Sorption characteristics in coal and shale: A review for enhanced methane recovery

The exploration and exploitation of hydrocarbon resources within coal and shale reservoirs is an engineering challenge. Well-developed internal micro-pore structures, complex sorption mechanism as well as numerous influencing factors affecting the gas flow are generally not well-accounted in the commercial life-cycle of shale gas and coalbed methane wells. Although large number of studies have been conducted to propose improved sorption models and study the influencing factors on adsorption and desorption characteristics of methane and CO2 in coal and shale reservoirs, a systematic review of such studies for efficient understanding of the accumulated literature is missing, especially with a focus towards coal and shale reservoirs. In that context, this study presents a review of sorption characteristics of methane in coal and shale. Firstly, theoretical mechanisms for methane sorption are introduced, followed by description of sorption models. Further, three factors influencing the sorption of gas in coal and shale are described: total organic carbon and clays, pore structures, and reservoir conditions. Finally, the preferential sorption characteristics of hydrocarbons and carbon dioxide are described, and the methods to promote methane desorption for enhanced recovery are discussed, which include technologies such as gas injection, microwave heating, and hydraulic fracturing. Cited as: Qin, X., Harpreet, S., Cai, J. Sorption characteristics in coal and shale: A review for enhanced methane recovery. Capillarity, 2022, 5(1): 1-11. https://doi.org/10.46690/capi.2022.01.01

Evaluation of CO2 Injection in Shale Gas Reservoirs Based on Numerical Simulation

Shale gas is natural gas that is found trapped within a typical sedimentary rock derived from clastic sources which is known as shale. The combination of two advanced technologies (horizontal drilling and hydraulic fracturing) has helped to access to a large volumes of shale gas that was previously not producible. In this paper, the performance of CO2 injection and CH4 recovery in shale gas reservoirs is evaluated. The study reveals that it is a complex function a several parameters such as CO2 injection time, soaking time, injection volume, reservoir permeability, reservoir porosity, and thickness. Numerical simulation is performed to model CO2 huff-n-puff process and multicomponent Langmuir isotherms in the Barnett Shale formation; two horizontal wells are performed for this case. Results show that CO2-EGR is very efficient for shale gas and a recovery factor of 82.6% can be obtained by this process. Besides, an analytical study is done in order to facilitate the estimation of recovery factor based on critical parameters obtained from simulation. A comparison study of the observed data from simulation and the estimated date from some production decline models such as LGA (Logistic Growth Analysis) model, and Ali model is also performed in this study

Heterogeneous Transport of Free CH4 and Free CO2 in Dual-Porosity Media Controlled by Anisotropic In Situ Stress during Shale Gas Production by CO2 Flooding: Implications for CO2 Geological Storage and Utilization.

Regarding shale gas production by CO2 flooding, few existing reports explain the performance of free CH4 and free CO2 in shale reservoirs controlled by anisotropic in situ stress, partly restricting the integrated recognition of shale-based CO2 geological storage and utilization (CGSU). In this work, a self-developed model embedded with thermo-hydro-mechanical coupling relationships is introduced to investigate how the anisotropic in situ stress determines the transport of free gases (CH4 and CO2) after CO2 is injected into the shale. Therefore, the stronger anisotropy of in situ stress enables more CO2 in the free phase to be trapped in the shale reservoir and is insignificant for the content of residual free CH4 compared to the situation under isotropic in situ stress. Along with CO2 injection into the shale, the matrix porosity decreases invariably, while the fracture porosity decreases first and then increases gradually. Therein, the variation amplitude of the matrix/fracture porosity is more distinct under a stronger anisotropic in situ stress. The simulations also suggest that the ratio of free CO2 relative to all free gases in shale is ∼65% at most after sufficient CGSU operation. Hopefully, this comprehensive work is helpful in enhancing the knowledge on the promising shale-based low-carbon CGSU technique.

Interferometric body-wave retrieval from ambient noise after polarization filtering: Application to shallow reflectivity imaging

Interferometric retrieval of body waves from ambient noise recorded at surface stations is usually challenged by the dominance of surface-wave energy, in particular in settings dominated by anthropogenic activities (e.g., natural resource exploitation, traffic, and infrastructure construction). As a consequence, ambient noise imaging of shallow structures such as sedimentary layers remains a difficult task for sparse and irregularly distributed receiver networks. We have determined how polarization filtering can be used to automatically extract steeply inclined compressional waves (P-waves) from continuous 3C recordings, and, in turn, it improves passive body-wave imaging. Being a single-station approach, the technique does not rely on a dense receiver array and is therefore well suited for data collected during surveillance monitoring for tasks such as reservoir hydraulic stimulation, CO2 sequestration, and wastewater disposal injection. We apply the method on a continuous data set acquired in the Wellington oilfield (Kansas, US), where local and regional seismicity and other forms of ambient noise provide an abundant source of surface- and body-wave energy recorded at 15 short-period receivers. We use autocorrelation (AC) to derive the shallow (<1 km) reflectivity structure below the receiver array and validate our workflow and results with well logs and active seismic data. Ray-tracing analysis and waveform modeling indicate that converted shear waves need to be taken into account for realistic ambient noise body-wave source distributions because they can be projected on the vertical component and might lead to misinterpretation of the P-wave reflectivity structure. Overall, our study suggests that polarization filtering significantly improves passive body-wave imaging on AC and interstation crosscorrelation. It reduces the impact of time-varying noise source distributions and is therefore also potentially useful for time-lapse ambient noise interferometry.

Origin of dawsonite-forming fluids in the Mihályi-Répcelak field (Pannonian Basin) using stable H, C and O isotope compositions: Implication for mineral storage of carbon-dioxide

Natural CO2 reservoirs provide an opportunity to study long-term fluid-rock interactions, which are essential to reassure the safety of mineral storage of carbon-dioxide. The Mihályi-Répcelak field (Pannonian Basin, Central Europe) is one of the largest natural CO2-bearing reservoirs in Europe (25 Mt). The CO2 was trapped in Neogene sandstones, which contain various carbonate minerals (dolomite, ankerite, siderite, dawsonite). To reveal the origin of the parent fluid, from which these minerals precipitated, dawsonite and siderite were separated by a new physical method to minimise the uncertainties in the analysis of their stable isotope composition. The δ13CDaw values range from +1.3‰ to +1.6‰ and the calculated δ13CCO2 values in equilibrium with dawsonite (−4.8‰ - –2.0‰) overlap with the carbon isotope compositions of the local CO2 and the European Subcontinental Lithospheric Mantle (−3.9‰ - –2.1‰). This indicates that the dawsonite-forming CO2 had a magmatic origin. The siderite data indicates that some formed from the magmatic CO2, possibly simultaneously with dawsonite (−6.0‰ - –3.9‰), whereas the rest (−8.4‰ - –6.1‰) formed either from a fractionated CO2 with magmatic origin or before the CO2 invasion. The hydrogen isotope composition of structural OH− of dawsonite (−57‰ to −74‰) was determined and was used to estimate the origin of the interacting porewater. The calculated porewater data (δD: −69‰ - –103‰ and δ18O: −1.4‰ - +4.7‰) indicate that the parent fluid was meteoric water modified by water-rock interaction. Our data allows estimation of the total amount of CO2 stored in the dawsonite-bearing sandstone reservoir to be 25 kg/m3, well in line with previous modelling works, which gives a total of 2.01 × 106 t of CO2, higher than previous estimates.We suggest that individual mineral analysis complemented by hydrogen isotope analysis is to be employed to effectively trace in-reservoir fluid-rock interactions in CO2 reservoirs and provide valuable input data for geochemical modelling for better predicting conditions for mineral storage of CO2.

Effects of impurities H2S and N2 on CO2 migration and dissolution in sedimentary geothermal reservoirs

CO2 geological storage is a key technology for reducing greenhouse gas emissions. But merely injecting CO2 into sedimentary strata has high costs and no direct economic benefit. A more cost-effective and environment-friendly way is storing and utilizing CO2 in deep sedimentary reservoirs for heat extraction. However, the effects of ubiquitous impurities on the sequestration safety and heat extraction performance of CO2 in high-temperature reservoirs, are unclear. In this study, we have performed numerical simulations of multiphase flow, solute transport, and heat conduction at reservoir temperature 50–250 °C, to reveal the effects of two common impurities (i.e., H2S and N2) on the migration, dissolution and thermal absorption of CO2 in sedimentary geothermal reservoirs. It is found that, for higher reservoir temperature, the impurity N2 leads to smaller promotion on the migration of gas phase, but retains the acceleration effect on the cooling in reservoir. Above 150 °C, the deviations on the gas phase migration velocity and the CO2 dissolution efficiency caused by 10 mol% N2 are less than 12%. Meanwhile, the 10 mol% N2 can increase the total heat absorbed from reservoir by about 40% at least. It means that N2 has the potential to benefit a CO2 plume geothermal system significantly without distinct risks. Besides, it is also found that the CO2 dissolution efficiency can be increased and decreased by H2S and N2 through partitioning phenomenon, respectively.

Calcium isotope compositions as a means to trace carbonate recycling.

Marine carbonate, an important CO2 reservoir, is continuously sent to the Earth's deep interior at subduction zones, forming an essential part of the global carbon cycle. The pros and cons of using calcium isotope compositions to trace marine carbonates recycled into the mantle are discussed in this Perspective.

Quantification of a subsea CO2 release with lab-on-chip sensors measuring benthic gradients

We present a novel approach to detecting and quantifying a subsea release of CO2 from within North Sea sediments, which mimicked a leak from a subsea CO2 reservoir. Autonomous lab-on-chip sensors performed in situ measurements of pH at two heights above the seafloor. During the 11 day experiment the rate of CO2 release was gradually increased. Whenever the currents carried the CO2-enriched water towards the sensors, the sensors measured a decrease in pH, with a strong vertical gradient within a metre of the seafloor. At the highest release rate, a decrease of over 0.6 pH units was observed 17 cm above the seafloor compared to background measurements. The sensor data was combined with hydrodynamic measurements to quantify the amount of CO2 escaping the sediments using an advective mass transport model. On average, we directly detected 43 ± 8% of the released CO2 in the water column. Accounting for the incomplete carbonate equilibration process increases this estimate to up to 61 ± 10%. This technique can provide long-term in situ monitoring of offshore CO2 reservoirs and hence provides a tool to support climate change mitigation activities. It could also be applied to characterising plumes and quantifying other natural or anthropogenic fluxes of dissolved solutes.

Quantification of solubility trapping in natural and engineered CO 2 reservoirs

Secure retention of CO 2 in geological reservoirs is essential for effective storage. Solubility trapping, the dissolution of CO 2 into formation water, is a major sink on geological timescales in natural CO 2 reservoirs. Observations during CO 2 injection, combined with models of CO 2 reservoirs, indicate the immediate onset of solubility trapping. There is uncertainty regarding the evolution of dissolution rates between the observable engineered timescale of years and decades, and the >10 kyr state represented by natural CO 2 reservoirs. A small number of studies have constrained dissolution rates within natural analogues. The studies show that solubility trapping is the principal storage mechanism after structural trapping, removing 10–50% of CO 2 across whole reservoirs. Natural analogues, engineered reservoirs and model studies produce a wide range of estimates on the fraction of CO 2 dissolved and the dissolution rate. Analogue and engineered reservoirs do not show the high fractions of dissolved CO 2 seen in several models. Evidence from natural analogues supports a model of most dissolution occurring during emplacement and migration, before the establishment of a stable gas–water contact. A rapid decline in CO 2 dissolution rate over time suggests that analogue reservoirs are in dissolution equilibrium for most of the CO 2 residence time. Supplementary material: Dissolution rate for all plots and exponential function curves for scenarios A and B are available at https://doi.org/10.6084/m9.figshare.c.5476199 Thematic collection: This article is part of the Geoscience for CO 2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

Investigation of mineral trapping processes based on coherent front propagation theory: A dawsonite-rich natural CO2 reservoir as an example

After nearly 30 years of research on geological carbon storage, studies providing an in-depth conceptual understanding of mineral trapping processes are still scarce. Based on the reaction front propagation theory, this paper developed a set of concise mathematical models for front propagation and mineral trapping in quasi-2D-advection-diffusion-dispersion-reaction-kinetics systems in the coherent state and explored the nature of long-term mineral trapping processes and the effects of hydrodynamic dispersion on the front propagation. A criterion based on dawsonite volume fraction and front propagation distance was established to determine whether the mineral trapping rate is reaction- or transport-controlled for the coherent reaction fronts. Combined with the numerical simulation, the mantle-magmatic-sourced CO2 invasion, and long-term CO2 storage was investigated for the Hailar natural CO2 reservoir. The conclusions include: (1) The relative errors of the concise models for coherent front propagation speed and mineral trapping rate are 3-10% and 0.5-10%, respectively. (2) For the complete reaction stage, mineral trapping rates are always transport-controlled and are proportional to Darcy velocity, the difference of total dissolved carbon concentrations, and the correction coefficient for the hydrodynamic dispersion; when reaction controlling, the mineral trapping rate is proportional to front propagation distance and upstream dawsonite precipitation rate. (3) Specific results related to propagation speed, front width, hydrodynamic dispersion and mineral trapping rate caused by changing the rate constants of minerals and albite volume fraction were also obtained.

Impacts of changes in mangrove ecosystems in the Ba and Rewa deltas, Fiji using multi-temporal Landsat data and social survey

ABSTRACT Despite the expected importance, the changes in mangrove ecosystems and the main causes in Fiji have not been well addressed. To address the issues, we collected data from multiple sources to assess mangrove ecosystem variation due to both natural factors and human impacts in the Ba and Rewa deltas, Fiji. Landsat satellite data were used to map the land use and cover of the study area from 2000 to 2020. Questionnaire surveys were conducted to identify the main uses of mangroves that could influence mangrove ecosystems. Over the period investigated, the mangrove area increased by 572 ha (by 12%) in Ba and decreased by 697 ha (by 9%) in Rewa. The social survey revealed that 45% of respondents in the Ba delta and 20% in the Rewa delta visited the mangrove area daily in search of food resources. The net annual economic loss and lost functioning of mangroves as an anthropogenic CO2 reservoir caused by mangrove degradation in the two deltas were estimated to be 335,000 USD and 202 t-C, respectively. Because local livelihoods are strongly linked with mangroves, the outcomes of this study will provide preliminary information for policy interventions to ensure the sustainability of the mangrove ecosystem.

Effect of permeability and fractures on oil mobilization of unconventional resources during CO2 EOR using nuclear magnetic resonance

CO2 EOR (enhanced oil recovery) will be one of main technologies of enhanced unconventional resources recovery. Understanding effect of permeability and fractures on the oil mobilization of unconventional resources, i.e. tight oil, is crucial during CO2 EOR process. Exposure experiments based on nuclear magnetic resonance (NMR) were used to study the interaction between CO2 and tight oil reservoirs in Chang 8 layer of Ordos Basin at 40 °C and 12 MPa. Effect of permeability and fractures on oil mobilization of exposure experiments were investigated for the different exposure time. The oil was mobilized from matrix to the surface of matrix and the oil recovery increased as the exposure time increased. The final oil recovery increased as the core permeability increased in these exposure experiments. Exposure area increased to 1.75 times by fractures resulting in that oil was mobilized faster in the initial stage of exposure experiment and the final oil recovery increased to 1.19 times from 28.8 to 34.2%. This study shows the quantitative results of effect of permeability and fractures on oil mobilization of unconventional resources during CO2 EOR, which will support CO2 EOR design in Chang 8 layer of Ordos Basin.

Monitoring well integrity at the Cranfield field phase III CO2 storage project

Two purpose-built monitoring wells monitoring wells at the Cranfield Field Phase III CO2 Storage Project were studied in detail using time-lapse well integrity logging, cased-hole sidewall core sampling, and point pressure testing to study the effect of CO2 on the materials used to construct wells for carbon capture, utilization, and storage (CCUS) projects. The wells were constructed using conventional steel casing, fiberglass casing, and conventional well cements. Both wells were heavily outfitted with monitoring equipment in the annulus between the formation and casing. The time-lapse comparison of the well integrity logs showed a general degradation of signal in many places along the well. Laboratory analyses of the sidewall cores showed that CO2 affected the cement in the well at every point that was cored, with significant alteration occurring adjacent to one of the monitoring control lines collected as part of a core. Additionally, the time-lapse logs and sidewall cores showed the fiberglass casing was strongly affected in the CO2 reservoir, causing the inside diameter of the casing to decrease. This study provides insight into how decisions regarding well materials and monitoring technology can impact well integrity over a period as short as 6 years.

Spatial variations in CO2 fluxes in a subtropical coastal reservoir of Southeast China were related to urbanization and land-use types

Carbon dioxide (CO2) emissions from aquatic ecosystems are important components of the global carbon cycle, yet the CO2 emissions from coastal reservoirs, especially in developing countries where urbanization and rapid land use change occur, are still poorly understood. In this study, the spatiotemporal variations in CO2 concentrations and fluxes were investigated in Wenwusha Reservoir located in the southeast coast of China. Overall, the mean CO2 concentration and flux across the whole reservoir were 41.85 ± 2.03 µmol/L and 2.87 ± 0.29 mmol/m2/h, respectively, and the reservoir was a consistent net CO2 source over the entire year. The land use types and urbanization levels in the reservoir catchment significantly affected the input of exogenous carbon to water. The mean CO2 flux was much higher from waters adjacent to the urban land (5.05 ± 0.87 mmol/m2/hr) than other land use types. Sites with larger input of exogenous substance via sewage discharge and upstream runoff were often the hotspots of CO2 emission in the reservoir. Our results suggested that urbanization process, agricultural activities, and large input of exogenous carbon could result in large spatial heterogeneity of CO2 emissions and alter the CO2 biogeochemical cycling in coastal reservoirs. Further studies should characterize the diurnal variations, microbial mechanisms, and impact of meteorological conditions on reservoir CO2 emissions to expand our understanding of the carbon cycle in aquatic ecosystems.

Geochemical reactions and their effect on CO2 storage efficiency during the whole process of CO2 EOR and subsequent storage

CO2-formation water-rock geochemical reactions can occur, affecting the reservoir physical properties and CO2 trapping forms during the CO2 enhanced oil recovery (CO2-EOR) process. In this paper, based on the properties of the reservoir and fluids in a high-temperature oil reservoir, a comprehensive model, including formation water evaporation, dissolution, diffusion of CO2 in water, CO2-water-rock geochemical reactions, and relation of porosity and permeability, is established to simulate the CO2 EOR process and subsequent geological storage. The geochemical reactions and CO2 trapping forms in these two stages are analyzed using this model, and the influences of reservoir heterogeneity and different CO2 injection schemes on EOR and CO2 geological storage are studied. The results show that the geochemical reactions can change reservoir porosity and permeability, but their effect is little, and most of the injected CO2 is trapped as the supercritical gas during the CO2 EOR process. During the stage of CO2 geological storage, the supercritical CO2 continuously converts into mineral form under the action of geochemical reactions, which results in the reservoir pressure decreasing. Whether the reservoir is heterogeneous or homogenous, the water-alternating-gas (WAG) scheme should be the first choice because it can enhance both oil recovery and CO2 storage efficiency in the EOR process. The injection of CO2 after the CO2 EOR process can improve the CO2 storage capacity and simultaneously maintain the reservoir pressure stabilization.