CO2 Dissolution Trapping Research Articles

Density-Driven CO2 Dissolution in Depleted Gas Reservoirs with Bottom Aquifers

Depleted gas reservoirs with bottom water show significant potential for long-term CO2 storage. The residual gas influences mass-transfer dynamics, further affecting CO2 dissolution and convection in porous media. In this study, we conducted a series of numerical simulations to explore how residual-gas mixtures impact CO2 dissolution trapping. Moreover, we analyzed the CO2 dissolution rate at various stages and delineated the initiation and decline of convection in relation to gas composition, thereby quantifying the influence of residual-gas mixtures. The findings elucidate that the temporal evolution of the Sherwood number observed in the synthetic model incorporating CTZ closely parallels that of the single-phase model, but the order of magnitude is markedly higher. The introduction of CTZ serves to augment gravity-induced convection and expedites the dissolution of CO2, whereas the presence of residual-gas mixtures exerts a deleterious impact on mass transfer. The escalation of residual gas content concomitantly diminishes the partial pressure and solubility of CO2. Consequently, there is an alleviation of the concentration and density differentials between saturated water and fresh water, resulting in the attenuation of the driving force governing CO2 diffusion and convection. This leads to a substantial reduction in the rate of CO2 dissolution, primarily governed by gravity-induced fingering, thereby manifesting as a delay in the onset and decay time of convection, accompanied by a pronounced decrement in the maximum Sherwood number. In the field-scale simulation, the injected CO2 improves the reservoir pressure, further pushing more gas to the producers. However, due to the presence of CH4 in the post-injection process, the capacity for CO2 dissolution is reduced.

Micro-scale experimental investigations of CO2-WAG injection and Ostwald ripening analysis in carbonate rocks with different pore structures

Carbonate reservoirs are rich in hydrocarbon resources. However, carbonate reservoirs usually have strong pore structure heterogeneity, and thus it is necessary to understand the effect of heterogeneity on pore-scale fluid distributions during CO2 enhanced oil recover (CO2-EOR) process. We thus conducted water-alternating-gas (CO2-WAG) injection in the three carbonate samples with strong to weak heterogeneity. The 15 sets of images showed that the fractured-vuggy carbonate with the strongest heterogeneity had the highest additional oil recovery. CO2-WAG injection significantly changed the residual oil distribution, and further oil production can be achieved by increasing the number of CO2-WAG cycles. For CO2 geological storage, with the increase of carbonate heterogeneity, CO2 capillary-trapping capacity decreased, CO2 dissolution trapping potentiality decreased, but the stability of CO2 capillary trapping enhanced. These key parameters related to CO2 storage were described through the saturation of trapped gas, the types of trapped gas, and the surface area-volume relationship of CO2 clusters. For example, the higher amount of network gas is detrimental to the stability of CO2 capillary trapping because the network gas is easily displaced when the hydrodynamic force is enhanced. The specific surface area of CO2 cluster determines dissolution dynamics, and a higher specific surface area is beneficial for CO2 dissolution. Another interesting phenomenon - Ostwald ripening, is related to long-term CO2 transport. The CO2 concentration gradient in aquifer generates a vertically upward diffusive flux, causing CO2 remobilization. By calculating the height of the transition zone and the time to reach gravity-capillary pressure equilibrium, we found that as carbonate heterogeneity increased, the height of transition zone (the area where CO2 is trapped by capillary pressure) increased, while equilibrium time shortened. By comparing the results of previous gas-water two-phase flow experiments, we believe that the relationship between CO2 storage characteristics and carbonate heterogeneity summarized in this paper are universally applicable.

Simulation of CO2 dissolution reactions in saline aquifers using lattice Boltzmann method

CO2 dissolution reactions in saline aquifers during geologic carbon sequestration (GCS) involve multiple physicochemical processes and pose a challenge for both experimental and numerical techniques to capture reactive transport features and complex porous structures. Benefited from advanced models in the lattice Boltzmann (LB) community, namely, multiphase flow LB model and multicomponent transport LB model with chemical reactions, dynamics of CO2-water interfaces, concentration evolution of ions, and heterogeneous and homogeneous reactions could be readily captured by the proposed LB model to characterize CO2 dissolution reactions applicable to far-field applications. This LB model can not only emphasize CO2 dissolution processes at the scale of a few pores but also elaborate on the corresponding controlling mechanisms in porous media during GCS in saline aquifers. LB simulations herein indicate that both diffusion coefficient and reactive interfacial length may change the total concentration gradient at the CO2-water interface to further affect CO2 dissolution reactions. It is also confirmed that CO2 dissolution is a non-equilibrium process, which could not be neglected in reservoir-scale simulations. The effect of reactive interfacial length on CO2 dissolution is negligible due to a narrower range of contact angles in saline aquifers. Influenced by the structural complexity of porous media, the dissolution of CO2 clusters shows a strong heterogeneous characteristic evaluated by the pore size distribution. The evolution of pH in homogeneous reactions is estimated via mass action equations. Besides, the effects of partial pressure, formation temperature, and salinity, as three key factors in the practical application, are thoroughly probed. The high pressure is conducive for sequestration because of its greater sequestration efficiency, but the temperature has both positive and negative impacts on CO2 dissolution trapping and should be carefully evaluated in a certain saline aquifer before the operation. High ion strength would decrease the efficiency of dissolution trapping to some extent.

Advanced modeling of enhanced CO2 dissolution trapping in saline aquifers

A consistent thermodynamic model based on a combination of the Peng-Robinson equation of state for gas components with an activity model for the aqueous phase is utilized in this study to accurately evaluate the performance of geologic CO2 storage in saline aquifers. To account for thermal effects, the phase enthalpy and conductivity are derived with consideration of CO2 dissolution in brines. Several conceptual numerical setups with variations in parameters and physics are constructed to investigate the enhanced CO2 dissolution in saline aquifers. In addition, a realistic geological structure is considered to evaluate the potential for storing CO2 at the field scale. Our simulation results show that the presence of a capillary transition zone (CTZ) can significantly reduce the onset time and enhance the dissolution rate. The temperature has a crucial effect on the dissolution trapping compared to isothermal assumptions. Thermal conduction and convection suspend the onset of mass convection, while the final mass flux is augmented. Heat conduction smears out the temperature difference between rocks and fluids, thus suppressing thermal fingering. We demonstrate that the dissolution trapping in realistic 3D aquifers can be significantly different from their 2D analogues. The field-scale simulation results show that the dissolution rate increases sharply once the convection dominates the CO2 migration which corresponds to a reduction of residual trapping. Besides, a high-resolution 3D model is required for accurately resolving the enhanced CO2 dissolution in realistic heterogeneous reservoirs. An efficient multi-scale framework for an accurate evaluation of CO2 trapping is proposed for this purpose.

Experimental study of impurity effects on convective mixing in Hele-Shaw cell with application to CO2 geological sequestration

Density-driven convective mixing, i.e., convective fingering, has been identified to enhance CO2 dissolution trapping in deep saline aquifers. In this study, a series of experiments in Hele-Shaw cells were conducted, and the effects of common impurities (N2, O2, and/or Ar) on the occurrence and development of the convective mixing process were compared and analyzed. It is demonstrated that the existence of these impurities, whether single or their mixture, delayed the onset of convection. However, the subsequent fingering development, including the mixing length and fingering footprint, exhibited different patterns and trends depending on the species and compositions of the impurities. The development and growth of the convective fingers displayed non-monotonic characteristics as a function of the concentration of single impurity Ar. Furthermore, the multiple impurities containing Ar had the potential to enhance the fingering development, especially during the advanced stage. Other than these cases involving Ar, the inclusion of other impurities (N2, and/or O2) was found to weaken the fingering development compared with the pure CO2 case. Nevertheless, the results of dissolved CO2 inventory suggested that the effects of different impurities at the same content, both single and multiple, might be similar to each other in the long term, except for the three-impurity case involving Ar. The laboratory experiments in the present study can help establishing a better understanding of the effects of multiple impurities in the application of impure CO2 geological sequestration.

Pore-scale study of capillary and dissolution trapping of CO2 in saline aquifers

Employing two-phase flow and species mass transfer simulations in natural rock samples, we establish a pore-scale description of CO2 migration pattern and its residual and dissolution trapping mechanisms in saline aquifers. Our results imply that wettability and fine-scale heterogeneity are key factors controlling the sweep efficiency of CO2 during the injection period. Furthermore, we elucidate how fractional-wettability controls the CO2 entrapment pattern and spatial distribution of residual CO2 ganglia during the post-injection period. It is found that the accumulation of CO2 ganglia and, as a result, their residual trapping, increases when the rock sample contains a larger portion of CO2-wet regions. We further characterize the physics of CO2 dissolution trapping occurring during the injection stage. Our study captures the underlying details of fine-scale characteristics that govern the dissolution behavior, such as the pore-scale distribution of species concentration, the evolution of phase distribution, CO2-brine interfacial area, and total mass of dissolved CO2 in brine. Finally, CO2 dissolution is quantified under different wettability conditions.

Change in Convection Mixing Properties with Salinity and Temperature: CO2 Storage Application.

In this study, we visualised CO2-brine, density-driven convection in a Hele-Shaw cell. Several experiments were conducted to analyse the effects of the salinity and temperature. The salinity and temperature of fluids were selected according to the storage site. By using charge coupled device (CCD) technology, convection finger formation and development were obtained through direct imaging and processing. The process can be divided into three stages: diffusion-dominated, convection-dominated and shutdown stages. Fingers were formed along the boundary at the onset time, reflecting the startup of convection mixing. Fingers formed, moved and aggregated with adjacent fingers during the convection-dominated stage. The relative migration of brine-saturated CO2 and brine enhanced the mass transfer. The effects of salinity and temperature on finger formation, number, and migration were analysed. Increasing the salinity accelerated finger formation but suppressed finger movement, and the onset time was inversely related to the salinity. However, the effect of temperature on convection is complex. The dissolved CO2 mass was investigated by calculating the CO2 mass fraction in brine during convection mixing. The results show that convection mixing greatly enhanced mass transfer. The study has implications for predicting the CO2 dissolution trapping time and accumulation for the geological storage of CO2.

Experimental investigation of assessment of the contribution of heterogeneous semi-confining shale layers on mixing and trapping of dissolved CO2 in deep geologic formations

In most conceptual models of dissolution trapping of CO2, it is assumed that mixing of dissolved supercritical CO2 and formation brine occurs through density-driven convection. In our previous modeling study, we showed that the presence of continuous low-permeability shale layers in the formations causes convective shutdown through disruption of fingers, which impacts the effectiveness of mixing and trapping processes. However, these layers are naturally heterogeneous due to variations in compositional and textural properties. In the present study, we investigate the potential effects of heterogeneity present within semi-confining low-permeability layers on the overall mixing and trapping of dissolved CO2. Since accurate field experimentation in deep geologic formations is difficult due to inability to adequately characterize geology and define boundary conditions of the formation, we designed well-controlled experiments using NaBr solution and water under laboratory conditions. We conducted intermediate-scale 3D laboratory experiments under two homogeneous and one heterogeneous multilayered sand packing configurations. The results of these experiments show that connectivity of relatively higher permeability material within the semi-confining low-permeability layers contributes to mixing through (1) brine leakage between upper and lower aquifers (reverse convection), and (2) trapping through diffusion and back diffusion of initially trapped mass due to reversed concentration gradients in the long term. These findings suggest that when estimating the effectiveness of dissolution trapping in CO2 sequestration one needs to consider possible deviations from the traditional convective mixing theory in homogeneous media. Under some conditions of natural formation heterogeneity, diffusion can contribute to trapping; in some others, it may not.

Study of convective-diffusive flow during CO2 sequestration in fractured heterogeneous saline aquifers

CO2 sequestration in naturally fractured saline aquifers is important to be studied. This paper investigates the convective-diffusive flow to study CO2 sequestration by dissolution in single and multi-fractured heterogeneous saline aquifers. The effect of fractures density, orientation, permeability, aperture and surface roughness on CO2 dissolution and propagation of density-driven fingers are investigated. The results indicate that a single fracture causes the growing density fingers to deviate towards the high permeability path of the fracture. Additionally, the higher vertical fracture orientation, aperture and permeability will lead to a higher CO2 storage. In the multiple fracture systems, matrix permeability has a dominant and positive effect on CO2 storage. Nevertheless, the effect of fractures properties such as fracture density, spacing, permeability, aperture and surface roughness on the CO2 storage are dependent on the fractures orientation. For instance, having fractures with rough surface can cause CO2 dissolution retardation in the aquifers with only vertical fractures. This study provides more insights into dissolution trapping of CO2 in fractured heterogeneous saline aquifers.

The effect of density difference on the development of density-driven convection under large Rayleigh number

In this study, we visualized density-driven convection in an improved Hele-Shaw cell at 25 °C and 0.1 MPa by examining the convection mixing process of analogous fluids and total dissolution rates in the high Rayleigh number (Ra) region. The high Ra number was inspired by data referring to the geological reservoir at the Sleipner site. We investigated the effect of the density difference between different fluid pairs on the initial moment and development of convective mixing. Though direct imaging and processing, we examined the convective mixing development regime, finger number density, onset time and movement speed of fingers. The fingering instability and mixing time of miscible fluids were also analysed and show that as the density difference increases, the density of the finger number increases and the fingers quickly merge. The onset time decreases sharply and then becomes stable and is inversely proportional to Ra2. The convection mixing time has the same tendency but is inversely proportional to Ra. Moreover, the finger speed linearly increases and is directly proportional to the characteristic velocity. The dissolution rate was investigated with the dimensionless convection flux Sherwood number (Sh) and shows a power law relationship with Ra. The density difference accelerates the dissolution rate. The experimental conditions were different from those of real CO2 geological storage, but this study has implications for predicting the CO2 dissolution trapping time and accumulation for geological storage of carbon dioxide.

Analysis of evolving capillary transition, gravitational fingering, and dissolution trapping of CO2 in deep saline aquifers during continuous injection of supercritical CO2

Dissolution trapping of supercritical CO2 (ScCO2) in deep saline aquifers undergoes different stages such as ScCO2 mixing with brine, buoyant convection and slumping of CO2 rich brine. The effects of permeability and capillarity on these stages were previously studied for (i) uniformly injected ScCO2 at the top of the aquifer and (ii) spreading from a finite initial patch of CO2 or after short injection duration of CO2. In this paper we discuss the results of continuous injection of CO2 from a well that is screened inside the aquifer between caprock at the top and bedrock at the bottom. For this case, the injection pressure significantly influences the spreading of CO2 plumes. The injection pressure decreases with time for constant mass injection flux of ScCO2 (m˙). Injected ScCO2 flows upward and spreads laterally below the caprock. An evolving capillary transition zone is one of the main features of this study. Below the capillary transition zone, the diffusive boundary layer of dissolved CO2 becomes unstable depending on the injection pressure (PScCO2), permeability (k) and capillary entry pressure (Pc0). The evolution of ScCO2 and dissolved CO2 plume and injection pressure history are directly proportional to m˙/k. The evolution rate and the onset time of gravitational instability are inversely proportional to k. For very small and large m˙/k ratio, gravitational instability may not occur. Higher permeability yields more effective CO2 dissolution trapping and dissolution flux increases after onset of convection. For continuous injection case, the plume tip position varies as the power law of time tn where 0.6 < n < 0.85 where t is in years and plume tip position is in meter.

Pore scale study of multiphase multicomponent reactive transport during CO2 dissolution trapping

Abstract Solubility trapping is crucial for permanent CO2 sequestration in deep saline aquifers. For the first time, a pore-scale numerical method is developed to investigate coupled scCO2-water two-phase flow, multicomponent (CO2(aq), H+, HCO3−, CO32− and OH−) mass transport, heterogeneous interfacial dissolution reaction, and homogeneous dissociation reactions. Pore-scale details of evolutions of multiphase distributions and concentration fields are presented and discussed. Time evolutions of several variables including averaged CO2(aq) concentration, scCO2 saturation, and pH value are analyzed. Specific interfacial length, an important variable which cannot be determined but is required by continuum models, is investigated in detail. Mass transport coefficient or efficient dissolution rate is also evaluated. The pore-scale results show strong non-equilibrium characteristics during solubility trapping due to non-uniform distributions of multiphase as well as slow mass transport process. Complicated coupling mechanisms between multiphase flow, mass transport and chemical reactions are also revealed. Finally, effects of wettability are also studied. The pore-scale studies provide deep understanding of non-linear non-equilibrium multiple physicochemical processes during CO2 solubility trapping processes, and also allow to quantitatively predict some important empirical relationships, such as saturation-interfacial surface area, for continuum models.

Pore scale study of multiphase multicomponent reactive transport during CO2 dissolution trapping

Solubility trapping is crucial for permanent CO2 sequestration in deep saline aquifers. For the first time, a pore-scale numerical method is developed to investigate coupled scCO2-water two-phase flow, multicomponent (CO2(aq), H+, HCO3−, CO32− and OH−) mass transport, heterogeneous interfacial dissolution reaction, and homogeneous dissociation reactions. Pore-scale details of evolutions of multiphase distributions and concentration fields are presented and discussed. Time evolutions of several variables including averaged CO2(aq) concentration, scCO2 saturation, and pH value are analyzed. Specific interfacial length, an important variable which cannot be determined but is required by continuum models, is investigated in detail. Mass transport coefficient or efficient dissolution rate is also evaluated. The pore-scale results show strong non-equilibrium characteristics during solubility trapping due to non-uniform distributions of multiphase as well as slow mass transport process. Complicated coupling mechanisms between multiphase flow, mass transport and chemical reactions are also revealed. Finally, effects of wettability are also studied. The pore-scale studies provide deep understanding of non-linear non-equilibrium multiple physicochemical processes during CO2 solubility trapping processes, and also allow to quantitatively predict some important empirical relationships, such as saturation-interfacial surface area, for continuum models.

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.

Effective constitutive relations for simulating CO2 capillary trapping in heterogeneous reservoirs with fluvial sedimentary architecture

Carbon dioxide (CO2) storage reservoirs commonly exhibit sedimentary architecture that reflects fluvial deposition. The heterogeneity in petrophysical properties arising from this architecture influences the dynamics of injected CO2. We previously used a geocellular modeling approach to represent this heterogeneity, including heterogeneity in constitutive saturation relationships. The dynamics of CO2 plumes in fluvial reservoirs were investigated during and after injection. It was shown that small-scale (centimeter–meter) features play a critical role in capillary trapping processes and have a primary effect on physical- and dissolution-trapping of CO2, and on the ultimate distribution of CO2 in the reservoir. Heterogeneity in saturation functions at that small scale enhances capillary trapping (snap-off), creates capillary pinning, and increases the surface area of the plume. The understanding of these small-scale trapping processes from previous work is used to develop effective saturation relationships that represent, at a larger scale, the integral effect of these processes. Though it is generally not computationally feasible to represent small-scale heterogeneity directly in a typical reservoir simulation, the effective saturation relationships for capillary pressure and relative permeability presented here, along with an effective intrinsic permeability, allow better representation of the total physical trapping at the scale of larger model grid cells, as typically used in reservoir simulations. Thus, the approach diminishes limits on cell size and decreases simulation time in reservoir simulations.

A field-scale investigation of residual and dissolution trapping of CO2 in a saline formation in Western Australia

In a typical carbon capture and storage project, residual and dissolution trapping is expected to play a significant role in minimising the risk associated with the containment. Various methods of maximising each trapping mechanism individually have been proposed in the literature. Yet, in actual projects, it is expected that these trapping mechanisms occur interactively. This area still remains unexplored.This paper presents the results of a numerical investigation of technically feasible engineering designs to co-optimise both trapping mechanisms. It integrates a geological model developed for an actual saline formation in Australia, along with other available reservoir engineering data using a compositional numerical reservoir simulator. The scenario involves injecting supercritical CO2 at a commercial rate into a thick, deep, layered, heterogeneous, high porosity, high-permeability sandstone formation that initially contains only water of high salinity. The design objective is to inject more than 200 million tonnes of CO2 over 40 years. Compositional interactions between formation water and injected CO2 are modelled using the Peng–Robinson equation of state. We examine different injection schemes to find out the scenario that maximises dissolution and residual trapping. These schemes include vertical wells, horizontal wells, water alternate gas injection (WAG) and simultaneous water and gas injection (SWAG).The results show that heterogeneity has a significant effect on the location of perforations. The middle region of the Wonnerup Member appears to be the optimum location for CO2 injection where desired amount of CO2 can be injected while a maximum benefit of residual trapping can be achieved. Horizontal wells appear to enhance residual trapping the most, whereas WAG enhances dissolution trapping the most. Furthermore, WAG scenarios result in the least mobile CO2, and therefore, show the highest cumulative potential for both residual and dissolution trapping. However, in terms of relative economics, vertical injection wells appear to be the most viable option.

Chemical and Hydrodynamic Mechanisms for Long-Term Geological Carbon Storage

Geological storage of CO2 (GCS), also referred to as carbon sequestration, is a critical component for decreasing anthropogenic CO2 atmospheric emissions. Stored CO2 will exist as a supercritical phase, most likely in deep, saline, sedimentary reservoirs. Research at the Center for Frontiers of Subsurface Energy Security (CFSES), a Department of Energy, Energy Frontier Research Center, provides insights into the storage process. The integration of pore-scale experiments, molecular dynamics simulations, and study of natural analogue sites has enabled understanding of the efficacy of capillary, solubility, and dissolution trapping of CO2 for GCS. Molecular dynamics simulations provide insight on relative wetting of supercritical CO2 and brine hydrophilic and hydrophobic basal surfaces of kaolinite. Column experiments of successive supercritical CO2/brine flooding with high-resolution X-ray computed tomography imaging show a greater than 10% difference of residual trapping of CO2 in hydrophobic media compare...

Ranking Importance of Uncertainties for the Assessment of Residual and Dissolution Trapping of CO2 on a Large-scale Storage Site

Abstract We propose an approach to assess the importance ranking of uncertainty sources, with regards to the behavior of the mobile CO 2 during the post-injection period: a variance-based global sensitivity analysis is performed on three output parameters characterizing the location and the quantity of mobile CO 2 , considering residual and dissolution trapping. The use of advanced meta-modeling techniques of ACOSSO-type allows circumventing two major difficulties: 1. The large number of computationally intensive reservoir-scale flow simulations; 2. The different nature of uncertainties whether linked to parameters (continuous variables) or to modeling assumptions (scenario-like variables). The feasibility of the approach is demonstrated using a potential site for CO 2 storage in the Paris basin (France), for which the amount, nature and quality of the data available at disposal and the associated uncertainties can be seen as representative to those of a storage project at the post-screening stage. A special attention has been paid to confront the results of the sensitivity analysis with the physical interpretation of the processes.

Combination of CO2 geological storage with deep saline water recovery in western China: Insights from numerical analyses

CO2 geological storage, when combined with deep saline water recovery (CO2-EWR), not only achieves the relatively secure storage of CO2 that was captured from the coal chemical industry, due to lower pressure, but also enhances saline water for drinking and industrial or agricultural utilization. This storage will undoubtedly become a win–win choice for the enhancement of energy security and for the promotion of regional development in China, particularly for western regions with a relative shortage of water resources and a more fragile ecological environment. In this paper, a three-dimensional injection–extraction model is established that uses the TOUGH2/ECO2N program according to typical formation parameters of a coal chemical industry in the Xinjiang Uyghur Autonomous Region. Numerical results showed that under the guarantee of sufficient water conditions, 1.73×108tons of saline water could be produced when the CO2-EWR is adopted. Well arrangements and formation parameters are also analyzed, and the following conclusions can be drawn: arrangements of pumping wells, such as pumping well number, pumping rate and distance, have considerable influences on the reservoir pressure, and in addition, the sensitivity of pressure on the distance and pumping rate decreases as their values increase. In view of these features, it is necessary to find an optimal point to achieve the best combination of pressure, the leakage time and the amount of dissolution. Formation parameters primarily control the mechanism of CO2 migration and dissolution. Salinity in the salt water has the greatest impact on CO2 dissolution trapping followed by permeability and porosity. The arrival time that is allowable for saline water production primarily depends on porosity followed by the permeability ratio and the arrangements of pumping wells. The reservoir pressure change that is caused by parameters is not obvious compared with setting pumping wells. Overall, CO2-EWR technology is a potential strategic choice for China, particularly in western regions. Additionally, the analysis results provide a reliable guide and reference for CO2 storage site selection, as well as the practical arrangements of wells.

Design of a two-well field test to determine in situ residual and dissolution trapping of CO2 applied to the Heletz CO2 injection site

Field testing is a critical step to improve our knowledge on in situ-trapping mechanisms of CO2 injected in geological formations and their relative importance. In this study, we present a two-well test sequence aimed at quantifying field values of both residual and dissolution trapping of CO2. Then, we apply it to the Heletz experimental CO2 injection site, using numerical modelling. The sequence includes a hydraulic test to measure residual scCO2 saturation and a novel tracer technique, together with measurements of abstracted fluid compositions for quantification of the rate of CO2 dissolution in the reservoir. The proposed tracer technique uses a tracer with negligible aqueous solubility, which is injected with the scCO2 and enriched in the scCO2 phase as CO2 dissolves. We show that this tracer can provide direct information about the dissolution of mobile scCO2. We also show that the rate of abstracted dissolved CO2 can be used to predict the total rate of CO2 dissolution, provided that the amount of dissolved CO2 in the formation stabilizes, and that this can be achieved with the proposed abstraction scheme. We conclude that the combination of these measurements is a promising tool for detailed field-scale characterization of residual and dissolution trapping processes.