Subsurface Geological Formations Research Articles

Impact of Regional Pressure Dissipation on Carbon Capture and Storage Projects: A Comprehensive Review

Carbon capture and storage (CCS) is a critical technology for mitigating greenhouse gas emissions and combating climate change. CCS involves capturing CO2 emissions from industrial processes and power plants and injecting them deep underground for long-term storage. The success of CCS projects is influenced by various factors, including the regional pressure dissipation effects in subsurface geological formations. The safe and efficient operation of CCS projects depends on maintaining the pressure in the storage formation. Regional pressure dissipation, often resulting from the permeability and geomechanical properties of the storage site, can have significant effects on project integrity. This paper provides a state-of-art of the impact of regional pressure dissipation on CCS projects, highlights its effects, and discusses ongoing investigations in this area based on different case studies. The results corroborate the idea that the Sleipner project has considerable lateral hydraulic connectivity, which is evidenced by pressure increase ranging from <0.1 MPa in case of an uncompartmentalized reservoir to >1 MPa in case of substantial flow barriers. After five years of injection, pore pressures in the water leg of a gas reservoir have increased from 18 MPa to 30 MPa at Salah project, resulting in a 2 cm surface uplift. Furthermore, artificial CO2 injection was simulated numerically for 30 years timespan in the depleted oil reservoir of Jurong, located near the Huangqiao CO2-oil reservoir. The maximum amount of CO2 injected into a single well could reach 5.43 × 106 tons, potentially increasing the formation pressure by up to 9.5 MPa. In conclusion, regional pressure dissipation is a critical factor in the implementation of CCS projects. Its impact can affect project safety, efficiency, and environmental sustainability. Ongoing research and investigations are essential to improve our understanding of this phenomenon and develop strategies to mitigate its effects, ultimately advancing the success of CCS as a climate change mitigation solution.

Identification of Lithology from Well Log Data Using Machine Learning

INTRODUCTION: Reservoir characterisation and geomechanical modelling benefit significantly from diverse machine learning techniques, addressing complexities inherent in subsurface information. Accurate lithology identification is pivotal, furnishing crucial insights into subsurface geological formations. Lithology is pivotal in appraising hydrocarbon accumulation potential and optimising drilling strategies.
 OBJECTIVES: This study employs multiple machine learning models to discern lithology from the well-log data of the Volve Field.
 METHODS: The well log data of the Volve field comprises of 10,220 data points with diverse features influencing the target variable, lithology. The dataset encompasses four primary lithologies—sandstone, limestone, marl, and claystone—constituting a complex subsurface stratum. Lithology identification is framed as a classification problem, and four distinct ML algorithms are deployed to train and assess the models, partitioning the dataset into a 7:3 ratio for training and testing, respectively.
 RESULTS: The resulting confusion matrix indicates a close alignment between predicted and true labels. While all algorithms exhibit favourable performance, the decision tree algorithm demonstrates the highest efficacy, yielding an exceptional overall accuracy of 0.98.
 CONCLUSION: Notably, this model's training spans diverse wells within the same basin, showcasing its capability to predict lithology within intricate strata. Additionally, its robustness positions it as a potential tool for identifying other properties of rock formations.

Reservoir Static Modelling towards Safe CO2 Storage in Depleted Oil Reservoirs of ‘CRK’ Field, Niger Delta, Nigeria

Geological carbon storage (GCS) is gradually gaining acceptance as the technology of highest repute for the mitigation against greenhouse gas effect. This involves the injection and long-term or permanent storage of carbon dioxide (CO2) in subsurface geological formations. The development of a robust 3D reservoir model then becomes necessary to understand the facies changes and the petrophysical properties distribution of candidate CO2 storage reservoirs. This study attempts the use of static modelling technique to investigate the depleted oil and gas reservoirs of ‘CRK’ field in the Niger Delta sedimentary Basin as a potential CO2 storage site. Using an integrated approach of 3D seismic and a suit of well logs from the study area, two reservoir sands (D10C0 and D6200) were mapped. 3D static modelling of discrete and continuous property distribution of the reservoirs revealed the reservoirs are composed majorly of clean sands with water saturation ranging from 8 to 75%. The average porosity and permeability values are between 20 to 29% and 250 to 890mD respectively. A theoretical storage capacity of 24.85Mt is estimated for the two reservoirs combined, while the median effective storage capacity is 6.22Mt. Comparison of the results obtained in the study with recommended standard values show the reservoirs are suitable for CO2 storage. The property models constructed can serve as primary input for dynamic simulation of the oilfield in future studies.

Artificial intelligence-based prediction of hydrogen adsorption in various kerogen types: Implications for underground hydrogen storage and cleaner production

Hydrogen offers significant potential as a sustainable energy source. However, its storage and transportation pose challenges due to its volatility and low density. Subsurface geological formations, such as shale, sandstone, and coal, have been investigated as potential storage sites for hydrogen. Precise estimation of hydrogen adsorption in these formations necessitates a thorough understanding of kerogens, organic-rich sedimentary rocks prevalent in unconventional formations. In this study, we introduce a novel approach employing Extreme Gradient Boosting (XGBoost), Random forest (RF), Light gradient Boosting (LGBM), and Natural gradient boosting (NGBM) algorithms to intelligently predict hydrogen adsorption in various kerogen types. Among these, the NGBM algorithm, utilizing three input features (temperature, pressure, and kerogen types), demonstrated the highest predictive accuracy, achieving an R2 value of 0.989, RMSE of 0.062, and MAE of 0.037 for all data samples. SHAP diagram analysis identified kerogen types as the most influential parameter within the NGBM model. The presented approach has implications beyond energy storage, highlighting the significance of advanced technologies in addressing complex energy challenges. Precise hydrogen adsorption estimation in subsurface formations is vital for developing sustainable energy solutions, and our approach has the potential to expedite progress in this field. Interdisciplinary collaboration among geologists, chemists, and data scientists is crucial for devising innovative solutions for sustainable energy.

A comparative study on transport and interfacial physics of H2/CO2/CH4 interacting with H2O and/or silica by molecular dynamics simulation

Underground H2 storage (UHS), i.e., injecting H2 into subsurface geological formation and its withdrawal when needed, is identified as a promising solution for large-scale and long-term storage of H2. In this study, molecular dynamics (MD) simulation was performed at a typical temperature 320 K with pressure up to 60 MPa to predict H2 transport properties and H2–H2O–rock interfacial properties, which are compared with those of CO2 and CH4. The MD results show that the CH4 profiles of property variations with pressure lie between those of H2 and CO2 and more comparable to CO2. The interaction of H2 with H2O/silica is much weaker than that of CH4 and CO2. It is found that the effect of H2 pressure on altering the water contact angle and interfacial tension is negligible under all conditions. Unlike the multi-adsorption layers of the confined CO2 and CH4, there is only one adsorption layer of H2 confined by silica nano-slit. The planar diffusion of H2 in the confined system is slower than that in the bulk system at pressures lower than 20 MPa. The data and findings of this study will be useful for modeling the multiphase flow dynamics of UHS on reservoir scale, optimizing UHS operation, and assessing the performance of a cushion gas, e.g., CO2 or CH4.

Electrical Resistivity - Tomography Studies In Determining Shallow Aquifer Potential Zones a Case Study in Different Terrains

The study documents the effectiveness of 2 D (Two Dimensional Electrical Resistivity Survey (Tomography) to map shallow subsurface geological formations namely recent Alluvium, Gondwana deposits and hard rock deposits. 2 dimensional Electrical Resistivity Survey (or tomography) was conducted at 2 locations at Valarpuram and one location Madurantakam areas. Gondwana deposits mask Valarpuram Thandalam, villages (at Kancheepuram District, Tamilnadu). To the east of Valarpuram alluvium thickness increases at shallow depths. Weathered and hard granitic gneiss and hard charnockite rocks overlie topsoil in Madurantakam areas (Chengalpettu District, Tamilandu). The survey was conducted to 12 m depth to decipher shallow permeable zones. Gondwana formations consisting of clays, siltstones are predominant in Valarpuram Thandalam and surrounding areas. Hard rock formations are found in Madurantakam areas. 2 Dimensional Resistivity Survey results indicate 2-D sections with very low resistivity values in the range of 2 to 4 ohm-m in Valarpuram Thandalam areas indicating predominant clay deposits with poor ground water potential. To the east of Valarpuram Thandalam resistivity data is of moderately higher value indicating sand deposits at shallow depths. These are ideal locations for dug wells. Low to moderate values at shallow depths indicating weathered thickness up to 12 m are observed at Madurantakam location. From 12m depth steep rise in resistivity values are observed in hard indicating presence of massive rock from 12m depth and below areas. Thus, 2 D Resistivity Imaging Technique are helpful in delineating shallow aquifer potential and this in turn helps in deciding the depth of open wells with high precision.

Transient Pressure Interference during CO2 Injection in Saline Aquifers

Summary CO2 injection in subsurface geological formations (e.g., deep saline aquifers) causes pressure perturbations over a large area surrounding the injection well. Observation wells are widely considered in geologic CO2 storage (GCS) projects where the pressure perturbation induced by CO2 injection is measured. In this work, we use analytical and numerical modeling tools along with field data to examine the pressure behavior in GCS projects before and after CO2 arrival at an observation well. Before CO2 arrival, a baseline pressure trend is established which corresponds to single-phase brine flow across the observation well (approximated by the Theis solution). Therefore, analysis of early time pressure data is straightforward, provides the single-phase flow characteristics (mobility and storativity), and helps in establishing a baseline pressure change that can be extended beyond the single-phase flow period at the observation well. Upon CO2 arrival, a departure from this baseline trend is expected. For the pressure to detect the CO2 arrival at an observation well, the departure from baseline pressure behavior must be significant and well above the background noise levels. We use existing analytical models to determine the strength of the expected pressure departure signal from the baseline trend upon CO2 arrival. The strength of the expected pressure departure is found to be directly proportional to the change in the mobility upon CO2 arrival. Larger change in the flow mobility—compared with single-phase brine mobility—results in a stronger pressure departure signal. In addition, the departure is found to be upward (downward) from the baseline pressure trend when the mobility ratio is less (more) than unity. We present a pressure analysis approach through application to synthetic and field data and show the characteristic pressure behavior before and after CO2 arrival. We show that while generally the pressure can be either above or below the expected baseline pressure trend, it would be likely above the baseline upon CO2 arrival. This is because the mobility ratio becomes less than unity after CO2 arrival. We show that depending on the reservoir characteristics, changes in the pressure trend may or may not be sufficient to detect the CO2 arrival.

Delineation of geological formations and their influence on groundwater in Umunneochi L.G.A of Abia State and Its Environs

For socioeconomic growth and environmental wellbeing of the populace, the availability and sustainable management of groundwater resources is very crucial. Effective management and use of this invaluable resources requires an understanding of the subsurface geological formations and their impact on groundwater. This study was carried out in other to delineate the subsurface geological formations and evaluate their impact on groundwater availability in Umunneochi Local Government Area (L.G.A) of Abia State and its environs using Vertical Electrical Sounding (VES) analysis. The study includes gathering resistivity data via a network of VES surveys carried out at twelve key locations in six selected town within Umunneochi L.G.A. The field data were subsequently interpreted using an iteration software IP2WIN to identify various geological formations and evaluate their potential as aquifers, which substantially impact groundwater occurrence and distribution. The research findings showed distinct geological units with resistivity values ranging from 0.37 - 2000Ωm, which is indicating a variety of lithological properties, with sand and gravel layers showing higher permeability representing potential aquifers. The results of this study have significant implications for groundwater management in Umunneochi L.G.A. and its surroundings, and also provide recommendations for the strategic placement of boreholes and wells to maximize groundwater extraction and long-term use.

Effect of cyclic hysteretic multiphase flow on underground hydrogen storage: A numerical investigation

Hydrogen storage in subsurface geological formation is a complex process requiring information at multiple scales for successful understanding and implementation. While some laboratory investigations and pore-scale studies showed evidence of hydrogen and water cyclic hysteresis, field-scale underground hydrogen storage considering the cyclic hysteretic effect received little or no attention and thus remains poorly understood. This is particularly important due to the fact that an underground hydrogen storage project typically involves repeated cycles of hydrogen injection and withdrawal. In this work, numerical simulations were performed to examine the impact of the cyclic hysteretic effect on the dynamics of hydrogen saturation and the associated efficiency of storage and withdrawal cycles. The results indicate that the cyclic hysteretic effect impedes the injected hydrogen distribution in the formation and results in a greater ultimate hydrogen recovery factor during subsequent withdrawal phases (∼77%). However, the accumulated hydrogen in the formation top will, in turn, increase the formation pressure. The impact of capillary pressure and H2 dissolution in brine were also investigated. The results suggest that capillary pressure and dissolution could alter the hydrogen saturation distribution while depicting a negligible effect on the ultimate hydrogen recovery factor. It was found that the loss of hydrogen due to dissolution can be compensated by avoiding more significant residual trapping in our study.

Residual trapping of CO2, N2, and a CO2-N2 mixture in Indiana limestone using robust NMR coreflooding: Implications for CO2 geological storage

Carbon capture and sequestration (CCS) in geological formations is a prominent solution for reducing anthropogenic carbon emissions and mitigating climate change. The capillary trapping of CO2 is a primary trapping mechanism governed by the pressure difference between the wetting and nonwetting phases in a porous rock, making the latter a key input parameter for dynamic simulation models. During the CCS operational process, however, the CO2 is prone to contamination by impurities from various sources such as surfaces (e.g., pipelines and tanks) and the subsurface (e.g., existing natural gas). Such contamination can strongly influence the overall CO2 wettability, storage capacity, and containment security. Hence, the present study uses the nuclear magnetic resonance (NMR) core flooding technique to investigate and compare the residual saturations of pure CO2, pure N2, and a 50:50 CO2/N2 mixture in an Indiana limestone. The longitudinal and transverse relaxation times (T1 and T2) are measured to examine the displacement process of the pore network, and the trapping mechanism is evaluated at the pore scale as a determinant of the field-scale flow behavior. The NMR T1-T2 and 2D maps are used to observe the fluid configurations in the pore network, and the T1/T2 ratios are used to evaluate the microscopic wettability of the limestone grains by the pore-space fluids following each drainage/imbibition process step. The results indicate substantial residual gas trapping in the rock for the CO2-brine, N2-brine, and CO2/N2-brine systems, corresponding to gas saturations of 25%, 27%, and 26%, respectively. In the CO2-brine system, the intermolecular interplay between the CO2-enriched brine and limestone grains results in a higher T1/T2 ratio and significantly reduces the hydrophilicity of the limestone. Furthermore, the NMR T2 distribution reveals the occurrence of preferential water displacement into the large pores (r > 1 µm) and from the intermediate pores (0.03 µm < r < 1 µm), whereas water remains immobile in the smaller pores (r < 0.03 µm). The insignificant difference in residual trapping saturation between pure CO2 and the CO2-N2 mixture indicates the potential to allow for impurities in the CO2 phase in CCS without reducing the residual trapping capacity. Thus, the present work provides comprehensive information on the impact of gas injection on residual gas trapping in subsurface geological formations at the pore scale, thereby aiding in the development of CCS and other potential applications in enhanced oil recovery (EOR).

Molecular dynamics simulation of interfacial tension of the CO2-CH4-water and H2-CH4-water systems at the temperature of 300 K and 323 K and pressure up to 70 MPa

Subsurface geologic formations such as depleted hydrocarbon reservoirs, deep saline aquifers and shale formations have been considered promising targets for carbon dioxide and hydrogen storage. A solid understanding of the interfacial properties of multiphase systems, including binary (pure gas-water) and ternary (gas mixtures and water), is vital to assess for reliability and storage capacity of the geological formations. However, most previous experimental and simulation studies for interfacial properties have mainly focused on binary systems at low-medium pressure. Only a few experimental and simulation studies investigated the interfacial tension at high pressure (above 20 MPa) for the CO2-CH4-H2O system, and no simulation data are available for the H2-CH4-H2O system. In this study, Molecular dynamics simulations were used to predict the interfacial tension (γ) for both the binary and ternary system at 300 K and 323 K for a wide pressure range (1.0 to 70 MPa). The study was first conducted for the binary systems (H2O-CO2; H2O-CH4 and H2OH2) and then followed by the ternary systems (CO2-CH4-H2O and H2-CH4-H2O). The γ results were also validated with previous studies by comparing them to experimental and simulation data. The findings of this study indicated that γ data of binary and ternary systems decreased with increasing pressure and temperature. However, at high pressure (above 50 MPa), the γ data at 300 K and 323 K showed a plateau or changed very slightly, apparently not depending significantly on temperature. Furthermore, at a fixed pressure, determined γ values for the ternary system (H2-CH4-H2O) are constantly larger than for the CH4-H2O and CO2-CH4-H2O systems. The results provide extending or new γ data in simulation for the binary and ternary systems and contribute to evaluating the stability and long-term viability of various key Carbon Capture and Storage (CCS) and Underground Hydrocarbon Storage (UHS) related processes in support of the large-scale implementation of a hydrogen economy.

Feasibility of source-free DAS logging for next-generation borehole imaging

Characterizing and monitoring geologic formations around a borehole are crucial for energy and environmental applications. However, conventional wireline sonic logging usually cannot be used in high-temperature environments nor is the tool feasible for long-term monitoring. We introduce and evaluate the feasibility of a source-free distributed-acoustic-sensing (DAS) logging method based on borehole DAS ambient noise. Our new logging method provides a next-generation borehole imaging tool. The tool is source free because it uses ever-present ambient noises as sources and does not need a borehole sonic source that cannot be easily re-inserted into a borehole after well completion for time-lapse monitoring. The receivers of our source-free DAS logging tool are fiber optic cables cemented behind casing, enabling logging in harsh, high-temperature environments, and eliminating the receiver repeatability issue of conventional wireline sonic logging for time-lapse monitoring. We analyze a borehole DAS ambient noise dataset to obtain root-mean-squares (RMS) amplitudes and use these amplitudes to infer subsurface elastic properties. We find that the ambient noise RMS amplitudes correlate well with anomalies in conventional logging data. The source-free DAS logging tool can advance our ability to characterize and monitor subsurface geologic formations in an efficient and cost-effective manner, particularly in high-temperature environments such as geothermal reservoirs. Further validation of the source-free DAS logging method using other borehole DAS ambient noise data would enable the new logging tool for wider applications.

Structural analysis of microbiomes from salt caverns used for underground gas storage

Salt caverns are a safe and well-proven reservoir for large-scale natural gas storage and hence, a potential hydrogen storage. Contrary to natural gas, hydrogen is a favorable energy source for many microorganisms. Microorganisms are ubiquitously abundant in the upper lithosphere and therefore expected to be present also in subsurface geological formations potentially selected for H2 storage, such as salt caverns. Thus, future salt cavern hydrogen storage requires monitoring of the cavern microbiome, to evaluate and prevent unwanted microbial activities. In this study, we analyzed the microbiomes of brines sampled from the bottom of five German natural gas storage caverns. All brines were colonized by microorganisms in considerable cell numbers ranging from 2 × 106 cells ml−1 to 7 × 106 cells ml−1. The structures of the microbiomes were characterized by 16S rRNA gene amplicon sequencing. A core community detected in all five studied caverns consists of members affiliated to the Halobacteria, Halanaerobiales and Balneolales. Further, a phylotype belonging to the extremely halophilic, lithoautotrophic and sulfate-reducing genus Desulfovermiculus was found. Examination of microbial activity also included measuring hydrochemical parameters in order to assess the salt concentrations and the availability of nutrients and potential microbial carbon sources or metabolites. NaCl (4.7 M) was the main salt and sulfate (at average 40.8 mM) the main electron acceptor; methanol (up to 37.5 mM) and ethanol (up to 6.9 mM) were of anthropogenic origin and found in higher concentrations. Some putative microbial metabolites were found in lower concentrations (butyrate, ≤0.7 mM; formate, ≤0.08 mM; acetate, ≤0.5 mM; lactate, ≤0.06 mM); their potential relation to microbial activity is discussed. We propose a guideline for sampling and subsequent chemical and molecular biological analysis for future characterization of microbial communities of salt cavern brines.

HESS Opinions: Chemical transport modeling in subsurface hydrological systems – space, time, and the “holy grail” of “upscaling”

Abstract. Extensive efforts over decades have focused on quantifying chemical transport in subsurface geological formations, from microfluidic laboratory cells to aquifer field scales. Outcomes of resulting models have remained largely unsatisfactory, however, largely because domain heterogeneity – characterized for example by porosity, hydraulic conductivity and geochemical properties – is present over multiple length scales, and “unresolved”, practically unmeasurable heterogeneities and preferential pathways arise at virtually every scale. While spatial averaging approaches are effective when considering overall fluid flow, wherein pressure propagation is essentially instantaneous, purely spatial averaging approaches are far less effective for chemical transport essentially because well-mixed conditions do not prevail. We assert here that an explicit accounting of temporal information, under uncertainty, is an additional but fundamental component in an effective modeling formulation. As an outcome, we further assert that “upscaling” of chemical transport equations – in the sense of attempting to develop and apply chemical transport equations at large length scales, based on measurements and model parameter values obtained at significantly smaller length scales – can be considered an unattainable “holy grail”. Rather, we maintain that it is necessary to formulate, calibrate and apply models using measurements at similar scales of interest.

Electrical resistivity tomography and induced polarization study for groundwater exploration in the agricultural development areas of Brunei Darussalam

Electrical resistivity tomography and induced polarization studies were conducted for groundwater exploration in eight selected agricultural sites in Brunei Darussalam. The study was undertaken to meet the growing demands for water supply in the agricultural sector, particularly for paddy field irrigation purposes. This study conducted nineteen surveys with lengths of up to 800 m and a depth of investigation of up to 150 m below ground level. 2D inverted resistivity and chargeability models delineated subsurface geological formations and aquifer zones in the study area. Aquifer zones were detected in all the investigated sites, with resistivity values ranging from 1 to 100 Ω m and chargeability values ranging from 0 to 10 ms. Based on the resistivity and chargeability interpretations, two new groundwater wells were drilled and constructed in two of the investigated sites. Borehole drilling encountered aquifer zones primarily in sand and sandstone layers. Hydraulic tests of the newly drilled boreholes revealed groundwater yields of 4.3 and 288 m3/day. Transmissivity values of the aquifer units estimated using the Cooper–Jacob time-drawdown solutions are 0.53 and 109 m2/day, while their hydraulic conductivity values are 0.05 and 2.75 m/day, respectively. Our findings suggest weak to moderate groundwater yield for withdrawal and distribution for irrigation purposes in the investigated sites. The present study helped decision-makers take suitable measures for placing future groundwater irrigation wells and achieve significant groundwater exploration results in the study area and other areas with similar geological settings.

GEOPHYSICAL POST-FOUNDATION STUDIES OF MINISRTY OF JUSTICE BUILDING, EKITI-STATE SECRETARIAT, ADO-EKITI, EKITI-STATE, NIGERIA

Geophysical investigations were carried out around the ministry of Justice building, Ekiti State Secretariat, Ado-Ekiti, with the aim of investigating the possible cause(s) of failures from the foundation of the building in the government Secretariat. The Electromagnetic and Electrical Resistivity methods were used for the investigation. The Electrical Resistivity method involves Vertical Electrical Sounding (VES) and 2D Electrical Resistivity Tomography using Schlumberger and dipole-dipole configuration. Three (3) VES points were conducted along the traverse within the investigated area with half current electrode separation (AB/2) varying from 2 m to a maximum of 65 m. The electromagnetic measurements were taken at distance separation of 10 m along the traverse line in the SE-NW direction. The VES curve types obtained in the area are H and HKH. Four geoelectric layers were delineated within the investigated area which includes the topsoil, lateritic clay, clay, and partly weathered basement which is a quartzite rock with resistivity values ranging from 47-280 ohm-m, 383-435 ohm-m, 47-78 ohm-m, and 239-4999 ohm-m and the thickness ranges from 1-1.6 m, 2.8-4.2 m, and 9.1-27.9 m respectively. The dipole-dipole results also delineate four geologic formations that revealed the differential settlement within the clayey zone of the building foundations. The electromagnetic method also delineated seven subsurface geologic formations which revealed that most of the materials characterizing the subsurface are of clayey composition. The integration of the results showed that the possible cause(s) of failures in part of the building is due to the presence of clayey formations and geologic fractures within the foundation of the studied area.

CO2 Storage in deep saline aquifers: impacts of fractures on hydrodynamic trapping

Natural or induced fractures are typically present in subsurface geological formations. Therefore, they need to be carefully studied for reliable estimation of the long-term carbon dioxide storage. Instinctively, flow-conductive fractures may undermine storage security as they increase the risk of CO2 leakage if they intersect the CO2 plume. In addition, fractures may act as flow barriers, causing significant pressure gradients over relatively small regions near fractures. Nevertheless, despite their high sensitivities, the impact of fractures on the full-cycle storage process has not been fully quantified and understood. In this study, a numerical model is developed and applied to analyze the role of discrete fractures on the flow and transport mechanism of CO2 plumes in simple and complex fracture geometries. A unified framework is developed to model the essential hydrogeological trapping mechanisms. Importantly, the projection-based embedded discrete fracture model is incorporated into the framework to describe fractures with varying conductivities. Impacts of fracture location, inclination angle, and fracture-matrix permeability ratio are systemically studied for a single fracture system. Moreover, the interplay between viscous and gravity forces in such fractured systems is analyzed. Results indicate that the fracture exhibits differing effects regarding different trapping mechanisms. Generally speaking, highly-conductive fractures facilitate dissolution trapping while weakening residual trapping, and flow barriers can assist dissolution trapping for systems with a relatively low gravity number. The findings from the test cases for single fracture geometries are found applicable to a larger-scale domain with complex fracture networks. This indicates the scalability of the study for field-relevant applications.

(Invited) Electrochemically Modulated Mitigation of Acid Gas Emissions

The alarming rise in anthropogenic CO2 levels in the atmosphere due primarily to fossil fuel emissions is leading to significant changes in global climate patterns and increasing ocean acidities, with their severe negative impacts on the health of our planet. The mitigation of these acid gas emissions is a daunting task, both because of the scale of the problem and because of the economic ramifications associated with the capture of the greenhouse gases and their subsequent utilization or subsurface storage. The traditional means for CO2 capture and release generally rely on either chemical or physical interactions with sorbents with subsequent temperature or pressure changes to release the captured CO2 and regenerate the sorbent. The captured CO2 can then be compressed for injection and sequestration in subsurface geological formations or converted to useful products. These processes require significant energy integration which adds complexity and cost to the overall capture operation. Isothermal operations that obviate or significantly reduce the heat integration requirements in these processes could have significant advantages over the traditional methods. One such approach is to exploit electrochemical technologies for the capture and release of CO2, which can be operated isothermally, and can rely only on renewable energy resources, if desired.The general principles underlying electrochemically modulated acid gas separation processes will be outlined, with an emphasis on the thermodynamic and transport considerations required for their effective implementation in carbon capture. In particular, we will consider electrochemical technologies for the capture of CO2 that are suitable for both point-of-use gas emissions mitigation, and for removal from the atmosphere and ocean waters, where CO2 has been accumulating for decades. Examples include an indirect approach, in which an electrochemically released species (e.g., copper ions, protons) displaces the CO2 bound to a chemical sorbent via competitive complexation; the CO2 and the sorbents are regenerated when the species is re-captured in the cathode chamber of an electrochemical cell, leaving the sorbent free to be cycled back to the absorber. An alternative approach exploits the complexation of an electroactive moiety directly with the CO2 upon activation by electrochemical reduction, and releases the CO2 when it is re-oxidized on reversal of the applied cell voltage. In general, these electrochemically based technologies hold promise for tackling the climate change challenges that we, and future generations, must face.

Joint geophysical prospecting for groundwater exploration in weathered terrains of South Guangdong, China.

Groundwater occurrence in a hard rock terrain is strongly controlled by the weathered/fractured zones. However, delineation of such zones is a challenging task given their structural heterogeneity. Traditionally, large numbers of well tests are conducted to assess the subsurface formation. But, such tests suffer from efficiency in terms of cost, time, and data coverage. Non-invasive geophysical methods can be the best alternative of expensive drilling methods. However, a geophysical method alone is ambiguous to interpret the highly heterogeneous subsurface formation. In this study, joint application of electrical resistivity tomography (ERT), magnetic method, and joint profile method (JPM) was conducted for groundwater exploitation in a weathered terrain of South Guangdong, China. ERT, magnetic, and JPM data were acquired along different geophysical profiles via a variety of survey parameters. The interpreted 2D models of electrical resistivity and magnetic data coupled with the local accessible boreholes and hydrogeological information constrain the subsurface geologic formation into four discrete layers with specific electrical resistivity range, i.e., topsoil cover, highly weathered saturated layer, semi-weathered saturated layer, and un-weathered substratum. Incorporation of JPM (ER, SP, and IP methods) with ERT and magnetic models reveal three faults (F1, F2, and F3) and several saturated intense fractures/discontinuities. The groundwater reserves associated with the weathered/fractured rock were estimated via hydraulic parameters, namely hydraulic conductivity and transmissivity. The results suggest that high-yield groundwater resources are found within the weathered/fractured zones. Geophysical results of this joint application fit pretty well to the local hydrogeological data of the study area. Our novel approach reduces any ambiguity caused in the geophysical interpretation and provides clearer insight of the subsurface formation with more confident solutions to the most challenging problems of the hard rock sites. This hydrogeophysical study provides important contributions to groundwater exploration in areas where weathering has significant effects on the hard rock aquifer system. Compared with traditional methods, this approach is more advantageous for assessment of groundwater resources in hard rock terrains.

Effects of salinity and shear stress on clay deformation: A molecular dynamics study

The deformation of clay minerals is an important phenomenon that is relevant to many problems, particularly those that occur in subsurface geological formations. The salinity of the formations and external shear stress applied to them are two important factors that contribute to the deformation of such porous media. To gain a deeper understanding of such phenomena, we have carried out extensive molecular dynamics simulations using the Na-montmorillonite (Na-MMT) structure as the model of clay minerals and have studied the effect of salt concentration on its swelling. As the NaCl concentration increases, so also does the basal spacing. We demonstrate the effect of the coupling between the applied shear stress and NaCl salinity on the swelling behavior of Na-MMT, namely, deformation of the interlayer space that results in swelling. According to the results, the extent of Na-MMT deformation depends on both the brine salinity and the shear rate.