Sequestration Reservoir Research Articles

Transition of CO2 from Emissions to Sequestration During Chemical Weathering of Ultramafic and Mafic Mine Tailings

Weather-enhanced sulphide oxidation accelerates CO2 release into the atmosphere. However, over extended geological timescales, ultramafic and mafic magmatic minerals may transition from being sources of CO2 emissions to reservoirs for carbon sequestration. Ultramafic and mafic mine tailings present a unique opportunity to monitor carbon balance processes, as mine waste undergoes instantaneous and rapid chemical weathering, which shortens the duration between CO2 release and absorption. In this study, we analysed 30 vanadium-titanium magnetite mine tailings ponds with varying closure times in the Panxi region of China, where ~60 years of mineral excavation and dressing have produced significant outcrops of mega-mine waste. Our analysis of anions, cations, saturation simulations, and 87Sr/86Sr; δ13C and δ34S isotopic fingerprints from mine tailings filtrates reveals that the dissolution load of mine tailings may depend significantly on early-stage sulphide oxidation. Despite the abundance of ultramafic and mafic minerals in tailings, dolomite dominates chemical weathering, accounting for ~79.2% of the cationic load. Additionally, due to sulphuric-carbonate weathering, the filtrates undergo deacidification along with sulphide depletion. The data in this study suggest that pristine V-Ti-Fe tailings ponds undergo CO2 emissions in the first two years but subsequently begin to absorb atmospheric CO2 along with the filtrates. Our results provide valuable insights into monitoring weathering transitions and carbon balance in ultramafic and mafic rocks.

Hypothetical CO2 leakage into, and hydrological plume management within, an underground source of drinking water at a proposed CO2 storage facility, Kemper County, Mississippi, USA

A large Geologic Carbon Sequestration (GCS) hub has been proposed in Kemper County, Mississippi. The target injection interval consists of numerous Cretaceous-aged deep saline aquifers overlain by a competent and extensive regional sealing layer. Above the seal, the deepest Underground Source of Drinking Water (USDW) at the site is the Eutaw aquifer of the Eutaw Group and McShan Formation, undifferentiated. To assess potential risks of leakage from the deep sequestration reservoir, a model of a portion of the Cretaceous Eutaw Group was constructed in this study. Simulations tested various permeabilities, hypothetical leakage rates, and plume mitigation strategies utilizing existing wells. Results suggest that, under the influence of regional groundwater flow fields, leaking CO2 would effectively bypass the existing wells, and to influence this migration would require very large water extraction rates. Therefore, to ensure plume detection, monitoring for leakage at the injection wells themselves is very important.

Experimental Evaluation of a Recrosslinkable CO2-Resistant Micro-Sized Preformed Particle Gel for CO2 Sweep Efficiency Improvement in Reservoirs with Super-K Channels.

A recrosslinkable CO2-resistant branched preformed particle gel (CO2-BRPPG) was developed for controlling CO2 injection conformance, particularly in reservoirs with super-permeable channels. Previous work focused on a millimeter-sized CO2-BRPPG in open fractures, but its performance in high-permeability channels with pore throat networks remained unexplored. This study used a sandpack model to evaluate a micro-sized CO2-BRPPG under varying conditions of salinity, gel concentration, and pH. At ambient conditions, the equilibrium swelling ratio (ESR) of the gel reached 76 times its original size. This ratio decreased with increasing salinity but remained stable at low pH values, demonstrating the gel's resilience in acidic environments. Rheological tests revealed shear-thinning behavior, with gel strength improving as salinity increased (the storage modulus rose from 113 Pa in 1% NaCl to 145 Pa in 10% NaCl). Injectivity tests showed that lower gel concentrations reduced the injection pressure, offering flexibility in deep injection treatments. Gels with higher swelling ratios had lower injection pressures due to increased strength and reduced deformability. The gel maintained stable plugging performance during two water-alternating-CO2 cycles, but a decline was observed in the third cycle. It also demonstrated a high CO2 breakthrough pressure of 177 psi in high salinity conditions (10% NaCl). The permeability reduction for water and CO2 was influenced by gel concentration and salinity, with higher salinity increasing the permeability reduction and higher gel concentrations decreasing it. These findings underscore the effectiveness of the CO2-BRPPG in improving CO2 sweep efficiency and managing CO2 sequestration in reservoirs with high permeability.

Role of intensive mariculture on CO2 absorption and carbon burial, and the carbon sink potential of Sanggou Bay, China

Suspended-raft cultivation of bivalves and seaweed significantly influences carbon transport and transformation pathways in bay ecosystems, ultimately affecting air–sea CO2 flux (FCO2) and sedimentary carbon burial. Previous studies focused on whether bivalve and seaweed individuals are a carbon source or sink, but have not discussed the effects of their large-scale cultivation on carbon dynamics from the perspective of a bay ecosystem. In this study, we compared seasonal and spatial variations in air–sea FCO2 and sedimentary carbon burial flux in seaweed culture, bivalve culture, bivalve–seaweed polyculture, and non-culture areas in Sanggou Bay, China, a bay with a long history of mariculture practices. We found that in spring and summer, the air–sea FCO2 was lowest in the seaweed area; it was lowest in the bivalve area in autumn and winter. Despite this, the seaweed area annually absorbed twice the amount of atmospheric CO2 compared to the bivalve area. The bivalve area had the highest sedimentary carbon burial flux and the non-culture area had the lowest. More inorganic carbon than organic carbon was buried in sediment. Atmospheric CO2 absorption was positively correlated with sedimentary carbon burial. Seaweed culture was the main contributor, absorbing around half of the atmospheric CO2 of the bay, whereas bivalve culture exhibited the highest carbon burial, contributing about 40 % to the bay's sediment. We constructed an annual carbon budget model, and the estimated values for atmospheric absorption, seawater dissolution, and sedimentary burial were 2.09 × 104, 1.02 × 104, and 0.78 × 104 t C a−1, respectively. Taken together, our results indicate that the large-scale mariculture of bivalves and seaweed affect both atmospheric CO2 absorption and sedimentary carbon burial, as well as their relationship. Seawater dissolution is also a substantial carbon sequestration reservoir that cannot be overlooked when assessing the biogeochemical carbon cycle of bay ecosystems.

The role of the production geologist in carbonate reservoir development

Carbonate production geologists are deployed to tackle field management and (re)development issues on increasingly mature fields. This paper shares examples of how geologists raise awareness of current and potential production opportunities with insights into permeability architecture and its uncertainty. The examples represent diverse fluid types and a range of depositional, diagenetic and structural settings. ‘Plumbing diagram’ examples communicate what matters to flow from the reservoir to the well perforations. In the Luconia gas fields, thin low-porosity horizons either accentuate or baffle vertical permeability to rising aquifers. In a Middle Eastern gas–oil gravity drainage development constrained by gas re-injection, well placement into unfractured intervals led to an oil redevelopment at a lower gas cut. In a Middle Eastern waterflood, dynamic characterization of permeability contrasts facilitates improved oil recovery through sweep management. The examples stress the importance of continued geological data analyses in brownfield settings, as well as active engagement during engineering-dominated discussions on fluid flow that inevitably follow field development plan execution and monitoring. A discussion examines why brownfield development opportunities delay to later field life, and into the energy transition, how production geologists might improve geothermal and sequestration reservoir developments.

Review on CO2–Brine Interaction in Oil and Gas Reservoirs

Carbon neutrality has become a global common goal. CCUS, as one of the technologies to achieve carbon neutrality, has received widespread attention from academia and industry. After CO2 enters the formation, under the conditions of formation temperature and pressure, supercritical CO2, formation water, and rock components interact, which directly affects the oil and gas recovery and carbon sequestration efficiency. In this paper, the recent progress on CO2 water–rock interaction was reviewed from three aspects, including (i) the investigation methods of CO2 water–rock interaction; (ii) the variable changes of key minerals, pore structure, and physical properties; and (iii) the nomination of suitable reservoirs for CO2 geological sequestration. The review obtains the following three understandings: (1) Physical simulation and cross-time scale numerical simulation based on formation temperature and pressure conditions are important research methods for CO2 water–rock interaction. High-precision mineral-pore in situ comparison and physical property evolution evaluation are important development directions. (2) Sensitive minerals in CO2 water–rock interaction mainly include dolomite, calcite, anhydrite, feldspar, kaolinite, and chlorite. Due to the differences in simulated formation conditions or geological backgrounds, these minerals generally show the pattern of dissolution or precipitation or dissolution before precipitation. This differential evolution leads to complex changes in pore structure and physical properties. (3) To select the suitable reservoir for sequestration, it is necessary to confirm the sequestration potential of the reservoir and the later sequestration capacity, and then select the appropriate layer and well location to start CO2 injection. At the same time, these processes can be optimized by CO2 water–rock interaction research. This review aims to provide scientific guidance and technical support for shale oil recovery and carbon sequestration by introducing the mechanism of CO2 water–rock interaction, expounding the changes of key minerals, pore structure, and physical properties, and summarizing the sequestration scheme.

Characterization of petrophysical and seismic properties for CO2 storage with sensitivity analysis

Saline aquifers are considered as highly favored reservoirs for CO2 sequestration due to their favorable properties. Understanding the impact of saline aquifer properties on the migration and distribution of CO2 plume is crucial. This study focuses on four key parameters—permeability, porosity, formation pressure, and temperature—to characterize the reservoir and analyse the petrophysical and elastic response of CO2. First, we performed reservoir simulations to simulate CO2 saturation, using multiple sets of these four parameters to examine their significance on CO2 saturation and the plume migration speed. Subsequently, the effect of these parameters on the elastic properties is tested using rock physics theory. We established a relationship of compressional wave velocity (Vp) and quality factor (Qp) with the four key parameters, and conducted a sensitivity analysis to test their sensitivity to Vp and Qp. Finally, we utilized visco-acoustic wave equation simulated time-lapse seismic data based on the computed Vp and Qp models, and analysed the impact of CO2 saturation changes on seismic data. As for the above numerical simulations and analysis, we conducted sensitivity analysis using both homogeneous and heterogeneous models. Consistent results are found between homogeneous and heterogeneous models. The permeability is the most sensitive parameter to the CO2 saturation, while porosity emerges as the primary factor affecting both Qp and Vp. Both Qp and Vp increase with the porosity, which contradicts the observations in gas reservoirs. The seismic simulations highlight significant variations in the seismic response to different parameters. We provided analysis for these observations, which serves as a valuable reference for comprehensive CO2 integrity analysis, time-lapse monitoring, injection planning and site selection.

Micromechanical property evolution and damage mechanism of coal subjected to ScCO2 treatment

Carbon dioxide (CO2) injection into deep coal seams holds considerable significance in both carbon mitigation and enhanced recovery of clean coalbed methane (CBM). The CO2-coal interaction leads to changes in the physicochemical properties, potentially impacting the stability of the target sequestration coal reservoir. This study applied the nanoindentation, scanning probe microscopy (SPM), X-ray diffraction (XRD), and electron probe X-ray Micro-Analyzer (EPMA) techniques to probe the micromechanical property change and damage mechanism of coal subjected to ScCO2 injection. The result shows that the load-displacement behavior of the tested coal changes with continuous ScCO2 treatment, and the mechanical strength significantly weakens after 4 days of ScCO2 treatment. The nanoindentation test indicates that the peak and creep displacements of tested coal increased by 168.79 % and 1046.32 % respectively with 4-day ScCO2 treatment, inferring that the coal deformation increases and changes from elastic to plastic after ScCO2 injection. As the ScCO2 treatment time increases, Young's modulus and hardness of coal exhibit an exponential decay trend and rapidly decrease by 77.77 %∼89.76 % and 68.37 %∼81.63 % respectively after the initial 3-day treatment, followed by a slow decrease with the ScCO2 duration prolonging. The XRD and EPMA results show that the carbonate minerals reacted preferentially with ScCO2, and silicate minerals experienced gradual dissolution, while the amorphous carbon fluctuated almost unchanged in the tested coal. Carbonate minerals in tested coal have decreased by 70.68 % within the first 1-day ScCO2 treatment, making the most significant contribution to the initial rapid weakening of coal mechanics. The mineral distribution is closely related to the mechanical anisotropy of coal, which is confirmed by the phenomenon that the coal anisotropy gradually weakens with the mineral reaction process. It is inferred that the mineral component and distribution of coal seams are important indicators for evaluating the intensity of physical and chemical reactions in ScCO2-injected reservoirs, which directly determines the mechanical damage behavior of sequestration reservoirs. This research provides basic support for site selection and safety assessment of carbon sequestration in deep coal seams.

Geo-mechanical and geo-morphology characterisation of the cap rocks in the Niger Delta for potential carbon capture and storage

The world has been battling climate change which is caused by an excessive amount of carbon dioxide in the atmosphere. Carbon sequestration in geological sinks was identified as one of the major methods to curb and reduce the amount of carbon dioxide in the atmosphere. The carbon dioxide stored in the geological formations must not escape back into the atmosphere. Characterisation of potential reservoirs for geological sequestration is pertinent for injectivity, capacity, and most importantly containment of carbon dioxide. The cap rock is one of the most important factors determining carbon sequestration success. This study focuses on the preliminary characterisation of the caprocks for evaluating potential subsurface storage of carbon dioxide based on their petrophysical characteristics. X-ray Diffraction and X-ray Fluorescence provided the mineralogical composition and geochemical data of the cap rocks while Scanning Electron Microscopy-EDS was used to identify the qualitative information on the micromorphological textural structure of the cap rocks in the Niger Delta. Conventional Triaxial experiments were used to determine the elastic and geo-mechanical strength of the caprocks. It was identified that the grain skeleton of caprocks in the region is predominated by Quartz, Albite, and Muscovite. The ratio of the tectosilicate and phyllosilicate minerals indicates that the cap rocks are silicate shale. The peak strength of the caprocks ranged from 48.85 to 80.50 MPa and are classified as strong rocks. The cap rocks showed a quasi-elastic behaviour after being subjected to compressive axial force. The elastic modulus of the caprocks was also observed. The characteristics exhibited by the shale caprock at the rock matrix level are highly favourable for sealing carbon dioxide and hence indicate capability for use in carbon sequestration.

Synergistic advantages of volcanic ash weathering in saline soils: CO2 sequestration and enhancement of plant growth

Carbon dioxide (CO2) is a primary greenhouse gas that has experienced a surge in atmospheric concentration due to human activities and lifestyles. It is imperative to curtail atmospheric CO2 levels promptly to alleviate the multifaceted impacts of climate warming. The soil serves as a natural reservoir for CO2 sequestration. The scientific premise of this study is that CO2 sequestration in agriculturally relevant, organically-deficient saline soil can be achieved by incorporating alkaline earth silicates. Volcanic ash (VA) was used as a soil amendment for CO2 removal from saline soil by leveraging enhanced silicate rock weathering (ERW). The study pursued two primary objectives: first, we aimed to evaluate the impact of various doses of VA, employed as an amendment for organically-deficient soil, on the growth performance of key cultivated crops (sorghum and mung bean) in inland saline-alkaline agricultural regions of northeastern China. Second, we aimed to assess alterations in the physical properties of the amended soil through mineralogical examinations, utilizing X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) analyses, quantifying the increase in inorganic carbon content within the soil. In the potting tests, mung bean plant height exhibited a noteworthy increase of approximately 41 % with the addition of 10 % VA. Sorghum plant height and aboveground and belowground biomass dry weights increased with VA application across all tested doses. At the optimal VA application rate (20 %), the sorghum achieved a CO2 sequestration rate of 0.14 kg CO2·m−2·month−1. XRD and SEM-EDS analyses confirmed that the augmented inorganic carbon in the VA-amended soils stemmed primarily from calcite accumulation. These findings contribute to elucidating the mechanism underlying VA as an amendment for organically-deficient soils and provide an effective approach for enhancing the carbon sink capacity of saline soils.

Assessing the potential for CO2 storage in shallow coal seams using gas geochemistry: A case study from Qinshui Basin, North China

Shallow coal seams are a target reservoir for CO2 sequestration. So far efforts to trace CO2 sequestration by coal beds have largely focussed on short-term monitoring or theoretical modelling. To evaluate the feasibility of safe long-term storage of CO2 in shallow coal beds, we report the first systematic gas geochemical study at a shallow injection site in Shizhuang, Qinshui Basin, one year after CO2 injection ceased. The injected CO2 is isotopically distinctive and appears to be enriched in heavy noble gases (Kr and Xe). The high CO2 content and light δ13CCO2 of gases from production wells confirm the migration of injected gas to the northeast and east of the main injection well previously tracked by geophysical studies. The migration of injected gas to the southeast has not been identified before, and exceeds that to the northeast, implying that gas geochemistry provides a more robust method of tracking migration. Less than 1 % of the total injected CO2 has been extracted from production wells in the year since injection ceased. We estimate that the target coal seam in the study area can store about 0.08 billion tonnes of CO2. Scaling up we estimate that the Qinshui Basin has the potential to store about 19 billion tonnes of CO2. This is significantly more than previous estimates. This study demonstrates the importance of geochemical tracers in quantifying and monitoring CO2 sequestration and also implies that unmined coal seams have high potential for the long-term storage of CO2.

Exploring CO2 storage potential in Lithuanian deep saline aquifers using digital rock volumes: a machine learning guided approach

The increasing significance of carbon capture, utilization and storage (CCUS) as a climate mitigation strategy has underscored the importance of accurately evaluating subsurface reservoirs for CO2 sequestration. In this context, digital rock volumes, obtained through advanced imaging techniques such as micro-Xray computed tomography (MXCT), offer intricate insights into the porous and permeable structures of geological formations. This study presents a comprehensive methodology for assessing CO2 storage viability within Lithuanian deep saline aquifers, namely Syderiai and Vaskai, by utilizing petrophysical properties estimated from digital rock volumes of samples from analogous formations. It also demonstrates the potential of integrating advanced imaging techniques, machine learning, and numerical modeling for accurate assessment and effective management of subsurface CO2 storage.

Exploring CO2 storage potential in Lithuanian deep saline aquifers using digital rock volumes: a machine learning guided approach

The increasing significance of carbon capture, utilization and storage (CCUS) as a climate mitigation strategy has underscored the importance of accurately evaluating subsurface reservoirs for CO2 sequestration [1]. In this context, digital rock volumes, obtained through advanced imaging techniques such as micro-Xray computed tomography (MXCT), offer intricate insights into the porous and permeable structures of geological formations [2]. This study presents a comprehensive methodology for assessing CO2 storage viability within Lithuanian deep saline aquifers, namely Syderiai and Vaskai, by utilizing petrophysical properties estimated from digital rock volumes [3, 4]. These petrophysical properties were derived from core samples collected from these formations. Utilizing machine learning algorithms, porosity was estimated while the Lattice Boltzmann method (LBM) was applied to determine permeability [5]. The methodology employed for estimating these petrophysical parameters was initially validated using samples from formations analogous to Lithuanian formations. Subsequently, it was applied to rock samples specifically obtained from Lithuanian formations. The estimated petrophysical properties were compared with peer-reviewed data from published literature. When fluids such as CO2 or H2 are injected into sub-surface reservoirs, they can alter pore and grain characteristics. Therefore, it is crucial to extract representative element volumes (REVs) from segmented volumes to study the impact of fluids on porosity and their distribution [6]. These mini models, representing small portions of the larger formation, assist in predicting fluid flow within the formation, which is vital for assessing the efficiency and safety of carbon capture and storage (CCS) operations. Subsequently, numerical modelling was conducted using the petrophysical parameters as inputs to assess the storage capacity of the Lithuanian formations using tNavigator software [7]. This research contributes to an enhanced understanding of pore space distribution and its role in various aspects of long-term CO2 storage. It also demonstrates the potential of integrating advanced imaging techniques, machine learning, and numerical modeling for accurate assessment and effective management of subsurface CO2 storage. This study shall aid in enhanced understanding of pore space distribution and their contribution towards various aspects of long-term storage. The results can be extended to study the geochemical reactions and geo-mechanical behaviour of the rocks. Such studies shall further facilitate identification of reservoir(s) wherein sequestration potential can be reliably explored.

Carbon sequestration reservoir at Rock Springs Uplift, Wyoming, USA

The Rock Springs Uplift (RSU) located in southwest Wyoming USA as indicated in Figure 1 is one of several sites across the country to be characterized for suitability for CO2 sequestration. The RSU was identified for several reasons: proximity to a large CO2 emissions point source, the overall geometry of the RSU structure, and geologic formations recognized to have properties required for long term storage of CO2. Original rock cores of the Weber Sandstone formation and the dolomite facies of the Madison Limestone formation from the RSU were prepared into 25-mm diameter rock samples. These samples were vacuum-saturated to 100% saturation with synthetic brine, aged with brine for 800 hours at in-situ temperatures and initial confining pressure of 3.5 MPa, and aged with CO2-rich brine (400 hours first with brine and another 400 hours with compressed CO2-saturated brine) at in-situ temperatures and initial confining pressure of 3.5 MPa. During the aging process, the pore and confining pressure were increased in small steps, typically 0.69 to 1.38 MPa at a time using high precision syringe pumps (Teledyne ISCO 260D, Lincoln, NE, USA) until the test conditions were met without fluid flow. Laboratory hydrostatic and triaxial experiments were performed at in-situ reservoir conditions with the temperatures of 90°C for Weber Sandstone and and 93°C for Madison Limestone under three differential pressures of 6.9 MPa, 34.5 MPa and 55.2 MPa.
 Geomechanical results of the aged rock samples are presented and discussed. Under the triaxial compression, Weber Sandstone exhibited brittle failure strengths while Madison Limestone exhibited a brittle-ductile transition behavior. For the sandstone samples with similar initial porosity, the Young’s moduli increased and Poisson’s ratios decreased as the result of CO2 under the linear elastic regime. No relationship between stress (or strain) data and CO2 in the nonlinear plastic regime was observed. However, the confining pressure has a greater effect than CO2 on geomechanical behaviors. For the limestone samples, the change of elastic constants due to CO2 is more significant than that of sandstone samples. However, no consistent trend was observed on the limestone samples,
 and the effect of CO2 is not the dominant factor influencing the plastic properties of rocks. DH1c with the lowest initial porosity and differential pressure exhibits the highest peak strength, Young’s modulus and Poisson’s ratio, indicating the effect of initial porosity. Considering the effect of CO2 on Mohr failure envelopes, the decreasing trend of cohesion and increasing friction angle was observed in both sandstone and limestone samples. Table 1 summarizes the maximum deviatoric stress, elastic properties, and shear strength parameters of Weber Sandstone and Madison Limestone samples.

Experimental Measurements and Molecular Simulation of Carbon Dioxide Adsorption on Carbon Surface

Summary Geological sequestration of carbon dioxide (CO2) in depleted gas reservoirs represents a cost-effective solution to mitigate global carbon emissions. The surface chemistry of the reservoir rock, pressure, temperature, and moisture content are critical factors that determine the CO2 adsorption capacity and storage mechanisms. Shale-gas reservoirs are good candidates for this application. However, the interactions between CO2 and organic content still need further investigation. The objectives of this paper are to (i) experimentally evaluate the adsorption isotherm of CO2 on activated carbon, (ii) quantify the nanoscale interfacial interactions between CO2 and the activated carbon surface using Monte Carlo (MC) and molecular dynamic (MD) simulations, (iii) evaluate the modeling reliability using experimental measurements, and (iv) quantify the influence of temperature and geochemistry on the adsorption behavior of CO2 on the surface of activated carbon. These objectives aim at obtaining a better understanding of the behavior of CO2 injection and storage in the kerogen structure of shale-gas formations, where activated carbon is used as a proxy for thermally mature kerogen. We performed experimental measurements, grand canonical Monte Carlo (GCMC) simulations, and MD simulations of CO2 adsorption and diffusion on activated carbon. The experimental work involved measurements of the high-pressure adsorption capacity of activated carbon using pure CO2 gas at a temperature of 300 K. The simulation work started with modeling and validating an activated carbon structure by calibrating the GCMC simulations with experimental CO2 adsorption measurements. Then, we extended the simulation work to quantify the adsorption isotherms at a temperature range of 250–500 K and various surface chemistry conditions. Moreover, CO2 self-diffusion coefficients were quantified at gas pressures of 0.5 MPa, 1 MPa, and 2 MPa using MD simulations. The experimental results showed a typical CO2 excess adsorption trend for the nanoporous structures, with a density of the sorbed gas phase of 504.76 kg/m3. The simulation results were in agreement with experimental adsorption isotherms with a 10.6% average absolute relative difference. The self-diffusion results showed a decrease in gas diffusion with increasing pressure due to the increase in the adsorbed gas amount. Increasing the simulation temperature from 300 K to 400 K led to a decrease in the amount of adsorbed CO2 molecules by about 87% at 2 MPa pressure. Finally, the presence of charged functional groups (e.g., hydroxyl–OH and carboxyl–COOH) led to an increase in the adsorption of CO2 gas to the activated carbon surface. The outcomes of this paper provide new insights about the parameters affecting CO2 adsorption and sequestration in depleted shale-gas reservoirs. This in turn helps in screening the candidate shale-gas reservoirs for carbon capture, sequestration, and storage to maximize the CO2 storage capacity.

Effects of CO2 on creep deformation in sandstones at carbon sequestration reservoir conditions: An experimental study

We conducted four hydrostatic creep/flow-through experiments to investigate the evolution of rock hydraulic and mechanical properties due to fluid-rock interactions in the context of underground CO2 utilization and storage. The experiments were performed on the macroporosity-dominated lithofacies of the Morrow B sandstone. The Morrow B is an early Pennsylvanian fluvial/marine deposit, and the target reservoir of an ongoing CO2-enhanced oil recovery project at the Farnsworth Unit oilfield, Ochiltree County, Texas. Core samples were held at 38.5 MPa effective stress and 100 °C while flowing through reservoir brackish solution (control) or CO2-enriched reservoir brackish solution at different flow rates (0.01, 0.02, and 0.1 ml/min). We monitored the evolution of the outlet fluid chemistry, sample axial strain, permeability, and bulk modulus, and measured pre- and post-experiment porosities and ultrasonic wave velocities. Results showed mineral dissolution, but no enhanced creep deformation in the presence of CO2. Samples experienced fluctuations in the bulk modulus of elasticity with up to 18% and 3% increases for flow-through with reservoir brackish solution and CO2-enriched reservoir solution, respectively. Permeability decreased monotonically during flow-through with brackish solution, but fluctuated with CO2-rich brackish solution. All samples experienced a decrease in the post-test ultrasonic wave velocities, where the decrease was proportional to the test duration. Chemical reactions were not rate limited, and ion concentrations varied as the fluid volume through the core for all experiments irrespective of flow rate, duration, or experiment type. We believe that the observed fluctuation in sample bulk modulus was due to dissolution/microcracking leading to rock softening versus rock consolidation/pressure solution leading to rock stiffening, which is supported by petrographic observations. CO2-driven dissolution of disseminated ankerite cements did not cause enhanced compaction because of their low occurrence and the presence of the more abundant unreactive pre-compaction quartz cements forming long contacts with the framework grains. Porous sandstone reservoirs whose framework grains are supported by early diagenetic (pre-compaction) quartz cements make good targets for the long-term safe underground storage of CO2.

Geomechanical risk and mechanism analysis of CO2 sequestration in unconventional coal seams and shale gas reservoirs

With global greenhouse gas emissions hitting record highs in 2021, CO2 geological sequestration (CGS) is the most realistic and feasible technology to ensure large-scale carbon reduction to achieve global carbon capping and carbon neutrality goals. Both coalbed methane and shale gas have the characteristics of self-generation and self-storage, which is considered to be a valuable target reservoir for geological sequestration of CO2. After a high volume of CO2 is injected into unconventional coal seams and shale gas reservoirs, many geomechanical issues may be induced, resulting in leakage. Therefore, it is crucial to evaluate the geomechanical risks of CO2 geological sequestration. In this article, global CO2 emissions and geological resources available for sequestration are teased out. The effects of coupled CO2-water-rock-driven geomechanical, geophysical, and geochemical interactions on the evolution of rock physical properties and pore characteristics, as well as caprock sealing, are discussed. The caprock failure and its inducing mechanism are analyzed, and the criteria for predicting the occurrence of risk are summarized, which is necessary for pressure management and risk prevention. To serve as a benchmark for CO2 sequestration in unconventional coal seams and shale gas reservoirs.

Paragenetic controls on CO2-fluid-rock interaction and weakening in a macroporous-dominated sandstone

The injection and storage of anthropogenic CO2 in the subsurface is being deployed as a climate change mitigation tool; however, diagenetic-paragenetic heterogeneity in sandstone reservoirs often contributes to interval specific chemomechanical changes that affect injection and can increase leakage risk. Here, we address reservoir heterogeneities’ impact on chemomechanical changes in a macroporous-dominated lithofacies of Morrow B sandstone, a formation containing several diagenetically-distinct hydraulic facies while undergoing enhanced oil recovery (EOR) and carbon dioxide (CO2) sequestration. We performed three flow-through experiments using a CO2-charged or uncharged formation water combined with four indirect tensile strength tests per post-test sample. We then used the microstructure and paragenetic sequence to understand chemomechanical weakening with key observations as follows: dissolution of carbonates and feldspars changed porosity; increased permeability led to reclassifying each sample in a different hydraulic flow unit; decreased ultrasonic velocity; and did not lead to a loss of tensile strength. Tensile strength maintenance occurred due to the low abundance and minor dissolution of siderite, the stability of quartz, and the relative position of diagenetic ankerite within feldspar. This macroporous-dominated lithofacies is the primary reservoir for the Morrow B Sandstone, and is analogous to other porous sandstone reservoirs. It represents an end-member of a chemomechanically low-risk siliceous CO2 sequestration and CO2-EOR reservoir.

Coal microstructural and mechanical alterations induced by supercritical CO2 exposure: Role of water

Injection of CO2 into deep unminable coal reservoirs is regarded as an effective method to reduce greenhouse gas emissions. Coal seams with burial depth > 800 m are the potential reservoirs for CO2 sequestration, and CO2 is stored in the form of supercritical state. The interactions between supercritical CO2 (ScCO2) and coal are a complex process, which has influence on coal structures and strength. Recent studies are rarely reported regarding the impact of ScCO2-coal-water reactions on coal microstructures and mechanical strength. In this paper, comparative analysis was conducted on the microstructural changes and mechanical degradation of coal induced by ScCO2 treatment and ScCO2-water saturation. The results show that water has great influence on coal pore structure, functional groups, mineral compositions and mechanical properties during the reaction. ScCO2 saturation only triggers a small decline of 11.27–17.65% in aliphatic groups in coal, while approximately 62.75–85.92% of aliphatic groups are extracted by ScCO2-water. The largest hysteresis loops are found in samples saturated by ScCO2-water, followed by the specimens treated by ScCO2, illustrating that the complexity of coal pore structure is enhanced by ScCO2 interaction, and the presence of water can further increase the pore roughness. The UCS is reduced by < 13% after ScCO2 saturation on dry coal, but it is greatly decreased by 26.58–44.21% in the presence of water. In the process of interaction, water accelerates the hydrocarbon extraction and mineral dissolution to aggravate the degradation of coal strength. This work is conducive to understanding the process of CO2 geological sequestration.

Fault Slip Tendency Analysis for a Deep-Sea Basalt CO2 Injection in the Cascadia Basin

Offshore basalts, most commonly found as oceanic crust formed at mid-ocean ridges, are estimated to offer an almost unlimited reservoir for CO2 sequestration and are regarded as one of the most durable locations for carbon sequestration since injected CO2 will mineralize, forming carbonate rock. As part of the Solid Carbon project, the potential of the Cascadia Basin, about 200 km off the west coast of Vancouver Island, Canada, is investigated as a site for geological CO2 sequestration. In anticipation of a demonstration proposed to take place, it is essential to assess the tendency of geologic faults in the area to slip in the presence of CO2 injection, potentially causing seismic events. To understand the viability of the reservoir, a quantitative risk assessment of the proposed site area was conducted. This involved a detailed characterization of the proposed injection site to understand baseline stress and pressure conditions and identify individual faults or fault zones with the potential to slip and thereby generate seismicity. The results indicate that fault slip potential is minimal (less than 1%) for a constant injection of up to ~2.5 MT/yr. This is in part due to the thickness of the basalt aquifer and its permeability. The results provide a reference for assessing the potential earthquake risk from CO2 injection in similar ocean basalt basins.