Evaluating the CO2 geological storage suitability of coal-bearing sedimentary basins in China.

The geological storage of CO2 is potentially one of the most effective methods to reduce the CO2 concentration in the atmosphere. Coalbeds are possibly suitable storage reservoirs, meaning that evaluating the suitability of individual coalbeds and associated engineering and construction is an important step in developing geological CO2 storage. This evaluation requires the development of a reasonable evaluation index system and associated weightings. This paper focuses on coal-bearing basins in China and outlines a technical process whereby the CO2 storage suitability of these basins can be comprehensively evaluated. This study uses an earth system science approach to determine the uncertainties involved in identifying ideal CO2 storage sites, develops an index that outlines the conditions related to the suitability of Chinese coal-bearing basins for geological CO2 storage, and incorporates this index into a hierarchical index system model for geological CO2 storage suitability that allows the comprehensive evaluation of coal-bearing basins and includes 5 aspects, 23 indexes, and 5 index levels. The weighting assigned to each evaluation index was determined using the analytic hierarchy process (AHP) and was subsequently incorporated into a fuzzy logic-based comprehensive evaluation approach. This approach was applied to assess the suitability of the Qinshui Basin for geological CO2 storage, revealing that this basin is indeed a suitable coalbed reservoir. The comprehensive geological CO2 storage evaluation model outlined here can also assess the CO2 storage suitability and capacity of other coal-bearing basins elsewhere in China and globally.

CO2 geological storage: A bibliometric analysis of research trends.

Geological sequestration of carbon dioxide is a critical strategy to combat global warming, playing a significant role in the reduction of greenhouse gas emissions and preservation of the global ecosystem. Over more than three decades, this domain has expanded to encompass myriad research avenues and nuanced sub-fields. Proficiency in navigating the dynamic developments and prominent challenges in this arena is imperative for promoting scholarly advancement. In this investigation, bibliometric techniques are applied to perform a comprehensive qualitative and quantitative investigation of the progression of studies on CO2 geological sequestration. The analysis incorporates a thorough review and synthesis of the accumulated literature, comprising 34,392 articles sourced from the Web of Science Core Collection. The assessment primarily scrutinizes the chronological dispersal of research outputs, geographical and institutional representation, principal journals of publication, and patterns of authorship to highlight burgeoning areas of concentrated research endeavors and prospective future research frontiers. The data reveals a pronounced surge in academic literature focusing on CO2 geological storage post-2009, which underscores the increasing value of this research sector. Investigations of CO2 geological sequestration are characterized by widespread international engagement, with notable contributions from the United States, China, and the United Kingdom substantially steering the research discourse. The core investigative themes comprise comprehensive inquiries into the physical and chemical dynamics of CO2 containment, environmental repercussions, safety assessments, evaluation methods, and technological assessments of carbon storage, along with stringent scrutiny of geological contexts for their viability and efficacy as sequestration sites.

Geomechanics contribution to CO2 storage containment and trapping mechanisms in tight sandstone complexes: A case study on Mae Moh Basin.

Recognized as a not-an-option approach to mitigate the climate crisis, carbon dioxide capture and storage (CCS) has a potential as much as gigaton of CO2 to sequestrate permanently and securely. Recent attention has been paid to store highly concentrated point-source CO2 into saline formation, of which Thailand considers one onshore case in the north located in Lampang - the Mae Moh coal-fired power plant matched with its own coal mine of Mae Moh Basin. Despite a large basin and short transport route from the source, target sandstone reservoir buried at deeper than 1000 m is of tight nature and limited data, while question on storing possibility has thereafter risen. The current study is thus aimed to examine the influence of reservoir geomechanics on CO2 storage containment and trapping mechanisms, with co-contributions from geochemistry and reservoir heterogeneity, using reservoir simulator - CMG-GEM. With the injection rate designed for 30-year injection, reservoir pressure build-ups were ∼77 % of fracture pressure but increased to ∼80 % when geomechanics excluded. Such pressure responses imply that storage security is associated with the geomechanics. Dominated by viscous force, CO2 plume migrated more laterally while geomechanics clearly contributed to lesser migration due to reservoir rock strength constraint. Reservoir geomechanics contributed to less plume traveling into more constrained spaces while leakage was secured, highlighting a significant and neglected influence of geomechanical factor. Spatiotemporal development of CO2 plume also confirms the geomechanics-dominant storage containment. Reservoir geomechanics as attributed to its respective reservoir fluid pressure controls development of trapping mechanisms, especially into residual and solubility traps. More secured storage containment after the injection was found with higher pressure, while less development into solubility trap was observed with lower pressure. The findings reveal the possibility of CO2 storage in tight sandstone formations, where geomechanics govern greatly the plume migration and the development of trapping mechanisms.

Potential assessment of CO2 source/sink and its matching research during CCS process of deep unworkable seam.

It is of great significance for the engineering popularization of CO2-ECBM technology to evaluate the potential of CCUS source and sink and study the matching of pipeline network of deep unworkable seam. In this study, the deep unworkable seam was taken as the research object. Firstly, the evaluation method of CO2 storage potential in deep unworkable seam was discussed. Secondly, the CO2 storage potential was analyzed. Then, the matching research of CO2 source and sink was carried out, and the pipe network design was optimized. Finally, suggestions for the design of pipe network are put forward from the perspective of time and space scale. The results show that the average annual CO2 emissions of coal-fired power plants vary greatly, and the total emissions are 58.76 million tons. The CO2 storage potential in deep unworkable seam is huge with a total amount of 762 million tons, which can store CO2 for 12.97 years. During the 10-year period, the deep unworkable seam can store 587.6 million tons of CO2, and the cumulative length of pipeline is 251.61 km with requiring a cumulative capital of $ 4.26 × 1010. In the process of CO2 source-sink matching, the cumulative saving mileage of carbon sink is 98.75 km, and the cumulative saving cost is $ 25.669 billion with accounting for 39.25% and 60.26% of the total mileage and cost, respectively. Based on the three-step approach, the whole line of CO2 source and sink in Huainan coalfield can be completed by stages and regions, and all CO2 transportation and storage can be realized. CO2 pipelines include gas collection and distribution branch lines, intra-regional trunk lines, and interregional trunk lines. Based on the reasonable layout of CO2 pipelines, a variety of CCS applications can be simultaneously carried out, intra-regional and inter-regional CO2 transport network demonstrations can be built, and integrated business models of CO2 transport and storage can be simultaneously built on land and sea. The research results can provide reference for the evaluation of CO2 sequestration potential of China's coal bases, and lay a foundation for the deployment of CCUS clusters.

Predicting wettability of mineral/CO2/brine systems via data-driven machine learning modeling: Implications for carbon geo-sequestration.

Effectively storing carbon dioxide (CO2) in geological formations synergizes with algal-based removal technology, enhancing carbon capture efficiency, leveraging biological processes for sustainable, long-term sequestration while aiding ecosystem restoration. On the other hand, geological carbon storage effectiveness depends on the interactions and wettability of rock, CO2, and brine. Rock wettability during storage determines the CO2/brine distribution, maximum storage capacity, and trapping potential. Due to the high CO2 reactivity and damage risk, an experimental assessment of the CO2 wettability on storage/caprocks is challenging. Data-driven machine learning (ML) models provide an efficient and less strenuous alternative, enabling research at geological storage conditions that are impossible or hazardous to achieve in the laboratory. This study used robust ML models, including fully connected feedforward neural networks (FCFNNs), extreme gradient boosting, k-nearest neighbors, decision trees, adaptive boosting, and random forest, to model the wettability of the CO2/brine and rock minerals (quartz and mica) in a ternary system under varying conditions. Exploratory data analysis methods were used to examine the experimental data. The GridSearchCV and Kfold cross-validation approaches were implemented to augment the performance abilities of the ML models. In addition, sensitivity plots were generated to study the influence of individual parameters on the model performance. The results indicated that the applied ML models accurately predicted the wettability behavior of the mineral/CO2/brine system under various operating conditions, where FCFNN performed better than other ML techniques with an R2 above 0.98 and an error of less than 3%.

Effects of the mechanical response of low-permeability sandstone reservoirs on CO2 geological storage based on laboratory experiments and numerical simulations.

Carbon dioxide (CO2) geological storage (CGS) is an effective way for reducing greenhouse emissions. The injection of CO2 into the deep formation changes the pore pressure and effective stresses in the reservoir, thus leading to changes in stress-dependent porosity and permeability. These changes give feedback to the injection rate, migration, storage amount of CO2 in the target reservoir. In this study, we focus on the Liujiagou reservoir, one of the first demonstration CGS project in saline aquifers in the Ordos Basin, China. The mathematical model that defines the relationship between the permeability and the injection pressure (or effective stress) was obtained by laboratory experiments. On this basis, the permeability-stress law was successfully integrated into the thermo-hydro-mechanical (THM) coupled simulator TOUGH2Biot to simulate the feedback between the flow and mechanical response. The improved simulator was used to analyze the effects of reservoir mechanical response on CO2 geological storage efficiency. The modeling results indicated that the mechanical response of the reservoir had little effect on reservoir pore pressure and porosity, but it had a significant effect on reservoir permeability and the migration distance, injection rate, and total storage amount of CO2. The maximum increases in the lateral migration distance of CO2 caused by the reservoir mechanical response reached 13.1% using 5 MPa injection pressure. In addition, the total CO2 storage amount increased by 11.6% after 5 years of continuous CO2 injection. Furthermore, when the injection pressure was greater, the reservoir mechanical response had stronger enhancement effects on CGS. Overall, the results suggested that the reservoir mechanical response during CO2 injection was beneficial for increasing CGS efficiency and emphasized the importance of considering the mechanical response in CGS.

Pore-scale visualization on a depressurization-induced CO2 exsolution.

The pore-scale behavior of the exsolved CO2 phase during the depressurization process in CO2 geological storage was investigated. The reservoir pressure decreases when the injection stops or when a leaking event or fluid extraction occurs. The exsolution characteristics of CO2 affect the migration and fate of CO2 in the storage site significantly. Here, a micromodel experimental system that can accommodate a large pressure variation provides a physical model with homogeneous porous media to dynamically visualize the nucleation and growth of exsolved CO2 bubbles. The pressure decreased from 9.85 to 3.95MPa at different temperatures and depressurization rates, and the behavior of CO2 bubbles was recorded. At the pore-scale, the nuclei became observable when the CO2 phase density was significantly reduced, and the pressure corresponding to this observation was slightly lower than that of the severe expansion pressure region. The lower temperature and faster depressurization rate produced more CO2 nuclei. The exsolved CO2 bubble preferentially grew into the pore body instead of the throat. The progress of smaller CO2 bubbles merging into a larger CO2 bubble was first captured, which validated the existence of the Ostwald ripening mechanism. The dispersed CO2 phase after exsolution shows similarity with the residually trapped CO2. This observation is consistent with the low mobility and high residual trapping ratio of exsolved CO2 measured in the core-scale measurement, which is considered to be a self-sealing mechanism during depressurization process in CO2 geological storage.

Behavior of CO2/water flow in porous media for CO2 geological storage.

A clear understanding of two-phase fluid flow properties in porous media is of importance to CO2 geological storage. The study visually measured the immiscible and miscible displacement of water by CO2 using MRI (magnetic resonance imaging), and investigated the factor influencing the displacement process in porous media which were filled with quartz glass beads. For immiscible displacement at slow flow rates, the MR signal intensity of images increased because of CO2 dissolution; before the dissolution phenomenon became inconspicuous at flow rate of 0.8mLmin-1. For miscible displacement, the MR signal intensity decreased gradually independent of flow rates, because supercritical CO2 and water became miscible in the beginning of CO2 injection. CO2 channeling or fingering phenomena were more obviously observed with lower permeable porous media. Capillary force decreases with increasing particle size, which would increase permeability and allow CO2 and water to invade into small pore spaces more easily. The study also showed CO2 flow patterns were dominated by dimensionless capillary number, changing from capillary finger to stable flow. The relative permeability curve was calculated using Brooks-Corey model, while the results showed the relative permeability of CO2 slightly decreases with the increase of capillary number.

Short-term effects of CO2 leakage on the soil bacterial community in a simulated gas leakage scenario.

The technology of carbon dioxide (CO2) capture and storage (CCS) has provided a new option for mitigating global anthropogenic emissions with unique advantages. However, the potential risk of gas leakage from CO2 sequestration and utilization processes has attracted considerable attention. Moreover, leakage might threaten soil ecosystems and thus cannot be ignored. In this study, a simulation experiment of leakage from CO2 geological storage was designed to investigate the short-term effects of different CO2 leakage concentration (from 400 g m-2 day-1 to 2,000 g m-2 day-1) on soil bacterial communities. A shunt device and adjustable flow meter were used to control the amount of CO2 injected into the soil. Comparisons were made between soil physicochemical properties, soil enzyme activities, and microbial community diversity before and after injecting different CO2 concentrations. Increasing CO2 concentration decreased the soil pH, and the largest variation ranged from 8.15 to 7.29 (p < 0.05). Nitrate nitrogen content varied from 1.01 to 4.03 mg/Kg, while Olsen-phosphorus and total phosphorus demonstrated less regular downtrends. The fluorescein diacetate (FDA) hydrolytic enzyme activity was inhibited by the increasing CO2 flux, with the average content varying from 22.69 to 11.25 mg/(Kg h) (p < 0.05). However, the increasing activity amplitude of the polyphenol oxidase enzyme approached 230%, while the urease activity presented a similar rising trend. Alpha diversity results showed that the Shannon index decreased from 7.66 ± 0.13 to 5.23 ± 0.35 as the soil CO2 concentration increased. The dominant phylum in the soil samples was Proteobacteria, whose proportion rose rapidly from 28.85% to 67.93%. In addition, the proportion of Acidobacteria decreased from 19.64% to 9.29% (p < 0.01). Moreover, the abundances of genera Methylophilus, Methylobacillus, and Methylovorus increased, while GP4, GP6 and GP7 decreased. Canonical correlation analysis results suggested that there was a correlation between the abundance variation of Proteobacteria, Acidobacteria, and the increasing nitrate nitrogen, urease and polyphenol oxidase enzyme activities, as well as the decreasing FDA hydrolytic enzyme activity, Olsen-phosphorus and total phosphorus contents. These results might be useful for evaluating the risk of potential CO2 leakages on soil ecosystems.