Our research is centered on the Gandhar oil field, which was discovered in 1983, where daily oil production has declined significantly over the years. The primary objective was to evaluate the feasibility of carbon dioxide (CO₂) storage through its injection into the siliciclastic reservoirs of Ankleshwar Formation. We aimed to obtain high-resolution acoustic impedance data to estimate porosity employing model-based poststack seismic inversion. We conducted an analysis of the density and effective porosity in the target zone through geostatistical techniques and probabilistic neural networks. Simultaneously, the work also involved geomechanical analysis through the computation of pore pressure and fracture gradient using well-log data, geological information, and drilling events in the Gandhar field. Our investigation unveiled spatial variations in effective porosity within the Hazad Member of the Ankleshwar Formation, with an effective porosity exceeding 25% observed in several areas, which indicates the presence of well-connected pore spaces conducive to efficient CO₂ migration. Geomechanical analysis showed that the vertical stress (Sv) ranged from 55 MPa to 57 MPa in Telwa and from 63.7 MPa to 67.7 MPa in Hazad Member. The pore pressure profile displayed variations along the stratigraphic sequence, with the shale zone, particularly in the Kanwa Formation, attaining the maximum pressure gradient (approximately 36 MPa). However, consistently low pore pressure values (30-34 MPa) considerably below the fracture gradient curves were observed in Hazad Member due to depletion. The results from our analysis provide valuable insights into shaping future field development strategies and exploration of the feasibility of CO₂ sequestration in Gandhar Field.

This study characterized the AA-01 depleted hydrocarbon reservoir in the KOKA field, Niger Delta, using a multidimensional approach. This investigation involved data validation analysis, evaluation of site suitability for CO₂ storage, and compositional simulation of hydrocarbon components. The primary objective was to determine the initial components and behavior of the hydrocarbon system required to optimize the injection of CO₂ and accompanying impurities, establishing a robust basis for subsequent sequestration efforts in the six wells in the depleted KOKA AA-01 reservoir. The process, simulated using industry software such as ECLIPSE, PVTi, SCAL, and Petrel, included a compositional fluid analysis to confirm the pressure volume temperature (PVT) hydrocarbon phases and components. This involved performing a material balance on the quality of the measured data and matching the initial reservoir pressure with the supplied data source. The compositional PVT analysis adopted the Peng-Robinson equation of state to model fluid flow in porous media and estimate the necessary number of phases and components to describe the system accurately. Results from this investigation indicate that the KOKA AA-01 reservoir is suitable for CO₂ sequestration. This conclusion is based on the reservoir’s good quality, evidenced by an average porosity of 0.21 and permeability of 1 111.0 mD, a measured lithological depth of 9 300 ft, and characteristic reservoir - seal properties correlated from well logs. The study confirmed that volumetric behavior predictions are directly linked to compositional behavior predictions, which are essential during reservoir initialization and data quality checks. Additionally, it highlighted that a safe design for CO₂ storage relies on accurately representing multiphase behaviour across wide-ranging pressure-temperature-composition conditions.

Excessive carbon emissions have resulted in the greenhouse effect, causing considerable global climate change. Marine carbon storage has emerged as a crucial approach to addressing climate change. The Qiantang Sag (QTS) in the East China Sea Shelf Basin, characterized by its extensive area, thick sedimentary strata, and optimal depth, presents distinct geological advantages for carbon dioxide (CO₂) storage. Focusing on the lower section of the Shimentan Formation in the Upper Cretaceous of the QTS, this study integrates seismic interpretation and drilling data with core and thin-section analysis. We reveal the vertical variation characteristics of the strata by providing a detailed stratigraphic description. We use petrophysical data to reveal the development characteristics of high-quality carbon-storage layers and favorable reservoir-caprock combinations, thereby evaluating the geological conditions for CO₂ storage in various stratigraphic sections. We identify Layer B of the lower Shimentan Formation as the most advantageous stratum for marine CO₂ storage. Furthermore, we analyze the carbon emission trends in the adjacent Yangtze River Delta region. Considering the characteristics of the source and sink areas, we suggest a strong correlation between the carbon emission sources of the Yangtze River Delta and the CO₂ storage area of the QTS, making the latter a priority area for conducting experiments on marine CO₂ storage.

The gas field in the Bohai Bay Basin is a fractured metamorphic buried-hill reservoir with dual-media characteristics. The retrograde vaporization mechanism observed in this type of gas condensate reservoir differs significantly from that observed in sand gas condensate reservoirs. However, studies on improving the recovery of fractured gas condensate reservoirs are limited; thus, the impact of retrograde vaporization on condensate within fractured metamorphic buried-hill reservoirs remains unclear. To address this gap, a series of gas injection experiments are conducted in pressure-volume-temperature (PVT) cells and long-cores to investigate the retrograde vaporization effect of condensate using different gas injection media in fractured gas condensate reservoirs. We analyze the variation in condensate volume, gas-to-oil ratio, and condensate recovery during gas injection and examine the influence of various gas injection media (CO₂, N₂, and dry gas) under different reservoir properties and varying gas injection times. The results demonstrate that the exchange of components between injected gas and condensate significantly influences condensate retrograde vaporization in the formation. Compared with dry gas injection and N₂ injection, CO₂ injection exhibits a superior retrograde vaporization effect. At a CO₂ injection volume of 1 PV, the percentage shrinkage volume of condensate is 13.82%. Additionally, at the maximum retrograde condensation pressure, CO₂ injection can increase the recovery of condensate by 22.4%. However, the condensate recovery is notably lower in fractured gas condensate reservoirs than in homogeneous reservoirs, owing to the creation of dominant gas channeling by fractures, which leads to decreased condensate recovery. Regarding gas injection timing, the effect of gas injection at reservoir pressure on improving condensate recovery is superior to that of gas injection at the maximum retrograde condensation pressure. This research provides valuable guidance for designing gas injection development plans and dynamic tracking adjustments for fractured gas condensate reservoirs.

Mineral carbonation, which precipitates dissolved carbon dioxide (CO₂) as carbonate minerals in basaltic groundwater environments, is a potential technique for negative emissions. The Leizhou Peninsula in southwest Guangdong province has extensive basalt, indicating a promising potential for CO₂ storage through rapid mineralization. However, understanding of the basic geological setting, potential, and mechanisms of CO₂ mineralization in the basalts of the Leizhou Peninsula is still limited. The mineralization processes associated with CO₂ storage at two candidate sites in the area are investigated in this paper: Yongshi Farm and Tianyang Basin (of the dried maar lake). Petrography, rock geochemistry, basalt petrophysical properties, and groundwater hydrochemistry analyses are included in the study. Numerical simulation is used to examine the reaction process and its effects. The results show that basalts in the study areas mainly comprise plagioclase, pyroxene, and Fe-Ti oxides, revealing a total volume fraction exceeding 85%. Additionally, small amounts of quartz and fayalite are available, with volume fractions of 5.1% and 1.0%, respectively. The basalts are rich in divalent metal cations, which can form carbonate minerals, with an average of approximately 6.2 moles of metal cations per 1 kg of rock. The groundwater samples have a pH of 7.5-8.2 and are dominated by the Mg-Ca-HCO₃ type. The basalts demonstrate a porosity range of 10.9% to 28.8%, with over 70% of interconnected pores. A 20-year geochemical simulation revealed that CO₂ injection dissolves primary minerals, including anorthite, albite, and diopside, while CO₂ mineralization dissolves precipitation secondary minerals, such as calcite, siderite, and dolomite. Furthermore, a substantial rise in pH from 7.6 to 10.6 is observed in the vicinity of the injected well, accompanied by a slight reduction in porosity from 20% to 19.8%. Additionally, 36.8% of the injected CO₂ underwent complete mineralization within five years, revealing an increasing percentage of 66.1% if the experimental period is extended to 20 years. The presence of abundant divalent metal cations in basalts and water-bearing permeable rocks in the Leizhou Peninsula supports the potential for mineral carbonation in basalts, as indicated by the geochemical simulation results. Additional research is necessary to identify the factors that influence the CO₂ mineralization, storage, and sensitivity analysis of basalt in the Leizhou Peninsula.

The geological storage of carbon dioxide (CO₂) is a crucial technology for mitigating climate change. Offshore deep saline aquifers have elicited increased attention due to their remarkable potential for storing CO₂. During long-term storage, CO₂ migration in a deep saline aquifer needs special attention to prevent it from reaching risk points and leading to security issues. In this paper, a mechanism model is established according to the geological characteristics of saline aquifers in an offshore sedimentary basin in China. The CO₂ migration over 100 years is simulated considering geological changes such as permeability, dip angle, thickness, and salinity. The effects of injection conditions on the CO₂ migration range are also investigated. Results reveal that the migration range of CO₂ in the injection period exceeds 70%, even if the post-injection period’s duration is five times longer than that of the injection period. As the values of the above geological parameters increase, the migration range of CO₂ increases, and permeability has a particularly substantial influence. Moreover, the influences of injection rate and well type are considerable. At high injection rates, CO₂ has a greater likelihood of displacing brine in a piston-like scheme. CO₂ injected by long horizontal wells migrates farther compared with that injected by vertical wells. In general, the plane migration range is within 3 000 m, although variations in the reservoir and injection parameters of the studied offshore saline aquifers are considered. This paper can offer references for the site selection and injection well deployment of CO₂ saline aquifer storage. According to the studied offshore aquifers, a distance of at least 3 000 m from potential leakage points, such as spill points, active faults, and old abandoned wells, must be maintained.

Offshore carbon dioxide (CO₂) geological storage (OCGS) represents a significant strategy for addressing climate change by curtailing greenhouse gas emissions. Nonetheless, the risk of CO₂ leakage poses a substantial concern associated with this technology. This study introduces an innovative approach for establishing OCGS leakage scenarios, involving four pivotal stages, namely, interactive matrix establishment, risk matrix evaluation, cause-effect analysis, and scenario development, which has been implemented in the Pearl River Estuary Basin in China. The initial phase encompassed the establishment of an interaction matrix for OCGS systems based on features, events, and processes. Subsequent risk matrix evaluation and cause-effect analysis identified key system components, specifically CO₂ injection and faults/features. Building upon this analysis, two leakage risk scenarios were successfully developed, accompanied by the corresponding mitigation measures. In addition, this study introduces the application of scenario development to risk assessment, including scenario numerical simulation and quantitative assessment. Overall, this research positively contributes to the sustainable development and safe operation of OCGS projects and holds potential for further refinement and broader application to diverse geographical environments and project requirements. This comprehensive study provides valuable insights into the establishment of OCGS leakage scenarios and demonstrates their practical application to risk assessment, laying the foundation for promoting the sustainable development and safe operation of ocean CO₂ geological storage projects while proposing possibilities for future improvements and broader applications to different contexts.

Marine carbon sequestration is an important component of carbon dioxide capture, utilization and storage (CCUS) technology. It is crucial for achieving carbon peaking and carbon neutralization in China. However, CO₂ leakage may lead to seabed geological disasters and threaten the safety of marine engineering. Therefore, it is of great significance to study the safety monitoring technology of marine carbon sequestration. Zhanjiang is industrially developed and rich in carbon sources. Owing to the good physical properties and reservoirs and trap characteristics, Zhanjiang has huge storage potential. This paper explores the disaster mechanism associated with CO₂ leakage in marine carbon sequestration areas. Based on the analysis of the development of Zhanjiang industry and relevant domestic monitoring technologies, several suggestions for safety monitoring of marine carbon sequestration are proposed: application of offshore aquaculture platforms, expansion and application of ocean observation networks, carbon sequestration safety monitoring and sensing system. Intended to build a comprehensive and multi-level safety monitoring system for marine carbon sequestration, the outcome of this study provides assistance for the development of marine carbon sequestration in China’s offshore areas.

To ensure project safety and secure public support, an integrated and comprehensive monitoring program is needed within a carbon capture and storage (CCS) project. Monitoring can be done using many well-established techniques from various fields, and the seismic method proves to be the crucial one. This method is widely used to determine the CO₂ distribution, image the plume development, and quantitatively estimate the concentration. Because both the CO₂ distribution and the potential migration pathway can be spatially small scale, high resolution for seismic imaging is demanded. However, obtaining a high-resolution image of a subsurface structure in marine settings is difficult. Herein, we introduce the novel Hcable (Harrow-like cable system) technique, which may be applied to offshore CCS monitoring. This technique uses a high-frequency source (the dominant frequency>100 Hz) to generate seismic waves and a combination of a long cable and several short streamers to receive seismic waves. Ultrahigh-frequency seismic images are achieved through the processing of Hcable seismic data. Hcable is then applied in a case study to demonstrate its detailed characterization for small-scale structures. This work reveals that Hcable is a promising tool for time-lapse seismic monitoring of oceanic CCS.

Offshore carbon capture, utilization, and storage (OCCUS) is regarded as a crucial technology for mitigating greenhouse gas emissions. Quantitative monitoring maps of sealed carbon dioxide are necessary in a comprehensive OCCUS project. A potential high-resolution method for the aforementioned purpose lies in the full-waveform inversion (FWI) of time-lapse seismic data. However, practical applications of FWI are severely restricted by the well-known cycle-skipping problem. A new time-lapse FWI method using cross-correlation-based dynamic time warping (CDTW) is proposed to detect changes in the subsurface property due to carbon dioxide (CO₂) injection and address the aforementioned issue. The proposed method, namely CDTW, which combines the advantages of cross-correlation and dynamic time warping, is employed in the automatic estimation of the discrepancy between the seismic signals simulated using the baseline/initial model and those acquired. The proposed FWI method can then back-project the estimated discrepancy to the subsurface space domain, thereby facilitating retrieval of the induced subsurface property change by taking the difference between the inverted baseline and monitor models. Numerical results on pairs of signals prove that CDTW can obtain reliable shifts under amplitude modulation and noise contamination conditions. The performance of CDTW substantially outperforms that of the conventional dynamic time warping method. The proposed time-lapse full-waveform inversion (FWI) method is applied to the Frio-2 CO₂ storage model. The baseline and monitor models are inverted from the corresponding time-lapse seismic data. The changes in velocity due to CO₂ injection are reconstructed by the difference between the baseline and the monitor models.

The phase change of CO₂ has a significant bearing on the siting, injection, and monitoring of storage. The phase state of CO₂ is closely related to pressure. In the process of seismic exploration, the information of formation pressure can be response in the seismic data. Therefore, it is possible to monitor the formation pressure using time-lapse seismic method. Apart from formation pressure, the information of porosity and CO₂ saturation can be reflected in the seismic data. Here, based on the actual situation of the work area, a rockphysical model is proposed to address the feasibility of time-lapse seismic monitoring during CO₂ storage in the anisotropic formation. The model takes into account the formation pressure, variety minerals composition, fracture, fluid inhomogeneous distribution, and anisotropy caused by horizontal layering of rock layers (or oriented alignment of minerals). From the proposed rockphysical model and the well-logging, cores and geological data at the target layer, the variation of P-wave and S-wave velocity with formation pressure after CO₂ injection is calculated. And so are the effects of porosity and CO₂ saturation. Finally, from anisotropic exact reflection coefficient equation, the reflection coefficients under different formation pressures are calculated. It is proved that the reflection coefficient varies with pressure. Compared with CO₂ saturation, the pressure has a greater effect on the reflection coefficient. Through the convolution model, the seismic record is calculated. The seismic record shows the difference with different formation pressure. At present, in the marine CO₂ sequestration monitoring domain, there is no study involving the effect of formation pressure changes on seismic records in seafloor anisotropic formation. This study can provide a basis for the inversion of reservoir parameters in anisotropic seafloor CO₂ reservoirs.

Offshore carbon dioxide (CO₂) storage is an effective method for reducing greenhouse gas emissions. However, when using traditional seismic wave methods to monitor the migration of sequestration CO₂ plumes, the characteristics of wave velocity changes tend to become insignificant beyond a certain limit. In contrast, the controllable source electromagnetic method (CSEM) remains highly sensitive to resistivity changes. By simulating different CO₂ plume migration conditions, we established the relevant models and calculated the corresponding electric field response characteristic curves, allowing us to analyze the CSEM’s ability to monitor CO₂ plumes. We considered potential scenarios for the migration and diffusion of offshore CO₂ storage, including various burial depths, vertical extension diffusion, lateral extension diffusion, multiple combinations of lateral intervals, and electric field components. We also obtained differences in resistivity inversion imaging obtained by CSEM to evaluate its feasibility in monitoring and to analyze all the electric field (E_x, E_y, and E_z) response characteristics. CSEM has great potential in monitoring CO₂ plume migration in offshore saltwater reservoirs due to its high sensitivity and accuracy. Furthermore, changes in electromagnetic field response reflect the transport status of CO₂ plumes, providing an important basis for monitoring and evaluating CO₂ transport behavior during storage processes.

The risk of gas leakage due to geological flaws in offshore carbon capture, utilization, and storage, as well as leakage from underwater oil or gas pipelines, highlights the need for underwater gas leakage monitoring technology. Remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) are equipped with high-resolution imaging sonar systems that have broad application potential in underwater gas and target detection tasks. However, some bubble clusters are relatively weak scatterers, so detecting and distinguishing them against the seabed reverberation in forward-looking sonar images are challenging. This study uses the dual-tree complex wavelet transform to extract the image features of multibeam forward-looking sonar. Underwater gas leakages with different flows are classified by combining deep learning theory. A pool experiment is designed to simulate gas leakage, where sonar images are obtained for further processing. Results demonstrate that this method can detect and classify underwater gas leakage streams with high classification accuracy. This performance indicates that the method can detect gas leakage from multibeam forward-looking sonar images and has the potential to predict gas leakage flow.