There is a critical situation that a huge amount of greenhouse gases is emitted into the atmosphere from natural coal fires in underground coal seams and spontaneous combustions in coal piles in the world. To reduce the emissions of combustion gases, such as CO₂, CH₄, etc., some technical challenges and researches are required related to CO₂ flux measurements, developing an effective chemical water-solution extinguishes coal fires and inhibits spontaneous combustion, constructing a numerical simulation model to treat the aging effect in coal heating rate. In this article, firstly, the previous studies were reviewed to explain measurement results of CO₂ emission from coal fire/spontaneous combustion fields and the aging model used to simulate coal heating rate. Secondary, a trial development of some chemical solutions has been introduced to inhibit microscopic and macroscopic coal oxidations by forming a gel. Especially, sodium meta silicate solution showed a relatively good performance shifting the critical self-ignition temperature (CSIT) of a low rank coal to be 4 to 5℃ higher than that of water by forming gel with CO₂ gases in the air and combustion gases emitted from coals. The brooking effect in macro scale by forming gel is also expected to extinguish coal seam fires by reducing aperture width in fractures connecting to the atmosphere. A numerical simulation using ANSI FLUENT has showed an extinguishing fire process of an underground coal seam including a horizontal fracture that is connected inlet and outlet fractures being air paths for air and combustion gas, respectively. Finally, the the research challenges required for technical developments have been proposed to carry a project extinguishing the coal seam fires and spontaneous combustions of coal piles with reducing CO₂ emissions.

Elucidating the sealing capacity of caprock is very important because CO₂ sequestered in deep aquifers can remain there for several hundred years in the CO₂ sequestration. Threshold capillary pressure is a key property affecting sealing capacity, and therefore, the ability to measure the threshold pressure of a target rock conveniently is highly sought after. In this study, data collected during the CO₂ injection process in triaxial compression tests of mudstone supposed to be a caprock were used to measure threshold pressure via the dynamic method. These triaxial tests were conducted to study the mechanical properties of mudstone below depths of 1000 m. The threshold pressure for 23 out of 26 test samples was successfully measured. Measured values ranged from 0.54 to 1.57 MPa, which was within a reasonable range compared to the results reported by three other studies that used similar mudstone specimens. A small amount of shrinkage induced by a pore pressure decrease, which was caused by a threshold pressure, was observed in the experiment. The threshold pressure showed a positive correlation with the shrinkage, implying that this pressure might be estimated from the shrinkage. Thus, a volumetric elastic constant was calculated from the shrinkage assuming that the decrease in mean pore pressure was half of the threshold pressure. The mean volumetric elastic constant was 0.92 GPa, which was about 40% lower than that of the rocks from which the test samples were obtained. These results indicate that the shrinkage value cannot be directly used to estimate the threshold pressure, and that a correction of this shrinkage value is necessary.

In this review, the status quo of social acceptance of CCS is discussed in relation with international efforts to establish understanding on CCS. The author analyzed suspended 43 CCS project cases out of total 133 cases, and identified major impacting factors: economic reasons, legislative reasons, political reasons and social protests were 50%, 25%, 10% and 12% respectively. All of these impacting factors are within the range of social acceptance considerations, and could be solved by stakeholder engagement efforts. The author reviewed ISO/TC265 CCS standards which encourages to establish preferable social acceptance on CCS project by way of stakeholder engagement process and risk communication process. The author summarized recommendations from CCS stakeholder engagement process. Then, the author surveyed reports on public perceptions on CCS and CCS outreach works. Finally, the author researched literally succeeded and failed cases in of social acceptance gaining among communities in vicinity of CCS site and discussed the issue.

A project of CO₂ injection in the Ishikari coal basin (Yubari-shi, Hokkaido, Japan) since 2002 has shown that injection of CO₂ into coal seams affected their permeability. Adsorption of CO₂ caused swelling of the coal matrix and lowered the permeability of coal seams, probably because of closure of cleats and pores in the coal seams. However, a temporary recovery of permeability was observed after subsequent injection of N₂, suggesting that some amount of adsorbed CO₂ was replaced by N₂ and the closure of cleats and pores were mitigated. However, the changes of mechanical properties of coal in the process are not yet fully understood. In this study, we injected CO₂ and N₂ into coal specimens recovered from the Ishikari coal basin and observed the changes of coal strength under hydrostatic pressure and temperatures typical of a coal basin. We conducted triaxial compressive strength tests on the coal specimens to clarify the effects of CO₂ and N₂ on the mechanical properties. Our results are summarized below.

1) We verified that swelling and shrinkage of the coal were associated with adsorption and desorption, respectively, of CO₂.

2) The triaxial compressive strength tests showed that injection of CO₂ caused decreases of peak strength and Young's modulus.

3) Both coal strength and Young's modulus recovered when previously injected CO₂ was replaced by N₂.

Heterogeneity in mudstone/shale layers has significant effects on seal layer integrity. The presence of intralayer sandstone channels in a seal layer may allow the buoyant CO₂ to escape from the reservoir, even if the globally averaged permeability of the seal layer seems low enough. On the other hand, multi-layered structures are known to work often as baffles for the upward migration of CO₂ in formations. In this paper, we investigate the storage capacity of multilayer formations with discontinuous seals. Numerical simulations are carried out to study the effects of seal layer discontinuity on the long-term behaviour of CO₂ injected into deep saline aquifers. To represent a seal layer composed of low permeability rocks intersected by sandstone channels, ‘MINC' doubleporosity model is adopted. Also conducted is sensitivity analysis to investigate the effects of key parameters such as capillary pressure, relative permeability, temperature, and the thickness of the formations. The results show that CO₂ injection into a sufficiently deep multi-layered reservoir enables CO₂ to be stored and trapped in and around the reservoir without reaching to a shallow aquifer, even though seal layers have discontinuities. The upward movement of CO₂ is greatly affected by capillary pressure of sandstone channels in seal layers. The relative permeability and the temperature-dependent CO₂ properties have a significant effect on the final plume spread and the amount of CO₂ dissolved or fixed by residual gas trapping.

In this study, pressure response at A monitoring well in early stage of CO₂ geological storage were predicted against pressure build-up at CO₂ injector after starting CO₂ injection into a deep saline aquifer to design the monitoring well distance from the injector and resolution or sensitivity of a pressure transmitter installed in the monitoring well. The numerical simulations on pressure distributions and expanding CO₂ plume front were carried out using a reservoir simulator, CMG-STARS, for the aquifer (10 km in radius, 50m in height) with open boundary under 1,000 m from the ground or seabed level. The ratio of pressure response at the monitoring well against the pressure build-up at the injector have been presented for various monitoring locations (500 to 5,000 m from a injector) and homogeneous and heterogeneous models of horizontal permeability distribution in the aquifer (Base Model and Model 1, 2 & 3) and CO₂ injection patters during 100 days (Scheme 1, 2 & 3). It has been presented from the numerical simulation results that the monitoring well radial distance from the injector is recommended to be 2,000 to 4,000 m or less than 1000 m when the resolution or sensitivity of the pressure transmitter is 1kPa or 10kPa, respectively.

Toward the implementation of geological CO₂ storage, cost reduction of CO₂ monitoring during and after injection into targeted reservoirs is the key issue. The continuous gravity measurement using a superconducting gravimeter, which has been applied to the demonstration project site in Japan ahead of the rest of the world, has a potential to resolve the issue from the viewpoint of a complement to the high-cost seismic survey. The concept is such that the continuous gravity measurement is substituted for constant monitoring and that the use of seismic survey could be limited only when any anomaly is detected. The comparison of the monitoring cost after the completion of CO₂ injection indicated that applying gravity measurement can reduce costs by up to 15 to 50% of the seismic cost. The current detection limit of gravity change is around 1 μGal based on the application result in the coastal area in Japan. However, the time series analysis using pseudo gravity data suggested the possibility of anomaly detection even below the detection limit. Further improvement of analysis precision would lead not only to more rapid and more reliable anomaly detection but also to contribution to the cost reduction during the CO₂ injection period.

Studying the mechanisms embedded in the immiscible displacement of the two-phase fluids has many engineering and natural implications, like the CO₂ geological storage, the secondary or ternary oil and gas recovery and the NAPL remediation in underground formations. The pore-scale phenomena such as Haines jump and capillary filling are recognized in the literatures to have a strong impact on the displacement phenomena. Haines jump is characterized by an abrupt jump of some certain two-phase interfaces while rearranging the configuration of the meniscus of the surrounding pores. In contrast, the capillary filling induces a flat interface movement, where non-wetting phase fluid favors the larger pores due to capillary forces, and vice versa. In this article, we have simulated the drainage process in randomly distributed pore structures with varied pore throat diameters. The Haines jump and capillary filling phenomena are recognized to play a significant role in the saturation and frontal position curves. As a result, Haines jump tends to induce a hop of frontal position given the same saturation, whereas the capillary filling evades pore spaces with a fixed frontal position. In addition, the influence of capillary number and viscosity ratio are thoroughly analyzed. The result shows that higher capillary number and higher viscosity ratio are deemed favorable for the entrapment of non-wetting phase fluid (like CO₂). The energy equilibrium study shows around 48% of the external input work is dissipated through these instantaneous irreversible events. Ways to reduce the occurrence of these events will undoubtedly increase the displacement energy efficiency. This work has a direct implication for CO₂ geological storage.

Dissolution of gas into flowing water inside a porous medium is important in various hydrogeology processes, such as geological carbon sequestration (GCS). The dissolution rate of CO₂ controls the amount of CO₂ injection and the safety of the GCS process. In this report, we investigated the dissolution behavior of four gas species, i.e., CO₂, O₂, N₂, and Ar, into flowing water inside a porous medium by using micro-tomography. The trapped gas characteristics can be observed, and thus interfacial area and mass transfer coefficient can be calculated. Compared with the other gases, we found that the dissolution behavior of CO₂ gas is unique, and the mass transfer coefficient is one order of magnitude lower than other gases. We believe that the high dissolution ratio of CO₂ gas could have generated a layer of the high solute region around the trapped CO₂ gas, disrupting the mass transfer process.