Tae-Hyuk Kwon

Site characterization and geotechnical aspects on geological storage of CO2 in Korea

The long-term storage of carbon dioxide (CO2) in deep geological formations, known as geological CO2 storage (GCS), has the potential to reduce CO2 emissions by 20%, a figure considered necessary to stabilize atmospheric CO2 levels over the next century. The purpose of this paper is to present the current state and future direction of geological CO2 sequestration in Korea. This study reviewed current storage technologies and strategies related to GCS worldwide, and the most suitable basins for GCS in Korea were selected from current available geophysical and geological research results. Finally, scientific questions and technical challenges were discussed in relation to the injection, storage, and monitoring processes from geotechnical engineering perspectives.

Observations of pore-scale growth patterns of carbon dioxide hydrate using X-ray computed microtomography

Natural and artificial gas hydrates with internal pores of nano to centimeters and weak grain-cementation have been widely reported, while the detailed formation process of grain-cementing hydrates remains poorly identified. Pore-scale morphology of carbon dioxide (CO2) hydrate formed in a partially brine-saturated porous medium was investigated via X-ray computed microtomography (X-ray CMT). Emphasis is placed on the pore-scale growth patterns of gas hydrate, including the growth of dendritic hydrate crystals on preformed hydrate and water-wetted grains, porous nature of the hydrate phase, volume expansion of more than 200% during the water-to-hydrate phase transformation, preference of unfrozen water wetting hydrophilic minerals, and the relevance to a weak cementation effect on macroscale physical properties. The presented pore-scale morphology and growth patterns of gas hydrate are expected in natural sediment settings where free gas is available for hydrate formation, such as active gas vents, gas seeps, mud volcanoes, permafrost gas hydrate provinces, and CO2 injected formation for the sake of geologic carbon storage; and in laboratory hydrate samples synthesized from partially brine-saturated sediments or formed from water-gas interfaces.

Thermal dissociation behavior and dissociation enthalpies of methane-carbon dioxide mixed hydrates.

Replacement of methane with carbon dioxide in hydrate has been proposed as a strategy for geologic sequestration of carbon dioxide (CO(2)) and/or production of methane (CH(4)) from natural hydrate deposits. This replacement strategy requires a better understanding of the thermodynamic characteristics of binary mixtures of CH(4) and CO(2) hydrate (CH(4)-CO(2) mixed hydrates), as well as thermophysical property changes during gas exchange. This study explores the thermal dissociation behavior and dissociation enthalpies of CH(4)-CO(2) mixed hydrates. We prepared CH(4)-CO(2) mixed hydrate samples from two different, well-defined gas mixtures. During thermal dissociation of a CH(4)-CO(2) mixed hydrate sample, gas samples from the head space were periodically collected and analyzed using gas chromatography. The changes in CH(4)-CO(2) compositions in both the vapor phase and hydrate phase during dissociation were estimated based on the gas chromatography measurements. It was found that the CO(2) concentration in the vapor phase became richer during dissociation because the initial hydrate composition contained relatively more CO(2) than the vapor phase. The composition change in the vapor phase during hydrate dissociation affected the dissociation pressure and temperature; the richer CO(2) in the vapor phase led to a lower dissociation pressure. Furthermore, the increase in CO(2) concentration in the vapor phase enriched the hydrate in CO(2). The dissociation enthalpy of the CH(4)-CO(2) mixed hydrate was computed by fitting the Clausius-Clapeyron equation to the pressure-temperature (PT) trace of a dissociation test. It was observed that the dissociation enthalpy of the CH(4)-CO(2) mixed hydrate lays between the limiting values of pure CH(4) hydrate and CO(2) hydrate, increasing with the CO(2) fraction in the hydrate phase.

Rheological properties of cemented tailing backfill and the construction of a prediction model

Workability is a key performance criterion for mining cemented tailing backfill, which should be defined in terms of rheological parameters such as yield stress and plastic viscosity. Cemented tailing backfill is basically composed of mill tailings, Portland cement, or blended cement with supplementary cement material (fly ash and blast furnace slag) and water, among others, and it is important to characterize relationships between paste components and rheological properties to optimize the workability of cemented tailing backfill. This study proposes a combined model for predicting rheological parameters of cemented tailing backfill based on a principal component analysis (PCA) and a back-propagation (BP) neural network. By analyzing experimental data on mix proportions and rheological parameters of cemented tailing backfill to determine the nonlinear relationships between rheological parameters (i.e., yield stress and viscosity) and mix proportions (i.e., solid concentrations, the tailing/cement ratio, the specific weight, and the slump), the study constructs a prediction model. The advantages of the combined model were as follows: First, through the PCA, original multiple variables were represented by two principal components (PCs), thereby leading to a 50% decrease in input parameters in the BP neural network model, which covered 98.634% of the original data. Second, in comparison to conventional BP neural network models, the proposed model featured a simpler network architecture, a faster training speed, and more satisfactory prediction performance. According to the test results, any error between estimated and expected output values from the combined prediction model based on the PCA and the BP neural network was within 5%, reflecting a remarkable improvement over results for BP neural network models with no PCA.

Evaluation of Particulate Materials using Wave-Based Techniques

Elastic and electromagnetic waves provide important information about particulate materials. In order to facilitate the application of wave-based techniques to soil characterization, fundamental soil properties are first reviewed, and then experiments are performed for both elastic and electromagnetic waves. The first application is related to characterization of particulate materials using shear wave, concentrated on the change of effective stress in consolidation process, multi-phase phenomenon with relation to capillarity, and microscale characteristics of grain particles. The second application is relevant to the electromagnetic wave, focused on stratigraphy detection in layered soils, estimation of void ratio and its spatial distribution, and conduction in unsaturated soils. Experimental results suggest that the shear wave measurements allow studying the evolution of effective stress in unsaturated soils as well as in saturated soils while the electromagnetic wave measurements give an insight into conduction process and allow estimating void ratio and its spatial distribution.

Ultrasonic P-Wave Reflection Monitoring of Soil Erosion for Erosion Function Apparatus

Erosion of soils in river, lake, and seabeds is an important component for scour estimation and design of underwater structures. This is because the scour can cause severe structural damage to underwater foundations or embankments. The erosion function apparatus (EFA) method is widely used to estimate the erosion rate of soils in the laboratory, where a soil protrusion of 1 mm thick is exposed to water flow and the time taken to erode this protrusion is measured. However, determining this erosion time is a difficult task because it is only visually inspected, and this can cause considerable measurement errors. Therefore, this study explored the feasibility of using an ultrasonic P-wave reflection monitoring method to more quantitatively assess the erosion rate that otherwise has been measured by visual inspection. The erosion rates were monitored using ultrasonic transducers mounted above a soil surface during the EFA testing on the prepared soil samples containing different clay fractions. Via the P-wave monitoring results, several important semi-quantitative observations were made: an increase in erosion resistance with an increase in the clay fraction, a discontinuous erosion behavior of fine-grained soils with sudden removal of soil lumps by water flows, a continuous erosive action of coarse-grained soils, and inherent heterogeneous erosion even at a specimen scale (i.e., the scale of milli-to-centimeter). While both the P-wave monitoring method and the visual inspection showed similar estimation on the erosion rate, the former was found to provide overall better quantitative assessment, particularly in conditions of very slow or rapid erosion and in the conditions with high turbidity water, unevenly eroded sample surfaces, or limited control on the soil protruding thickness.

Effect of slit-type barrier on characteristics of water-dominant debris flows: small-scale physical modeling

Slit-type barriers, one of open-type barriers, are widely used as active measures to mitigate potential risk and damage by debris flows, and those are designed and installed to reduce the flow energy by only passing relatively small debris. However, the mechanisms of slit-type barriers in reducing the debris flow velocity and debris volume remain poorly understood because of the lack of well-controlled and reliable physical modeling results. This study explored the influence of various arrangements of slit-type barriers, including P-type barriers in which each rectangular barrier was placed in parallel and V-type barriers where the barriers were placed in a V-shape, on characteristics of water-dominant debris flows via small-scale model experiments. The debris flow events were reproduced against the slit-type barriers, where the velocity reduction and trap ratio were monitored, varying the angle and shape of barrier arrangements. The velocity reduction and trap ratio appeared to increase as the angle of the barrier wall decreased because of the decreased opening ratio. The V-type barriers resulted in higher velocity reduction and trap ratio than the P-type, primarily because of the smaller effective opening ratio and the more backwater effect. In addition, as the debris contained more boulders, the extent of velocity reduction and debris trap became greater in all barrier types. Two types of opening ratios, the projected and effective opening ratios, were correlated to the interactions between debris and walls. The obtained results provide baseline data for the optimum design of slit-type barriers against debris flow and suggest that the slit-type barriers can effectively manage the risk of damage by debris flows.

Depressurization-Induced Fines Migration in Sediments Containing Methane Hydrate: X-Ray Computed Tomography Imaging Experiments

Author(s): Han, G; Kwon, TH; Lee, JY; Kneafsey, TJ | Abstract: ©2018. American Geophysical Union. All Rights Reserved. Depressurization of hydrate-bearing sediments (HBS) can cause the movement of fine particles, and in turn, such fines migration affects fluid flow and mechanical behavior of sediments, ultimately affecting long-term hydrocarbon production and wellbore stability. This study investigated how and to what extent depressurization of HBS causes fines migration using X-ray computed tomography (CT) imaging. Methane hydrate was synthesized in sediments with 10% fines content (FC), composed of sands with silt and/or clay, and the hydrate-bearing samples were stepwisely depressurized while acquiring CT images. The CT images were analyzed to quantify the spatial changes in FC in the host sediment and thus to capture the fines migration during depressurization. It was found that the FC changes began occurring from the hydrate dissociation regions. This confirms that the multiphase flow caused by depressurization accompanies fines migration. Depressurization of HBS with a hydrate saturation of ~20–40% caused FC reduction from ~10% to ~6–9%, and the extent of fines migration differed with the particle sizes of the host sands and the types of fines. It was found that fines migration was more pronounced with coarse sands and with silty fines. Such observed level of FC reduction is estimated to increase sediment permeability by several factors based on the Kozeny-type permeability model. Our results support the notion that the extent of fines migration and its effect on fluid flow behavior need to be assessed in consideration of physical properties of host sediment and fine particles to identify optimum depressurization strategies.

Biosurfactant as an Enhancer of Geologic Carbon Storage: Microbial Modification of Interfacial Tension and Contact Angle in Carbon dioxide/Water/Quartz Systems

Injecting and storing of carbon dioxide (CO2) in deep geologic formations is considered as one of the promising approaches for geologic carbon storage. Microbial wettability alteration of injected CO2 is expected to occur naturally by microorganisms indigenous to the geologic formation or microorganisms intentionally introduced to increase CO2 storage capacity in the target reservoirs. The question as to the extent of microbial CO2 wettability alteration under reservoir conditions still warrants further investigation. This study investigated the effect of a lipopeptide biosurfactant—surfactin, on interfacial tension (IFT) reduction and contact angle alteration in CO2/water/quartz systems under a laboratory setup simulating in situ reservoir conditions. The temporal shifts in the IFT and the contact angle among CO2, brine, and quartz were monitored for different CO2 phases (3 MPa, 30°C for gaseous CO2; 10 MPa, 28°C for liquid CO2; 10 MPa, 37°C for supercritical CO2) upon cultivation of Bacillus subtilis strain ATCC6633 with induced surfactin secretion activity. Due to the secreted surfactin, the IFT between CO2 and brine decreased: from 49.5 to 30 mN/m, by ∼39% for gaseous CO2; from 28.5 to 13 mN/m, by 54% for liquid CO2; and from 32.5 to 18.5 mN/m, by ∼43% for supercritical CO2, respectively. The contact angle of a CO2 droplet on a quartz disk in brine increased: from 20.5° to 23.2°, by 1.16 times for gaseous CO2; from 18.4° to 61.8°, by 3.36 times for liquid CO2; and from 35.5° to 47.7°, by 1.34 times for supercritical CO2, respectively. With the microbially altered CO2 wettability, improvement in sweep efficiency of injected and displaced CO2 was evaluated using 2-D pore network model simulations; again the increment in sweep efficiency was the greatest in liquid CO2 phase due to the largest reduction in capillary factor. This result provides novel insights as to the role of naturally occurring biosurfactants in CO2 storage and suggests that biostimulation of biosurfactant production may be a feasible technique for enhancement of CO2 storage capacity.

P and S wave responses of bacterial biopolymer formation in unconsolidated porous media

Author(s): Noh, DH; Ajo-Franklin, JB; Kwon, TH; Muhunthan, B | Abstract: