Yasuhide Sakamoto

Laboratory-Scale Experiments of the Methane Hydrate Dissociation Process in a Porous Media and Numerical Study for the Estimation of Permeability in Methane Hydrate Reservoir

An experimental study of the dissociation of methane hydrate (MH) by hot-water injection and depressurization was carried out at the National Institute of Advanced Industrial Science and Technology (AIST). These experiments helped us understand some important aspects of MH behavior such as how temperature, pressure, and permeability change during dissociation and gas production. In order to understand the experimental results, a model of MH dissociation in a porous media was designed and implemented in a numerical simulator. In the model, we treated the MH phase as a two-component system by representing the pore space occupied by MH as a separate component. Absolute permeability and relative permeability were formulated as a function of MH saturation, porosity, and sand grain diameter and introduced into the numerical model. Using the developed numerical simulator, we attempted history matching of laboratory-scale experiments of the MH dissociation process. It was found that numerical simulator was able to reproduce temperature change, permeability characteristics, and gas production behavior associated with both MH formation and dissociation.

EXPERIMENTAL STUDY OF ENHANCED GAS RECOVERY FROM GAS HYDRATE BEARING SEDIMENTS BY INHIBITOR AND STEAM INJECTION METHODS

The inhibitor and steam injection methods have been examined using a laboratory-prepared methane hydrate bearing sediment. New experimental apparatuses have been designed and constructed. In the case of inhibitor injection, the measurement of gas production vs. time suggested that the inhibitor increased dissociation rate. Core temperature decreased upon the inhibitor injection, in contrast to that in the case of pure water injection. The observed pressure differentials between the inlet and outlet of the core sample suggest that the inhibitor effectively prevented the hydrate reformation within the dissociating core sample. In the case of steam injection coupled with depressurization, it can be seen that the effect of steam (or hot water) injection was clear in the later stage of dissociation, compared with that in the case of depressurization alone. The inner (core) temperature change indicates that the coupling of depressurization and steam injection induces MH dissociation from upstream and downstream to the center of the sample. However, it starts from an upstream region and continues downstream steadily in the case of steam (hot water) injection alone.

Evaluation of Natural Degradation of Persistent Organic Chemicals in Acid Sulfate Soils Distributed in a Coastal Area

Abstract—Coastal acid sulfate soils are naturally generated sediments, mainly containing iron sulfides. This study investigated the natural distribution of acid sulfate soils at the mouth of a river, and their remediation ability of persistent organic chemicals. These soils were distributed on a small scale in southern Japan; Iriomote Island was used as the study site. Coastal acid sulfate soils were found to be restricted to the surface in the downstream portions but distribution depth increased upstream. The shallow, surficial portion of acid sulfate soils in upstream areas had already oxidized and leached sulfuric acid. The degradation ability of sampled acid sulfate soils for dieldrin was confirmed in laboratory experiments and the degradation ability was found to increase with iron sulfide content. These reactions were controlled by the chemical reactivity of iron sulfides in natural systems but independent of microbial activity.

Risk Evaluation of Toxic Chemicals in the Geo‐environment by Site‐Specific Risk Assessment Model

A risk assessment model which can be evaluated risk levels of toxic chemicals for human beings living in contaminated both onsite and offsite have been developed. The case studies of the risk assessment by using this model were carried out and the exposure rates, the distribution of exposure paths and the risk level of toxic chemicals in the geo‐environment were estimated. The main exposure pathways of trichloroethylene (TCE) at the onsite were inhalation air, groundwater intake and crops intake, which indicates that TCE easily volatilize into the air and migrate into the pore water from the soil. The exposure rate of the inhalation air at the offsite was not so high, because the concentration of TCE in the air at the offsite was decreased by the fresh air. However, the exposure rate of the groundwater intake at the offsite had a high risk when no degradation and no adsorption of TCE occurred in the groundwater environment. Considering the adsorption or the degradation of TCE, the risk level from the groundwater intake at the offsite was not so high. To raise the precision of the evaluation, it is very important to obtain these site‐specific parameters.

Experimental Study on Improvement of Permeability and Gas Production Behavior by the Simultaneous Injection of Hot Water and Nitrogen-Estimation of Permeability in Methane Hydrate Reservoir, Part3-

Methane hydrate is one of the potential resources of natural gas in the near future, because it exists in marine sediments or in permafrost regions worldwide. Some extraction methods of methane hydrates from the reservoir has been proposed, such as depressurization, thermal stimulation and inhibitor injection. These are all based on the in-situ dissociation process of methane hydrates that is transformed into methane methane and water. Therefore, it is very important to clarify the physical phenomena of gas-water multiphase flow in porous media and the properties of formation and dissociation of methane hydrates.We have carried out the experimental study on hot water injection as one of the thermal stimulation methods. From the results, it was found that (1) when temperature in downstream zone of sand column was lower than equilibrium condition, additional hydrate growth was promoted at downstream zone due to migration of cooled water and dissociated gas, (2) as a result, differential pressure increased exponentially, and water permeability of sand column decreased drastically.For inhibition of hydrate growth and improvement of permeability in hydrate reservoir, we conducted further experimental work on the simultaneous injection process of nitrogen and hot water. Nitrogen has the effect as an inhibitor as well as methanol and salts. In this experiment, firstly, nitrogen was injected into sand column to displace free methane gas, and then hot water injection was started. In the progress of dissociation, temperatures in the sand column and differential pressures, production rate of dissociated gas were measured. Additionally, based on measuring data, water permeability in dissociation process was estimated. Due to the inhibitor effect of nitrogen, it was possible to continue water injection without permeability reduction. Thus dissociated gas production was completed earlier in comparison with normal hot water injection process.