Analysis of productivity and stability behaviors according to the characteristics of gas hydrate-bearing sediment through the thermal-hydraulic-mechanical coupled algorithm

Abstract

however, the stability tended to deteriorate. Although the effect of the depressurization rate on total productivity and stability was quite small, gradually increasing the depressurization rate can ensure initial stability, which prevents the sand production phenomenon. The results of this study are expected to be used as basic data for establishing a production strategy for the planned field production tests in the Ulleung Basin after 2024.

few have considered the productivity and stability in long-term production.In this dissertation, effective permeability and stiffness models were derived from permeability experiments and high-pressure triaxial tests using the artificial specimen of the Ulleung Basin, which were then applied to THM algorithms. Productivity and stability analyses were performed based on the characteristics of the site. The analysis confirmed that the stiffness model did not significantly affect the productivity and stability. However, in the case of the derived permeability model, the total productivity of gas and water was less than that of the previous model. The initial hydrate saturation was analyzed as the ground characteristic that had the greatest influence on productivity and stability.An interface model was derived using the results of the direct shear test and consolidation test that simulated the interface between the production wellbore and sediments of the Ulleung Basin. A stability evaluation algorithm for the production wellbore was proposed, which considered the slippage of the interface between the production wellbore and sediments. As a result of the stability evaluation of the production wellbore, axial stress was generated in the production wellbore according to the ground behavior. However, when excessive ground subsidence occurred, the development of additional axial stress did not occur because of ground shear failure at the interface.Long-term production behavior according to the initial hydrate saturation was analyzed through long-term production analysis of the Ulleung Basin for three years. The productivity and stability according to the depressurization strategy in short-term production, and production efficiency were evaluated. The productivity analysis according to the production period showed that the lower the initial hydrate saturation, the higher the short-term productivity and production efficiency. The longer the production period, the lower the productivity and production efficiency. These results confirm that the higher the initial hydrate saturation, the higher the total productivity and production efficiency in long-term production.For the depressurization method, the lower the bottom hole pressure, the higher the productivity

thus, the analysis had to be performed by applying models reported in the literature. In most studies, the sediments were analyzed as homogeneous ground. Previous studies have also focused on estimating the water and gas productivity for a short-term production period

hence, research is mainly conducted through numerical analysis. Reasonable input parameters must be used to obtain accurate numerical modeling results. Permeability is a geotechnical property with the greatest impact on productivity. The permeability of hydrate-bearing sediments depends on the geotechnical properties of the sediments and the hydrate saturation. However, in previous studies, there was no permeability and stiffness model reflecting the characteristics of the Ulleung Basin

Previous studies have confirmed approximately 0.6 billion tons of methane hydrate reserves in the Ulleung Basin, East Sea, Korea. Although gas production from hydrate-bearing sediments is being studied worldwide, the production mechanism is a complex phenomenon in which thermal-hydraulic-mechanical (THM) phenomena occur simultaneously. A depressurization method, inhibitor or hot water injection method, and molecular exchange method are being actively studied for hydrate production. Among these, depressurization is considered a reliable production method in Korea.In the production of methane gas using the depressurization method, the increase in effective stress due to decompression, and the decrease in the strength of sediments due to the dissociation of solid hydrates may cause ground subsidence, while vertical displacement of the sediments affects the stability of the production wellbore. In addition, when the hydrate is dissociated, the temperature change of the sediments due to the endothermic reaction changes the hydrate phase-change characteristics, which affects the gas dissociation rate. Therefore, for the production and utilization of gas hydrates, an in-depth study of gas productivity and stability according to the production period is required.Studies for estimating gas productivity and stability are difficult to analyze on a laboratory scale