The Production-Induced Geomechanical Property Changes during Gas Production from Gas Hydrate Deposits

摘要

Gas hydrates are widespread, occurring in both permafrost and deep sea sediments. The large estimated areas of gas hydrate reservoirs suggest that the high potential of gas hydrates as an energy resource if economically viable production methods were developed. The production of natural gas from gas hydrate deposits poses challenges such as assessing hydrate recovery rates from physical properties and geological structure of the hydrate reservoir, securing the economic viability of produced gas from a particular resource, and keeping process safe from geomechanical impacts from hydrate dissociation. During the hydrate dissociation and the subsequent gas production from dissociated gas hydrate, geomechanical property changes due to the sediment deformation, the changes in hydrate saturations, and fine migrations. In this study, extensive laboratory studies have been conducted to quantify these issues and the implications of these changes to the gas production from gas hydrate deposits have been investigated. Strength, stiffness, permeability changes due to gas hydrate saturations were examined in high-pressure oedometric system and tri-axial system. Fine migrations characteristics and the subsequent property changes were examined with many different experimental systems. The experimental system includes core-flooding system with X-ray CT monitoring, oedometric system, triaxial system, and one-dimensional fine migration experiment system. The sediment used in this study is synthesized gas hydrate-bearing sediments and the mean grain size of the sediments lies in fine sands. Hydrate saturation ranges from 10 to 50%. Fine fraction ranges also from 10 to 50%. Sediment deformation from compressive stress concentration generally increases stiffness and decreases permeability. Hydrate saturation decrease induced from gas hydrate production generally decrease strength and stiffness and increase permeability. The property changes are not linearly related to gas hydrate saturations and the relations differ depending on the character of deposits. Fine migrations induced by gas hydrate production alter fine contents in producing intervals and also would change geomechanical properties. Moving particles generally concentrates near well-bore but the locus of concentration depends on the character of the producing interval, such as grain size distributions and flow rate. Even a small fraction of fine particles can induce significant changes in physical properties. In fine-concentrated zones, stiffness generally increases and permeability generally decreases. The quantifications of these phenomena based on the systematic and extensive experimental studies are the essential steps before the development of THM numerical simulation code for gas hydrate production. For near future the quantitative relations in this study will be implemented to THM simulation code for gas hydrate production.