Novel concepts of mechanical technology for gas recovery from marine hydrate reservoir

According to the characteristics of marine natural gas hydrate,China has proposed the solid-state fluidization exploitation technology or natural gas hydrate,with subsea exploitation being key to the commercial recovery of gas.In this paper,two new integrated tools are proposed for breaking and collecting natural gas hydrate,and their working principles and steps are illustrated.Finite element analysis,three-dimensional modeling,and simulations were conducted for both exploitation tools to verify their technological feasibility.The results show that the two exploitation tools can effectively improve the efficiency of hydrate exploitation and ensure the stability of the hydrate reservoir.This provides a reference for further research on the solid-state fluidization exploitation technology of marine gas hydrates.

Sensitivity analysis of reservoir risk in marine gas hydrate drilling

It is easy to change the original temperature state of marine gas hydrate reservoir by drilling,which leads to uncontrollable decomposition of gas hydrate and release of large amount of gas.The decomposition gas will further escape and expand,and the reservoir will break and collapse due to its weak cementation characteristic,which will easily lead to a series of other potential risks.Therefore,in this study,based on the drilling process of marine gas hydrate,we establish the theoretical model and numerical calculation method of wellbore temperature field,analyze the influence on wellbore temperature of drilling fluid displacement,density,viscosity and injection temperature,and seawater depth.Then the sensitivity laws of reservoir risk in marine gas hydrate drilling are obtained.The results show that with the increase of drilling fluid displacement,density,viscosity and injection temperature,the temperature in lower well section and bottom hole will increase,making the increasing amplitude of temperature in hydrate reservoir larger and the scope of influence on hydrate reservoir stability bigger.Moreover,drilling is more likely to raise the temperature of reservoirs in shallow seawater depth,posing greater risks.Thus,engineering measures to avoid risks caused by rising reservoir temperature in marine gas hydrate reservoir drilling are presented.This study is of great significance to ensure the safety of marine gas hydrate reservoir drilling.

Application of deep-towed multichannel seismic system for gas hydrate on mid-slope of northern Cascadia margin

The Deep-towed Acoustics and Geophysics System (DTAGS) is a high frequency (220-820 Hz) multichannel seismic system towed about 300 m above seafloor.Compared to the conventional surface-towed seismic system,the DTAGS system is characterized by its shorter wavelength (<6 m),smaller Fresnel zone,and greater sampling in wavenumber space,so it has unique advantages in distinguishing fine sedimentary layers and geological structures.Given the near-bottom configuration and wide high-frequency bandwidth,the precise source and hydrophone positioning is the basement of subsequent seismic imaging and velocity analysis,and thus the quality of array geometry inversion is the key of DTAGS data processing.In the application of exploration for marine gas hydrate on mid-slope of northern Cascadia margin,the DTAGS system has shown high vertical and lateral resolution images of the sedimentary and structural features of the Cucumber Ridge (a carbonate mound) and Bullseye Vent (a cold vent),and provided abundant information for the evaluation of gas hydrate concentration and the mechanism of fluid flow that controls the formation and distribution of gas hydrate.

Effect of Agents on Hydrate Formation and Low-Temperature Rheology of Polyalcohol Drilling Fluid

In order to ensure safe drilling in deep water and marine gas hydrate bearing sediments, the needed characteristics of drilling fluid system were analyzed. Moreover, the effect of different agents on hydrate formation and the low-temperature rheology of designed polyalcohol drilling fluid were tested, respectively. The results show that clay can promote gas hydrate growth, while modified starch and polyalcohol can inhibit hydrate formation to some extent, and PVP K90 has a good performance on hydrate inhibition. The influence of clay on low-temperature rheology of polyglycols drilling fluid is notable. Therefore, the clay-free polyalcohol drilling fluid is suitable for deep water and marine gas hydrate drilling under optimal conditions.