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.

Assessment of natural gas hydrate reservoirs at Site GMGS3-W19 in the Shenhu area,South China Sea based on various well logs

To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19,extensive geophysical logging data and cores were analyzed to assess the reservoir properties.Sediment porosities were estimated from density,neutron,and nuclear magnetic resonance(NMR)logs.Both the resistivity and NMR logs were used to calculate gas hydrate saturations,the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data.The density porosity was closely in agreement with the core-derived porosity,and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates.The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity.For the gas hydrate reservoirs at GMGS3-W19,the porosity,gas hydrate saturation and free gas saturation was 52.7%,42.7%and 10%,on average,respectively.The various logs provide different methods for the comprehensive evaluation of hydrate reservoir,which supports the selection of candidate site for gas hydrate production testing.

Nuclear magnetic resonance study of the formation and dissociation process of nature gas hydrate in sandstone

In this work,the authors monitored the formation and dissociation process of methane hydrate in four different rock core samples through nuclear magnetic resonance(NMR)relaxation time(T_(2))and 2D imaging measurement.The result shows that the intensity of T_(2) spectra and magnetic resonance imaging(MRI)signals gradually decreases in the hydrate formation process,and at the same time,the T_(2) spectra move toward the left domain as the growth of hydrate in the pores of the sample accelerates the decay rate.The hydrate grows and dissociates preferentially in the purer sandstone samples with larger pore size and higher porosity.Significantly,for the sample with lower porosity and higher argillaceous content,the intensity of the T_(2) spectra also shows a trend of a great decrease in the hydrate formation process,which means that high-saturation gas hydrate can also be formed in the sample with higher argillaceous content.The changes in MRI of the sample in the process show that the formation and dissociation of methane hydrate can reshape the distribution of water in the pores.

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.

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.

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.