Phase Equilibrium Conditions And Physico-mechanical Properties Of Seafloor Hydrate-bearing Sediments

Energy resources play crucial roles in for the development of human society and economy.Under the background of global energy transformation,natural gas hydrate(“hydrate” hereinafter)is considered as one of the most potential strategic alternative energy resources in the 21 st century,due to its rich resources,wide distribution,and efficient energy storage and clean combustion,and has been attracting more and more attentions from industrial sectors and research institutions around the world.In the exploitation of submarine hydrate,however,the mechanical properties of the reservoir is deteriorated,triggering engineering problems and geological hazards such as drilling platform damage,production well failures and even ecological and geological disasters.Therefore,determination and characterization of the engineering properties of hydrate bearing sediments is dispensiably important for evaluation and control of production-induced hazards.This research is focused on the characterization of the phase equilibrium conditions and mechanical properties of hydrate-bearing sediments and the evolution of these properties during hydrate extraction.The obtained important conclusions are summarized as follows.(1)Based on the LF-NMR technique,the temporal variations of individual bulk component contents during the decomposition of methane hydrate in sediments were measured and analyzed;a method is proposed to effectively determine the average hydration number.It is shown that the hydrate decomposition in small pores is prior to that in macropores,which is related to the thermodynamic potential energy of its occurrence environment.It is also shown that the experimental conditions,including initial pressure,density,initial water saturation and salt content,have insignificant effect on the hydration number during the hydrate decomposition in sediments.(2)The phase equilibrium conditions of methane hydrate in sediments with wide pore-size distribution were determined via the isochoric heating method,and the effects of density,initial water saturation,fine particle content and salt content on the phase equilibrium conditions and unhydrated water were analyzed.It is shown that the phase equilibrium condition of pore methane hydrate,when presented on the P-T plane,depends upon the initial experimental conditions;in a contrast,however,a unique relationship exists among temperature shift,unhydrated water and the amount of dissolved salt.The underlying reason for the difference bwteen the hydration characteristic curve of salt-free sand and fine-grained soil is different.The former is attributed to the fact that the potential energy of unhydrated water is practically unaffected by pore structure in sand,whereas the latter is closely related to water-adsorption effect in fine soil.For sand samples,the generalized phase equilibrium model can very well describe the relationship between unhydrated water and temperature shift without introducing any other parameters.For fine-grained soil samples,however,the effect of salt content on soil suction should be taken into account in the model prediction.(3)Methane hydrate was synthesized in fine sand via the excess gas method and the frost seeding method,and the effect of water saturation on the P-wave velocity and the mechanical properties of hydrate-bearing sand was studied.The results show that the influence of hydrate occurrence on the P-wave velocity,shear strength and stress-strain behavior of hydrate-bearing sand depends on water saturation,hydrate synthesis procedure and hydrate saturation.Water-saturated sand samples containing hydrate can degrade the cementation effect of pore hydrate,depending upon the method of sample preparation and hydrate saturation.It is suggested that the combination of frost seeding method and water saturation method can be effectively used to prepare the hydrate-bearing sand samples,as substitution for those occurring under seafloor water-rich environments.(4)A series of isotropic and triaxial compression tests were performed on the methane hydrate-bearing sediments prepared by the excess gas method.The results show that with the increase of hydrate saturation,the swelling index decreases and the yield stress increases linearly,whereas the compression index is practically independent of hydrate saturation.The hydrate-bearing fine-grained sediment generally shows strain-hardening behavior,though strain softening may occur at low confining net stress and high hydrate saturation.The degree to which hydrate enhances the stiffness and strength of the specimen and the effect of hydrate on the dilatancy curve depend on the confining net stress.Based on the test results,a strength model and a mathematical expression which can characterize the shear expansion characteristics of hydrate cementation damage caused by confining net stress in hydrate-bearing fine-grained sediment are established.(5)In decomposition of pore hydrate,the shear strength of the hydrate-bearing sediment follows an exponential attenuation law,depends upon initial hydrate saturation and decomposition degree.The shear strength of the sample with partial hydrate decomposition is significantly lower than that of the samples without hydrate decomposition at the same hydrate saturation.These results can be attributed to the formation,damage or degradation of hydrate cementing characteristics at particle contact and the reduction of soil suction.Based on the generalized equation of phase equilibrium,a strength criterion is proposed for the hydrate-bearing soils with hydrate decomposition.