Coupling DEM Simulation And Stability Study On Grain-cementing Type MHBS Under Hydrate Dissociation

Methane hydrate also known as combustible ice is an ice-like crystal substance,in which methane is trapped within a cage structure composed of water molecules under high pressure and low temperature conditions.It is widely distributed in deep sea sedi-mentary layers within 0-1500 meters and Arctic permafrost regions.However,the sub-marine environment of methane hydrate is often affected by geological configuration,and artificial activities（exploitation,etc.）,resulting in the dissociation of methane hy-drate.This further causes the increase of excess pore pressure and the weakening of hydrate bond cementation,resulting in the decrease of the strength of methane hydrate bearing sediments（MHBS）,which leads to large-scale submarine landslide,tsunami,and catastrophic damage to submarine engineering structure（such as oil and gas pipe-lines,submarine cables and other engineering facilities）.Therefore,the mechanical properties of grain-cementing type MHBS under different temperature,pressure and salinity are studied in this thesis.The specific research contents are summarized as fol-lows:（1）Given that several multi-field coupling simulators fails to accurately describe large deformation such as particle migration,we construct a practical multi-filed method T+H+PFC（TOUGH+HYDRATE+PFC）for methane hydrate bearing sedi-ments（MHBS）based on an improved Distinct Element Method（DEM）commercial software PFC and a numerical code T+H（TOUGH+HYDRATE）targeting the Mul-tiphase flow analysis of hydrate-bearing geologic systems.The coupling simulation can be achieved by the information exchange between porosity updated by mechanical module DEM and temperature,pressure,and salinity updated by T+H.Then,the method is validated by conducting numerical modeling of one-dimensional（1D）con-solidation and 1D heat conduction tests.（2）A 1D production model was established by using the hydrate model parame-ters obtained by comparing the results of gas production and temperature from available laboratory test.Then,T+H+PFC is used to simulate the dynamic responses of MHBS and weakening of hydrate bond under one-dimensional gas production tests（depressur-ization,heat stimulating,chemical injection and combined exploitation methods）.The results suggest that due to the weakening of MHBS strength during methane hydrate exploitation there is a linear correlation between soil deformation and bond breakage regardless of exploitation method.This finding is in good agreement with the experi-mental results,which proves the reliability of this coupling simulation method.Mean-while,depressurization exploitation will cause large deformation due to the increase of effective stress;the effects of temperature and salinity on strength and deformation are similar.For different exploitation methods,the depressurization method will cause a larger decrease in temperature,especially for the specimen with lower initial tempera-ture.So,heat simulating method or inhibitor injection method should be combined to improve the exploitation efficiency.（3）Based on the gas production trial in Nankai Trough in Japan,the parameter selection and fitting to actual geographic information,the simulation conducted can properly reflect the multi field coupling correlation and the evolutions of temperature,pressure and gas production rate in the process of production.Then the evolution of mechanical properties and deformation of MHBS under actual gas production is ana-lyzed.The change of temperature,deformation,water pressure and methane hydrate cementation breakage develop rapidly,and the change of them is closely related.The mechanical properties of the soil around the wellbore are rapidly weakened,the settle-ment of sediments develops unevenly during depressurization stage especially under large depressurization.Meanwhile,the increase of effective stress caused by depressur-ization is also the main factor leading to the sediments deformation.The stress path at each point of sediments develops towards k₀ line in the process of hydrate decomposi-tion.Finally,when the hydrate cementation is completely broken,the stress path falls to the strength envelope line of the pure sand.At the same time,the horizontal pressure acts on the wellbore also increases,so attention should be paid to the effect of horizontal pressure on the stability of the wellbore.