Acoustic Emission Detection-based CO₂ Hydrate Fromation Method In Sandy Sediments

The deep sea-seated natural gas hydrate is a clean energy with large reserves,and its sucessful utilization definitely produce a profound impact on improving China's coal-based energy consumption structure.However,the deep sea-seated hydrates are prone to decompose uncontrollably under a disturbance from exploitation activities.This can cause landslides of the sea floor and even the collapse of offshore platforms.A development of monitoring strategy of hydrate formation and decomposition process applicable to deep sea environment is significant for realizing risk warning and disaster prevention.The correspondent studies are urgently required.Based on the current frozen hollow cylinder test system,a test method considering the field condition characterized by K₀ consolidation,freezing under applied stress and gas permeation was developed.A strategy on gas permeation along the radial direction of the hollow cylinder was propsed.This measure lead to a short gas permeation path and a great contact area between gas and sediments.Then,a novel gas flow and pressure control apparatus was manufactured.A comprehensive test system can achive the condition for horizontal gas permeation and the matched test method were developed to study the hydrate in sediments.Standard sand was selected as the sediment and CO₂ was selected as the gas source.CO₂ hydrate was generated for the first time considering a horizontal gas permeation environment.Hydrate formation-induced acoustic emission?AE?in sandy sediments is a typical local transient event dominated by phase transition.With the continuous formation of hydrates in sediments,the AE signal intensity was enhanced and the distribution range of hydrate in sediments was expanded.The measured AE signal amplitude during the formation process of hydrates increased by over 30%compared with the sandy sediments without hydrate,the AE energy and duration have two-fold increase.The presence of hydrate obviously improved the viscoplastic bebavior of the sediment and increased the strength of the sediment.However,the intensity of AE signials was significantly higher than the hydrate formation process due to the occlusion and breakage of sand particles under the confinment.The AE energy and amplitude excited in sediments with hydrate during triaxial shearing were reduced by nearly 30%and 14%due to the lubrication and buffering effects,respectively,compared to those in sediments without hydrate.Acoustic emission characteristics associated in the formation process of hydrates in sandy sediments will provide a scientific basis for the development of dynamic monitoring techniqe for decomposition of deep sea-seated hydrate.This thesis includes 142 Figures,9 Tables and 74 References.