## 1 Introduction  
Gas hydrate is polycrystalline, non-stoichiometric, and clathrate solid consisting of water molecules and small gas molecules, wherein water molecules form hydrogen bonds in cages, then gas molecules as guest molecules are wrapped in them [1]. Typical guest molecules include: methane, ethane, propane, and carbon dioxide, etc. Currently, hydrate blockage has become a hot issue in flow-assurance in the petroleum industry [2] since [PERSON] [3] discovered hydrate blockage in gas pipelines for the first time. Controlling methods of hydrate blockage include traditional inhibition [4; 5; 6; 7] and risk-control techniques [8; 9; 10; 11; 12]. As exploration and production is going into deeper water, the focus of flow-assurance strategies are shifting from traditional methods to risk-control techniques [2]. Hydrate slurry flow has gradually gained the attention of researchers [13; 14; 15; 16; 17; 18; 19; 20], who have referred to it as a form of hydrate risk control. However, studies on hydrate slurry are still in the period of experimental investigation with preliminary theoretical development [21]. In addition, the hydrate slurry might take place in decomposition reactions in the pipeline, when it is met with extraordinary operating conditions, resulting in transportation problems or hydrate blockage accidents. Consequently, it is important to investigate the hydrate slurry flow behavior under diverse operating conditions (e.g., particle agglomeration phenomena during the process of hydrate slurry decomposition). In order to eliminate the risk of hydrate blocking and to prevent secondary aggregation of hydrate particles in the decomposition process, we not only had to grasp the mechanism of hydrate formation, but also needed to master the mechanism of hydrate decomposition and its morphological changes of hydrate particles. Studies on hydrate slurry dissociation morphology can obtain valuable information through observing the state and change of hydrate interfaces [22]. This method supplements structural research (to analyze the hydrate structure and components) and molecular simulation modeling (to explain the formation and dissociation mechanism from the molecular level), with a direct and visualized acquiring and recording process [23]. Studies [24; 25; 26; 27; 28; 29; 30] had have found from dissociation morphology experiments the following: The irregular variation on the dissociating surface of the hydrate particle was controlled by the uniformity of system temperature, and it would not change with guest gas molecules. Hydrate lattice imperfection would lead to an asymmetrical pattern (partial decomposition) during the dissociation process. Guest gas molecules evolved from the cage lattice on the surface, while the produced water adsorbed on the undecomposed part to form a thin aqueous layer. This aqueous layer strengthened the wettability of the particle surface and enlarged the particle's own stickiness and the probability of agglomeration. The dissociation rate on the surface was inversely proportional to the escape resistance against guest gas out of the hydrate shell, and was proportional to the dissociation driving force. The hydrate dissociation in a flowing system would be far more complicated, compared to these above morphological studies [24; 25; 26; 27; 28; 29; 30] of a single hydrate particle under static conditions. Both decomposed and original particles coexist in a flowing system, increasing the probability of particle collision, agglomeration, and breakage under the shear effect. This would greatly influence the stability of hydrate slurry and the decomposition kinetics. However, research in the flowing field [9] is still insufficient so that further exploration and experimentation is required. In this paper, natural gas hydrate slurry decomposition experiments were investigated on a high-pressure hydrate experimental loop. The loop was equipped with the latest on-line particle analyzer focused beam reflectance measurement (FBRM) and particle video microscope (PVM).