Study on the Decomposition of Natural Gas Hydrate Induced by Cavitation Damage
Authors: Huang Haochen , Zhang Yiqun , Wu Xiaoya , Hui Chengyu , Tan Yawen , Li Gensheng 
Published: September 2023

ABSTRACT
Natural gas hydrate (NGH) is a clean and abundant emerging energy source that develops in pores of soil deposits in permafrost regions and deep seabed, which are difficult to exploit. Cavitation bubbles contain powerful energy that can effectively break hydrate and may bring higher exploitation efficiency. However, the decomposition of hydrate induced by cavitation damage is still unclear, and the damage characteristics of hydrate need further research. In this paper, a study is conducted based on experiments of laser induced cavitation damage to methane hydrate to evaluate the influence of cavitation on the decomposition of hydrate under different cavitation parameters. The results show that: 1) Cavitation exerts a strong promotion effect on the decomposition of hydrate, and microjet induced by bubble collapse is a key factor accelerating hydrate dissociation; 2) During hydrate decomposition, the generated methane bubbles have an influence on the cavitation damage effect. 3) Larger cavitation energies can lead to stronger hydrate damage, while closer standoff distances do not always lead to more sustained cavitation damage. This paper conducts a preliminary study on the influence of cavitation bubbles on the decomposition of hydrate, which provides a theoretical guidance for further research on the mechanism of cavitation effects on hydrate.

INTRODUCTION
Natural gas hydrate is an important unconventional energy source, which is considered to be one of the most promising alternative energy sources in the 21st century. The proven reserves of natural gas hydrates are up to 80 billion tons of oil equivalent and are predominantly distributed under seabed (Fitzgerald, G. C. et al., 2012; Li et al., 2020; Sloan, 2003; Song et al., 2021). Currently, there are many problems in the process of gas hydrate exploitation by conventional methods, such as low efficiency of gas production, poor sustainability and risk of reservoir destabilization and collapse. Thus, commercial extraction of gas hydrate cannot be achieved for the moment based on the existing technology and equipment (Luo et al., 2021; Terzariol et al., 2017; Zhang et al., 2022). The cavitation phenomenon is a destructive effect with enormous energy and was first discovered in the shipbuilding industry (Lauterborn, W. & Bolle, H. J., 1975). The high temperature and pressure generated during the collapse of a bubble can cause significant damage to the surrounding wall (Li et al., 2007; Zhou et al., 2018), which has a strong damaging effect on hydrated soft sediments (Zhang et al., 2022). In recognition of the powerful destructive effect of cavitation, Li et al. proposed a new idea of integrated method of drilling radial horizontal wells by cavitation jets with screen tubing completion to extract hydrates based on the characteristics of unconformable gas hydrate resources in the South China Sea (Li et al., 2020). In particular, the hydro-jet erosion effect plays an important role in improving the efficiency of jet breaking underwater and increasing the production of hydrate. Peng et al. optimized a contraction-expansion cavitation nozzle and verified the cavitation erosion ability of cavitation jets based on laboratory experiments (Peng, K., 2018). Zhang et al. conducted a macroscopic study for the cavitation jet's erosion performance on natural gas hydrates and verified that the cavitation jet has a stronger destructive effect than classic water jets (Zhang et al., 2020). Existing studies and results argue for the effectiveness of cavitation jets for efficient gas hydrate fragmentation, while most of the work is conducted based on macroscopic jet experiments, and the results are difficult to characterize the specific effects of cavitation on hydrate. Few experimental studies have been conducted to investigate the effect of the bubble collapse process on the hydrate breakage.