As a clean fossil fuel with great reserves, natural gas hydrate (NGH) is widely regarded as an important future alternative energy source. NGH is widely distributed in onshore tundra and shallow sedimentary layer s in the deep sea. These sedimentary layers typically exits shallow burial depth, poor diagenesis and low strength characteristics; moreover, the decomposition of NGH can also greatly reduce reservoir strength. Therefore, NGH development can easily cause s many geomechanical problems , including reservoir instability, sand production and seabed landslide s, etc., which may further trigger a series of environmental disasters such as tsunami s, natural gas leakage and the acceleration of global warming. This study mainly reviews the research progress regarding geomechanical issues in NGH development , including mechanical properties of NGH -bearing sediments, borehole stability, hydraulic fracturing, sand production, reservoir settlement and seabed landslide s. In addition to previous research achievements regarding geomechanical problems in NGH exploitation , the limitations and challenges are also discussed , and several questions and insightful suggestions are put forward for future research from our point of view.

Introduction

With rapid economic and social development, the demand for energy has witnessed sustainable growth and the exploitation of conventional fossil fuels encountered a bottleneck; meanwhile, scholars are paying more attention on unconventional energy such as natural gas hydrate (NGH). NGH is a type of ice -like compound formed by the combination of water and gas molecules at low -temperature , high -pressure conditions (Sloan Jr and Koh, 2007). In the natural world, because of large reserves and its high energy density, NGH is regarded as one of the most important alternative energy sources in the 21st century and now attracts widespread attention from the scholars all over the world (Chong et al., 2016; Makogon et al., 2007;Nair et al., 2016). Approximately 1 ,500 billion ton s carbon is stored in NGH, which occupies estimated 25% of mobile organic carbon in global land -ocean -atmosphere system s (Beaudoin et al., 2014) , almost twice that of proven conventional mineral fuels such as coal, oil and natural gas (Kvenvolden, 1998; Yan et al., 2017). Because the massive amount of NGH on the seafloor has been found through deep sea drilling (Huene et al., 1980; Moore et al., 1982), many countries have successively formulated research and development plans of NGH, with the ultimate aim of achieving commercial development of NGH ( Dickens, 2004; China Geological Survey Bureau, 2017; JOGMEC, 2017; Chen et al., 2018; Wu and Wang, 2018).