###### Abstract
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fields in the near wellbore zone, and conducting stability analysis of horizontal well drilling in hydrate layers are of great significance for the safe and efficient exploitation of hydrates. During the drilling process of the horizontal well in the hydrate reservoir at sea, the drilling fluid invading the reservoir could make the solid hydrate particles decompose into gas and liquid in two phases. Consequently, the mechanical properties of the hydrate reservoir may change significantly, and the stress distribution state of the reservoir may change accordingly. So, an accurate assessment of the variations of hydrate reservoir mechanical properties during the drilling process is an important prerequisite for analyzing the wellbore stability. [PERSON] et al. (2007) (2007) investigated the effects of reservoir rock type, hydrate saturation, and effective confining pressure on the mechanical strength of hydrate reservoir by rock triaxial tests. [PERSON] et al. (2013) (2013) carried out stiffness experiments on samples of water and gas-saturated hydrate deposits and found that the stiffness and strength of hydrate deposits saturated with gas were significantly higher than those saturated with water. Through experiments, [PERSON] et al. (2017) reached the findings that in the formation of argillaceous siltstone, hydrate particles, when formed, would occupy the original space of the rock particles and be distributed in uneven micro-fractures in a belt or lenticular zone. This finding is very similar to the results of laboratory X-CT scanning and field logging. But in the sandy deposits with hydrate, things are completely different, either in the form of hydrate filling the pores or in the way of rock cementation. Therefore, argillaceous siltstone and sandy sediments containing gas hydrate differ widely in mechanical properties, porosity, and permeability. [PERSON] et al. (2014) (2014) investigated the shear strength and deformation characteristics of hydrate deposits during hydrate decomposition by depressurization and heating. Researchers at home and abroad have carried out a lot of studies on the mechanical properties of rocks containing hydrate, but few studies have covered the variation patterns of mechanical characteristics of the argillaceous siltstone reservoirs with weak cementation in the South China Sea under the disturbance by wellbore temperature and pressure in the drilling process. In the process of drilling and mining of hydrate reservoirs, once the wellbore loses stability, many difficult problems could arise. As a result, researchers have conducted numerous studies on the stability of wellbores in hydrate formation. After simplifying the multi-phase flow caused by the hydrate, [PERSON] et al. (2019) established a wellbore stability evaluation model by coupling temperature, pressure, and mechanical fields. By extending the Biot consolidation model, [PERSON] et al. (2019) simulated the mechanical response characteristics of hydrate reservoirs in the process of horizontal well drilling and production, and the results provided a theoretical reference for the study of the stability of horizontal wellbores in hydrate reservoirs during drilling and production. Based on the theory of overloading and secondary loading surface, [PERSON] et al. (2019) built a mechanical constitutive model of a hydrate reservoir and simulated the formation settlement characteristics during the depressurization extraction of hydrate; moreover, they simulated the mechanical behavior of an argillaceous siltstone hydrate reservoir in the first year of depressurization extraction. [PERSON] et al. (2020) considered the evolution of reservoir mechanical strength, wellbore temperature, and pressure parameters in depth; established a wellbore stability analysis model; and analyzed the stability evolution law of the wellbore area during shallow seabed drilling. [PERSON] et al.