The experimental testing and analysis of strength and deformation characteristics of hydrate reservoirs is an integral part of natural gas hydrate exploitation. However, studies so far have failed to deeply explore samples from the South China Sea. Especially, there is a lack of a simple and applicable method to estimate their mechanical behaviors. Thus, based on test data, an improved Duncan-Chang model is established in this paper to characterize the strength and deformation of reconstituted samples with various hydrate saturation and stress states from this area. This model can accurately describe the strain-hardening characteristics, and failure strength is estimated by the improved Drucker-Prager criterion with high fitting accuracy. The initial elastic modulus and failure ratio are given by the proposed empirical models, which are obtained from experimental data and fitting methods. Generally, this model has several advantages including simple structure, favorable performances, and a limited number of model parameters. Therefore, it could be widely used in strength and deformation analysis. This study can support the prevention and control of geological risks during natural gas hydrate exploitation in the South China Sea.

To solve the problems of rock strength increase caused by high in-situ stress, the stress release method with rock slot in the bottom hole by an ultra-high-pressure water jet is proposed. The stress conditions of bottom hole rock, before and after slotting are analyzed and the stress release mechanism of slotting is clarified. The results show that the stress release by slotting is due to the coupling of three factors: the relief of horizontal stress, the stress concentration zone distancing away from the cutting face, and the increase of pore pressure caused by rock mass expansion; The stress concentration increases the effective stress of rock along the radial distance from 0.6R to 1R (R is the radius of the well), and the presence of groove completely releases the stress, it also allows the stress concentration zone to be pushed away from the cutting face, while significantly lowering the value of stresses in the area the drilling bit acting, the maximum stress release efficiency can reach 80%. The effect of slotting characteristics on release efficiency is obvious when the groove location is near the borehole wall. With the increase of groove depth, the stress release efficiency is significantly increased, and the release range of effective stress is enlarged along the axial direction. Therefore, the stress release method and results of simulations in this paper have a guiding significance for best-improving rock-breaking efficiency and further understanding the technique.