松散沉积物中降压幅度和饱和度对天然气水合物分解过程的影响

为了弄清楚降压幅度和饱和度对于天然气水合物（以下简称水合物）分解过程的影响规律这一事关水合物工业开采的核心问题，基于我国南海北部神狐海域沉积物粒径特征配置出多孔介质样品，在实验室模拟试采区现场钻孔压力、温度、盐度、饱和度条件，开展了松散沉积物中两种饱和度范围（Sh,Ⅰ=23%～26%，以下简称体系Ⅰ；Sh,Ⅱ=46%～50%，以下简称体系Ⅱ）和4种降压幅度（12 MPa、9 MPa、6 MPa、3 MPa）条件下水合物降压分解实验。研究结果表明：①降压幅度为12 MPa条件下产气集中于分解前期，分解前期产气量随饱和度增大占产气总量比例升高；②分解时间（开发期）随降压幅度的增大呈线性减小趋势，降压幅度增加9 MPa，体系Ⅰ与体系Ⅱ的分解时间分别缩短为原来的28.39%和44.97%；③高饱和度体系水合物瞬时产气速率波动较为剧烈，其在降压幅度12 MPa条件下瞬时产气速率峰值、阶段产气速率峰值为最大，降压开采效果较好。结论认为：①所做实验和南海试采结果均表明，产气速率峰值在降压开采前期出现，可能引发储层和井筒失稳，需在水合物降压开采进一步试验和现场工程中加强关注；②后续研究需借助较大尺度水合物降压开采模拟装置，明确尺寸效应对水合物降压开采产气规律的影响。

Influence of depressurization amplitude and saturation in loose sediments on the dissociation process of natural gas hydrates

The influence law of depressurization amplitude and saturation on the dissociation process of natural gas hydrates (NGH) is a core issue of industrial NGH production. In order to clarify this issue, this paper prepared porous medium samples based on the grain size characteristics of the sediments in the Shenhu sea area of the northern South China Sea. Then, experiments of depressurization induced NGH dissociation were carried out on loose sediments under two ranges of saturations (Sh,Ⅰ=23–26%, hereinafter referred to as system Ⅰ; Sh,Ⅱ=46–50%, hereinafter referred to as system Ⅱ and four depressurization amplitudes (12 MPa, 9 MPa, 6 MPa and 3 MPa) by simulating the in-situ bore pressure, temperature, salinity and saturation of the production area in the laboratory. And the following research results were obtained. First, under the depressurization amplitude of 12 MPa, gas production mainly happens in the early stage of dissociation. The proportion of the gas yield in the early stage of dissociation in the total gas yield increases with the increase of saturation. Second, the dissociation duration (development period) decreases linearly with the increase of depressurization amplitude. When the depressurization amplitude is 9 MPa, the dissociation duration of system 1 and 2 is shortened into 28.39% and 44.97%, respectively. Third, the instantaneous gas production rate of high saturation NGH fluctuates more violently. Under the depressurization amplitude of 12 MPa, the peak instantaneous gas production rate and the peak intermediate gas production rate are the highest and the effect of depressurization production is better. In conclusion, the experimental results and the production test results in the South China Sea show that peak gas production rate occurs in the early stage of depressurization production, which may lead to reservoir and wellbore instability, so it shall be concerned further in the following experiments and field engineering of NGH depressurization production. And in the follow-up studies, it is necessary to make use of the NGH depressurization production simulation device of larger scale to clarify the influence of size effect on the gas production laws in NGH depressurization production.