水合物储层伺服降压开采模型试验研究

深海水合物赋存于一定的温度和压力环境下，降压开采时降压速率对分解产气速率和储层变形特性影响显著。利用浙江大学自主研发的水合物降压开采试验装置，通过伺服控制降压速率，初步开展了水合物储层模型降压开采试验，研究了储层温度场、孔压场、产气量等的响应特性，探讨了降压速率对产气效率和储层变形特性的影响规律。试验表明：水合物竖井降压开采时，开采井周围储层温度率先下降，分解域由井周逐步向周围发展。适当提高降压速率能够提高储层开采效率，但降压速率过快时易导致水合物重生成，反而不利于水合物高效持续稳定开采，开采时应选择合理的降压速率以达到最优产气效率。开采过程中根据储层孔隙与外界连通程度，储层孔隙状态可分为完全封闭型、局部封闭型和开放型3种类型。储层开采试验完成后，浅层土体出现 3 种不同变形特征的区域：I 区为井周土层，呈漏斗型下陷；II 区土层平坦，无明显扰动痕迹；III 区为边界土层，该处水气产出受阻导致部分气体向上迁移引起土丘状隆起带出现。这些变形特征与气体在储层中的迁移路径和运移模式相关。通过相似性分析，给出了模型与原型分解时间和产气量等的对应关系。

Experimental study of behavior of hydrate-bearing sediments during servo depressurization

Natural gas hydrate in deep sea exists in a certain temperature and pressure condition. The depressurization rate during hydrate dissociation by depressurization has a great impact on the gas production rate and hydrate-bearing sediment deformation characteristics. In order to investigate the influence of depressurization rate on temperature field, pore pressure field, deformation characteristics, and gas production rate of hydrate-bearing sediment, a group of depressurization tests with different depressurization rates was carried out on the apparatus independently developed by Zhejiang University that can perform linear gradient servo depressurization for simulating the hydrate decomposition process. The results show that the temperature decreases first from the perimeter of the shaft where the decomposition region starts, and then gradually spreads to the surrounding sediment at the initial stage of depressurization. Increasing the depressurization rate appropriately can improve the production efficiency of the reservoir, but the higher depressurization rate may cause the hydrate regeneration, which is not conducive to gas production. Optimal gas production efficiency can be obtained by selecting a specific depressurization rate. In the process of hydrate exploitation, the pore shape of the hydrate-bearing sediment can be divided into three types, according to the connection degree between pores and the surrounding area: completely sealed, partially sealed, and open. After hydrate exploitation, the shallow surface soil of reservoir can be divided into three areas based on the deformation characteristics: Zone I is the soil layer around the shaft, showing a funnel-shaped subsidence structure; the soil layer in Zone II is flat with no obvious disturbance; Zone III is the boundary soil layer, where the upward migration of water and gas production is blocked, leading to a mound like uplift zone. These deformation characteristics are related to the migration paths and modes of gas production in hydrate-bearing sediment. Through similarity analysis, the corresponding relationships between the decomposition time and gas production of the model and prototype are given.