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THE REACTIVATION MECHANISM OF ANCIENT OCEAN LANDSLIDES DURING HYDRATE PRODUCTION: A PRELIMINARY STUDY
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摘要: 我国南海北陆坡水合物富集区广泛发育古滑坡，若水合物开采不当可能导致古滑坡再次滑动。为了探究水合物开采诱发古滑坡再启滑机制，针对含下卧型水合物藏和伴生型水合物藏的两个典型古滑坡体，在边坡极限平衡分析框架内考虑了水合物开采过程中的瞬态孔压与土体抗剪强度变化，分析了水合物开采过程中不同古滑坡体的稳定性演变与失稳模式。研究表明，水合物分解导致所赋存土体的胶结强度弱化，同时逸出气体可能被阻滞于渗透性较低的古滑坡体下方，从而形成横向扩展的高压区。下卧型储层边坡的潜在滑移面贯穿古滑移面，一般表现为滑动型滑坡；开采初期因孔压积聚而导致边坡稳定性降低，开采中后期因二次水合物生成可能导致边坡稳定性有所回升，在本文计算条件下未触发古滑坡复活。伴生型储层边坡的稳定性受土体强度劣化与孔压积聚的共同影响，水合物开采导致古滑坡重新滑动，表现为滑塌型滑坡。Abstract: Ancient landslides are widely developed in hydrate-rich areas in the northern continental slope of the South China Sea. Imprudent hydrate production may result in the reactivation of the ancient submarine landslides. In order to explore the mechanism of the ancient landslide reactivation induced by hydrate production, we analyzed the slope stability and instability modes of two typical ancient landslides: the underburden-type and the associated-type. The analysis accounted for the changes of the transient pore pressure and the soil shear strength during hydrate production within the limit equilibrium analysis framework. The results suggest that hydrate dissociation results in the reduction of the cementing strength and meanwhile, the released gas may be trapped below the ancient landslide body with low permeability, giving rise to a laterally extending high-pressure zone. The potential slip surface of the underburden-type reservoir goes through the ancient slip surface, showing a slide pattern. In the early stage of production, the slope stability decreases due to the pore pressure build-up. Then, during the middle and late stages of production, the slope stability recovers because of the secondary hydrate formation. The production would not trigger the ancient reactivation with the calculation configuration in this study. The slope stability of the associated-type reservoir is affected by both the soil strength reduction and the pore pressure build-up. Hydrate production from an associated-type reservoir may trigger the reactivation of the ancient landslide, showing a slump pattern.
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图 1 下卧型与伴生型水合物储层成藏机理和开采逃逸气体被阻滞于古滑移面的示意图
Figure 1. Illustration of the formation mechanism of the under￣burden- and associated-type hydrate reservoirs and the escaping gas trapped in the ancient slip surface from hydrate production
图 2 计算边坡模型及不同类型边坡的地层设置
Figure 2. The calculation slope model and the stratum settings of different types of slopes
图 3 水合物分解锋面的扩展：(a~c)下卧型储层；(d~f)伴生型储层
Figure 3. The propagation of hydrate dissociation front in: (a~c) underburden-type reservoir; (d~f) associated-type reservoir
a. 435 d; b. 1200 d; c. 3019 d; d. 155 d; e. 1139 d; f. 3439 d
图 4 有效黏聚力的演变：(a~c)下卧型储层；(d~f)伴生型储层
Figure 4. The evolution of effective cohesion in: (a~c) underburden-type reservoir; (d~f) associated-type reservoir
a. 435 d; b. 1200 d; c. 3019 d; d. 155 d; e. 1139 d; f. 3439 d
图 5 超孔压的演变：(a~c)下卧型储层；(d~f)伴生型储层
Figure 5. The evolution of excess pore pressure in: (a~c) underburden-type reservoir; (d~f) associated-type reservoir
a. 435 d; b. 1200 d; c. 3019 d; d. 155 d; e. 1139 d; f. 3439 d
图 6 初始潜在滑移面：(a)下卧型储层；(b)伴生型储层
Figure 6. The initial potential slip surface of: (a) underburden-type reservoir; (b) associated-type reservoir
图 7 边坡稳定安全系数演变：(a)下卧型储层；(b)伴生型储层
Figure 7. The evolution of the slope stability factor of safety of: (a) underburden-type reservoir; (b) associated-type reservoir
图 8 滑移面有效黏聚力演变：(a)下卧型储层；(b)伴生型储层
Figure 8. The evolution of the effective cohesion on the potential slip surface of: (a) underburden-type reservoir; (b) associated-type reservoir
图 9 滑移面超孔压演变：(a)下卧型储层；(b)伴生型储层
Figure 9. The evolution of the excess pore pressure on the potential slip surface of: (a) underburden-type slope; (b) associated-type slope
表 1 计算模型
Table 1. Models used in this study
参数 计算模型 热导率(Moridis et al., 2005) $ {\lambda _\mathit{\Theta }}{\rm{ = }}\left({\sqrt {{\mathit{S}_{\rm{H}}}} + \sqrt {{\mathit{S}_{\rm{A}}}} } \right)\left({{\mathit{\lambda }_{{\rm{SW}}}} - {\mathit{\lambda }_{{\rm{sd}}}}} \right) + {\mathit{\lambda }_{{\rm{sd}}}}$ λsd/W·(mK)-1
λsw/W(mK)-1
SH
SA干热导率
湿热导率
水合物饱和度
液相饱和度毛细压力(Genuchten，1980) $ {P_{\rm{c}}} = {P_0}{\left[ {{{\left({\frac{{{S_{\rm{A}}} - {S_{{\rm{irA}}}}}}{{1 - {S_{{\rm{irA}}}}}}} \right)}^{ - 1/\xi }} - 1} \right]^\xi }$ ξ
SirA
P0/Pa孔隙尺寸分布指标
残余液相饱和度
进气压强值含水合物沉积物渗透率(Moridis et al., 2014) $ k = {k_0}{\left[ {\frac{{\varphi \left({1 - {S_{\rm{H}}}} \right) - {\varphi _{\rm{c}}}}}{{\varphi - {\varphi _{\rm{c}}}}}} \right]^n}$ k0/mD
φ
φc
n固有渗透率
实际孔隙率
临界孔隙率
衰减指数液相(A)与气相(G)相对渗透率(Moridis，2014) $ {k_{{\rm{r}}\alpha }} = {\left[ {\frac{{\left({{S_{\rm{ \mathsf{ α} }}} - {S_{{\rm{ir}}\mathit{\alpha }}}} \right)}}{{1 - {S_{{\rm{ir}}\mathit{\alpha }}}}}} \right]^{{n_\alpha }}}, \mathit{\alpha }{\rm{ \equiv A, G}}$ nA，nG
SA，SG
SirA
SirG经验指数
液相与气相饱和度
液相残余饱和度
气相残余饱和度
下载: 导出CSV
表 2 地层的物理力学参数
Table 2. Physical and mechanical parameters of the strata
参数* 泥质粉砂 古滑坡主体 古滑移面 含水合物层 ρ/kg·m-3 2650 2650 2650 2650 c′/kPa 20 20 0 1000 φ′/(°) 25 25 25 25 λd/W·(mK)-1 1.0 1.0 1.0 1.0 λw/W·(mK)-1 3.1 3.1 3.1 3.1 P0/Pa 105 104 104 104 ξ 0.45 0.15 0.45 0.45 nA 3.5 5.0 3.5 3.5 nG 2.5 3.0 2.5 2.5 SirA 0.25 0.55 0.25 0.25 SirG 0.01 0.05 0.01 0.01 φ 0.4 0.1 0.6 0.4 φc 0.02 0.005 0.05 0.02 k0/mD 75 10-5 — 75 *命名：ρ为土粒密度；c′为有效黏聚力；φ′为有效内摩擦角
下载: 导出CSV
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