水平井降压法和热激法水合物开采对海底边坡稳定性的影响
SUBMARINE SLOPE STABILITY DURING DEPRESSURIZATION AND THERMAL STIMULATION HYDRATE PRODUCTION WITH HORIZONTAL WELLS
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摘要: 天然气水合物广泛赋存在深海沉积物孔隙中, 被认为是具有巨大开发潜力的未来绿色能源之一, 引起全球的关注. 深海水合物开采将造成含水合物储层的强度劣化, 可能产生孔压积聚, 诱发海床失稳. 本文基于边坡稳定极限平衡分析框架, 引入考虑水合物开采热-流-化学耦合过程的数值分析模型, 研究水合物开采对海底边坡稳定性的影响. 采用TOUGH+HYDRATE热-流-化学耦合分析程序, 模拟了采用水平井降压法和热激法开采深海水合物的过程, 分析了水合物分解锋面扩展和瞬态孔压演变的规律, 并通过SLOPE/W程序采用极限平衡分析方法计算水合物开采过程及停采后的海底边坡安全系数, 分析开采井位置和开采方法对海底边坡稳定性的影响. 研究表明, 对于存在致密盖层的细砂储层陡坡, 单水平井降压开采过程中, 由于孔压降低, 土体有效应力增加, 边坡稳定性显著提高, 当开采井布设在坡体中部时, 边坡稳定性提高最为明显; 停采后, 由于水合物分解导致土体黏聚强度降低, 且孔压逐渐回升到静水压状态, 导致边坡稳定性下降, 最危险滑弧通过水合物分解区. 若采用双水平井热激法开采, 开采过程与停采后的最危险滑弧始终通过水合物分解区, 由于开采过程中温度升高, 井周孔压显著上升, 导致边坡安全系数明显下降, 存在诱发滑坡的风险.Abstract: Gas hydrate is extensively found in the voids of sediments under deep sea. It is considered one of the promising sources of future clean energy, and has attracted global attentions of research and development. Submarine slope instability could be triggered during gas production from oceanic hydrate reservoirs due to reduction in soil strength and build-up of pore pressure. This paper investigates the impact of hydrate extraction on submarine slope stability within the framework of the limit equilibrium slope stability analysis considering the thermo-hydro-chemical (THC) coupled processes of hydrate dissociation. A THC coupled simulator TOUGH + HYDRATE was employed to simulate the process of hydrate production under depressurization and thermal stimulation with horizontal wells. The simulation captured the propagation of hydrate dissociation front and the evolution of pore pressure during hydrate production. The safety factor of the slope stability during and after production was then acquired from the limit equilibrium slope analysis by means of SLOPE/W. A parametric study was conducted to investigate the effect of well locations and production methods on the slope stability. The results indicate that, for a steep slope with a sandy reservoir covered by a tight overburden layer, the slope is stabilized during single-well depressurization production due to increase in the effective stress caused by depressurization. Compared to wells installed elsewhere, a depressurization well installed at the mid-height of the slope provides the highest safety factor during production. As the pore pressure recovers back to hydrostatic conditions and the cohesion of the host soils decreases, the slope becomes less stable after production. For scenarios using double-well thermal stimulation production, the critical slip surface passes through dissociated zones, and the safety factor of the slope significantly drops due to high pressure build-up developed around the injection well. Submarine landslides could be triggered by hydrate production with thermal stimulation.