降压幅度和出砂堵塞对天然气水合物开采产能的影响分析

为研究降压幅度和出砂堵塞对天然气水合物开采产能的影响，使用天然气水合物多相流数值模拟软件TOUGH＋HYDRATE进行水合物降压开采模拟，通过不同情况下的水合物分解速率、产气速率、产气量和产水量分析了降压幅度和出砂堵塞对天然气水合物开采产能的影响，并通过不同情况下的储层压力、储层温度和水合物饱和度分布分析了其影响机理。数值模拟结果表明：①随着降压幅度的增大，储层中压力降低范围逐渐增大，而且压力降低幅值逐渐增大，储层与开采井之间的压力梯度越大，导致相同时间时的水合物分解速率、产气速率、产气量和产水量都逐渐增大；降压幅度的增大对短期开采的累积产气量有明显提高，而对长期开采的产气量影响不大，而降压幅度的增大可能导致出砂堵塞以及水合物二次生成，因此实际开采时应设定一个合理的降压幅度并辅助升温等其它措施；②随着出砂堵塞的加剧，井周附近的渗透率逐渐降低，储层中压力降低范围逐渐减小，而且压力降低幅值逐渐减小，储层与开采井之间的压力梯度越小，另外井周渗透率的降低还会导致气体的流速的降低，从而导致相同时间时的水合物分解速率、产气速率、产气量和产水量逐渐减小；出砂堵塞会对产气量持续产生影响，导致产气量随时间成比例减少，因此实际开采时应进行储层改良减轻出砂问题或采取防砂措施避免出砂堵塞。

Analysis of the influence of depressurization amplitude and sand blockage on gas hydrate production capacity

To study the influence of depressurization amplitude and sand blockage on gas hydrate production capacity, hydrate antihypertensive extraction simulation was performed by gas hydrate multiphase flow numerical simulation software TOUGH+HYDRATE. The effects of depressurization amplitude and sand blockage on the hydrate dissociation rate were analyzed by gas production rate, gas production and water production, and the influence mechanisms of depressurization amplitude and sand blockage on gas hydrate production capacity were analyzed by the distributions of reservoir pressure, reservoir temperature and hydrate saturation under different conditions. The numerical simulation results show that: (1) With the increase of the depressurization amplitude, the pressure reduction range in the reservoir gradually increased, the pressure reduction amplitude gradually increased, and the greater the pressure gradient between the reservoir and the mining well was, which resulted in the hydrate decomposition rate, gas production rate, gas production and water production all gradually increased at the same time. The increase of depressurization amplitude significantly raised the cumulative gas production for the short-term mining, but had little impact on the gas production for the long-term mining, while the increase of depressurization amplitude might cause sand blockage and secondary generation of the hydrate. Therefore, a reasonable depressurization amplitude should be set and assisted by other measures such as heating in the actual mining. (2) With the aggravation of the sand blockage, the permeability around the well circumference gradually decreased, the pressure reduction range in the reservoir gradually decreased, the pressure reduction amplitude gradually decreased, and the smaller the pressure gradient between the reservoir and the mining well was, moreover, the decrease of permeability around the well circumference resulted in the decrease of the gas flow rate, which resulted in the hydrate decomposition rate, gas production rate, gas production and water production all gradually decreased at the same time. Sand blockage had a continuous impact on gas production, which caused the gas production decreased in proportion over time. Therefore, the reservoir should be improved to reduce the sand production problem or the prevention measures of sand production should be taken to avoid sand blockage in the actual mining.