一种新型CO_2置换CH_4水合物的开采方法

天然气水合物储量巨大,是未来极具开发潜力的清洁能源。CO_2置换法兼具能源开采与温室气体封存的双重功效,但通常CO_2对CH_4的置换速率非常低。为此,结合抑制剂存在条件下CH_4水合物和CO_2水合物具有不同的热力学稳定性这一特点,提出并通过实验证实了一种可用于开采天然气水合物的新方法,它将CO_2置换法与注热力学抑制剂的工艺相结合,实现了CH_4水合物分解过程的加速。通过岩心驱替实验,对比考察了两类3种常见CH_4水合物热力学抑制剂(甲醇、氯化钠和氯化镁)的作用效果。实验结果表明:在甲醇溶液作用下,CH_4水合物分解速率高达0.011 94 mol/h,远高于电解质盐溶液的作用效果(分别为0.000 86 mol/h和0.001 41 mol/h)。选择甲醇溶液作为水合物分解加速剂,通过前期注入甲醇溶液段塞、后期连续注入CO_2的方式,使得CH_4水合物分解率超过92%,且实现了CO_2气体以水合物形式的封存固定,最终CO_2水合物的生成量占到初始甲烷水合物总量的16%~27%。

A new method for the replacement of CH4 with CO2 in natural gas hydrate production

Natural gas hydrate (NGH), as a clean energy, has a great development potential in the future for its immense reserves. The CO₂ replacement method has dual effects in energy exploitation and greenhouse gas (GHG) sequestration, but its CH₄ replacement rate is too low. When an inhibitor is added, CH₄ hydrate and CO₂ hydrate are different in terms of their thermodynamic stability. In this regard, a new method was proposed and experimentally proved for NGH production. This new method combines CO₂ replacement method with thermodynamic hydrate inhibitor technology to accelerate the decomposition of CH₄ hydrate. Three common thermodynamic CH₄ hydrate inhibitors (methanol, sodium chloride and magnesium chloride) in two categories were compared by means of core displacement experiments. It is shown that, when methanol solution is used as the inhibitor, the decomposition rate of CH₄ hydrate reaches 0.011 94 mol/h, which is much higher than that of electrolyte solution (0.000 86 mol/h and 0.001 41 mol/h respectively). Therefore, methanol solution is used to accelerate hydrate decomposition. The decomposition ratio of CH₄ hydrate was over 92% when methanol solution plug was injected at the early stage and CO₂ was continuously injected at the late stage. Moreover, CO₂ was sequestrated in the form of hydrate. Eventually, the generated CO₂ hydrate accounted for 16–27% of the initial total CH4 hydrate.