水合物开发储层温度场扰动研究

Temperature disturbance of formation during the exploitation of hydrate

天然气水合物的开采过程中,水合物会发生相变,不仅会降低岩石的强度,同时相变分解会吸收大量的热,从而对储层的温度场产生扰动。这与多孔介质中的冻融现象本质相同,根据Gibbs-Thomson方程,孔隙中的相变温度不再是一个定值,而是与孔隙的尺度有关,即小孔中的固态晶体先于大孔发生相变。同时,相变过程中热量以潜热形式转化,并不会引起温度的变化。针对这些特点,将理论公式中的潜热与比热考虑成等效热容,从而建立了带相变的热传导控制方程。控制方程中含冰量与温度间的关系可由孔径分布曲线得到。在这个理论模型的基础上,考虑了热对流边界条件,模拟了地层岩石温度随时间的变化规律。模拟显示储层的升温曲线有明显的平稳段,即温度随时间几乎不变化。分析同时进一步表明,该温度平段的位置和长度是由孔隙大小分布决定的。

The phase transition occurs during the exploitation of gas hydrate, which will reduce the strength of sediment.Moreover, the endothermic effect during the dissociation of gas hydrate will lead to the temperature disturbance. Similar to the freeze/thaw process in porous media, the temperature of phase transition in pores is no longer a fixed value but is related to the pore size according to Gibbs-Thomson equation. During the phase transition process, heat is transformed no longer in the form of specific heat but in the form of latent heat, which does not alter the temperature. Considering these characteristics, a governing equation of heat conduction with phase transition was established;and the specific heat and latent heat terms were combined into an “equivalent heat capacity” term in the model. The relationship between ice content and temperature was assessed by the pore size distribution curve which was obtained from mercury intrusion porosimetry data. Considering a convectional boundary condition, the evolution of temperature in formation was simulated according to the governing equation. The temperature evolution curve was featured by a plateau regime. That is, temperature remains quasi-constant with time. The analysis demonstrates that the position and length of this temperature plateau is associated with the pore size distribution.