不同开采方法下深海能源土离散元模拟

Distinct element simulations of exploitation of methane hydrate bearing sediments with different methods

天然气水合物以胶结及孔隙填充等形式存在于深海能源土中,开采时因其分解会劣化地层力学特性进而引发海底事故,使得人们对能源土开采过程进行中力学特性的变化愈发重视。在前期室内试验的基础上,将一个温度-水压-力学二维微观胶结模型引入离散元商业软件PFC2D中,通过对排气、排水性较好的土体进行升温及降压法开采进行数值模拟,并将模拟结果与相同条件下的室内试验结果对比,验证了该胶结模型的适用性。进一步分析了颗粒接触分布与颗粒平均纯转动率(averaged pure rotation rate,APR)在水合物分解时的变化情况。升温分解过程中随温度升高,颗粒总接触分布各向异性程度增大;胶结接触逐渐减少并始终保持主方向为水平方向,无胶结接触增多并始终保持主方向为竖直方向;APR值逐渐增大且正负值分布逐渐趋于集中。降压分解过程中随反(水)压降低,颗粒总接触由各向同性分布逐渐发展为主方向为竖直方向的各向异性,APR值较小且分布均匀;恢复反压后,试样进一步破坏,颗粒总接触各向异性更加明显,APR值增大且正负值呈集中分布。

Methane hydrate bearing sediments(MHBS) are usually found undersea, which is generally categorized into different types, i.e. bond, pore-filling and so on. The mechanical properties of MHBS will be deteriorated as methane hydrate(MH) dissociates and many marine accidents will be caused; thus a lot of attention has been attracted in investigating MHBS properties changes during exploitation. A newly proposed 2D temperature-water pressure-mechanical bond model is implemented into PFC2 D, the commercial software of the distinct element method(DEM), to simulate the dissociation of MH by thermal recovery and depressurization using soil in good exhaust and drainage conditions. By comparison with laboratory test results, it is validated that the newly proposed model is applicable to simulate the mechanical behavior of MHBS. At the same time, the micromechanical properties are also analyzed. With temperature increasing during thermal exploitation, the anisotropic degree of total contact distribution increases; the amount of bond contact, whose principal direction is horizontal all the time, decreases and the amount of unbonded contact, whose principal direction is vertical all the time, increases; the value of averaged pure rotation rate(APR) and its concentration degree increases all the time. With back(water) pressure decreasing during exploitation by depressurization, the total contact distribution changes from isotropic to anisotropic with a vertical principal direction, and the value of APR is small and uniformly distributed. After the recovery of back water pressure, the sample is damaged further, and the anisotropic degree of total contact distribution increases. Besides, the value of APR increases and the concentration degree of positive and negative value increases.