砂土沉积物中甲烷水合物降压分解渗流阵面实验

地层渗流特性是天然气水合物（以下简称水合物）开采评价的重要内容之一，但是目前与水合物地层渗流特性密切相关的孔隙压力演化数据积累明显不足。为此，进行了砂土沉积物中甲烷水合物降压分解实验。首先进行了实验设计，并标定检验了实验系统测量仪器（压力传感器、温度探头和气体流量计）；按照设计的实验步骤，进行了3次降压分解实验，根据实验获得的孔隙压力等重要数据，分析了孔隙压力演化和渗流阵面传播的规律以及影响因素，探讨了孔隙压力演化曲线出现波动段和平稳段的原因。基于实验的基本条件，得到如下结论：①砂土沉积物中渗流阵面传播距离与时间的平方根成近似线性关系，绝对渗透率相近时，初始水合物饱和度越高，渗流阵面传播速度越慢；②甲烷水合物分解导致砂土沉积物的孔隙压力演化曲线存在明显的波动段和平稳段，波动可能是由于样品渗流通道堵塞、贯通交替进行或者气液饱和度交替增大所引起的，平稳段应该是砂土沉积物中等效轴向渗流通道内甲烷水合物分解产气与气体渗流达到动态平衡的间接反映。

An experimental study of seepage front due to methane hydrate dissociation by depressurization in sandy sediments

Seepage characteristic of hydrate bearing sediments is one of the key parameters of natural gas hydrate exploitation. However, there are few literatures about experimental data of pore pressure which are closely related to seepage characteristic at present. In view of this, experiments of methane hydrate (MH) dissociation by depressurization in sandy sediments were carried out. First, an experiment design was made and calibration checks were performed of the experimental measurement devices including pressure sensors, temperature sensors and gas flow meters. According to the designed experiment procedures, three tests were performed to obtain important data like pore pressure, on this basis, evolution rules of pore pressure and gas seepage front were also analyzed as well as their influencing factors, and the reason for the waves and stable periods in the pore pressure evolution curves were discussed. Based on these experimental conditions, main conclusions were made as follows. First, a propagation range of seepage front is approximately linear with the square root of time；the propagation velocity of seepage front becomes slower with higher saturation of MH when absolute permeability of sandy sediments is similar. Second, waves and stable periods in evolution curves of pore pressure are caused by MH dissociation: waves may be induced by blocked seepage channels or alternately increased saturations of gas and water while stable periods by dynamic balance between gas production resulting from MH dissociation and gas outflow in the equivalent seepage channel.