基于激电法评价含水合物沉积物渗透率数值模拟研究

Numerical Study on Permeability Evaluation for Hydrate-Bearing SedimentsBased on Induced Polarization Principle

含水合物沉积物样品渗透率的实验测量过程中存在成本高、周期长、水合物相态变化等问题,水合物开采过程中缺乏可对储层渗透率动态变化进行实时监测的技术。提出基于激电响应评价含水合物沉积物渗透率的新方法,构建含水合物多孔介质的流场与电场有限元数值模型,探讨水合物饱和度、骨架颗粒尺寸等因素对复电导率和渗透率的影响规律及机理,建立基于复电导率参数的含水合物多孔介质渗透率评价模型。研究结果表明:(1)随着多孔介质骨架颗粒粒径及等效孔径的增大,复电导率谱的弛豫时间增大,复电导率虚部极大值所对应频率降低,双电层极化主导频率范围内的复电导率虚部增大,多孔介质有效渗透率增大;(2)随着孔隙填充型水合物的饱和度增加,双电层极化主导的频率范围内复电导率虚部减小,界面极化主导的频率范围内复电导率虚部增大,含水合物多孔介质有效渗透率及其变化率持续减小;(3)双电层极化主导的频率范围内复电导率虚部与渗透率之间存在确定性关系,在饱和水条件下选取的固定频率处复电导率虚部与渗透率呈幂函数关系,结合饱和与非饱和条件下复电导率之间的关系以及归一化渗透率与饱和度之间的关系可建立基于复电导率虚部的含水合物多孔介质有效渗透率评价模型。研究结果可为开发基于激电法的含水合物沉积物样品渗透率实验测量技术以及天然气水合物储层渗透率动态监测技术提供理论与模型基础。

There are significant problems such as high cost,long period and phase-state change of hydrate in the experimental process for measuring the permeability of hydrate-bearing samples.Moreover,real-time monitoring technologies for the dynamic evolution of reservoir permeability in hydrate exploitation processes are still lacking.A new method for evaluating the permeability of hydrate-bearing sediments based on induced polarization(IP)responses was proposed,and finite-element numerical models of flow field and electrical field for hydrate-bearing porous media were constructed.The effects and corresponding mechanisms of hydrate saturation and skeleton particle size on the complex conductivity and permeability were analyzed.Finally,a permeability evaluation model of hydrate-bearing porous media based on complex conductivity was established.It was demonstrated that:(1)with the increase of the skeleton particle size and the equivalent pore diameter of porous medium,the relaxation time of the complex-conductivity spectrum increases,the frequency corresponding to the maximum imaginary complex conductivity decreases,the imaginary complex conductivity within the frequency range of electrical-double-layer(EDL)polarization increases,and the effective permeability of the porous medium increases;(2)with the increase of the pore-filling-hydrate saturation,the imaginary complex conductivity within the frequency range of EDL polarization decreases,the imaginary complex conductivity within the frequency range of interface polarization increases,and the effective permeability and its changing rate of the hydrate-bearing porous medium decrease continuously;(3)there is a definite relationship between the imaginary complex conductivity and permeability within the frequency range of EDL polarization;the imaginary complex conductivity at a fixed frequency under water-saturated conditions exhibits a power-law relationship with permeability;combining the relationship between the complex conductivity under saturated/unsaturated conditions and that between the normalized permeability and saturation,an evaluation model for the effective permeability of hydrate-bearing porous media can be established based on the imaginary complex conductivity.This study can provide a theoretical and model basis for the development of an experimental measurement technology for the permeability of hydrate-bearing samples based on IP principle and a dynamic monitoring technology for the permeability of natural gas hydrate reservoirs.