Pore-scale modelling on complex-conductivity responses of hydrate-bearing clayey sediments: Implications for evaluating hydrate saturation and clay content

Abstract

A majority of the accumulated gas hydrates exist in fine-grained unconsolidated sediments with clays, which pose challenges to reservoir evaluation with resistivity-based techniques. Characteristic electrical parameters derived from induced polarization (IP) measurements have potentials to describe complex formations of various lithology, pore-water salinities, and different depths from the borehole. However, there is still a knowledge gap in complex-conductivity properties of the hydrate-bearing clayey sediments. We present a pore-scale numerical study on modelling the low-frequency (<1 kHz) complex-conductivity spectra of clayey sediments containing hydrates based on the finite-element approach, with an emphasis on evaluating the hydrate saturation (sh) and clay content. Firstly, the influences of clay type, distribution form, content and sh on the complex conductivity of sediments containing hydrates were examined systematically. Secondly, power-law and linear correlations were established for evaluating the hydrate saturation and clay content, respectively, based on complex-conductivity parameters. (Effects of clay type) The in-phase conductivity of hydrate-bearing smectite is significantly higher than that of hydrate-bearing illite and kaolinite due to the higher cation exchange capacity (CEC) of smectite. Higher peak frequency and quadrature conductivity appear for the hydrate-bearing kaolinite case because the mobility of counterions in the Stern layer of kaolinite is about ten times of that for smectite and illite. (Effects of clay distribution form) The coating-clay case has much lower in-phase and quadrature conductivities than the dispersed- and laminated-clay cases because the coating clay isolates sand particles from the pore water and no electrical double layer (EDL) forms around the sand particles. (Effects of clay content) With an increasing content of the structural clay up to 60%, the in-phase conductivity decreases and increases in the frequency bands lower and higher, respectively, than the peak frequency corresponding to the EDL polarization. The effective dielectric constant increases consistently with the clay content due to the much higher CEC of clays than that of sands. (Effects of hydrate saturation) The in-phase conductivity decreases consistently with an increasing sh up to 0.40 due to the negligible conductivity of hydrates and blockage effect on conduction currents. Both the quadrature conductivity and effective dielectric constant in the EDL-polarization-dominant frequency band decrease with an increasing sh. In this work, it has been evidenced that complex-conductivity responses of hydrate-bearing clayey sediments can be understood theoretically and modelled numerically based on the interpretation of electrical conduction and electrochemical polarization mechanisms of EDLs. This study provides a theoretical and modelling foundation for the development of new IP-based geophysical techniques for hydrate-reservoir evaluation and monitoring in both the exploration and exploitation stages.