Electrical Conductivity of Hydrate-Bearing Rocks Studied by Pore-Scale Modeling

Abstract

Gas hydrates have the potential to significantly disturb global climate change and alter subsurface stability, particularly in the context of production due to their extensive presence and widespread distribution in marine deposits. The electrical conductivity of hydrate-bearing sediments (HBS) serves as a crucial parameter for hydrate reservoir prospection. However, the electrical conductivity of HBS is influenced not only by hydrate saturation but also by the hydrate distribution within the pore space. This study presents a numerical approach for quantifying the relationship between the hydrate volume, distribution, and conductivity of HBS using pore network modeling (PNM). We use two distinct hydrate distributions in pores, ideal grain-contacting and pore-filling. Their electrical conductivities, in relation to hydrate saturation, were simulated on the pore scale using the finite element method. Regardless of the hydrate distribution, the electrical conductivity of the pore network models decreases with increasing hydrate saturation. At the same saturation, the electrical conductivity of PNM with grain-contacting hydrates is higher than that of pore-filling hydrates. While the resistivity index of the hydrate-bearing PNM exhibits a variation pattern consistent with Archie’s formula, the saturation exponent is not a fixed value. The experimental samples represent a closed system where significant local fluid salinity changes would occur due to hydrate formation, strongly influencing the bulk conductivity. The numerical simulation results considering the salinity effect confirm the plausibility of grain-contacting hydrate while challenging the existence of an ideal pore-filling hydrate when compared to the measured data as the conductivity associated with such a uniformly distributed pore-filling hydrate contradicts the experimental measurements. Our research indicates that the variability of the saturation exponent, highlighting the complex nature of hydrate distributions within sediments, calls for refined electrical saturation models to enhance the evaluation of marine hydrate reservoirs.