Modeling of the influence of gas hydrate content on the electrical properties of porous sediments

Abstract

The geophysical investigation of gas hydrate accumulations within marine sediments and the sediments of permafrost regions suffer from a lack of information on the influence of the hydrate content on the physical sediment properties. The estimation of the hydrate content using downhole electrical measurements based on Archie's law requires the knowledge of the saturation exponent. The saturation exponent is an empirical parameter that includes influences from the internal rock structure such as pore shape, connectivity and constrictivity of the pore network, and the distribution of the conducting phase. Based on different models that account for the different morphological forms of gas hydrates found during gas hydrate sampling in various research wells, the influence of gas hydrate content on the electrical properties of the hydrate bearing sediment was investigated. For all studied forms of hydrate occurrence, disseminated in the pore space, nodular, and layered, the saturation exponent depends on the sediment properties and on saturation itself. The growth of gas hydrate nodules, lenses, and layers is a process that is assumed to result in the displacement and compaction of the surrounding sediment. Because of this change of sediment properties during hydrate generation, the saturation exponent for these forms of hydrate occurrence depends strongly on the relationship between porosity and formation resistivity factor, expressed in the form of Archie's cementation exponent. For the case that hydrate occurs disseminated in the pore space and the assumption that capillary effects are important for hydrate generation, the saturation exponent depends on grain size and grain size sorting. For the parameters chosen for these model calculations, the saturation exponent varies between 0.5 and 4. The use of a constant mean value for the saturation exponent of approximately 2 can result in both underestimation and overestimation of the hydrate content.