Effect of interlayer mixed zone and effective stress on permeability anisotropy of NGH turbidite reservoir

Abstract

Resulting from the bedding structure of turbidite reservoir, permeability anisotropy is an essential parameter for natural gas hydrate (NGH) exploitation. Fundamentally, the evolution of permeability anisotropy under increasing effective stress are investigated in this study. Taking the turbidite sediments in northern Cascadia as an example, a series of experimental tests and co-simulations of discrete element method (DEM) and computational fluid dynamics method (CFD) are conducted. For layered turbidite sediments, an interlayer mixed zone will be formed in the interface between fine-grained layer and coarse-grained layer after loading. Fine particles will migrate to the coarse layer, blocking the horizontal preponderance flow paths and further reduce the permeability anisotropy. Moreover, the development of interlayer mixed zone mainly occurs in initial compaction stage and slows down in higher stress level, for which the permeability anisotropy shows a staged reduction with increasing effective stress. Considering the effect of interlayer mixed zone, a predictive model of permeability anisotropy is proposed and verified by the experimental results, which has higher accuracy than traditional “layered cake” model. The finding of this research also confirms the mixing effect in deep sediments, which may affect the geological history records in target sedimentary sequence.