Numerical simulations of sand migration during gas production in hydrate-bearing sands interbedded with thin mud layers at site NGHP-02-16

Abstract

The geomechanical behavior of gas hydrate-bearing sands during gas production is complex, involving changes in hydrate-dependent mechanical characteristics and also redistribution of stresses carried by the hydrate. When an interbedded mud layer is present, the complex response of the hydrate-bearing sediments is more pronounced because hydrate dissociation tends to occur preferentially in the sands adjacent to the hydrate-free mud layer, which can act as a heat source. This leads to non-uniform sediment deformation and potentially to shearing deformation along the sand/mud layers. In addition, since the hydrate-bearing sand permeability becomes far greater than the mud permeability with hydrate dissociation, a greater hydraulic gradient may exist along the interface between the sand/mud layers. These two behaviors may contribute to sand migration and thus a thorough study on the effect of a thin interbedded mud layer on the extent of sand migration is necessary. This paper presents numerical sand migration modeling of sand/mud interbedded hydrate-bearing sediments found at Site NGHP-02-16, one of 25 sites drilled as a part of National Gas Hydrate Program Expedition 02 (NGHP-02) in the offshore of India, subjected to depressurization. For a comparison and better understanding, simulation of homogeneous hydrate-bearing sediments is also conducted. It is found that greater sand migration occurs when the interbedded mud layer is present because of greater shearing deformation and greater pressure gradients at sand-mud interface. • This is the first work to analyze in detail the effect of presence of interbeds on sand migration. • Sediment deformation is mostly volumetric when homogeneous but becomes more deviatoric when interbedded. • Interbedded mud layers induce non-uniform hydrate dissociation, leading to greater sand migration in magnitude and duration.