Numerical simulation of sand production based on a grain scale microcosmic model for natural gas hydrate reservoir

Key words:  natural gas hydrate  sand production simulation  sand production prediction  grain size scale  sand production morphology

Abstract:

A sand particle structure model was built in terms of grain size scales for natural gas hydrate reservoir, and the microcosmic sand production process and morphology of gas hydrate reservoir were simulated, in which the sand production mechanism, sand production morphology and the influencing factors were studied. In the particle structure model, the particle diameter, sphericity coefficient and position inclination angle were randomly determined according to the particle size distribution of the hydrate formation sands. Particle cementation strength, hydrate saturation, inter-grain stress and their random heterogeneous distribution were characterized according to the well logging curve using a stochastic method. A hydrate strength ratio coefficient and its influence coefficient were used to characterize the effect of hydrate decomposition on the bonding strength of sands. Based on the constructed particle exfoliation discriminant model, and combined with a hydrate decomposition model. Numerical simulations of particle exfoliation channel expansion and sand production morphology were conducted. The results show that, for reservoirs with high hydrate saturation and low cementation strength, sand production can start from the boundary of the pore wall, then it can expand unevenly along the weak cementation surface to form a wormhole like path, and finally it can form a composite sand production morphology, in which the front end looks like a wormhole and the back end will continuously collapses. The decomposition of hydrate can greatly reduce the strength of the reservoir, and the sand production front and the decomposition front become very close during the production process. With the continuation of initial production, the maximum exfoliated sand particle size can be reduced from 16 μm to 6 μm, and the speed of exfoliated sand production showed an upward trend. Hydrate saturation and its decomposition have a significant impact on the size distribution of the sands produced and sand production rate. The lower the hydrate saturation is, the less impact of its decomposition has on the overall cohesion of the reservoir, and the less possible for sand out. Compared with an average hydrate saturation of 0.48, for hydrate saturation of 0.4,0.3 and 0.18, the exfoliation rate of sand particles can be decreased to about 80％, 69％ and 58％ of average value, respectively.