Linking basin-scale and pore-scale gas hydrate distribution patterns in diffusion-dominated marine hydrate systems

The goal of this study is to computationally determine the potential distribution patterns of diffusion-driven methane hydrate accumulations in coarse-grained marine sediments. Diffusion of dissolved methane in marine gas hydrate systems has been proposed as a potential transport mechanism through which large concentrations of hydrate can preferentially accumulate in coarse-grained sediments over geologic time. Using one-dimensional compositional reservoir simulations, we examine hydrate distribution patterns at the scale of individual sand layers (1-20 m thick) that are deposited between microbially active fine-grained material buried through the gas hydrate stability zone (GHSZ). We then extrapolate to two-dimensional and basin-scale three-dimensional simulations, where we model dipping sands and multilayered systems. We find that properties of a sand layer including pore size distribution, layer thickness, dip, and proximity to other layers in multilayered systems all exert control on diffusive methane fluxes toward and within a sand, which in turn impact the distribution of hydrate throughout a sand unit. In all of these simulations, we incorporate data on physical properties and sand layer geometries from the Terrebonne Basin gas hydrate system in the Gulf of Mexico. We demonstrate that diffusion can generate high hydrate saturations (upward of 90%) at the edges of thin sands at shallow depths within the GHSZ, but that it is ineffective at producing high hydrate saturations throughout thick (greater than 10 m) sands buried deep within the GHSZ. Furthermore, we find that hydrate in fine-grained material can preserve high hydrate saturations in nearby thin sands with burial

Turbulence modulation by large ellipsoidal particles: Concentration effects

We use laboratory measurements to study how suspended ellipsoidal particles affect the velocity statistics of a turbulent flow. The ellipsoids have size, time, and velocity scales corresponding to the inertial subrange of the turbulence and are nearly neutrally buoyant. These characteristics make them likely candidates for two-way interactions with the fluid (i.e.; they influence the flow and are influenced by it). We vary the volume fraction of suspended ellipsoids and observe the effects on one- and two-point velocity statistics in the fluid phase. Measurements at two different heights indicate that particle buoyancy (0.5 % denser than the ambient fluid) significantly changes volume fraction. We see that particles' effect on turbulent kinetic energy is a non-monotonic function of the volume fraction. We also find that particles' presence causes a redistribution of velocity variance from large scales to small scales within the inertial subrange, i.e.; the slope of power spectra is flatter than in the single-phase case. © 2013 Springer-Verlag Wien