Stony Brook AMS - Downloadable Preprints - 2008

The dependence of oil permeability on oil throughput determines how long it takes for a production well to "clean up" or restore productivity after a gel treatment. Consequently, we were interested in how the gel dehydration process progresses as a function of oil throughput. This paper describes a new study where pore-scale XMT images were obtained at a variety of oil (hexadecane) throughput values after gel placement (involving a pore-filling Cr(III)-acetate- HPAM gel). For each pore in our image volume, we followed oil and water saturations as a function of oil throughput. These studies were performed both in water-wet Berea sandstone and in hydrophobic porous polyethylene. In hydrophobic porous polyethylene, oil saturations increased and gel was destroyed (presumably dehydrated) quite quickly in the smallest pores (10-6 mm3). Also, oil saturations increased and gel was destroyed quickly in the largest pores (<0.005 mm3). In contrast, oil saturations rose much more gradually for the most common or intermediate-sized pores (around 5x10-5 mm3, the peak in the pore size distribution). The minimum in oil saturation versus pore size may result from a balance between gel dehydration by oil film growth versus gel extrusion. Presumably, during oil injection after gel placement, an oil film forms with a thickness that is about the same on all polyethylene surfaces. However, because the ratio of film thickness to pore or throat size increases with decreased pore size, gel dehydration occurs faster and more effectively in the smallest polyethylene pores, causing oil saturation to increase with decreased pore size. This effect may lose its signicance for pore sizes above 10-4 mm3. As pore size increases above 10-4 mm3, gel extrusion from the pore becomes more lIKELY - explaining why oil saturation increased with increased pore size for the largest pores.

In contrast in water-wet Berea sandstone, increases in oil saturation occurred evenly over all pore sizes (10-6 to 0.02 mm3) for all oil throughput values. Consistent with imbibition and drainage studies performed before gel placement, oil apparently had equal access to Berea pores of all sizes, and thus uniformly dehydrated gel in pores of all sizes. Gel extrusion did not appear to be significant in the Berea pores.

To be presented at the 2008 SPE Improved Oil Recovery Symposium, Tulsa, OK, Apr. 19-23, 2008.
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SUNYSB-AMS-08-02	Dominance Hierarchies as a Model of Network Formation in Small Groups	I.D. Chase and W.B. Lindquist

Virtually all human societies show elements of hierarchical organization - top to bottom arrangements in which some individuals have greater access to material goods, participate more in group activities, receive more attention, or exert more power and influence than do other individuals. The near universality of hierarchies indicates that they are of fundamental significance for the social organization of groups and societies. Some of the core questions concerning the development of these social structures are: What mechanisms generate hierarchies? Do these mechanisms reflect differences among individuals or their actions during hierarchy formation? What theoretical models exist to explain the structural forms of hierarchies, and what are the levels of support for the different models? Can understanding the development of hierarchies provide more general insight into the evolution of other kinds of networks?

We explore these questions in an unusual way for sociology: we first look at theoretical models for the explanation of dominance hierarchies ("pecking orders") in small groups of animals before considering hierarchies in small groups of humans. We describe dominance relationships and dominance hierarchies in animal groups, evaluate the various theoretical models that biologists have proposed to explain the structural forms of these hierarchies, outline a new type of model for animal hierarchies that avoids some of the problems inherent in the earlier models, and then compare current models for human small group hierarchies with those of animal hierarchies. We conclude that both our new model and current experimental work on animals indicate, ironically, that animals generate their hierarchies through richer and more sociologically sophisticated dynamics than those proposed in the current models for humans.

To appear in The Oxfold Handbook of Analytic Sociology, P. Hedstrom and P. Bearman (Eds.) Oxford University Press
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SUNYSB-AMS-08-03	Tomographic Analysis of Reactive Fluid Induced Pore Structure Changes in Flow Column Experiments	R. Cai, W.B. Lindquist, W. Um, and K.W. Jones,

Dissolution followed by precipitation is a major reaction mechanism in forming secondary precipitates in most porous sediments. Secondary precipitation is of interest as a structure modifying mechanism tp control contaminant transport in the subsurface environment. We employed synchrotron X-ray computed microtomography in combination with flow-column experiments to capture and quantify dissolution and secondary precipitation changes in the microstructure of Hanford sediments exposed to simulated caustic waste. Dissolution induced changes include: an increase in the number of larger pores. Precipitation induced changes include reduction in the number of small pores and closure of small throats, with accompanying reduction in pore coordination numbers and reduction in the number of pore pathways. Pathway tortuosities however, are not dramatically affected.
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SUNYSB-AMS-08-04	Supporting Online Material for Tomographic Analysis of Reactive Fluid Induced Pore Structure Changes in Flow Column Experiments	R. Cai, W.B. Lindquist, W. Um, and K.W. Jones

This is the Materials and Methods section for SUNYSB-AMS-08-03.
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SUNYSB-AMS-08-05	Continuous Shifts in the Active Set of Spinal Interneurons During Changes in Locomotor Speed	D.L. McLean, M.A. Masino, I.Y.Y. Koh, W.B. Lindquist and J.R. Fetcho

The classic 'size principle' of motor control describes how increasingly forceful movements arise by the recruitment of motoneurons of progressively larger size and force output into the active pool. Here, we explore the activity of pools of spinal interneurons in larval zebrafish and find that increases in swimming speed are not associated with the simple addition of cells to the active pool. Instead, the recruitment of interneurons at faster speeds is accompanied by the silencing of those driving movements at slower speeds. This silencing occurs both between and within classes of rhythmically-active premotor excitatory interneurons. Thus, unlike motoneurons, there is a continuous shift in the set of cells driving the behavior, even though changes in the speed of the movements and the frequency of the motor pattern appear smoothly graded. We conclude that fundamentally different principles may underlie the recruitment of motoneuron and interneuron pools.

To appear in Nature Neuroscience
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SUNYSB-AMS-08-06	Direct Observations of Three Dimensional Growth of Hydrates Hosted in Porous Media	P. Kerakr, K.W. Jones, R. Kleinberg, W.B. Lindquist, S. Tomov, H. Feng, and D. Mahajan

We present the first visualization of time-resolved 3-D growth of tetrahydrofuran (THF) hydrates in porous media using X-ray computed microtomography (CMT). The 1119?m x 1630?m x 1443?m volume rendered from a stack of images shows patchy hydrates formed from excess THF in aqueous solution. Hydrate growth is found convex away from the grains, showing that liquid, not hydrate, is the wetting phase. This is similar to ice growth in porous media in which mineral grains are coated with unfrozen water films. Hydrates formed at several locations in the system before the first images were taken, 28 hours after the system was cooled to hydrate-forming temperature. The size and shape of hydrate patches varied within a sample of glass spheres of uniform size, consistent with the random nature of nucleation and growth. Tracking individual grains in tomoscans taken over a three day period indicated grain movement during the hydrate growth. No container-wall effect was observed. The extension of the observed growth behavior to methane hydrate could have implication in understanding the role of hydrate in seafloor stability and climate change.

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SUNYSB-AMS-08-07	Subgrid Models for Mass and Thermal Diffusion in Turbulent Mixing	H. Lim, Y. Yu, J. Glimm, X.-L. Li and D. H. Sharp

We are concerned with the chaotic flow fields of turbulent mixing. Chaotic flow is found in an extreme form in multiply shocked Richtmyer-Meshkov unstable flows. The goal of a converged simulation for this problem is twofold: to obtain converged solutions for macro solution features, such as the trajectories of the principal shock waves, mixing zone edges, and mean densities and velocities within each phase, and also for such micro solution features as the joint probability distributions of the temperature and species concentration. We introduce parameterized subgrid models of mass and thermal diffusion, to define LES that replicate the micro features observed in the DNS. The Schmidt numbers and Prandtl numbers are chosen to represent typical liquid, gas and plasma parameter values. Our main result is to explore the variation of the Schmidt, Prandtl and Reynolds numbers by three orders of magnitude, and the mesh by a factor of 8 per linear dimension (up to 3200 cells per dimension), to allow exploration of both DNS and LES regimes and verification of the simulations for both macro and micro observables. We find mesh convergence for key properties describing the molecular level of mixing, including chemical reaction rates between the distinct fluid species. We find results nearly independent of Reynolds number for Re ~ 300,~ 6000, ~600K. Methodologically, the results are also new. In common with the shock capturing community, we allow and maintain sharp solution gradients, and we enhance these gradients through use of front tracking. In common with the turbulence modeling community, we include subgrid scale models with no adjustable parameters for LES. To the authors' knowledge, these two methodologies have not been previously combined. In contrast to both of these methodologies, our use of Front Tracking, with DNS or LES resolution of the momentum equation at or near the Kolmogorov scale, but without resolving the Batchelor scale, allows a feasible approach to the modeling of high Schmidt number flows.

Submitted to Physics of Fluids
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