HR: 17:45h
AN: NG12C-08    [PDF]
TI: Nonlinear Effects of Dynamic Stress on Particle Behavior in Fluids and Porous Media
AU: * Roberts, P M
EM: proberts@lanl.gov
AF: Los Alamos National Laboratory, EES-11, MS D443, Los Alamos, NM 87545
AB: Stress (seismic) waves have long been known to induce observable changes in porous flow behavior in the Earth's crust. Different types and degrees of flow changes will occur with different combinations of stress wave parameters, such as frequency, amplitude, wave mode and duration, and with the medium properties, such as permeability, elastic moduli, geometry, mineralogy, saturation and ionic strength. Numerous international research projects are investigating physical mechanisms to understand the stress-induced flow phenomenon over a size range of nano- to macro-scale. One of the mechanisms being studied in detail is nonlinear coupling of dynamic stress energy to sub-pore size particle behavior. This is an important mechanism because the distribution of particles in a porous medium will directly affect the permeability. Initial results will be presented from a new laboratory experimental project aimed at characterizing stress/particle interactions at two scales: microscopic (sub-micron) and bench-top (cm to m). Individual particle behavior in an ultrasonic field is being characterized with a microscopic, video image-processing system, focused on a glass flow-visualization cell that contains known particle/fluid suspensions. Using 3- and 1-micron diameter polystyrene spheres suspended in non-flowing water, three types of behavior are observed when the glass cell is excited with acoustic energy at 0.1-10 MHz: 1) Particle conglomeration and alignment at specific locations in the cell, 2) Induced fluid convection and particle entrainment, and 3) Changes in particle attachment to and detachment from the glass cell walls. Experimental examples of all three types of behavior will be shown, as well as initial results of characterizing the combinations of physical forces involved and simulating the behaviors numerically. Effects of stress/particle interactions on meso-scale porous flow are being investigated in the seismic range of frequencies (1-1000 Hz), using a core holder apparatus that mechanically strains 1-inch diameter porous rock samples during constant-rate fluid flow. Experiments have shown that axial stress oscillations at 10-100 Hz can cause both enhancement and reduction of bulk fluid flow through sandstone core samples. At 50 Hz and approximately 10e-04 strain amplitude, in-situ clay particles fouling the pore throats of a Berea sandstone sample were re-mobilized and expelled, causing a 20% increase in permeability. New experiments are being performed where known particle supensions are injected into a clean porous core sample and the effects on permeability are measured with and without stress stimulation and under different initial flow conditions. Particle flux and retention data will be compared with similar data from the microscopic visualization experiments above. This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Geosciences Division.
DE: 3909 Elasticity and anelasticity
DE: 3994 Instruments and techniques
DE: 5114 Permeability and porosity
DE: 5139 Transport properties
DE: 6045 Physics and chemistry of materials
SC: Nonlinear Geophysics [NG]
MN: 2003 Fall Meeting