HR: 0800h
AN: H51G-0858    [Abstracts]
TI: Stress/Strain Cycling Effects on Colloid Transport in a Synthetic Porous Medium
AU: * Roberts, P M
EM: proberts@lanl.gov
AF: Los Alamos National Laboratory, PO Box 1663 MS-D443, Los Alamos, NM 87545, United States
AU: Ibrahim, R H
EM: reem@lanl.gov
AF: Los Alamos National Laboratory, PO Box 1663 MS-D443, Los Alamos, NM 87545, United States
AU: Abdel-Fattah, A I
EM: amr2450@lanl.gov
AF: Los Alamos National Laboratory, PO Box 1663 MS-D443, Los Alamos, NM 87545, United States
AB: Laboratory experiments on porous rock cores have shown that seismic-band (100 Hz or less) mechanical stress/strain cycling of the rock matrix can mobilize sub-pore-size particles (colloids) trapped in the pore space and allow them to be expelled during steady-state water flow. This coupling of dynamic stress to colloid mobility is a potential key mechanism whereby seismic waves may alter formation permeability and porous mass transport in Earth's crust. Prior experiments where colloid suspensions were injected into Fontainebleau sandstone demonstrated that colloid size and the ionic strength of the suspending fluid are major parameters that will control the ability of the colloids to attach to pore walls or to form particle bridges at pore throats. Both effects can lead to significant changes in permeability. A unique core-holder apparatus that applies low-frequency mechanical stress/strain to 2.54-cm-diameter porous rock samples during constant-rate fluid flow was used for those experiments. It was found that dynamic stress at 25 to 50 Hz could mobilize these trapped colloids only when the ionic strength is low. These earlier experiments on natural rocks were difficult to interpret in terms of how the colloids distributed themselves throughout the heterogeneous pore space and what interactions were occurring between the colloids and the solid matrix. Observed permeability changes appeared to be confined to the first 5-10 cm of the rock where the colloids were injected, yet significant transport of colloids was observed along the entire length of the sample. The "natural rock" system is too complex geometrically at the pore scale to allow quantification of mass transport properties along its entire length. To remedy this problem, new colloid transport experiments were performed with a synthetic glass-bead pack. Unconsolidated 1-mm-diameter borosilicate beads were packed into a confining sleeve to a length of 30 cm. Sufficient radial and axial confinement pressures were applied to the sleeve to create a fluid seal and to introduce rigidity to the sample. Suspensions of 2-um polystyrene microspheres in deionized water were injected into the bead pack while maintaining constant flow through the sample. Sample permeability and effluent microsphere production were measured before, during, and after low-frequency stress cycling, as in the previous experiments on rocks. Because the core is composed of uniform sized beads, the detailed microsphere breakthrough curves combined with precise permeability measurements as a function of position along the bead pack allow identification of regions where applied dynamic stress has the largest effects on colloid mobility. Preliminary results of these new experiments will be presented.
DE: 1832 Groundwater transport
DE: 1835 Hydrogeophysics
DE: 3653 Fluid flow
DE: 5114 Permeability and porosity
DE: 5139 Transport properties
SC: Hydrology [H]
MN: 2007 Fall Meeting