lab

THE SEDIMENT & ROCK MECHANICS LAB AT PENN STATE

Equipment | Example Lab Data | Projects | Papers

EQUIPMENT

Back Row: Steve Swavely, Ki-Bok Min, Jon Samuelson, Matt Ikari, Rob Skarbek, Bryan Kaproth, Demian Saffer, Derek Elsworth, Igor Faoro, Josh Taron; Front Row: Ryan Jacoby, Hui Long, Insun Song, Andy Rathbun, Peter “Tejas” Flemings, Chris Marone, Brett Carpenter, Andre Niemeijer

We maintain a state-of-the-art sediment mechanics laboratory, which hosts a high-pressure consolidation and permeability system, as well as several computers for laboratory data reduction. It is housed in a large shared rock/sediment mechanics laboratory at Penn State, along with a biaxial shearing apparatus and true triaxial system developed and maintained by Chris Marone. Our facility is equipped for uniaxial sediment consolidation studies at loads up to 50 kN (corresponding to stresses up to 100 MPa on 25 mm diameter samples), for high-precision constant-flow rate permeability measurements on 25 mm, 38 mm and 50 mm diameter core samples, and for triaxial deformation and permeability measurements at confining pressures up to 70 MPa. The systems are optimized for high-precision, long-term measurement of hydraulic and mechanical properties, in which long duration stability of pressure/volume, temperature, and load frame control are essential (for example, flow-through permeability and/or consolidation experiments on fine-grained mudstones and shales that typically require several days to a few weeks to complete).

All of the equipment can operate under either manual or computer control; typical tests are run in computer control. We measure (or control) fluid volumes to +/- 1 mm³ (1 microliter), displacements to +/- 1 micron or less, and displacement rates to a precision of +/- 10^-2 micron/min. We typically measure or control stress to +/- 1 kPa, but this can be modified by swapping load cells and pressure transducers. For uniaxial deformation experiments, the sample cell is a high-pressure fixed-ring oedometer vessel designed jointly by Saffer and a private vendor (below). It consists of a cell base and top, which bolt together at a mid-point allowing for easy sample insertion and access. The vessel includes two fluid ports at the top, and two at the base. One pressure/volume controller connected at the top and the other at the base – in order to conduct singly or doubly drained consolidation experiments and flow-through permeability measurements. The sample cell is rated to 10 MPa internal pore pressure.

For triaxial experiments, we use three different cells: a low-pressure cell rated to 3 MPa confining pressure (shown below), a medium-pressure cell rated to 14 MPa, and a triaxial core holder rated for 70 MPa confining pressure and up to 150 degrees C. The vessels are all interchangeable within the load frame, and all allow measurement of permeability during deformation, but differ slightly in their construction. The low-pressure vessel is constructed from plexiglass, and is designed for use with water or oil as a confining fluid. It has 4 ports for fluid or electrical lines to pass into the vessel. These are typically used for confining pressure and pore pressure, which can be controlled separately at the sample base and top. The high pressure vessel is constructed of 3-16 stainless steel, and is designed for use with oil as a confining fluid. It has 8 ports for fluid or electrical lines, which allow the use of additional measurement elements, including an internal load cell for measurement of the axial stress without interference from the seal friction (although this can be calibrated and is typically small) and a radial strain gauge. The high pressure core holder is modified from a commercial vendor; axial pressure is typically applied hydraulically, but can also be applied mechanically (using a load frame).

In our experiments, samples are typically jacketed in latex with o-ring seals at the top and base of the sample, and silicon oil is used as a confining fluid. Any desired stress path within the apparatus maximum pressure and stress capability can be achieved under (1) stress feedback control (controlled mean and differential stress, resulting strain is monitored), (2) strain control (controlled shortening rate and/or rate of volume change, resulting stresses are monitored), or (3) manual control.

EXAMPLES OF LAB DATA

We routinely measure permeability of marine sediments, fault gouge, and wall rock as part of our studies. We obtain permeability data from flow-through experiments (constant head or constant flow), and also through constant rate of strain (CRS) tests. In the latter, a sample undergoes uniaxial consolidation at a constant displacement rate, and we monitor the pore pressure within the sample. The displacement rate is chosen such that excess pore pressures are a small fraction (typically 2-5 %) of the effective stress. The example below shows permeability for a suite of marine mudstones sampled on the incoming oceanic plate outboard of the Nankai Trench during ODP Leg 190. Small dots are data from CRS tests; filled colored symbols are from flow-through tests, and gray circles are from Gamage & Screaton (2006), shown for comparison.

We also measure deformation behavior as part of our research, in order to define sediment, fault gouge, and country rock rheology. As one example, we conduct consolidation experiments on marine sediments in order to define compression properties. Clay-rich sediments are especially interesting because they retain a “memory” of their maximum past burial stress. Upon reloading to effective stresses below the maximum past burial stress (known as preconsolidation stress or Pc’), samples follow a primarily elastic recompression trajectory (below). At stresses beyond Pc’, the stress-strain behavior changes and deformation is dominantly plastic. If specimens are handled carefully enough during sampling and test preparation in the lab, these data can be used to estimate in situ effective stress and pore pressure. The plot below shows data for two samples from the Nankai Trough, illustrating Pc’, recompression behavior, and plastic compression behavior (described by the compression index Cc).

SOME OF OUR ONGOING PROJECTS IN THE LAB

Our ongoing lab studies of permeability, deformation, and frictional behavior include studies of subduction zone sediment and fault materials, in order to understand the rock properties that govern the strength and sliding stability of subduction megathrusts. For this project, our approach is to study a unique suite of natural samples that span P-T conditions from seafloor conditions up to depths of ~12 km and temperatures of 250 C (schematic above). These samples include material from ODP boreholes offshore Costa Rica (CR) and at the Nankai Trough (NT), in addition to outcrop samples from exhumed accretionary complexes and strike-slip faults.

We also use permeability and consolidation measurements as a key component of our research on pore pressure in subduction zones. We use the laboratory permeability data to constrain numerical models of fluid flow, and consolidation and triaxial tests to provide estimates of in situ pore pressure.

We are also conducting measurements on core samples and cuttings from the SAFOD borehole and on outcrop samples representing lithologic units within the 3-D crustal volume containing the San Andreas Fault and SAFOD hole. This work is described here. Photos to right are from field outcrop sampling, showing serpentinite from New Idria (top right), and en echelon cracks in a road that crosses the creeping portion of the San Andreas Fault North of Paso Robles, CA (bottom right).

Our laboratory studies also include a range of studies that investigate the effects of stress state on permeability, the role of deformation bands in reducing the permeability of sandstone reservoirs and aquifers, the relationships between mudrock porosity and stress in complex stress settings (meaning non-uniaxial deformation), and most recently, we are ramping a collaboration to study the permeability of coal and the effects of flooding with water and CO₂.

Relevant publications & selected meeting abstracts from our lab work:

Long, H., Flemings, P.B., Germaine, J.T., and Saffer, D.M., Consolidation characteristics of sediments from IODP Expedition 308, Ursa Basin, Gulf of Mexico, in press, Ocean Drill. Prog. Sci. Results, 308.

Saffer, D.M. (2007), Pore pressure within underthrust sediments in subduction zones, in Dixon, T. et al. (Eds.), The Seismogenic Zone of Subduction Thrust Faults, Columbia University Press, p. 171-209.

Marone, C., and Saffer, D.M. (2007), Fault friction and the upper transition from seismic to aseismic faulting, in Dixon, T. et al. (Eds.), The Seismogenic Zone of Subduction Thrust Faults, Columbia University Press, p. 346-369.

Ikari, M.J., Saffer, D.M., and Marone, C. (2007), Effect of hydration state on the frictional properties of montmorillonite-based fault gouge, J. Geophys. Res., 112, B06423, doi:10.1029/2006JB004748.

McKiernan, A.W., and Saffer, D.M. (2005), Data Report: Permeability and consolidation characteristics of sediments collected during ODP Leg 205, Costa Rica, online publication, Ocean Drill. Prog. Sci. Results, 205, 2005.

Saffer, D.M., and McKiernan, A.W. (2005), Permeability of underthrust sediments at the Costa Rican margin: Scale dependence and implications for dewatering, Geophys. Res. Lett., 32, L02302, doi:10.1029/2004GL021388.

Saffer, Demian M., and Marone, Chris J. (2003), Comparison of smectite- and illite-rich gouge frictional properties: Implications for the updip limit of the seismogenic zone along subduction megathrusts, Earth Planet. Sci. Lett., v. 215, p. 219-235.

Saffer, Demian M. (2003), Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with Northern Barbados and Nankai, J. Geophys. Res., 108 (B5), 2261, doi: 10.1029/2002JB001787.

Saffer, D.M., Frye, K., Marone, C., and Mair, K. (2001), Laboratory results indicating weak and potentially ustable frictional behavior of smectite clay, Geophsical Research Letters, 28, p. 2297-2300.

Saffer, Demian M., et al. (2000), Inferred pore pressures at the Costa Rica subduction zone: Implications for dewatering processes, Earth and Planet. Sci. Lett., 177, 193-207.

Skarbek, R.M., and Saffer, D.M., Pore Pressure Development in Sub-Decollement Sediments in Subduction Zones: Insights From Laboratory Data and Numerical Modeling, AGU Fall meeting, 2007.

Carpenter, B.M., Marone, C.J., and Saffer, D.M., Frictional Behavior of Materials in the 3D SAFOD Volume, AGU Fall meeting, 2007.

Ikari, M.J., Marone, C., and Saffer, D.M., Stability of Clay-rich Fault Gouge at Intermediate to High Shear Strain, Euro-conference of "Rock Physics and Geomechanics" on Natural hazards: Thermo-hydro-mechanical processes in rocks, 2007.

Ikari, M., Marone, C., Saffer, D.M., and Samuelson, J., Shear Induced Pore Pressure Generation in Montmorillonite-Based Fault Gouge, AGU Fall meeting, 2006.

Saffer, D.M., and McKiernan, A.W., Constraints on Pore Pressure in Subduction Zones From Geotechnical Tests and Physical Properties Data, AGU Fall meeting, 2005.

Ikari, M.J., Marone, C., Saffer, D.M., and McKiernan, A.W., Effect of Hydration State on the Frictional Properties of Montmorillonite-based Fault Gouge, AGU Fall meeting, 2005.