H22B-01 INVITED 10:20h
Laboratory Equipment and Methodology for Physically Based Upscaling of Heterogeneous Porous Media
The accurate characterization of flow and transport in near-surface aquifers and hydrocarbon reservoirs requires the detailed knowledge of subsurface structures and flow paths. Enormous resources are invested in exploration and characterization using 3-D geophysical surveys, well tests, geophysical logging, core measurements, etc. Unfortunately, much of the information acquired is lost to compromises and simplifications made in constructing numerical grids for the simulators used to predict field performance and economic viability. In the interest of computational efficiency, recognized heterogeneities are simplified, averaged out, or entirely ignored in spite of recent studies that recognize that (1) structural and lithologic heterogeneities exist at all scales in rocks and, (2) small heterogeneities influence and can control the physical properties of rocks. In this work we demonstrate the use of laboratory equipment designed to measure fine scale heterogeneity of rocks and soils. We then discuss the development of a methodology that uses these measurements to develop reservoir and aquifer models. These models are calibrated to the measured heterogeneous data and can be upscaled in a way that is consistent with the transport physics and the efficient use of environmental and hydrocarbon geophysics. This methodology provides more accurate interpretation and representation of the subsurface for both environmental engineering and oil and gas applications. We show through examples, (i) the important influence of even subtle heterogeneity in the interpreting of petrophysical data, and (ii) how physically based upscaling can lead to a different and more accurate description of a heterogeneous system, when compared to a more traditional upscaling approach that combines averaging and the application of core-based models.
H22B-02 10:40h
The Effect Of Scale On Seismic Measurements Of Fracture Properties
Remotely determining the hydraulic properties of a fractured rock is very important for characterization and monitoring of shallow fractured aquifers. Laboratory studies have shown that the hydraulic properties of a fractured medium are related to the specific stiffness of a fracture. In addition, fracture specific stiffness can be quantified from seismic measurements of attenuation and velocity. However, quantifying fracture specific stiffness from seismic methods is a scale-dependent process because there are length scales associated with the fracture geometry that controls stiffness, with the wavelength of the seismic probe and with the size of the source/receiver illumination of the fracture plane. We have developed an acoustic lens system to investigate the aforementioned length scales that affect geophysical interpretation of fracture properties. Using compressional mode piezoelectric transducers (1Mhz) in conjunction with a set of acoustic lenses, the acoustic system produces a collimated wavefront with diameters of 15 mm, 30 mm, 45 mm and 60 mm. The seismic wavelength varies over one order of magnitude. To study the scaling behavior of the fractured medium, the same region of a sample is scanned with the collimated wavefronts with different diameters. The number of measurements depends on the size of the collimated wavefront. The recorded signals are analyzed using wavelet transformation to examine wave attenuation, dispersion and velocity as a function of scale. In this study, cylindrical carbonate rocks and acrylic samples measuring 150 mm in diameter by 76 mm in height were used. After characterizing the intact rock samples, a fracture was induced using Brazil testing. For the acrylic samples, diffraction gratings were milled into the acrylic. For homogeneous samples, the average seismic properties were scale independent. However, heterogeneous fracture geometry that produced strong scattering resulted in scale dependent seismic properties. Scale dependent seismic attributes will affect the interpretation of the hydraulic properties of the fracture from seismic measurements. Acknowledgments: Geosciences Research Program, Office of Basic Energy Sciences US Department of Energy. LJPN wishes to acknowledge University Faculty Scholar program at Purdue University
H22B-03 10:55h
Infiltration Flow Path Distributions in Unsaturated Rocks
Spatial distributions of infiltration flow paths through rock formations are complex networks that determine flow velocities, control rates of natural geochemical reactions in the subsurface, as well as rates of contaminant transport to underlying groundwater. Despite these important consequences, distributions of infiltration paths and locally fast seepage rates through rocks are not well understood. Laboratory-based studies on fractured rocks cannot easily be conducted on systems large enough to include sufficient fracture network complexity, so that inferences of field-scale flux distributions cannot be reliably made. Field-based studies to date have permitted quantification of only a small fraction of the flow distribution, typically while imposing extremely high fluxes, and therefore have not allowed comprehensive delineation of flow distributions expected under natural recharge. Based on hydraulic scaling considerations, we hypothesize that unsaturated flow path distributions in rock deposits will be similar to those occurring in fractured rock formations under low overall infiltration rates. Talus rock deposits and mine waste rock piles control flow and transport into their respective underlying groundwaters. All of these reasons motivated infiltration experiments in rock packs. Experiments have been conducted on 4 different rock types and system scales ranging from 1 to 46 rock layers. Our experiments showed that infiltration through rocks conforms to no previously reported behavior in soils, and that flow paths do not progressively converge into fewer and fewer flow paths. Instead, a fundamentally different hydraulic structure develops, having an exponential (geometric) flux distribution, with the characteristic scale determined by the characteristic rock size. Although the phenomena are very different, the evolution of flow path distributions and local seepage rate distributions is predictable based on a statistical mechanical model for energy distributions. Our experiments and model are consistent with the available data on natural recharge flow paths in deep unsaturated fracture rocks at Yucca Mountain. Funding of this study was provided by the Geosciences Research Program, Basic Energy Sciences, U.S. Department of Energy.
H22B-04 11:10h
Heat Transfer through Rockfall
Thermally induced rock stresses at the potential high-level nuclear waste (HLW) repository at Yucca Mountain, Nevada, can degrade the drifts, possibly causing rockfall onto the dripshield overlying the emplaced waste packages. Thermal-hydrological processes will be altered by changes in thermal conductivity, ventilation, radiation, and convection resulting from such rockfalls. Determining the effects of collapse materials on repository performance is difficult because heat and mass transfer through the engineered barrier and highly heterogeneous collapse materials is a complex and coupled process. Measurements and estimates of thermal conductivity of intact rock samples from geologic units at Yucca Mountain are available, but collapsed rock will be a mixture of broken rock and air. The purpose of this investigation is to assess the thermal conductivity of crushed tuff from the Topopah Spring lower lithophysal unit at Yucca Mountain and identify the important heat and mass transfer mechanisms in the collapsed rock for the range of conditions expected at a potential geologic HLW repository. A laboratory apparatus was used to directly measure steady-state bulk thermal conductivity of crushed tuff for a range of temperatures up to 197°C and thermal gradients as large as 990°C/m. Measured thermal conductivities varied from 0.38 to 0.56 W/m-K. In general, thermal conductivity increased with either increased temperature or increased thermal gradient. This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC. The NRC staff views expressed herein are preliminary and do not constitute a final judgement of determination of the matters addressed or the acceptability of a license application for a geologic repository at Yucca Mountain.
H22B-05 11:25h
The Effect of Mineral Deposition on the Hydraulic and Seismic Properties of Fractures
Fractures and other subsurface discontinuities can be altered over time from geochemical interaction with the pore fluids. Alteration of the pore space in a fracture will affect the seismic and hydraulic properties of a fracture. Mineral deposition can alter the fracture specific stiffness by changing the size and strength of the contact area and/or filling-in the void space which will reduce the flow rate. We performed acoustic imaging experiments on single fractures in granite to determine the effect of mineral deposition on the seismic and hydraulic properties of single fractures. Prior to and after mineral precipitation (CaCO3), the samples (110 mm x 104 mm x 70 mm) were imaged over a 64 mm by 64 mm region using transmitted compressional waves (~1 Mhz). Eight ports were distributed around the perimeter of the fracture to measure the variation in flow as a function of position, as well as prior to and after chemical invasion. After mineral precipitation, the flow rate decreased by approximately 90%. The decrease in flow rate can be attributed to reduction of the fracture aperture and blocking of flow paths by mineralization. We observed that the initial void geometry controlled the amount and spatial distribution of mineral deposition within the fracture. The most reliable seismic indicator that the fracture had been altered was a reduction in the variance of the frequency distribution of the received signal. The reduced variance indicates that the fracture is homogenized by mineral deposition in the fracture voids (i.e. tending toward a move uniform fracture stiffness). Homogenization occurs because the mixing predominantly takes place in the dominant flow paths within the fracture, which tend to have lower fracture stiffness. The results indicate that acoustic imaging techniques are required in this characterization because they provide statistical indicators that help monitor changes in fracture geometry caused by mineral deposition. Acknowledgments: Geosciences Research Program, Office of Basic Energy Sciences US Department of Energy. LJPN wishes to acknowledge University Faculty Scholar program at Purdue University
H22B-06 11:40h
Effect of Stress on Water-Filled Inclusions in Viscoplastic Soils
The relationships between applied stress, deformation and internal pressure of water filled inclusions embedded in soft soils were studied to improve understanding of pore-scale mechanical behavior and towards development of methods for in situ measurement of key mechanical properties. Analytical expressions for inclusion internal fluid pressure as a function of biaxial remote stresses and material properties were developed and tested with Finite Element calculations and experimental results. We found that the applied mean stress, matrix yield stress and Poisson's ratio influence pressure in a water filled inclusion. Internal pressure tends to be higher than applied mean stress, in contrast with established effective stress theory for saturated soils. Changes in inclusion shape is mainly controlled by matrix shear properties (changes in volume are negligible due to water incompressibility). The results of this study provide the framework for development of sensors for in-situ measurement of soil mechanical and rheological properties required for linking mechanical and hydraulic properties.
H22B-07 11:55h
Elastic Waves Push Residual Organic Fluids From Saturated Rock
With world oil reserves dwindling and production shifting to increasingly forbidding environments, the emphasis is greater than ever on the more efficient extraction of the existing oil. Yet typically up to two-thirds of the U. S. domestic oil is abandoned underground. Elastic waves have been observed to increase productivity of oil wells, although the reason for the vibratory motion mobilizing the residual organic fluids has remained unclear. Residual oil is entrapped as blobs or ganglia in narrow pore constrictions due to the resisting capillary forces that prevent free motion of non-wetting fluids driven by water. A finite external pressure gradient, exceeding an "unplugging" threshold, is needed to carry the residual ganglia through. We show that vibrations help overcome the resistance of capillary forces by adding an oscillatory inertial forcing to the external gradient; when the vibratory forcing acts along the gradient and the threshold is exceeded, instant "unplugging" occurs. This mechanism predicts the mobilization effect to be proportional to the amplitude and inversely proportional to the frequency of vibrations. We observe this dependence in a laboratory experiment, in which residual saturation of an organic fluid is created in a glass micromodel, and mobilization of the dyed ganglia is monitored using digital photography. We also directly demonstrate the release of an entrapped ganglion from a pore constriction by the application of vibrations in a computational fluid-dynamics simulation. The technologies that can utilize this phenomenon are not limited to enhanced oil recovery, but also apply to the remediation of groundwater contaminated by leaks from underground storage tanks and surface spills of organic fluids.