Clay A. Cooper

Experimental investigation of the stability boundary for double-diffusive finger convection in a Hele-Shaw cell

Double-diffusive convection may be an important transport phenomenon in subsurface porous media and fractures. The classic linear stability analysis derived for a porous medium with two components stratified such that each affects the vertical density gradient in an opposing manner predicts double-diffusive finger instability to occur when Rs1 + Rs2 ≥ Rsc, where Rs1 and Rs2 are the Rayleigh numbers of the faster and slower diffusing components, respectively, and Rsc is a critical value dependent upon the boundary conditions (0 ≤ Rsc ≤ 4π2). For cases where Rsc/|Rs1| ≪ 1, the above result can be simplified to −Rρ < 1/т, where Rρ is the buoyancy ratio of the fluid and т is the ratio of diffusivities (0 < т < 1). We experimentally tested the applicability of both stability criteria for situations where a narrow transition zone exists bounded above and below by constant concentrations and within a domain of uniform permeability. Experiments were conducted in a Hele-Shaw cell using a digital imaging technique which provided pixel-scale (∼0.2 mm) resolution of the evolving concentration field during convection. Within experimental error, our experiments support both criteria within their predicted ranges of applicability.

Double-diffusive Finger Convection in a Hele–Shaw Cell: An Experiment Exploring the Evolution of Concentration Fields, Length Scales and Mass Transfer

We present the results of an experiment conducted to explore the temporal and spatial development of double-diffusive finger convection in a Hele–Shaw cell. Two solutions each containing a different density affecting component were layered in a density stable configuration (sucrose solution over a more dense salt solution) with a nearly perturbation-free interface between. The mismatch of diffusive time scales for the two components leads to local density instabilities that generate upward and downward convecting fingers. Throughout the course of the experiment, a full-field quantitative light transmission technique was used to measure concentration fields of a dye tracer dissolved in the salt solution. Analysis of these fields yielded the temporal evolution of length scales associated with the vertical and horizontal finger structure as well as mass transfer. Distinct developmental stages are identified with strong correlation between all measures. These data provide a baseline that can be used to develop and evaluate both process-level models that simulate the full complexity of the evolving flow field and large-scale effective models that integrate over small-scale behavior.

Experimental evidence for seismically initiated gas bubble nucleation and growth in groundwater as a mechanism for coseismic borehole water level rise and remotely triggered seismicity

Changes in borehole water levels and remotely triggered seismicity occur in response to near and distant earthquakes at locations around the globe, but the mechanisms for these phenomena are not well understood. Experiments were conducted to show that seismically initiated gas bubble growth in groundwater can trigger a sustained increase in pore fluid pressure consistent in magnitude with observed coseismic borehole water level rise, constituting a physically plausible mechanism for remote triggering of secondary earthquakes through the reduction of effective stress in critically loaded geologic faults. A portion of the CO2 degassing from the Earths crust dissolves in groundwater where seismic Rayleigh and P waves cause dilational strain, which can reduce pore fluid pressure to or below the bubble pressure, triggering CO2 gas bubble growth in the saturated zone, indicated by a spontaneous buildup of pore fluid pressure. Excess pore fluid pressure was measured in response to the application of 0.1–1.0 MPa, 0.01–0.30 Hz confining stress oscillations to a Berea sandstone core flooded with initially subsaturated aqueous CO2, under conditions representative of a confined aquifer. Confining stress oscillations equivalent to the dynamic stress of the 28 June 1992 Mw 7.3 Landers, California, earthquake Rayleigh wave as it traveled through the Long Valley caldera, and Parkfield, California, increased the pore fluid pressure in the Berea core by an average of 36 ± 15 cm and 23 ± 15 cm of equivalent freshwater head, respectively, in agreement with 41.8 cm and 34 cm rises recorded in wells at those locations.

Evaporation from Fractures Exposed at the Land Surface: Impact of Gas‐Phase Convection on Salt Accumulation

A mechanism is investigated by which surface-exposed fractures could be a source of aquifer salinization in low-permeability fractured formations under arid conditions. It is hypothesized that evaporation of pore water within surface-exposed fractures is enhanced by convective air circulation within those fractures that vents moisture to the atmosphere. This evaporation also simultaneously enhances lateral movement of pore water from the adjacent matrix towards the fracture surface, permitting dissolved solutes to precipitate on the surface and form a crust. The salt crust can then dissolve during infiltration events and be flushed downward to the aquifer. Theoretical analysis shows that convective venting is expected during cool nights when atmospheric air is denser than the fracture air. Laboratory experiments support the hypothesis of rapid salt-crust formation in the presence of convectively moving air across a fracture face. A numerical model is developed and used to quantify the buildup of salt on a fracture face.

A Geologically Based Markov Chain Model for Simulating Tritium Transport With Uncertain Conditions in a Nuclear- Stimulated Natural Gas Reservoir

Summary Nuclear-stimulation technology, which used subsurface nuclear detonation to increase permeability of tight natural gas reservoirs, was evaluated in the late 1960s and early 1970s. The Rulison site, located in the Piceance basin, Colorado, is one of three sites in the US where the technology was tested. An increase in exploration and production for natural gas in the basin has led to a need to quantify the extent of radionuclide (mainly tritium) migration after the detonation and potential migration under likely production scenarios. To meet this need, a numerical model was developed to simulate gas flow and tritium transport toward a hypothetical production well. A crucial problem in the model development is that limited on-site data are too sparse to quantify uncertainty of subsurface properties. This problem is partly resolved by using indirect data and information, such as parameter measurements from a nearby site and geological information regarding lithofacies geometry. In particular, a geologically based Markov chain model was developed to simulate spatial distribution of the sandstone lithofacies. This paper presents an application of the numerical model for simulating tritium transport from the nuclear chimney toward the production well at a likely location producing at a rate typical for the basin. The results show that under the circumstances considered in this paper, tritium will not reach the production well with a confidence level of 95%. The results also show that the lithofacies structure is more critical in controlling tritium transport than parameters of the sandstone and hydraulically fractured sandstone. The parameters become important only when the connectivity of sandstone lenses exists to support tritium transport from the chimney to the production well. The developed modeling framework can be updated as additional subsurface data are collected. The framework can be used to support establishment of drilling restrictions that protect public health and the environment for different production well scenarios.

Transient pore pressure response to confining stress excursions in Berea sandstone flooded with an aqueous solution of CO2

We measured the pore pressure response due to carbon dioxide (CO2) gas bubble nucleation and growth in a Berea sandstone core flooded with an initially subsaturated aqueous solution of CO2, in response to a rapid drop in confining stress, under conditions representative of a confined aquifer. A portion of the CO2 in the Earths crust, derived from volcanic, magmatic, and biogenic sources, dissolves in groundwater. Sudden reductions in confining stress in the Earths crust occur due to dilational strain generated by the propagation of seismic Rayleigh and P waves, or aseismic slip in the near field of earthquakes. A drop in confining stress produces a proportional drop in pore fluid pressure. When the pore fluid contains dissolved CO2, the pore pressure responds to a drop in confining stress like it does in the dissolved gas-free case, until the pore pressure falls below the bubble pressure. Gas bubble nucleation and diffusive growth in the pore space trigger spontaneous, transient buildup of the pore fluid pressure, and reduction of effective stress. We measured the rate of pore fluid pressure buildup in the 100 s immediately following the confining stress drop, as a function of the saturation with respect to CO2 at the lowest pore pressure realized during the confining stress drop, using five different CO2 partial pressures. The rate scales with the saturation with respect to dissolved CO2, from 10 kPa/min at 1.25 to 166 kPa/min at 1.8. The net pore pressure rise was as large as 0.7 MPa (100 psi) over 5 h.

On the conversion of tritium units to mass fractions for hydrologic applications

We develop a general equation for converting laboratory-reported tritium levels, expressed either as concentrations (tritium isotope number fractions) or mass-based specific activities, to mass fractions in aqueous systems. Assuming that all tritium is in the form of monotritiated water simplifies the derivation and is shown to be reasonable for most environmental settings encountered in practice. The general equation is nonlinear. For tritium concentrations c less than 4.5×1012 tritium units (TU) – i.e. specific tritium activities<5.3×1011 Bq kg−1 – the mass fraction w of tritiated water is approximated to within 1 part per million by w ≈ c×2.22293×10−18, i.e. the conversion is linear for all practical purposes. Terrestrial abundances serve as a proxy for non-tritium isotopes in the absence of sample-specific data. Variation in the relative abundances of non-tritium isotopes in the terrestrial hydrosphere produces a minimum range for the mantissa of the conversion factor of [2.22287; 2.22300].

Injection of Nuclear Rocket Exhaust and Water into a Deep Unsaturated Zone

Abstract Nuclear rocket engine technology is being considered as a means of interplanetary vehicle propulsion for a manned mission to Mars. Significant technological research and development are required before nuclear-based rocket propulsion can be integrated into an interplanetary vehicle, including the firing of full-scale nuclear rocket engines in a test and evaluation facility. Testing of nuclear engines in the 1950s and 1960s was accomplished by directing engine exhaust gases into the atmosphere, a practice that is no longer acceptable. Testing nuclear rocket engines by injection of associated radioactive exhaust gases and water vapor into deep unsaturated zones may be a way to sequester radionuclides and will require comprehensive design of a nuclear engine test facility. We conducted numerical simulations to determine the ability of an unsaturated zone with the hydraulic properties of Yucca Flat alluvium at the Nevada National Security Site to contain gas-phase radionuclides. In these simulations, gas and water vapor (from water sprayed into the exhaust for cooling) were injected for two hours at a temperature of 600°C and with rates of 14.5 kg s–1 and 15 kg s–1, respectively, in varying thicknesses of alluvium with an intrinsic permeability of 10–11 m2 and porosity of 0.35. These simulations suggest that following the test of an engine, gaseous radionuclides injected below 200 m will not migrate to the land surface. The simulations show that the gaseous/vapor injectate will cool and condense within several meters of the injection point, although there will be limited, if any, downward drainage of liquid. However, the nearly horizontal hydraulic groundwater gradient present in Yucca Flat should limit lateral migration of any condensate that may drain downward and reach the water table.

Radionuclide Migration at the Rio Blanco Site, A Nuclear-stimulated Low-permeability Natural Gas Reservoir

The U.S. Department of Energy and its predecessor agencies conducted a program in the 1960s and 1970s that evaluated technology for the nuclear stimulation of low-permeability gas reservoirs. The third and final project in the program, Project Rio Blanco, was conducted in Rio Blanco County, in northwestern Colorado. In this experiment, three 33-kiloton nuclear explosives were simultaneously detonated in a single emplacement well in the Mesaverde Group and Fort Union Formation, at depths of 1,780, 1,899, and 2,039 m below land surface on May 17, 1973. The objective of this work is to estimate lateral distances that tritium released from the detonations may have traveled in the subsurface and evaluate the possible effect of postulated natural-gas development on radionuclide migration. Other radionuclides were considered in the analysis, but the majority occur in relatively immobile forms (such as nuclear melt glass). Of the radionuclides present in the gas phase, tritium dominates in terms of quantity of radioactivity in the long term and contribution to possible whole body exposure. One simulation is performed for {sup 85}Kr, the second most abundant gaseous radionuclide produced after tritium.

Rapid Migration of Radionuclides Leaked from High-Level Water Tanks; A Study of Salinity Gradients, Wetted Path Geometry and Water Vapor Transport

The basis of this study was the hypothesis that the physical and chemical properties of hypersaline tank waste could lead to wetting from instability and fingered flow following a tank leak. Thus, the goal of this project was to develop an understanding of the impacts of the properties of hypersaline fluids on transport through the unsaturated zone beneath Hanfords Tank Farms. There were three specific objectives (i) to develop an improved conceptualization of hypersaline fluid transport in laboratory (ii) to identify the degree to which field conditions mimic the flow processes observed in the laboratory and (iii) to provide a validation data set to establish the degree to which the conceptual models, embodied in a numerical simulator, could explain the observed field behavior. As hypothesized, high ionic strength solutions entering homogeneous pre-wetted porous media formed unstable wetting fronts atypical of low ionic strength infiltration. In the field, this mechanism could for ce flow in vertical flow paths, 5-15 cm in width, bypassing much of the media and leading to waste penetration to greater depths than would be predicted by current conceptual models. Preferential flow may lead to highly accelerated transport through large homogeneous units, and must be included in any conservative analysis of tank waste losses through coarse-textured units. However, numerical description of fingered flow using current techniques has been unreliable, thereby precluding tank-scale 3-D simulation of these processes. A new approach based on nonzero, hysteretic contract angles and fluid-dependent liquid entry has been developed for the continuum scale modeling of fingered flow. This approach has been coupled with and adaptive-grid finite-difference solver to permit the prediction of finger formation and persistence form sub centimeter scales to the filed scale using both scalar and vector processors. Although laboratory experiments demonstrated that elevated surface tens ion of imbibing solutions can enhance vertical fingered flow, this phenomenon was not observed in the field. Field tests showed that the fingered flow behavior was overwhelmed by the variability in texture resulting from differences in the depositional environment. Field plumes were characterized by lateral spreading with an average width to depth aspect ratio of 4. For both vertical fingers and lateral flow, the high ionic strength contributed to the vapor phase dilution of the waste, which increased waste volume and pushed the wetting from well beyond what would have occurred if the volume of material had remained unchanged from that initially released into the system. It was also observed that following significant vapor-phase dilution of this waste simulants that streams of colloids were ejected from the sediment surfaces. It was shown that due to the high-sodium content of the tank wastes the colloids were deflocculated below a critical salt concentration in Hanford sediments. Th e released colloids, which at the site would be expected to carry the bulk of the sorbed heavy metals and radioisotopes, were mobile though coarse Hanford sediments, but clogged finer layers. The developments resulting from this study are already being applied at Hanford in the nonisothermal prediction of the hypersaline, high pH waste migration in tank farms and in the development of inverse methods for history matching under DOEs Groundwater/Vadose Zone Integration Project at Hanford.