Numerical simulations of the Macondo well blowout reveal strong control of oil flow by reservoir permeability and exsolution of gas.

In response to the urgent need for estimates of the oil and gas flow rate from the Macondo well MC252-1 blowout, we assembled a small team and carried out oil and gas flow simulations using the TOUGH2 codes over two weeks in mid-2010. The conceptual model included the oil reservoir and the well with a top boundary condition located at the bottom of the blowout preventer. We developed a fluid properties module (Eoil) applicable to a simple two-phase and two-component oil-gas system. The flow of oil and gas was simulated using T2Well, a coupled reservoir-wellbore flow model, along with iTOUGH2 for sensitivity analysis and uncertainty quantification. The most likely oil flow rate estimated from simulations based on the data available in early June 2010 was about 100,000 bbl/d (barrels per day) with a corresponding gas flow rate of 300 MMscf/d (million standard cubic feet per day) assuming the well was open to the reservoir over 30 m of thickness. A Monte Carlo analysis of reservoir and fluid properties provided an uncertainty distribution with a long tail extending down to 60,000 bbl/d of oil (170 MMscf/d of gas). The flow rate was most strongly sensitive to reservoir permeability. Conceptual model uncertainty was also significant, particularly with regard to the length of the well that was open to the reservoir. For fluid-entry interval length of 1.5 m, the oil flow rate was about 56,000 bbl/d. Sensitivity analyses showed that flow rate was not very sensitive to pressure-drop across the blowout preventer due to the interplay between gas exsolution and oil flow rate.

Numerical investigation of air intrusion and aerobic reactions in municipal solid waste landfills.

Air intrusion into municipal solid waste landfills can cause a localized switch from anaerobic to aerobic biodegradation adjacent to the intrusion. The purpose of this study was to explore the effects on temperature and gas composition of air intrusion into an idealized anaerobic landfill. Two scenarios of air intrusion and injection were simulated using a mechanistic landfill model built into TOUGH2. The modeled landfill geometry and properties are based on an actual U.S. landfill. The simulation results show that air intrusion can cause a quick switch from anaerobic to aerobic conditions and as a result, cause a fast increase in temperature of up to 30 °C associated with stimulation of aerobic biodegradation reactions. Associated with the change to aerobic conditions is a decrease in CH4/CO2 (v/v) ratio in the landfill gas. Depending on the air flow rate intruding or injecting into the landfill, localized aerobic biodegradation is stimulated and as a result heat generation rate of 10 to 150 W/m3 leads to temperature increase. Temperature increase near a temporary air intrusion lasts no longer than a few weeks while the high temperatures in deep layers could last up to one year.

Interpreting velocities from heat-based flow sensors by numerical simulation.

We have carried out numerical simulations of three-dimensional nonisothermal flow around an in situ heat-based flow sensor to investigate how formation heterogeneities can affect the interpretation of ground water flow velocities from this instrument. The flow sensor operates by constant heating of a 0.75-m-long, 5-cm-diameter cylindrical probe, which contains 30 thermistors in contact with the formation. The temperature evolution at each thermistor can be inverted to obtain an estimate of the ground water flow velocity vector using the standard interpretive method, which assumes that the formation is homogeneous. Analysis of data from heat-based flow sensors installed in a sand aquifer at the Former Fort Ord Army Base near Monterey, California, suggested an unexpected component of downward flow. The magnitudes of the vertical velocities were expected to be much less than those of the horizontal velocities at this site because the sensors were installed just above a clay aquitard. Numerical simulations were conducted to examine how differences in thermal conductivities may lead to spurious indications of vertical flow velocities. We found that a decrease in the thermal conductivity near the bottom of the sensor can perturb the temperature profiles along the instrument in such a manner that analyses assuming homogeneous thermal conductivity could indicate a vertical flow component even though flow is actually horizontal. This work demonstrates how modeling can be used to simulate instrument response to formation heterogeneity and shows that caution must be used in interpreting data from such devices.

Comparison of aerobic and anaerobic biotreatment of municipal solid waste.

To increase the operating lifetime of landfills and to lower leachate treatment costs, an increasing number of municipal solid waste (MSW) landfills are being managed as either aerobic or anaerobic bioreactors. Landfill gas composition, respiration rates, and subsidence were measured for 400 days in 200-L tanks filled with fresh waste materials to compare the relative effectiveness of the two treatments. Tanks were prepared to provide the following conditions: (1) air injection and leachate recirculation (aerobic), (2) leachate recirculation (anaerobic), and (3) no treatment (anaerobic). Respiration tests on the aerobic wet tank showed a steady decrease in oxygen consumption rates from 1.3 mol/day at 20 days to 0.1 mol/day at 400 days. Aerobic wet tanks produced, on average, 6 mol of carbon dioxide (CO2)/kg of MSW as compared with anaerobic wet tanks, which produced 2.2 mol methane/kg of MSW and 2.0 mol CO2/kg methane. Over the test period, the aerobic tanks settled on average 35%, anaerobic tanks settled 21.7%, and the no-treatment tank settled 7.5%, equivalent to overall mass loss in the corresponding reactors. Aerobic tanks reduced stabilization time and produced negligible odor compared with anaerobic tanks, possibly because of the 2 orders of magnitude lower leachate ammonia levels in the aerobic tank. Both treatment regimes provide the opportunity for disposal and remediation of liquid waste.

Fault-matrix interactions in nonwelded tuff of the Paintbrush Group at Yucca Mountain.

To investigate the potential for fast flow through altered tuff of the nonwelded unit of the Paintbrush Group (PTn) at Yucca Mountain, Nevada, we carried out in situ field experiments using water released directly into the matrix and along a minor subvertical normal fault at Alcove 4 in the Exploratory Studies Facility (ESF). During the experiments, changes in moisture content were monitored within the test bed, and a slot excavated below the test bed was visually inspected for seepage. Our field tests suggest that the dry porous PTn matrix is capable of attenuating episodic percolation fluxes in localized areas (such as around faults) where fast flow would be expected to dominate. Once wetted, the matrix is able to retain the moisture over a period of months. As saturation increases in the matrix, less water imbibes along the fault and more water travels farther along the fault. From this observation, we infer that a sequence of infiltration events separated by periods of up to a few months could convey water over increasing distances along the fault.

Flow and transport in unsaturated fractured rock: effects of multiscale heterogeneity of hydrogeologic properties.

The heterogeneity of hydrogeologic properties at different scales may have different effects on flow and transport processes in a subsurface system. A model for the unsaturated zone of Yucca Mountain, Nevada, is developed to represent complex heterogeneity at two different scales: (1) layer scale corresponding to geologic layering and (2) local scale. The layer-scale hydrogeologic properties are obtained using inverse modeling, based on the available measurements collected from the Yucca Mountain site. Calibration results show a significant lateral and vertical variability in matrix and fracture properties. Hydrogeologic property distributions in a two-dimensional, vertical cross-section of the site are generated by combining the average layer-scale matrix and fracture properties with local-scale perturbations generated using a stochastic simulation method. The unsaturated water flow and conservative (nonsorbing) tracer transport through the cross-section are simulated for different sets of matrix and fracture property fields. Comparison of simulation results indicates that the local-scale heterogeneity of matrix and fracture properties has a considerable effect on unsaturated flow processes, leading to fast flow paths in fractures and the matrix. These paths shorten the travel time of a conservative tracer from the source (repository) horizon in the unsaturated zone to the water table for small fractions of total released tracer mass. As a result, the local-scale heterogeneity also has a noticeable effect on global tracer transport processes, characterized by an average breakthrough curve at the water table, especially at the early arrival time of tracer mass. However, the effect is not significant at the later time after 20% tracer mass reaches the water table. The simulation results also verify that matrix diffusion plays an important role in overall solute transport processes in the unsaturated zone at Yucca Mountain.