Numerical Modeling of Methane Leakage from a Faulty Natural Gas Well into Fractured Tight Formations

Horizontal drilling and hydraulic fracturing have enabled hydrocarbon recovery from unconventional reservoirs, but led to natural gas contamination of shallow groundwaters. We describe and apply numerical models of gas‐phase migration associated with leaking natural gas wells. Three leakage scenarios are simulated: (1) high‐pressure natural gas pulse released into a fractured aquifer; (2) continuous slow leakage into a tilted fractured formation; and (3) continuous slow leakage into an unfractured aquifer with fluvial channels, to facilitate a generalized evaluation of natural gas transport from faulty natural gas wells. High‐pressure pulses of gas leakage into sparsely fractured media are needed to produce the extensive and rapid lateral spreading of free gas previously observed in field studies. Transport in fractures explains how methane can travel vastly different distances and directions laterally away from a leaking well, which leads to variable levels of methane contamination in nearby groundwater wells. Lower rates of methane leakage (≤1 Mcf/day) produce shorter length scales of gas transport than determined by the high‐pressure scenario or field studies, unless aquifers have low vertical permeabilities (≤1 millidarcy) and fractures and bedding planes have sufficient tilt (～10°) to allow a lateral buoyancy component. Similarly, in fractured rock aquifers or where permeability is controlled by channelized fluvial deposits, lateral flow is not sufficiently developed to explain fast‐developing gas contamination (0‐3 months) or large length scales (～1 km) documented in field studies. Thus, current efforts to evaluate the frequency, mechanism, and impacts of natural gas leakage from faulty natural gas wells likely underestimate contributions from small‐volume, low‐pressure leakage events.     

Naïve Simplicity: The Overlooked Piece of the Complexity‐Simplicity Paradigm

Concepts of simplicity and complexity in modeling have been explored in papers, editorials, and talks. The concept is not well understood because there are at least two flavors of simplicity. Modelers envision simplicity (i.e., elegant simplicity) as the sought‐after goal in modeling, but naïve simplicity, which is the focus of this paper, is commonly unrecognized and dangerous. The problem is that naïve or simple ideas are often mistaken for settled science and come with the prospect of being more wrong than right. The concept of the so‐called simplicity cycle, in relation to classical problems of carbon‐14 age and salinity in closed‐basin lakes, is used to illustrate these points. The emerging problems of water‐mosquitoes‐diseases show the value of mapping new problems to the simplicity cycle. Researchers can “know what they do not know” and avoid the dangers of naïve simplicity.     

Chromospheric and coronal Alfvénic oscillations in non-vertical magnetic fields

We generalize previous studies of Alfvénic oscillations in the solar atmosphere to geometries in which the background magnetic field is not parallel to the gravitational acceleration. A uniform but inclined field produces only subtle changes in the mathematics, and virtually no changes to the coronal energy flux, over previous vertical field studies. We show that simple, two-layer models agree remarkably well with more sophisticated, multi-layer calculations. In addition, we derive several useful and accurate analytic results with which we highlight many features and parameter dependences. We also study a model with a spreading field geometry. For low magnetic fields (- 10 G) the corona still appears WKB to the oscillations and we do not find any significant deviations from the uniform field calculations. This is not the case for higher magnetic fields in active regions (- 3000 G) where we confirm previous results which suggest an order of magnitude increase in the coronal flux. Again, we derive useful analytic approximations.Now at: Mathematics Department, Monash University, Clayton, Victoria, Australia.     

Influence of basin characteristics on baseflow and stormflow chemistry in the Great Smoky Mountains National Park,USA

Relationships between stream chemistry and elevation, area, Anakeesta geology, soil properties, and dominant vegetation were evaluated to identify the influence of basin characteristics on baseflow and stormflow chemistry in eight streams of the Great Smoky Mountains National Park. Statistical analyses were employed to determine differences between baseflow and stormflow chemistry, and relate basin‐scale factors governing local chemical processes to stream chemistry. Following precipitation events, stream pH was reduced and aluminium concentrations increased, while the response of acid neutralizing capacity (ANC), nitrate, sulfate, and base cations varied. Several basin characteristics were highly correlated with each other, demonstrating the interrelatedness of topographical, geological, soil, and vegetative parameters. These interrelated basin factors uniquely influenced acidification response in these streams. Streams in higher‐elevation basins (>975 m) had significantly lower pH, ANC, sodium, and silicon and higher nitrate concentrations (p < 0.05). Streams in smaller basins (<10 km²) had significantly lower nitrate, sodium, magnesium, silicon, and base cation concentrations. In stormflow, streams in basins with Anakeesta geology (>10%) had significantly lower pH and sodium concentrations, and higher aluminium concentrations. Chemical and physical soil characteristics and dominant overstory vegetation in basins were more strongly correlated with baseflow and stormflow chemical constituents than topographical and geological basin factors. Saturated hydraulic conductivity, of all the soil parameters, was most related to concentrations of stormflow constituents. Basins with higher average hydraulic conductivities were associated with lower stream pH, ANC, and base cation concentrations, and higher nitrate and sulfate concentrations. These results emphasize the importance of soil and geological properties influencing stream chemistry and promote the prioritization of management strategies for aquatic resources. Copyright © 2012 John Wiley & Sons, Ltd.     

The Low Compaction Grading Technique on Steep Reclaimed Slopes: Soil Characterization and Static Slope Stability

Since the Surface Mining and Control Reclamation Act of 1977, US coal mining companies have been required by law to restore the approximate ground contours that existed prior to mining. To ensure mass stability and limit erosion, the reclaimed materials have traditionally been placed with significant compaction energy. The Forest Reclamation Approach (FRA) is a relatively new approach that has been successfully used to facilitate the fast establishment of native healthy forests. The FRA method specifies the use of low compaction energy in the top 1.2–1.5 m of the contour, which may be in conflict with general considerations for mechanical slope stability. Although successful for reforestation, the stability of FRA slopes has not been fully investigated and a rational stability method has not been identified. Further, a mechanics-based analysis is limited due to the significant amount of oversize particles which makes the sampling and measurement of soil strength properties difficult. To investigate the stability of steep FRA slopes (steeper than 20°), three reclaimed coal mining sites in the Appalachian region of East Tennessee were investigated. The stability was evaluated by several methods to identify the predominant failure modes. The infinite slope method, coupled with the estimation of the shear strength from field observations, was shown to provide a rational means to evaluate the stability of FRA slopes. The analysis results suggest that the low compaction of the surface materials may not compromise the long-term stability for the sites and material properties investigated.     

Gamma-Ray Line Observations of the 2000 July 14 Flare and SEP Impact on the Earth

The HXS and GRS detectors on Yohkoh observed the 14 July 2000, X5.7 flare, beginning at ∼ 10:20 UT, ∼ 4 min before the peak in soft X-rays. The hard X-rays and γ-rays peaked ∼ 3 min later at ∼ 10:27 UT. Solar γ-ray emission lasted until ∼ 10:40 UT. Impact of high-energy ions at the Sun is revealed by the γ-ray lines from neutron capture, annihilation radiation and de-excitation that are visible above the bremsstrahlung continuum. From measurement of these lines we find that the flare-averaged spectrum of accelerated protons is consistent with a power law ge10 MeV with index 3.14±0.15 and flux 1.1×10³² protons MeV⁻¹ at 10 MeV. We estimate that there were ∼1.5×10³⁰ erg in accelerated ions if the power law extended without a break down to 1 MeV; this is about 1% of the energy in electrons > 20 keV from measurements of the hard X-rays. We find no evidence for spectral hardening in the hard X-rays that has been suggested as a predictor for the occurrence of solar energetic particle (SEP) events. This was the third largest proton event above 10 MeV since 1976. The GRS and HXS also observed γ-ray lines and continuum produced by the impact of SEP on the Earth's atmosphere beginning about 13 UT on 14 July. These measurements show that the SEP spectrum softened considerably over the next 24 hours. We compare these measurements with proton measurements in space.     

Coring artifacts and contaminant inventories in lake sediment

Sediments of Lake 382, Experimental Lakes Area, Canada, were sampled at six sites using a 5-cm Wildco KB core sampler (KB), a similar device incorporating a ball check valve (BC), and a 0.2 m by 1.2 m flat-faced aluminum freeze core sampler (FC). Cores were sectioned at 1-cm intervals to a depth of 15 cm. Contaminant (²¹⁰Pb and ¹³⁷Cs) concentrations (Bq g^-1) were measured by gamma spectroscopy, and inventories (Bq cm^-2) were calculated following standard methods. Sediments collected using FC, BC and KB had similar contaminant concentrations, however, cores collected by FC and BC had lower estimated inventories than KB cores. Differences between estimates appear to be caused by differences in the water content (WC) of core material. Laboratory studies confirm that FC sediments have higher WC than tube-cored sediment. We hypothesize that ice crystal formation increases the WC of freeze cores, resulting in lower contaminant inventories. Loss of surficial sediment caused by a bow wave may have a similar effect on BC samples. We conclude that KB core gear is appropriate for sampling sediments to measure contaminant concentrations and inventories in recently deposited sediments.     

Rhessi and Trace Observations of the 21 April 2002 x1.5 Flare

Solar Physics - Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer...     

1300 micron continuum and C18O line mapping of giant molecular cloud cores. II. W3, NGC 2264, NGC 6334I, RHO Ophiuchi and S140

In this paper we present nearly simultaneous 1300 microns continuum and J = 2-1 C18O maps of the cores of five molecular clouds, W3, NGC 2264, NGC 6334I, rho Oph, and S140. The purpose of this experiment was to compare these two column density tracers. We find that dust continuum and C18O emission are equally effective tracers of column density in molecular cloud cores and give a good indication of cloud structure. When the maps are analyzed in terms of the quantity q = Q/[a rho RX(C18O)], we find that q does not vary by much more than an order of magnitude either within objects or from object to object, implying that nominal dust parameters of absorption efficiency, radius, and gas-to-dust ratio and CO abundance are on average correct in a variety of sources. We did detect source-to-source variations in q. This variation could be either in the dust-to-CO number density ratio or in grain parameters. These variations are not well correlated with total source luminosity, average or typical temperature, or total column density. The best example of this variation appears to be rho Oph where q is about a factor of 7 lower than is typically found. Our approach is analogous to the study of the A nu to CO ratio and is probably equivalent to extending this study to large A nu if the same grains are responsible for both optical opacity and far-infrared to millimeter-wave emission. There is no fundamental reason to expect A nu/NCO or q to be constant and, in fact, we have found that it is not constant in even a small source sample.     

Solar and cosmic X-rays above 7.7 keV

Since its launch on March 8, 1967, the OSO-III has continuously observed solar and cosmic X-rays over the 7.7–210 keV range. The sun emits many impulsive X-ray bursts having fluxes several orders of magnitude above the background level of 8 × 10^–9 ergs(cm²-sec)^–1 at 7.7 keV and characteristic times on the order of 5 min. Ninety-five such events having fluxes >3 × 10^–5 ergs(cm²-sec)^–1 were detected in the period from March 8 to June 15, 1967. The cosmic X-ray source Lupus XR-1 has been observed to have a power law spectral form and no significant time variations over a 40-day period. Upper limits have been obtained on the hard X-ray flux of the peculiar galaxy M 87.