The Hydraulic Effects of Lost Circulation and the Implications for Contaminant Migration During Drilling

The impact of lost circulation during rotary drilling near an existing monitoring well cluster was evaluated by periodic measurements of water levels and contaminant concentrations at the well cluster. Due to regulatory concerns, changes in water levels or VOC concentration in the well cluster during drilling would trigger monitoring well redevelopment. The borehole was drilled approximately 30 feet northeast of four nested monitoring wells that screen Devonian and Silurian carbonate bedrock at depths of 15, 60, 130, and 190 feet. Following complete circulation loss at depths of 177 and 1 S3 feet in the borehole, a rapid decrease in water levels was observed in the upper three monitoring wells. The water level in the well that was screened through the lost circulation zones increased slightly.
Decreasing water levels in formations located above the point of circulation loss appear to occur in response to a sudden decrease in borehole fluid pressure caused by the flow of drilling fluid into the formation. The relative contribution of contaminated formation water lo the borehole can be estimated by using the time-drawdown relationship and estimates of transmissivity. At the point of circulation loss, significant dilution of contaminant concentrations occurs from the loss of drilling fluid into the contaminated zone. Contaminated formation water entering the borehole during periods of complete lost circulation may mobilize contaminants from upper lo lower formations. Lost circulation into a formation would be signaled by a water level increase in monitoring wells. The wells would subsequently require development to remove the volume of fluid lost to the formation, including both drilling fluid and contaminated formation water. Monitoring wells exhibiting declining water levels following lost circulation would not require development since drilling water has not entered the zones screened by these wells.     

Heterogeneous enriched mantle materials and dupal-type magmatism along the SW margin of the São Francisco craton, Brazil

The nature and restricted range of Dupal-type Sr, Nd and Pb isotopic compositions of Cretaceous kimberlites, tuffaceous diatremes of kamafugitic affinity and carbonatite complexes which intrude the southwestern São Francisco craton margin in Brazil, indicate that these magmas either interacted extensively with, or were derived from, a light rare earth element (LREE) enriched homogeneous lithospheric mantle source isotopically similar to the “enriched mantle I” (EMI) component. The shallow-derived alkalic rocks contain a greater proportion of this EMI-like component, whereas the lower time-averaged Rb/Sr, Nd/Sm and Pb/U ratios of the kimberlites compared to the other rock types suggest mixing of the EMI-like mantle material with variable amounts of mantle with a high ²³⁸U/²⁰⁴Pb (HIMU-like) component. Systematic variations in rock types and geochemistry on a regional scale are believed to be indicative of vertical geochemical heterogeneities which are translated into lateral heterogeneities by different depths of melting. It is proposed that HIMU- and EMI-like signatures in particular, are concentrated in laterally extensive but vertically distinctive portions of the mantle beneath the São Francisco craton. The EMI-type signatures appear to be restricted to shallow-derived volcanism, whereas the HIMU-type signatures may originate from a source that started melting deeper in the mantle. The Nd signatures of the EMI-type volcanics follow the evolution path defined by the NeoProterozoic crustal sequences which overlie and flank the craton margin. This suggests that the source of the EMI-type mantle signatures might be related to the tectono-thermal processes which led to the formation and evolution of such crustal sequences. The isotopic similarity of the sources of the studied rocks and of the high-Ti basalts of the northern Paraná basin to those of some Ocean Island Basalts with Dupal signatures in the South Atlantic (viz. in Walvis Ridge) is ascribed to processes by which continental lithosphere became firstly delaminated, and then contaminated a zone of South Atlantic asthenosphere from which hotspot islands have been erupting.     

Defence and the environment: An economic approach

Abtract The environmental impacts of defence is a neglected and under-researched area for economists. This article presents a conceptual approach outlining some of the economic issues involved in the environmental impacts of defence spending and the activities of the armed forces. It reviews the economics and politics of externalities and then considers beneficial and harmful externalities and concludes by identifying some of the environmental costs of disarmanent.     

A review of the geochemistry of methane in natural gas hydrate

The largest accumulations on Earth of natural gas are in the form of gas hydrate, found mainly offshore in outer continental margin sediment and, to a lesser extent, in polar regions commonly associated with permafrost. Measurements of hydrocarbon gas compositions and of carbon-isotopic compositions of methane from natural gas hydrate samples, collected in subaquatic settings from around the world, suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of CO₂ from sedimentary organic matter. Typically, these hydrocarbon gases are composed of > 99% methane, with carbon-isotopic compositions (δ¹³C_PDB) ranging from − 57 to − 73‰. In only two regions, the Gulf of Mexico and the Caspian Sea, has mainly thermogenic methane been found in gas hydrate. There, hydrocarbon gases have methane contents ranging from 21 to 97%, with δ¹³C values ranging from − 29 to − 57‰. At a few locations, where the gas hydrate contains a mixture of microbial and thermal methane, microbial methane is always dominant. Continental gas hydrate, identified in Alaska and Russia, also has hydrocarbon gases composed of > 99% methane, with carbon-isotopic compositions ranging from − 41 to − 49‰. These gas hydrate deposits also contain a mixture of microbial and thermal methane, with thermal methane likely to be dominant. Published by Elsevier Science Ltd     

Elemental composition of individual chondrules from ordinary chondrites

Sequential non-destructive neutron activation analysis was used to determine the bulk abundance of Fe, Al, Na, Mn, Or, Sc, Co and Ir in approximately 300 individual chondrules from 16 chondrites representing the H (3–5), L4 and LL(3–6) compositional and petrologic classes. For some of the chondrules, Si, Ni, Ca and V were also determined. The histograms indicate that the most probable abundances for lithophilic elements, except Cr, are enriched in the chondrules, while the siderophilic elements are depleted in the chondrules compared to the whole chondrite. Some of the abundance populations, such as Al and Fe, appear to be multimodal. Systematic variations in the composition of the chondrules with increasing petrologic type were observed; most consistent are an increasing Na-Al and Cr-Al correlation, a decreasing Na-Mn correlation, increasing Na abundance and decreasing Na and Mn dispersions among chondrules. The systematic compositional variations with increasing petrologic type are consistent with an increasing approach to equilibrium between chondrules and matrix.Observed elemental correlations are generally consistent with mineralogical controls expected on the basis of geochemical affinities suggested by the mineral assemblages present in the chondrules. However, a prevalent Al-Ir correlation was observed, and is most pronounced for a group of chondrules belonging to a population high in Al. A Sc-Ir correlation was observed. Also, an anti-correlation between chondrule masses and Al (and Ir for some chondrules) content of the chondrules was observed. These correlations are attributed to a fractionation during condensation or chondrule formation and cannot be attributed to classical geochemical similarities i.e. these correlations result from a cosmochemical fractionation. From the compositional evidence, it is suggested that there may be two mechanisms for chondrule production. Some high Al chondrules which exhibit the Al-Ir correlation are believed to be remelted primitive high-temperature aggregates. The elemental composition of the chondrules from the lower Al abundance population is consistent with a preferential remelting of pre-existing silicates.     

Aragonite crystallization from strained calcite at reduced pressures and its bearing on aragonite in low-grade metamorphism

Aragonite, the dense form of CaCO₃, grew hydrothermally at 100–300° C and dry at 300–400° C at very low pressures from calcite strained by grinding. Nearly complete inversion to aragonite occurred in some runs with Ca-Mg chloride solutions at 0–2.4 kb and 100–200° C on strained calcite having a (10¯14) reflection with a half-width of 0.48° 2 Cu K. A little aragonite grew dry at one atm. from the ground calcite at 300–400° C in a few hrs. Simultaneous shear during recrystallization of calcite in a rotating squeezer resulted in significant aragonite at 300–400° C several kb. below the stability field. No inversion occurred in any ground calcite when previously annealed in CO₂ at 500° C for a few hrs. Thermochemical data show that at least 200 cal/mole of strain energy can be produced in calcite by mild deformation. This much stored energy would lower the pressure requirements of aragonite, relative to the strained calcite by more than 3 kb, and our observation that aragonite growth was faster than strain recovery of calcite indicates that aragonite can grow in nature at reduced pressures from strained calcite.Some experiments were also carried out on highly magnesian calcites with the thought that aragonite might also form at the expense of this metastable material. No aragonite was produced, but the possibility that this mechanism could be operative in nature cannot be discounted.The microtexture of aragonitic deformed marbles from NW Washington (prehnite-pumpellyite facies rocks, courtesy of J. A. Vance) as well as electron probe microanalysis of these rocks indicates that aragonite selectively replaced highly strained calcite. The calcite-aragonite transition is thus a questionable indicator of high-pressure in certain metamorphic rocks.     

The relationship between anomalies of interannual net radiation and 37 GHz polarization difference over North Africa

We show evidence that interannual variations in the net radiation balance at the top-of-atmosphere over North Africa may be linked to variations at the land surface within the Sahelian zone. By utilizing concurrent monthly time series of earth radiation budget and passive microwave measurements obtained from the Nimbus 7 satellite over a five year period from 1979 to 1983, we have examined the spatial distribution of the amplitude of interannual variability of the net radiation at the top of the atmosphere. A strong association is found between the radiation budget anomalies derived from ERB-WFOV measurements and the anomalies of 37 GHz polarized brightness temperature difference derived from SMMR measurements. Previous research has shown that the interannual anomalies in the top-of-atmosphere net radiation balance in the Sahelian zone mostly arise from changes taking place at the surface, not from variations in cloudiness which brings variable rainfall throughout the African Sahel. Since microwave polarization difference is related in part to vegetation cover, we suggest that the main control of the modulation of North Africa's radiation balance may be year to year fluctuations of vegetation cover within the Sahelian zone, likely stemming from fluctuations in the rainfall. This relationship is relevant to the problem of radiation control and feedback on global climate through processes taking place within the biosphere of a semi-arid zone.     

Assessing the impact of weather and climate in Australia

Weather and climate have a significant impact on the Australian society and environment. While the impacts of major changes in the Australian climate likely to occur from the enhanced greenhouse effect have been reported in the literature fewer studies have addressed the impact of the climate's natural variability. Ways in which weather and climate and its variability impact on the decision making process are described and methods available to assess the sensitivity of the economy and the natural environment are also presented. Methods which are being developed to determine the value of weather and climate information to decision makers are briefly described. The El Niño Southern Oscillation is a strong contributor to the variability of the Australian climate. It is used to illustrate the impact of climate variability on both the economy and natural environment of Australia. Finally, the extent to which there is political awareness of the impact of climate variability on the Australian society and environment is briefly examined.     

Upstream stagnation points in stratified flow past obstacles

An experimental study has been made of stagnation points and flow splitting on the upstream side of obstacles in uniformly stratified flow. A range from small to large values of Nh/U (where N is the buoyancy frequency, h_m is the maximum obstacle height and U is the undisturbed fluid velocity) has been covered, for three obstacle shapes which are, respectively, axisymmetric, and elongated in the across-stream and in the downstream directions. Upstream stagnation for the first two of these models does not occur until Nh_m/U > 1.05, where it occurs at z ≈ h_m/2. On the central line below this point the flow descends and diverges, and we term this ‘flow splitting’. For the third model (elongated in the downstream direction), stagnation upstream first occurs at Nh_m/U ≈ 1.43, at z ≈ 0. Results for this obstacle are not consistent with the ‘Sheppard criterion’, and this upstream flow stagnation is not apparently related to lee wave overturning, in contrast to flow over two-dimensional obstacles.