An improved permeability model of coal for coalbed methane recovery and CO2 geosequestration

An alternative approach is proposed to develop an improved permeability model for coalbed methane (CBM) and CO₂-enhanced CBM (ECBM) recovery, and CO₂ geosequestration in coal. This approach integrates the textural and mechanical properties to describe the anisotropy of gas permeability in coal reservoirs. The model accounts for the stress dependent deformation using a stress–strain correlation, which allows determination of directional permeability for coals. The stress–strain correlation was developed by combining mechanical strain with sorption-induced strain for any given direction. The mechanical strain of coal is described by the general thermo-poro-elastic constitutive equations for solid materials under isothermal conditions and the sorption-induced strain is approximated by treating the swelling/shrinkage of coal matrix equivalent to the thermal contraction/expansion of materials. With directional strains, the permeability of coal in any given direction can be modeled based on the theory of rock hydraulics. In this study, the proposed model was tested with both literature data and experiments. The experiments were carried out using a specially designed true tri-axial stress coal permeameter (TTSCP). The results show that the proposed model provides better predictions for the literature data compared with other conventional coal permeability models. The model also gives reasonable agreement between the predicted and measured stress–strains and directional permeabilities under laboratory conditions.     

Quantifying the effects of three-dimensional subsurface heterogeneity on Hortonian runoff processes using a coupled numerical,stochastic approach

The impact of three-dimensional subsurface heterogeneity in the saturated hydraulic conductivity on hillslope runoff generated by excess infiltration (so-called Hortonian runoff) is examined. A fully coupled, parallel subsurface–overland flow model is used to simulate runoff from an idealized hillslope. Ensembles of correlated, Gaussian random fields of saturated hydraulic conductivity are used to create uncertainty in spatial structure. A large number of cases are simulated in a parametric manner with the variance of the hydraulic conductivity varied over orders of magnitude. These cases include rainfall rates above, equal and below the geometric mean of the hydraulic conductivity distribution. These cases are also compared to theoretical representations of runoff production based on simple assumptions regarding (1) the rainfall rate and the value of hydraulic conductivity in the surface cell using a spatially-indiscriminant approach; and (2) a percolation-theory type approach to incorporate so-called runon. Simulations to test the ergodicity of hydraulic conductivity on hillslope runoff are also performed. Results show that three-dimensional stochastic representations of the subsurface hydraulic conductivity can create shallow perching, which has an important effect on runoff behavior that is different than previous two-dimensional analyses. The simple theories are shown to be very poor predictors of the fraction of saturated area that might runoff due to excess infiltration. It is also shown that ergodicity is reached only for a large number of integral scales (∼30) and not achieved for cases where the rainfall rate is less than the geometric mean of the saturated hydraulic conductivity.     

Measurements of Carbon Monoxide and Nonmethane Hydrocarbons During POPCORN

During the field campaign POPCORN (Photo oxidant formation by plant emitted compounds and OH radicals in North-eastern Germany) in Pennewitt (Mecklenburg-Vorpommern, Germany) in August 1994, carbon monoxide and nonmethane hydrocarbons were measured over a large maize field by in-situ gas chromatography. Throughout the campaign CO and NMHC showed, even for a remote rural area, unexpectedly low mixing ratios. Except a few episodes, CO mixing ratios were around 120 ppb. Ethane was the only hydrocarbon showing mixing ratios exceeding 1 ppb. The mixing ratios of all other NMHC ranged between several hundred ppt and the lower limit of detection which was between 20 and 5 ppt depending on the compound. During three frontal passages CO and NMHC mixing ratios increased significantly, while between August 13 and 16, 1994, polar air masses were encountered with CO and NMHC mixing ratios dropping to values which are typical for North Atlantic background air. During this period average CO mixing ratios were 85 ppb and ethane as the most abundant hydrocarbon decreased to 650 ppt. The large-scale meteorological situation is reflected in an unusual frequency distribution of CO. The distribution shows three maxima which can be assigned to the periods of the frontal passages, to the observation of polar air masses and the rest of the campaign. Two-day backward trajectories were calculated in order to obtain information about the origin of the air masses transported to the site. The observed NMHC and CO data can be attributed to the origin of the air masses and the air mass trajectories. NMHC and CO mixing ratios were well correlated indicating that these compounds originated from similar mostly anthropogenic sources. An exception was isoprene which showed no correlation with CO. With values below 100 ppt the mixing ratio of isoprene, which is emitted by terrestrial vegetation, was also unexpectedly low during the first half of the campaign although the maximum temperatures were around 35°C.     

Carbon isotope characteristics of the diaromatic carotenoid, isorenieratene (intact and sulfide-bound) and a novel isomer in sediments

Messinian marls from evaporitic cycle IV of the Gessoso-solfifera formation (Italy) are known to contain in high abundance the diaromatic carotenoid isorenieratene of green sulfur bacterial (Chlorobiaceae) origin along with a second diaromatic carotenoid. The previous tentative assignment of the latter as a cis diastereomer of isorenieratene has now been confirmed; this indicates that double bond isomerization occurs during early diagenesis and provides further evidence for the pathway proposed previously to link isorenieratene to a number of aromatic isoprenoid hydrocarbon biomarkers. Stable carbon isotopic data confirm the Chlorobiaceae origin of the intact all-trans isorenieratene isomer and indicate that the isomerization is accompanied by little or no kinetic isotope effect. Comparison of the δ¹³C values of each of the isolated carotenoids (measured as isorenieratane after hydrogenation) with those of their sulfide-bound counterpart in the polar fraction of the extracts (also measured as isorenieratane) indicates that sulfurization resulted in little depletion in ¹³C.     

Quantifying the effects of subsurface heterogeneity on hillslope runoff using a stochastic approach

The role of heterogeneity and uncertainty in hydraulic conductivity on hillslope runoff production was evaluated using the fully integrated hydrologic model ParFlow. Simulations were generated using idealized high-resolution hillslopes configured both with a deep water table and a water table equal to the outlet to isolate surface and subsurface flow, respectively. Heterogeneous, correlated random fields were used to create spatial variability in the hydraulic conductivity. Ensembles, generated by multiple realizations of hydraulic conductivity, were used to evaluate how this uncertainty propagates to runoff. Ensemble averages were used to determine the effective runoff for a given hillslope as a function of rainfall rate and degree of subsurface heterogeneity. Cases where the water table is initialized at the outlet show runoff behavior with little sensitivity to variance in hydraulic conductivity. A technique is presented that explicitly interrogates individual realizations at every simulation timestep to partition overland and subsurface flow contributions. This hydrograph separation technique shows that the degree of heterogeneity can play a role in determining proportions of surface and subsurface flow, even when effective hillslope outflow is seen. This method is also used to evaluate current hydrograph separation techniques and demonstrates that recursive filters can accurately proportion overland and base-flow for certain cases.     

Carbon and nitrogen isotopic compositions of alkyl porphyrins from the Triassic Serpiano oil shale

The carbon and nitrogen isotopic compositions of seven of the most abundant alkylporphyrins from the Serpiano oil shale (marine, Triassic) were determined. For the C31 and C32 butanoporphyrins, values of delta 13CPDB and delta 15NAIR averaged -24.0% and -3.1%. In contrast, the C31 and C32 methylpropanoporphyrins, DPEP, and a C30 13-nor etioporphyrin had delta 13C and delta 15N values averaging -27.5 and -3.3%, respectively. Carbon and nitrogen isotopic values for kerogen averaged -30.8 and -0.9, whereas those for total extract averaged -31.6, and -4.0%. The butanoporphyrins apparently derive from a biological source different from that giving rise to the other porphyrins, their 13C enrichment not being related to carbon isotopic fractionation accompanying diagenetic reactions. The delta 15N values for all the porphyrins indicate that the depletion of 15N observed in the kerogen is of primary origin. Consistent with the very high abundance of hopanoids and methyl hopanoids in the aliphatic hydrocarbon fraction, it is suggested that cyanobacterial fixation of N2 may have been the main cause of 15N depletion.     

Intercomparison of instruments for tropospheric measurements using differential optical absorption spectroscopy

The results of an intercomparison campaign of eight different long path UV-visible DOAS instruments measuring NO₂, O₃ and SO₂ concentrations in a moderately polluted urban site are presented. For effective optical path lengths of 230 and 780 m the overall spread of these measurements (±1) are 5×10¹⁰, 6×10¹⁰ and 1×10¹⁰ molec·cm^-3 (2.0, 2.4, and 0.4 ppb) for these molecules respectively when all instruments used a common set of absorption cross sections. The remaining differences are not completely random and the systematic differences are attributed to the different retrieval methods used for each instrument.     

Peroxyacetyl Nitrate (PAN) Measurements During the POPCORN Campaign

During the POPCORN campaign between 3 and 24 August 1994 we measured peroxyacetyl nitrate (PAN) in a rural area of Mecklenburg-Vorpommern (North-Eastern Germany) above a corn field. A total of about 5000 PAN measurements were carried out within the three weeks of the campaign. Measured PAN mixing ratios ranged from below the detection limit of 10 ppt up to an afternoon maximum of 1 ppb. The mean value of all data was 140 ppt. The daily mean PAN mixing ratios were typically in the range of 50 to 250 ppt, but during a clean air episode PAN mixing ratios of well below 40 ppt were observed. The characteristic relative diurnal variation of the PAN mixing ratios with a late night/early morning minimum and an afternoon maximum persisted during these episodes. The daily averages of the PAN mixing ratios showed clear episodic variations which coincided with the duration of typical synoptic episodes of two to six days duration. Based on the measurements of the various parameters determining the PAN formation and destruction rates, the local budget for PAN was calculated. During daytime the calculated net photochemical formation rate of PAN was nearly always significantly higher than the observed change of the PAN concentration. This demonstrates that substantial amounts of PAN (often in the range of several hundred ppt/h) were exported from the corn field. The resulting removal of NOx to some extent effects the budget of nitrogen oxides (NOx), but the export of odd oxygen radicals in the form of PAN during daytime often amounted up to 30–50% of the OH-radical formation by ozone photolysis. Thus the importance of PAN as reservoir and transport medium for odd oxygen radicals can be very substantial and may have a significant impact on the budget and distribution of odd oxygen radicals.     

A piston-driven shock in the solar corona

A solar flare that occurred on the west limb at 1981, March 25, 2038 UT generated a massive, rapidly-expanding optical coronal transient, which moved outward with an approximately constant velocity of 800 km s^–1. An associated magnetohydrodynamic shock travelled out ahead of the transient with a velocity estimated to be approximately 1000 km s^–1. The optical and radio data on the transient and shock fit well with general theories concerning piston-driven shocks and with current MHD models for propagation of such shocks through the solar corona.