40 research outputs found
Fracture Propagation Driven by Fluid Outflow from a Low-permeability Aquifer
Deep saline aquifers are promising geological reservoirs for CO2
sequestration if they do not leak. The absence of leakage is provided by the
caprock integrity. However, CO2 injection operations may change the
geomechanical stresses and cause fracturing of the caprock. We present a model
for the propagation of a fracture in the caprock driven by the outflow of fluid
from a low-permeability aquifer. We show that to describe the fracture
propagation, it is necessary to solve the pressure diffusion problem in the
aquifer. We solve the problem numerically for the two-dimensional domain and
show that, after a relatively short time, the solution is close to that of
one-dimensional problem, which can be solved analytically. We use the relations
derived in the hydraulic fracture literature to relate the the width of the
fracture to its length and the flux into it, which allows us to obtain an
analytical expression for the fracture length as a function of time. Using
these results we predict the propagation of a hypothetical fracture at the In
Salah CO2 injection site to be as fast as a typical hydraulic fracture. We also
show that the hydrostatic and geostatic effects cause the increase of the
driving force for the fracture propagation and, therefore, our solution serves
as an estimate from below. Numerical estimates show that if a fracture appears,
it is likely that it will become a pathway for CO2 leakage.Comment: 21 page
Underground railroads: citizen entitlements and unauthorized mobility in the antebellum period and today
In recent years, some scholars and prominent political figures have advocated the deepening of North American integration on roughly the European Union model, including the creation of new political institutions and the free movement of workers across borders. The construction of such a North American Union, if it included even a very thin trans-state citizenship regime, could represent the most significant expansion of individual entitlements in the region since citizenship was extended to former slaves in the United States. With such a possibility as its starting point, this article explores some striking parallels between the mass, legally prohibited movement across boundaries by fugitive slaves in the pre-Civil War period, and that by current unauthorized migrants to the United States. Both were, or are, met on their journeys by historically parallel groups of would-be helpers and hinderers. Their unauthorized movements in both periods serve as important signals of incomplete entitlements or institutional protections. Most crucially, moral arguments for extending fuller entitlements to both groups are shown here to be less distinct than may be prima facie evident, reinforcing the case for expanding and deepening the regional membership regime
Calcium ion currents mediating oocyte maturation events
During maturation, the last phase of oogenesis, the oocyte undergoes several changes which prepare it to be ovulated and fertilized. Immature oocytes are arrested in the first meiotic process prophase, that is morphologically identified by a germinal vesicle. The removal of the first meiotic block marks the initiation of maturation. Although a large number of molecules are involved in complex sequences of events, there is evidence that a calcium increase plays a pivotal role in meiosis re-initiation. It is well established that, during this process, calcium is released from the intracellular stores, whereas less is known on the role of external calcium entering the cell through the plasma membrane ion channels. This review is focused on the functional role of calcium currents during oocyte maturation in all the species, from invertebrates to mammals. The emerging role of specific L-type calcium channels will be discussed
An 8-year record of gas geochemistry and isotopic composition of methane during baseline sampling at a groundwater observation well in Alberta (Canada)
Variability in baseline groundwater methane concentrations and isotopic compositions was assessed while comparing free and dissolved gas sampling approaches for a groundwater monitoring well in Alberta (Canada) over an 8-year period. Methane concentrations in dissolved gas samples (n = 12) were on average 4,380 ± 2,452 μg/L, yielding a coefficient of variation (CV) >50 %. Methane concentrations in free gas samples (n = 12) were on average 228,756 ± 62,498 ppm by volume, yielding a CV of 27 %. Quantification of combined sampling, sample handling and analytical uncertainties was assessed via triplicate sampling (CV of 19 % and 12 % for free gas and dissolved gas methane concentrations, respectively). Free and dissolved gas samples yielded comparable methane concentration patterns and there was evidence that sampling operations and pumping rates had a marked influence on the obtained methane concentrations in free gas. δ13CCH4 and δ2HCH4 values of methane were essentially constant (−78.6 ± 1.3 and −300 ± 3 ‰, respectively) throughout the observation period, suggesting that methane was derived from the same biogenic source irrespective of methane concentration variations. The isotopic composition of methane constitutes a robust and highly valuable baseline parameter and increasing δ13CCH4 and δ2HCH4 values during repeat sampling may indicate influx of thermogenic methane. Careful sampling and analytical procedures with identical and repeatable approaches are required in baseline-monitoring programs to generate methane concentration and isotope data for groundwater that can be reliably compared to repeat measurements once potential impact from oil and gas development, for example, may occur
Occurrence and origin of methane in groundwater in Alberta (Canada): Gas geochemical and isotopic approaches
To assess potential future impacts on shallow aquifers by leakage of natural gas from unconventional energy resource development it is essential to establish a reliable baseline. Occurrence of methane in shallow groundwater in Alberta between 2006 and 2014 was assessed and was ubiquitous in 186 sampled monitoring wells. Free and dissolved gas sampling and measurement approaches yielded comparable results with low methane concentrations in shallow groundwater, but in 28 samples from 21 wells methane exceeded 10mg/L in dissolved gas and 300,000 ppmv in free gas. Methane concentrations in free and dissolved gas samples were found to increase with well depth and were especially elevated in groundwater obtained from aquifers containing coal seams and shale units. Carbon isotope ratios of methane averaged -69.7 ± 11.1‰ (n=63) in free gas and -65.6 ± 8.9‰ (n=26) in dissolved gas. δ(13)C values were not found to vary with well depth or lithology indicating that methane in Alberta groundwater was derived from a similar source. The low δ(13)C values in concert with average δ(2)HCH4 values of -289 ± 44‰ (n=45) suggest that most methane was of biogenic origin predominantly generated via CO2 reduction. This interpretation is confirmed by dryness parameters typically >500 due to only small amounts of ethane and a lack of propane in most samples. Comparison with mud gas profile carbon isotope data revealed that methane in the investigated shallow groundwater in Alberta is isotopically similar to hydrocarbon gases found in 100-250 meter depths in the WCSB and is currently not sourced from thermogenic hydrocarbon occurrences in deeper portions of the basin. The chemical and isotopic data for methane gas samples obtained from Alberta groundwater provide an excellent baseline against which potential future impact of deeper stray gases on shallow aquifers can be assessed
Redox controls on methane formation, migration and fate in shallow aquifers
Development of unconventional energy resources such as shale gas and coalbed
methane has generated some public concern with regard to the protection of
groundwater and surface water resources from leakage of stray gas from the
deep subsurface. In terms of environmental impact to and risk assessment of
shallow groundwater resources, the ultimate challenge is to distinguish
(a) natural in situ production of biogenic methane, (b) biogenic or
thermogenic methane migration into shallow aquifers due to natural causes,
and (c)Â thermogenic methane migration from deep sources due to human
activities associated with the exploitation of conventional or unconventional
oil and gas resources. This study combines aqueous and gas (dissolved and
free) geochemical and isotope data from 372 groundwater samples obtained from
186 monitoring wells of the provincial Groundwater Observation Well Network
(GOWN) in Alberta (Canada), a province with a long record of conventional and
unconventional hydrocarbon exploration. We investigated whether methane
occurring in shallow groundwater formed in situ, or whether it migrated into
the shallow aquifers from elsewhere in the stratigraphic column. It was found
that methane is ubiquitous in groundwater in Alberta and is predominantly of
biogenic origin. The highest concentrations of biogenic methane
(>  0.01 mM or  >  0.2 mg<mspace width="0.125em" linebreak="nobreak"/>L<sup>−1</sup>), characterized by
<i>δ</i><sup>13</sup>C<sub>CH<sub>4</sub></sub> values  <  −55 ‰, occurred in anoxic
Na-Cl, Na-HCO<sub>3</sub>, and
Na-HCO<sub>3</sub>-Cl type groundwaters with negligible
concentrations of nitrate and sulfate suggesting that methane was formed
in situ under methanogenic conditions for 39.1 % of the samples. In only
a few cases (3.7 %) was methane of biogenic origin found in more
oxidizing shallow aquifer portions suggesting limited upward migration from
deeper methanogenic aquifers. Of the samples, 14.1 % contained methane
with <i>δ</i><sup>13</sup>C<sub>CH<sub>4</sub></sub> values  >  −54 ‰, potentially
suggesting a thermogenic origin, but aqueous and isotope geochemistry data
revealed that the elevated <i>δ</i><sup>13</sup>C<sub>CH<sub>4</sub></sub> values were caused by
microbial oxidation of biogenic methane or post-sampling degradation of low
CH<sub>4</sub> content samples rather than migration of deep thermogenic gas. A
significant number of samples (39.2 %) contained methane with
predominantly biogenic CÂ isotope ratios
(<i>δ</i><sup>13</sup>C<sub>CH<sub>4</sub></sub> <  −55 ‰) accompanied by elevated
concentrations of ethane and sometimes trace concentrations of propane. These
gases, observed in 28.1 % of the samples, bearing both biogenic
(<i>δ</i><sup>13</sup>C) and thermogenic (presence of C<sub>3</sub>) characteristics,
are most likely derived from shallow coal seams that are prevalent in the
Cretaceous Horseshoe Canyon and neighboring formations in which some of the
groundwater wells are completed. The remaining 3.7 % of samples were not
assigned because of conflicting parameters in the data sets or between
replicates samples. Hence, despite quite variable gas concentrations and a
wide range of <i>δ</i><sup>13</sup>C<sub>CH<sub>4</sub></sub> values in baseline groundwater
samples, we found no conclusive evidence for deep thermogenic gas migration
into shallow aquifers either naturally or via anthropogenically induced
pathways in this baseline groundwater survey. This study shows that the
combined interpretation of aqueous geochemistry data in concert with chemical
and isotopic compositions of dissolved and/or free gas can yield
unprecedented insights into formation and potential migration of methane in
shallow groundwater. This enables the assessment of cross-formational methane
migration and provides an understanding of alkane gas sources and pathways
necessary for a stringent baseline definition in the context of current and
future unconventional hydrocarbon exploration and exploitation