Faulting in prospective CO2 storage sites in the UK Southern North Sea

Post-depositional folding of Triassic strata, formed largely by the development of salt domes and pillows in the underlying Zechstein Group, led to the formation of numerous large anticlinal structures at the level of the Triassic aged Bunter Sandstone Formation (BSF). These structural closures, some of which have formed effective traps to natural gas, have been mapped across the UK Southern North Sea (SNS), and are currently of interest as potential prospects for the storage of anthropogenic CO2

Some thoughts on Darcy-type flow simulation for modelling underground CO 2 storage, based on the Sleipner CO 2 storage operation

We take three flow simulators, all based on Darcy’s Law but with different numerical solver implementations, to assess some of the issues surrounding their use to model underground CO2 storage. We focus on the Sleipner CO2 injection project, which, with its seismic monitoring datasets, provides unique insights into CO2 plume development during a large-scale injection operation. The case studies firstly compare simulator performance in terms of outputs and run-times on carefully matched model scenarios; then we compare numerical with analytical Darcy solutions to explore the potential for modelling simplification; finally we look at the effects of including conservation of energy in the simulations. The initial case-study used simplified axisymmetric model geometry to simulate the upward flux of CO2 through a heterogeneous reservoir, incorporating multiphase flow with coupled CO2 dissolution into formation brine. All three codes produced near-identical results with respect to CO2 migration velocity and total upward CO2 flux at the reservoir top. The second case-study involved 3D modelling of the growth of the topmost layer of CO2 trapped and migrating beneath topseal topography. Again the three codes showed excellent agreement. In the third case-study the simulators were tested against a simplified analytical solution for gravity currents to model the spreading of a single CO2 layer beneath a flat caprock. Neglecting capillary effects, the numerical models showed similar layer migration and geometry to the analytical model, but it was necessary to minimise the effects of numerical dispersion by adopting very fine cell thicknesses. The final case-study was designed to test the non-isothermal effects of injecting CO2 into a reservoir at non-ambient temperature. Only two of the simulators solve for conservation of energy, but both showed a near identical thermal anomaly, dominated by Joule-Thomson effects. These can be significant, particularly where reservoir conditions are close to the critical point for CO2 where property variations can significantly affect plume mobility and also seismic response. In conclusion, the three simulators show robust consistency, any differences far less than would result from geological parameter uncertainty and limitations of model resolution. In this respect the three implementations are significantly different in terms of computing resource requirement and it is clear that approaches with simplified physics will pay rich dividends in allowing more detailed reservoir heterogeneity to be included. Contrary to this, including conservation of energy is heavier on computing time but is likely to be required for storage scenarios where the injectant stream is significantly different in temperature to the reservoir and most critically for shallower storage reservoirs where CO2 is close to its critical point

CO 2 storage: setting a simple bound on potential leakage through the overburden in the North Sea Basin

So-called ‘gas chimneys’ are likely to provide the main geological risk for out-of-reservoir CO2 migration in thick post-rift overburden successions such as typify the central and northern North Sea. Here we postulate that, in the North Sea, such chimneys formed in the geological past, with a likely peak activity at the end of the ice-age, and are currently rather dormant. With this postulate we set a bound on possible bulk migration rates considering both advective and diffusive flow and based on a hypothetical CO2 storage site at 800 m depth. Calculated migration velocities into the overburden, by either advection or diffusion, are very low, at less than one metre per thousand years. Consequently flux rates are also very low, several orders of magnitude below the leakage thresholds that have been suggested as ensuring effective mitigation performance. Time-lapse seismic reflection data from the Sleipner storage site, which is located beneath some small chimney features, show no evidence of CO2 migration into the overburden. This cannot prove the postulate, because the time interval spanned by the seismic surveys is just a few years, but it is nevertheless consistent with it

Fighting for mates: the importance of individual size in mating contests in rocky shore littorinids

Studies of mating contests have reported how traits (e.g. body size) related to resource holding potential (RHP) and strategies to assess RHP and resource value influence contest outcome in many taxa but are rare in the Gastropoda. The influence of male size (as an index of RHP) and female size (as a measure of resource value) on contest outcome were investigated in two littorinid snails, Echinolittorina malaccana and E. radiata, in Hong Kong during May－June 2013. In these snails, contests between males take the form of a 'challenger' attempting to take over the copulation position occupied by a 'defender'. Both challengers and defenders were, generally, smaller than the females in both species. In both species, the larger the challenger relative to the defender, the more likely he would replace the defender in the copulation position. The challengers were, however, more successful in E. radiata, as they generally challenged defenders that were smaller than themselves, suggesting an ability to detect rival size before entering into a contest in this species. When sizes of the contestants were similar, defenders were more likely to win contests in E. malaccana but not in E. radiata. Evidence for pure self-assessment of RHP and the ability to assess resource value in challengers was found in E. malaccana. Different fighting strategies appear to have evolved in these congeneric marine snail species and decisions based on male and female sizes play an important role in determining male reproductive success

Critical properties of loop percolation models with optimization constraints

We study loop percolation models in two and in three space dimensions, in which configurations of occupied bonds are forced to form closed loop. We show that the uncorrelated occupation of elementary plaquettes of the square and the simple cubic lattice by elementary loops leads to a percolation transition that is in the same universality class as the conventional bond percolation. In contrast to this an optimization constraint for the loop configurations, which then have to minimize a particular generic energy function, leads to a percolation transition that constitutes a new universality class, for which we report the critical exponents. Implication for the physics of solid-on-solid and vortex glass models are discussed.Comment: 8 pages, 8 figure

The meaning of maxima and minima in first order reversal curves: Determining the interaction between species in a sample

First-order reversal curves (FORCs) are a characterization technique for magnetic materials used in a wide range of research fields. Since their first application in the Earth Sciences two decades ago, their importance in science has been continuously growing and new experimental techniques have been subsequently designed based on the original idea of FORCs. Nonetheless, very recent experimental works on very well designed and simple magnetic structures demonstrate that even for the most simple cases the interpretation of FORC data lacks understanding. In this work, we address this problem analytically, explaining the meaning of maxima, minima and noisy tails and set a strategy to extract the interaction field between magnetic structures. The origin of this interaction field is often the magnetostatic energy, however, we propose that this strategy could be applied for estimating exchange interactions too

Sampling scale can cause bias in positive assortative mating estimates: The first evidence in two intertidal snails

Assortative mating in the wild is commonly estimated by correlating between traits in mating pairs (e.g. size of males and females). Unfortunately such an approach may suffer from considerable sampling bias when the distribution of different expressions of a trait in the wild is non-random; for example, when segregation of different size classes of individuals occur in different microhabitats or areas. Consequently, any observed trait correlation in the wild can be an artifact of pooling heterogeneous samples of mating pairs from different microhabitats or areas rather than true non-random matings. This bias in estimating trait correlations due to sampling scale is termed the scale-of-choice effect (SCE). Here we use two intertidal littorinid species from Hong Kong to show how the SCE can bias size-assortative mating estimates from mating pairs captured in the wild, empirically demonstrating the influence of this effect on measures of positive assortative mating. This finding cautions that studies that have overlooked SCE may have misinterpreted the magnitude and the cause of assortative mating, and we provide a new analytical approach to protect against this potential bias in future studies

A vortex description of the first-order phase transition in type-I superconductors

Using both analytical arguments and detailed numerical evidence we show that the first order transition in the type-I 2D Abelian Higgs model can be understood in terms of the statistical mechanics of vortices, which behave in this regime as an ensemble of attractive particles. The well-known instabilities of such ensembles are shown to be connected to the process of phase nucleation. By characterizing the equation of state for the vortex ensemble we show that the temperature for the onset of a clustering instability is in qualitative agreement with the critical temperature. Below this point the vortex ensemble collapses to a single cluster, which is a non-extensive phase, and disappears in the absence of net topological charge. The vortex description provides a detailed mechanism for the first order transition, which applies at arbitrarily weak type-I and is gauge invariant unlike the usual field-theoretic considerations, which rely on asymptotically large gauge coupling.Comment: 4 pages, 6 figures, uses RevTex. Additional references added, some small corrections to the tex

Evolution of a Network of Vortex Loops in the Turbulent Superfluid Helium; Derivation of the Vinen Equation

The evolution a network of vortex loops due to the fusion and breakdown in the turbulent superfluid helium is studied. We perform investigation on the base of the "rate equation" for the distribution function $n(l)$ of number of loops in space of their length $l$. There are two mechanisms for change of quantity $n(l)$. Firstly, the function changes due to deterministic process of mutual friction, when the length grows or decreases depending on orientation. Secondly, the change of $n(l)$ occurs due to random events when the loop crosses itself breaking down into two daughter or two loops collide merging into one larger loop. Accordingly the "rate equation" includes the "collision" term collecting random processes of fusion and breakdown and the deterministic term. Assuming, further, that processes of random colliding are fastest we are in position to study more slow processes related to deterministic term. In this way we study the evolution of full length of vortex loops per unit volume-so called vortex line density ${\cal L}(t)$. It is shown this evolution to obey the famous Vinen equation. In conclusion we discuss properties of the Vinen equation from the point of view of the developed approach.Comment: Presentation at QFS2006, submitted to JLT