A theoretical model for gas adsorption-induced coal swelling

Swelling and shrinkage (volumetric change) of coal during adsorption and desorption of gas is a well-known phenomenon. For coalbed methane recovery and carbon sequestration in deep, unminable coal beds, adsorption-induced coal volumetric change may cause significant reservoir permeability change. In this work, a theoretical model is derived to describe adsorption-induced coal swelling at adsorption and strain equilibrium. This model applies an energy balance approach, which assumes that the surface energy change caused by adsorption is equal to the elastic energy change of the coal solid. The elastic modulus of the coal, gas adsorption isotherm, and other measurable parameters, including coal density and porosity, are required in this model. Results from the model agree well with experimental observations of swelling. It is shown that the model is able to describe the differences in swelling behaviour with respect to gas species and at very high gas pressures, where the coal swelling ratio reaches a maximum then decreases. Furthermore, this model can be used to describe mixed-gas adsorption induced-coal swelling, and can thus be applied to CO₂-enhanced coalbed methane recovery.     

Estuarine clay mineral distribution: Modern analogue for ancient sandstone reservoir quality prediction

The spatial distribution of clay minerals in sandstones, which may both enhance or degrade reservoir quality, is poorly understood. To address this, clay mineral distribution patterns and host‐sediment properties (grain size, sorting, clay fraction abundance and bioturbation intensity) have, for the first time, been determined and mapped at an unprecedentedly high‐resolution in a modern estuarine setting (Ravenglass Estuary, UK). Results show that the estuary sediment is dominated by illite with subordinate chlorite and kaolinite, although the rivers supply sediment with less illite and significantly more chlorite than found in the estuary. Fluvial‐supplied sediment has been locally diluted by sediment derived from glaciogenic drift deposits on the margins of the estuary. Detailed clay mineral maps and statistical analyses reveal that the estuary has a heterogeneous distribution of illite, chlorite and kaolinite. Chlorite is relatively most abundant on the northern foreshore and backshore and is concentrated in coarse‐grained inner estuary dunes and tidal bars. Illite is relatively most abundant (as well as being most crystalline and most Fe–Mg‐rich) in fine‐grained inner estuary and central basin mud and mixed flats. Kaolinite has the highest abundance in fluvial sediment and is relatively homogenous in tidally‐influenced environments. Clay mineral distribution patterns in the Ravenglass Estuary have been strongly influenced by sediment supply (residence time) and subsequently modified by hydrodynamic processes. There is no relationship between macro‐faunal bioturbation intensity and the abundance of chlorite, illite or kaolinite. Based on this modern‐analogue study, outer estuarine sediments are likely to be heavily quartz cemented in deeply‐buried (burial temperatures exceeding 80 to 100°C) sandstone reservoirs due to a paucity of clay grade material (<0·5%) to form complete grain coats. In contrast, chlorite‐enriched tidal bars and dunes in the inner estuary, with their well‐developed detrital clay coats, are likely to have quartz cement inhibiting authigenic clay coats in deeply‐buried sandstones.     

Wake studies of a 1/30th scale horizontal axis marine current turbine

A 0.4 m diameter (1:30th scale) horizontal axis marine current turbine (MCT) was tested in a circulating water channel. The turbine performance and wake characteristics were determined over a range of flow speeds and rotor thrust coefficients. Measurements of the water surface elevation profiles indicated increasing variation and surface turbulence with increasing flow speeds. Blockage-type effects (where the measured point velocity was greater than the inflow velocity) occurred around the sides of the rotor for all flow speeds. Although the effects were exaggerated at model scale, it is expected that reasonable variations in water level and flow velocity could also occur over a full scale MCT array.     

Constructing an idealized model of the North Atlantic Ocean using slippery sacks

This paper documents the continued development and testing of a new Lagrangian oceanic general circulation model. The slippery sacks ocean model (SSOM), which represents a body of water as a pile of conforming parcels, is improved and is used to simulate circulations in homogeneous oceans and in an idealized model of the North Atlantic Ocean.A method for including horizontal mixing in the SSOM is presented. A given sack’s nearest neighbors are identified in the positive and negative x- and y-directions, and the sack exchanges momentum and/or tracers with these neighbors. This formulation of mixing is straightforward to implement, computationally efficient, and it produces results similar to a standard Eulerian finite-difference representation of diffusion.The model’s ability to reproduce the Stommel and Munk solutions to the classical western boundary current problem is tested. When steps are taken to reduce the potential energy barrier to sacks crossing one another, the model generates circulations that are consistent with linear theory. In moderately non-linear regimes the model produces appropriate departures from linear solutions including a boundary current that continues along the northern boundary for a time.Taking advantage of the new mixing scheme and lessons learned from simulations of homogeneous oceans, the authors construct an idealized model of the North Atlantic Ocean. They compare simulations conducted with the SSOM to similar simulations conducted with the Massachusetts Institute of Technology general circulation model (MITgcm). The SSOM and the MITgcm produce similar wind-forced gyres, thermocline structure, and meridional overturning. The SSOM is also used to explore how circulations change in the limit when tracer diffusion goes to zero.     

Ecological assessment to detect imminent change in coral reefs of Admiral Cockburn Land and Sea National Park,Turks and Caicos Islands

Coral reefs of the Turks and Caicos Islands (TCIs) (Caribbean Sea) constitute some of the few pristine coral reef systems in the world and play a crucial role in the islands’ economy because they support rich fisheries catches and tourism development. Ambitious development plans involving increase in fishing and tourism pressures are about to bring changes in coastal zone resources of the TCIs associated with increased sediments and nutrients and reduced predation by herbivorous fish on coral reefs. Understanding change is critical when attempting to protect the resources that these coral reefs support and to adopt proper management strategies. Yet, an environmental assessment program to detect imminent human‐induced changes on the surrounding reefs of the TCIs is lacking. Thus, (i) we obtained baseline data on benthic composition and coral community structure at seven reef sites of representative reefs of the TCIs within the Admiral Cockburn Land and Sea National Park (ACLSNP) of South Caicos Island and (ii) performed a priori statistical power analysis to calculate replication requirements for safely and confidently detecting small (δ = 0.1), medium (δ = 0.3), and large (δ = 0.5) effect sizes for a number of relevant to anticipated changes, univariate, benthic indices and for power β = 0.95. The platforms of the margin reefs studied (9–12 m depth) appeared rather variable regarding benthic composition but quite homogeneous regarding hard coral community structure. Mean percent cover of algal functional groups was 0.1 ± 0.3 (mean ± sd) percent for coralline algae and Halimeda, 0.1 ± 0.6 (mean ± sd) percent for macroalgae, 21.7 ± 33 (mean ± sd) percent for turf algae and 4.8 ± 4.0 (mean ± sd) percent for hard coral cover. The dominant benthic component, however, was carbonate substrate (mean ± sd = 30.4 ± 34.3), thus indicating an accreting reef framework. Mean hard coral density, colony size and recruit density were 5.5 ± 1.8 (mean ± sd) corals per 20‐m line transect, 13.0 ± 2.3 (mean ± sd) cm maximum colony diameter, and 1.3 ± 1.4 (mean ± sd) recruits per square foot, respectively. Due to high natural variance, hard coral colony size and density were practically the most sensitive indices in detecting even small size changes on benthos. Also, the geometric mean of log‐transformed colony size‐frequency distributions of the most abundant hard coral taxa, i.e. Montastrea annularis, Agaricia spp., Siderastrea spp. and Porites asteroides were practically sensitive for the same purpose. We hope that the study will optimize the spatial component of a necessary environmental impact assessment program on coral reefs of the TCIs once the natural spatial variability of the system has been assessed and sensitive, benthic, univariate indices have been identified for representative reference coral reef sites of the TCIs.