Influence Of Small-scale Heterogeneity Research Articles

Mimicking Rock Heterogeneity in the Laboratory to Better Understand its Effects on CO2 Migration and Trapping

Small scale heterogeneity characteristic of clastic formations has been well known to affect CO2 migration pathways during geologic carbon sequestration. Especially far field from the injection sites, when buoyancy and capillary forces take over from the injection pressure gradients, the effects of such geological features on the CO2 plume movement become compounded. Numerical simulations have established that accumulation and trapping of the buoyant plume is primarily due to the contrast in capillary entry pressures of different rock layers. Capillary trapping can help maximize the storage capacity of reservoirs and hence it is essential to understand the dynamics of CO2 movement and trapping under the influence of small-scale heterogeneity and quantify its effects. In this work we propose buoyancy driven migration experiments, conducted at the meter scale using glass beads packed in a quasi 2D glass cell. We demonstrate a novel automated glass bead filler system that can generate 2D heterogeneous structures in a reproducible manner. A fluid pair that mimics the phase densities and viscosities of CO2-Brine at reservoir pressures and temperatures is employed. Light transmission technique is used for visualization, and to calibrate and quantify saturation of the trapped non-wetting fluid during the experiments. With the ability to generate different types of heterogeneous structures in a reproducible manner, a systematic investigation of the effect of heterogeneity on capillary trapping becomes possible.

Investigating the Influence of Geological Heterogeneity on Capillary Trapping of Buoyant CO2 Using Transmitted-light Flow Visualization Experiments

Small scale heterogeneity characteristic of clastic formations has been well known to affect CO2 migration pathways during geologic carbon sequestration. Especially far field from the injection sites, when buoyancy and capillary forces take over from the injection pressure gradients, the effects of such geological features on the CO2 plume movement become compounded. Numerical simulations have established that accumulation and trapping of the buoyant plume is primarily due to the contrast in capillary entry pressures of different rock layers. Capillary trapping can help maximize the storage capacity of reservoirs and hence it is essential to understand the dynamics of CO2 movement and trapping under the influence of small scale heterogeneity and quantify its effects. In this work we propose buoyancy driven migration experiments and simulations conducted at the meter scale using glass beads packed in a quasi 2D glass cell. Using a combination of two linear axis actuators, we demonstrate an automated glass bead filler system that can generate 2D heterogeneous structures in a reproducible manner. A fluid pair that mimics the phase densities and viscosities of CO2-Brine at reservoir pressures and temperatures is employed. Light transmission technique is used for visualization, and to calibrate and quantify saturation of the trapped non-wetting fluid during the experiments. Invasion Percolation is used to simulate the buoyancy driven flow. With the ability to generate different types of heterogeneous structures in a reproducible manner, and by comparing experiments and simulations, a systematic investigation of the effect of heterogeneity on capillary trapping becomes possible.

Hyporheic zone exchange fluxes and residence times inferred from riverbed temperature and radon data

Summary Vertical profiles of temperature, radon and electrical conductivity are used to characterise downwelling, neutral and upwelling hyporheic zones along a pool–riffle sequence in the Haughton River in north-eastern Australia. Water residence times and vertical fluxes are derived from temperature and radon data and then directly compared for downwelling profiles. Temperature and radon-derived fluxes in downwelling zones ranged from 0.02 to 24 m day −1 with a mean of 1.69 m day −1 while residence times across the study site ranged from tens of minutes to greater than 15 days. The radon approach has the lowest uncertainty for residence times between 0.1 and 15 days while the uncertainty of the temperature approach (using a diel river signal) is lowest for residence times that are less than a few days. For 83% of depths in downwelling profiles, radon-derived residence times were greater (some up to two orders of magnitude greater) than temperature-derived residence times. When the error bounds of the residence time estimates were accounted for, 57% of radon-derived residence times were considerably greater than temperature-derived residence times in downwelling profiles. We suggest that this disparity is due to the different influence of small scale heterogeneity on temperature and radon transport. These field based results indicate that small scale heterogeneity may play a far more important role than has been previously considered in groundwater–surface water interaction studies.

Influence of Small Scale Heterogeneity on CO2 Trapping Processes in Deep Saline Aquifers

The physical mechanism of CO2 trapping in porous media by capillary trapping (pore scale) incorporates a number of related processes, i.e. residual trapping, trapping due to hysteresis of the relative permeability, and trapping due to hysteresis of the capillary pressure. Additionally CO2 may be trapped in heterogeneous media due to difference in capillary pressure entry points for different materials (facies scale). The amount of CO2 trapped by these processes depends upon a complex system of non-linear and hysteretic relationships including how relative permeability and capillary pressure vary with brine and CO2 saturation, and upon the spatial variation in these relationships as caused by geologic heterogeneity.Geological heterogeneities affect the dynamics of CO2 plumes in subsurface environments. Recent studies have led to new conceptual and quantitative models for sedimentary architecture in fluvial deposits over a range of scales that are relevant to the performance of some deep saline reservoirs. We investigated how the dynamics of a CO2 plume, during and after injection, is influenced by the hierarchical and multi-scale stratal architecture in such reservoirs. The results strongly suggest that representing small scales features (decimeter to meter), including their organization within a hierarchy of larger-scale features, is critical to understanding trapping processes.

Modelling the impact of small‐scale heterogeneities on tree—grass coexistence in semi‐arid savannas

Summary Savanna ecosystems show codominance of trees and grasses, and the mechanisms involved in their coexistence remain unresolved. We investigated the possible influence of small‐scale heterogeneities and disturbances in determining tree spacing and tree‐grass coexistence in semi‐arid savannas, using a spatially explicit, grid‐based simulation model. We added factors such as seed clumping, locally improved moisture conditions, vegetation clearing and combinations of all three of these variables to a previously published model. We also examined the influence of changing the size and spatiotemporal correlation among individual heterogeneities. Increased seed availability in localized clumps, especially in combination with the other heterogeneity types, had the strongest impact on the long‐term tree‐grass coexistence in the model. Localized deposition of tree seeds in herbivore dung and underground seed caches built by seed‐collecting rodents may act as a determinant of the distribution of trees in savannas. Spatially autocorrelated small‐scale heterogeneities led to a more stable coexistence of trees and grasses than uncorrelated heterogeneities did, and at high levels of autocorrelation runs with all three variables led to long‐term coexistence in up to 60% of the rainfall scenarios tested. The size of individual heterogeneities, also played a significant role in determining whether trees would establish as lone individuals or in clumps. In simulations that included small‐scale heterogeneities, the number of isolated trees scattered throughout the landscape increased, whereas large heterogeneity patches and high spatiotemporal autocorrelation tended to promote tree clustering. Simulations based on rainfall data from the southern Kalahari produced a realistic density and spatial distribution for trees in this semi‐arid savanna for a realistic range of small‐scale heterogeneities. Small‐scale disturbances and heterogeneities, at least those that furnish better establishment conditions for tree seedlings, therefore act to increase the range of environmental conditions under which trees and grasses can coexist as savanna.