Discrete Flow Paths Research Articles

Discrete Dilatant Pathway Modeling of Gas Migration Through Compacted Bentonite Clay

A coupled multiphase fluid flow and discrete fracturing model is applied to simulate bench-scale gas migration experiments on compacted bentonite. The numerical modeling is based on the linking of the multiphase fluid flow simulator TOUGH2 with a Rigid-Body-Spring Network model, which enables a discrete (lattice) representation of elasticity and individual fractures. The evolution of a complex network of dilatant flow paths is modeled through opening and breakage of lattice interface bonds between porous-elastic matrix elements. To achieve a good match with the experimental results, including an abrupt gas breakthrough along with pressure and stress responses, it was necessary to calibrate model parameters for (1) air-entry pressure, (2) shear and tensile failure of lattice interface bonds, (3) moisture swelling/shrinkage effects on stress, and (4) aperture-dependent permeability of dilatant flow paths. Our best-fit conceptual model considers a pervasive network of discrete flow paths propagating from the gas injection point, whereas some of the experimental data indicate the potential for heterogeneous and unstable flow paths.

MeshIt—a software for three dimensional volumetric meshing of complex faulted reservoirs

Flow, mass and energy transport processes in natural reservoirs are controlled to a large degree by the presence of geological heterogeneities including structures such as fractures and fault zones embedded in a spatially varying three-dimensional (3D) porous matrix of the reservoir. Despite recent advances, currently, state-of-the-art models rely on a number of simplifications partly related to our inability to represent heterogeneities as observed in the field into dynamic model realizations. In this respect, an adequate geometric representation of the discrete system is a basic requirement. In this study, we show how fundamental concept from computational geometry can be assembled and used to bridge the gap between geological and dynamic forward models. The result is an automated, open source software solution (MeshIt) to generate quality 3D meshes suitable for the study of flow and transport processes in faulted and fractured reservoirs. The software enables us to integrate into a 3D volumetric representation dipping structures, comprising fault zones and fractures as well as inclined well paths. This permits us to correctly simulate interactions between discrete flow paths along these interacting components and the 3D flow within the reservoir matrix. The crucial factor that makes the approach applicable to real case reservoirs is that all algorithms are local and scalable parallel and have computing times increasing approximately linearly with data volumes. We test the performance and the robustness of the software against three different scenarios of increasing complexity and further discuss current limitations and range of applicability of the software. Although all examples describe geothermal applications, it is worth mentioning that the approach is equally valid for other applications in geoscience from oil and gas industry to carbon capture and sequestration issues.

Hydrological storage and transmission characteristics of an alpine talus

AbstractAlpine watersheds are the source region of some of the largest rivers in North America and elsewhere. Understanding of hydrological processes in alpine watersheds is important for understanding the response of river basins to meteorological forcing. Talus units in alpine watersheds have been suggested in the literature as potential reservoirs of groundwater, but relatively little is known about hydrological processes in talus. To develop conceptual understanding of alpine talus and determine its storage capacity and hydraulic properties, we investigated a talus unit in the Lake O'Hara watershed in the Canadian Rockies using ground‐penetrating radar, electrical resistivity tomography, measurements of talus discharge, tracer tests, and isotopic hydrograph separation. The study talus, consisting mainly of quartzite and carbonate rock fragments, had very high hydraulic conductivity (0·01–0·03 m s−1) and fast hydrograph recession (exponential decay coefficient of 1 d−1), suggesting that its storage capacity is limited to a time scale of less than a week. Groundwater flow through the talus occurs in a relatively thin (0·01–0·1 m) saturated zone at the base of the talus, which appears to have discrete flow paths rather than a single continuous sheet. A late‐lying snowpack, located at the top of the talus and cliff ledges above, sustains baseflow discharging from the talus, which provides moisture to alpine meadows downstream. Although this study indicates limited storage capacity of talus, further research is required to examine the storage and transmission characteristics of talus consisting of different types of geological materials or formed in different environments. Copyright © 2011 John Wiley & Sons, Ltd.

Evolution of the unsaturated zone testing at Yucca Mountain

The evaluation of the Yucca Mountain site has evolved from intensive surface-based investigations in the early 1980s to current focus on testing in underground drifts. Different periods of site characterization activities and prominent issues concerning the unsaturated zone (UZ) are summarized. Data collection activities have evolved from mapping of faults and fractures to estimation of percolation through tuff layers, and to quantification of seepage into drifts. Evaluation of discrete flow paths in drifts has led to fracture–matrix interaction and matrix diffusion tests over different scales. The effects of tuff interfaces and local faults are evaluated in fractured-welded and porous-nonwelded units. Mobilization of matrix water and redistribution of moisture are measured in thermal tests. Lessons learned from underground tests are used to focus on processes needed for additional quantification. Migration through the drift shadow zone and liquid flow through faults are two important issues that have evolved from current knowledge.

Development of discrete flow paths in unsaturated fractures at Yucca Mountain

We have carried out numerical modeling studies to investigate the development of discrete-fracture flow paths and flow-focusing phenomena in the unsaturated rock of the potential repository horizon at Yucca Mountain, Nevada. These studies are based on two- and three-dimensional (2-D and 3-D) numerical models using site-specific parameters. The 2-D and 3-D models use high-resolution spatial discretization to explicitly include effects of discrete fractures with stochastically developed fracture permeabilities and a continuum approach. The permeability field is generated based on air permeability measurements at various scales. For most of the cases considered, uniform infiltration with different average rates (1–500 mm/year) is prescribed at the top of the model, while variability in outflow at the bottom of the model is used to evaluate the degree of flow focusing. In addition, scenarios involving nonuniform infiltration at the top boundary, different permeability correlation lengths and different flow-allocation schemes were analyzed. The modeling results obtained from all of the cases showed a remarkably similar flow-focusing pattern at the repository horizon. Furthermore, tracer transport simulation results also revealed additional features of focused flow and transport through the fracture network.

Using Hydrochemical Fades to Delineate Ground Water Flowpaths in Fractured Shale

AbstractLarge differences in chemistry between sampling points separated In short vertical intervals are often observed in contaminant plumes in both granular and fractured aquifers. However, most regional models assume that such differences will be reduced by dispersive mixing during transport. At a field site located in a discharge area on the Oak Ridge Reservation, Tennessee, ground water flows along discrete flowpaths, as evidenced by the presence of four distinct water types—Ca‐HCO3, Ca‐Na‐HCO3, and Na‐Ca‐HCO3, and Na‐Ca‐HCO3‐S04—in samples collected from shallow (< 3D in) multilevel wells. The preservation of distinct chemical signatures suggests that ground water must he contained in discrete flow zones during much of its transport time. The chemical composition of the water types can be explained primarily by strata‐bound flow over varying flowpath lengths and secondarily by mixing of waters during cross‐formational flow in a discharge zone. The hydrochemical facies identified by correlation of water types between the boreholes indicate the general orientation of ground water How paths. These inferred flowpaths are oblique to the orientation of the measured hydraulic gradient and are more closely aligned with bedding and the calculated flow direction. Results of this study indicate that discrete multilevel sampling for analysis of major ions, in addition to information gathered from tracer tests, borehole flow tests. and visual core observations, can provide valuable information on flow directions and preferential flowpaths for contaminant transport.

The hydrochemistry of meltwaters draining a polythermal‐based, high Arctic glacier, south Svalbard: I. The ablation season

Solute and runoff time-series at Finsterwalderbreen, Svalbard, provide evidence for considerable basal routing of water and the existence of at least two contrasting subglacial chemical weathering environments. The hydrochemistry of a subglacial upwelling provides evidence for a snowmelt-fed subglacial reservoir that dominates bulk runoff during recession flow. High concentrations of Cl− and crustal ions, high pCO2 and ratios of [*SO2−4/(*SO2−4+HCO−3)] close to 0·5 indicate the passage of snowmelt through a subglacial weathering environment characterized by high rock:water ratios, prolonged residence times and restricted access to the atmosphere. At higher discharges, bulk runoff becomes dominated by icemelt from the lower part of the glacier that is conveyed through a chemical weathering environment characterized by low rock:water ratios, short residence times and free contact with atmospheric gases. These observations suggest that icemelt is routed via a hydrological system composed of basal/ice-marginal, englacial and supraglacial components and is directed to the glacier margins by the ice surface slope. Upwelling water flows relatively independently of icemelt to the terminus via a subglacial drainage system, possibly constituting flow through a sediment layer. Cold basal ice at the terminus forces it to take a subterranean routing in its latter stages. The existence of spatially discrete flow paths conveying icemelt and subglacial snowmelt to the terminus may be the norm for polythermal-based glaciers on Svalbard. Proglacial mixing of these components to form the bulk meltwaters gives rise to hydrochemical trends that resemble those of warm-based glaciers. These hydrochemical characteristics of bulk runoff have not been documented on any other glacier on Svalbard to date and have significance for understanding interactions between thermal regime and glacier hydrology. © 1998 John Wiley & Sons, Ltd.

The hydrochemistry of meltwaters draining a polythermal-based, high Arctic glacier, south Svalbard: I. The ablation season

Solute and runoff time-series at Finsterwalderbreen, Svalbard, provide evidence for considerable basal routing of water and the existence of at least two contrasting subglacial chemical weathering environments. The hydrochemistry of a subglacial upwelling provides evidence for a snowmelt-fed subglacial reservoir that dominates bulk runoff during recession flow. High concentrations of Cl− and crustal ions, high pCO2 and ratios of [*SO2−4/(*SO2−4+HCO−3)] close to 0·5 indicate the passage of snowmelt through a subglacial weathering environment characterized by high rock:water ratios, prolonged residence times and restricted access to the atmosphere. At higher discharges, bulk runoff becomes dominated by icemelt from the lower part of the glacier that is conveyed through a chemical weathering environment characterized by low rock:water ratios, short residence times and free contact with atmospheric gases. These observations suggest that icemelt is routed via a hydrological system composed of basal/ice-marginal, englacial and supraglacial components and is directed to the glacier margins by the ice surface slope. Upwelling water flows relatively independently of icemelt to the terminus via a subglacial drainage system, possibly constituting flow through a sediment layer. Cold basal ice at the terminus forces it to take a subterranean routing in its latter stages. The existence of spatially discrete flow paths conveying icemelt and subglacial snowmelt to the terminus may be the norm for polythermal-based glaciers on Svalbard. Proglacial mixing of these components to form the bulk meltwaters gives rise to hydrochemical trends that resemble those of warm-based glaciers. These hydrochemical characteristics of bulk runoff have not been documented on any other glacier on Svalbard to date and have significance for understanding interactions between thermal regime and glacier hydrology. © 1998 John Wiley & Sons, Ltd.

A Reinterpretation of Astropolithon Hindii Dawson 1878

ABSTRACT Astropolithon hindii was first described by Dawson (1878) from the Meguma Group of Nova Scotia and interpreted as a biogenic sedimentary structure. Subsequent workers in ichnology have generally accepted this interpretation and have utilized Astropolithon as a distinctive Cambrian ichnofossil and furthermore have even erected new ichnospecies. It is proposed here that A. hindii is a physical rather than biogenic sedimentary structure, merely representing one variation in a morphologically variable assemblage of sand volcanoes produced as a result of fluidization along discrete flow paths of rapidly deposited turbiditic or fluidized sheet sands. As such, A. hindii should be regarded as a pseudofossil and other biogenic structures assigned to it should necess rily be incorporatred into a new ichnogenus.