Estimating Effective Porosity in Bedrock Aquifers

Flow in many bedrock aquifers is through fracture networks. Point to point tracer tests using applied tracers provide a direct measure of time of travel and are most useful for determining effective porosity. Calculated values from these tests are typically between 10⁻⁴ and 10⁻² (0.01% to 1%), with these low values indicating preferential flow through fracture and channel networks. Tracer tests are not commonly used in site investigations, and specific yield is often used as a proxy for effective porosity. The most popular methods have used centrifuge measurements, water table fluctuations, pumping tests, and packer tests. Specific yield varies substantially with the testing method. No method is as reliable as tracer testing for providing estimates of effective porosity, but all methods provide complementary insights on aquifer structure. Temporal and spatial scaling effects suggest that bedrock aquifers have hierarchical structures, with a network of more permeable fractures and channels, which are connected to less permeable fractures and to the matrix. Consequences of the low effective porosities include groundwater velocities that often exceed 100 m/d and more frequent microbial contamination than in aquifers in unconsolidated sediments. The large uncertainty over the magnitude of effective porosity in bedrock aquifers makes it an important parameter to determine in studies where time of travel is of interest.     

Modeling of Methane Migration in Shallow Aquifers from Shale Gas Well Drilling

The vertical portion of a shale gas well, known as the “tophole” is often drilled using an air‐hammer bit that may introduce pressures as high as 2400 kPa (350 psi) into groundwater while penetrating shallow aquifers. A 3‐D TOUGH2 model was used to simulate the flow of groundwater under the high hydraulic heads that may be imposed by such trapped compressed air, based on an observed case in West Virginia (USA) in 2012. The model realizations show that high‐pressure air trapped in aquifers may cause groundwater to surge away from the drill site at observable velocities. If dissolved methane is present within the aquifer, the methane can be entrained and transported to a maximum distance of 10.6 m per day. Results from this study suggest that one cause of the reported increase in methane concentrations in groundwater near shale gas production wells may be the transport of pre‐existing methane via groundwater surges induced by air drilling, not necessarily direct natural gas leakage from the unconventional gas reservoir. The primary transport mechanisms are advective transport of dissolved methane with water flow, and diffusive transport of dissolved methane.     

Fractal analysis of 2-D fracture networks of geothermal reservoirs in south-western Turkey

Natural fracture patterns of producing geothermal formations in south-western Turkey are mapped at different scales. The fractal dimensions of different fracture network properties, such as spatial distribution, density, connectivity, orientation, and length are measured by different methods. Analysis of the natural fracture patterns from giga to microscales identifies the descending behavior of box-counting fractal dimension with respect to the scale. It is observed that the fracture networks represent scale-invariant properties, but fractal dimensions might notably differ when the mass dimension is measured applying different methods. Anisotropic nature of fracture networks is also included in the fractal analysis.     

Review of Modeling Approaches to Groundwater Flow in Deformed Carbonate Aquifers

We discuss techniques to represent groundwater flow in carbonate aquifers using the three existing modeling approaches: equivalent porous medium, conduit network, and discrete fracture network. Fractures in faulted stratigraphic successions are characterized by dominant sets of sub-vertical joints. Grid rotation is recommended using the equivalent porous medium to match higher hydraulic conductivity with the dominant orientation of the joints. Modeling carbonate faults with throws greater than approximately 100 m is more challenging. Such faults are characterized by combined conduit-barrier behavior. The barrier behavior can be modeled using the Horizontal Flow Barrier Package with a low-permeability vertical barrier inserted to represent the impediment of horizontal flow in faults characterized by sharp drops of the piezometric surface. Cavities can occur parallel to the strike of normal faults generating channels for the groundwater. In this case, flow models need to account for turbulence using a conduit network approach. Channels need to be embedded in an equivalent porous medium due to cavities a few centimeters large, which are present in carbonate aquifers even in areas characterized by low hydraulic gradients. Discrete fracture network modeling enables representation of individual rock discontinuities in three dimensions. This approach is used in non-heavily karstified aquifers at industrial sites and was recently combined with the equivalent porous medium to simulate diffusivity in the matrix. Following this review, we recommend that the future research combines three practiced modeling approaches: equivalent porous medium, discrete fracture network, and conduit network, in order to capture structural and flow aspects in the modeling of groundwater in carbonate rocks.     

Generalized Radial Flow in Synthetic Flow Systems

Traditional analysis methods used to determine hydraulic properties from pumping tests work well in many porous media aquifers, but they often do not work in heterogeneous and fractured‐rock aquifers, producing non‐plausible and erroneous results. The generalized radial flow model developed by Barker (1988) can reveal information about heterogeneity characteristics and aquifer geometry from pumping test data by way of a flow dimension parameter. The physical meaning of non‐integer flow dimensions has long been a subject of debate and research. We focus on understanding and interpreting non‐radial flow through high permeability conduits within fractured aquifers. We develop and simulate flow within idealized non‐radial flow conduits and expand on this concept by simulating pumping in non‐fractal random fields with specific properties that mimic persistent sub‐radial flow responses. Our results demonstrate that non‐integer flow dimensions can arise from non‐fractal geometries within aquifers. We expand on these geometric concepts and successfully simulate pumping in random fields that mimic well‐test responses seen in the Culebra Dolomite above the Waste Isolation Pilot Plant.     

Investigating Groundwater Flow Between Edwards and Trinity Aquifers in Central Texas

Understanding the nature of communication between aquifers can be challenging when using traditional physical and geochemical groundwater sampling approaches. This study uses two multiport wells completed within Edwards and Trinity aquifers in central Texas to determine the degree of groundwater inter‐flow between adjacent aquifers. Potentiometric surfaces, hydraulic conductivities, and groundwater major ion concentrations and Sr isotope values were measured from multiple zones within three hydrostratigraphic units (Edwards and Upper and Middle Trinity aquifers). Physical and geochemical data from the multiport wells were combined with historical measurements of groundwater levels and geochemical compositions from the region to characterize groundwater flow and identify controls on the geochemical compositions of the Edwards and Trinity aquifers. Our results suggest that vertical groundwater flow between Edwards and Middle Trinity aquifers is likely limited by low permeability, evaporite‐rich units within the Upper and Middle Trinity. Potentiometric surface levels in both aquifers vary with changes in wet vs. dry conditions, indicating that recharge to both aquifers occurs through distinct recharge areas. Geochemical compositions in the Edwards, Upper, and Middle Trinity aquifers are distinct and likely reflect groundwater interaction with different lithologies (e.g., carbonates, evaporites, and siliceous sediments) as opposed to mixing of groundwater between the aquifers. These results have implications for the management of these aquifers as they indicate that, under current conditions, pumping of either aquifer will likely not induce vertical cross‐formational flow between the aquifers. Inter‐flow between the Trinity and the Edwards aquifers, however, should be reevaluated as pumping patterns and hydrogeologic conditions change.     

Permeability profiles in granular aquifers using flowmeters in direct-push wells

Numerical hydrogeological models should ideally be based on the spatial distribution of hydraulic conductivity (K), a property rarely defined on the basis of sufficient data due to the lack of efficient characterization methods. Electromagnetic borehole flowmeter measurements during pumping in uncased wells can effectively provide a continuous vertical distribution of K in consolidated rocks. However, relatively few studies have used the flowmeter in screened wells penetrating unconsolidated aquifers, and tests conducted in gravel-packed wells have shown that flowmeter data may yield misleading results. This paper describes the practical application of flowmeter profiles in direct-push wells to measure K and delineate hydrofacies in heterogeneous unconsolidated aquifers having low-to-moderate K (10(-6) to 10(-4) m/s). The effect of direct-push well installation on K measurements in unconsolidated deposits is first assessed based on the previous work indicating that such installations minimize disturbance to the aquifer fabric. The installation and development of long-screen wells are then used in a case study validating K profiles from flowmeter tests at high-resolution intervals (15 cm) with K profiles derived from multilevel slug tests between packers at identical intervals. For 119 intervals tested in five different wells, the difference in log K values obtained from the two methods is consistently below 10%. Finally, a graphical approach to the interpretation of flowmeter profiles is proposed to delineate intervals corresponding to distinct hydrofacies, thus providing a method whereby both the scale and magnitude of K contrasts in heterogeneous unconsolidated aquifers may be represented.     

The fractal geometry of flow paths in natural fractures in rock and the approach to percolation

The distributions of contact areas in single, natural fractures in quartz monzonite (Stripa granite) are found to have fractal dimensions which decrease fromD=2.00 to values nearD=1.96 as stress normal to the fractures is increased from 3 MPa up to 85 MPa. The effect of stress on fluid flow is studied in the same samples. Fluid transport through a fracture depends on two properties of the fracture void space geometry. the void aperture; and the tortuosity of the flow paths, determined through the distribution of contact area. Each of these quantities change under stress and contribute to changes observed in the flow rate. A general flow law is presented which separates these different effects. The effects of tortuosity on flow are largely governed by the proximity of the flow path distribution to a percolation threshold. A fractal model of correlated continuum percolation is presented which quantitatively reproduces the flow path geometries. The fractal dimension in this model is fit to the measured fractal dimensions of the flow systems to determine how far the flow systems are above the percolation threshold.     

The unusual and large drawdown response of buried-valley aquifers to pumping

The buried-valley aquifers that are common in the glacial deposits of the northern hemisphere are a typical case of the strip aquifers that occur in many parts of the world. Pumping from a narrow strip aquifer leads to much greater drawdown and much more distant drawdown effects then would occur in a sheet aquifer with a similar transmissivity and storage coefficient. Widely used theories for radial flow to wells, such as the Theis equation, are not appropriate for narrow strip aquifers. Previously published theory for flow to wells in semiconfined strip aquifers is reviewed and a practical format of the type curves for pumping-test analysis is described. The drawdown response of strip aquifers to pumping tests is distinctive, especially for observation wells near the pumped well. A case study is presented, based on extensive pumping test experience for the Estevan Valley Aquifer in southern Saskatchewan, Canada. Evaluation of groundwater resources in such buried-valley aquifers needs to take into account the unusually large drawdowns in response to pumping.     

Conceptual models for carbonate aquifers. 1969

The very diverse types of ground‐water behavior in carbonate terrains can be classified by relating the flow type to a particular hydrogeologic environment each exhibiting a characteristic cave morphology. The ground water may move by diffuse flow, by retarded flow, or by free flow. Diffuse flow occurs in less soluble rocks such as extremely shaley limestones or crystalline dolomites. Integrated conduits are rare. Caves tend to be small, irregular, and often little more than solutionally widened joints. Retarded flows occur in artesian environments and in situations where unfavorable stratigraphy forces ground water to be confined to relatively thin beds. Network cave patterns are characteristic since hydrodynamic forces are damped by the external controls. Solution occurs along many available joints. Free flowing aquifers are those in which solution has developed a subsurface drainage system logically regarded as an underground extension of surface streams. These streams may have fully developed surface tributaries as well as recharge from sinkholes and general infiltration. Characteristic cave patterns are those of integrated conduit systems which are often truncated into linear, angulate, and branchwork caves. Free Flow aquifers may be further subdivided into Open aquifers lying beneath karst plains and Capped aquifers in which significant parts of the drainage net lie beneath an insoluble cap rock. Other geologic factors such as structure, detailed lithology, relief, and locations of major streams, control the details of cave morphology and orientation of the drainage network.     

Control of BTEX Migration Using a Biologically Enhanced Permeable Barrier

A permeable barrier system. consisting of a line of closely spaced wclls. was installed perpendicular to ground water flow to control the migration of a dissolved hydrocarhon plume. The wells were charged wiih concrete briquets that release oxygen and nitrate at a controlled rate. enhancing aerobic bio-degradation in the downgradient aquifer.
Laboratory batch reactor experiments were conducted to identify concrete mixtures that slowly released oxygcn over an extended time period. Concretes prepared with urea hydrogen peroxide were unsatisfactory, while concretes prepared with calcium peroxide and a proprietary formalation of magnesium peroxide (ORC®) gradually released oxygen at a steadily declining rate. The 21 percent MgO₂ conerete cylinders and briquets released oxygen at measurable rates for up to 300 days, while the 14 percent CaO₂ briquets were exhausted by 100 days.
A full-scale permeable barrier system using ORC was constructed at a gasoline-spill site. During the first 242 days of operation. total BTFX decreased from 17 to 3.4 mg/L. and dissolved oxygen increased from 0.4 to 1.8 mg/L. during transport through the barrier. Over time, BTEX treatment efficiencies declined. indicating the barrier system had becomc less effective in releasing oxygen and nutrients to the highly contaminated portion of the aquifer. Point dilution tests and sediment analyses performed at the conclusion of the project indicated that ihc aquifer in the vicinity of the remediation wells had been clogged by precipitation with iron minerals. This clogging is believed to result from high pH from the concrete and oxygen released by ihc ORC. Oxygen-releasing permeable barriers and other aerobic bioremediation processes should be used with caution in aquifers with high levels of dissolved iron.     

Configuring Detection Wells Near Landfills

A graphical method was devised for designing contaminant detection monitoring networks in aquifers. The approach eliminates bias in detection efficiency among well pairs, thereby improving the overall efficiency of a ground water monitoring network. In the equidistant configurations derived by the graphical approach, all wells are located the same distance from a landfill, but the distance is measured parallel to ground water flow, Measured perpendicular to ground water flow, there is also an equal spacing between wells in an equidistant network. A simulation model was used to compare an equidistant network to a peripheral monitoring configuration, in which wells were spaced evenly along the downgradient boundaries of a landfill. The equidistant network yielded a 12.4% higher detection efficiency and also facilitated earlier release detection. In practice, the graphical approach that yields equidistant configurations can be used to identify candidate monitoring networks to detect potential releases from landfills.     

Geochemistry of the Springfield Plateau aquifer of the Ozark Plateaus Province in Arkansas,Kansas, Missouri and Oklahoma,USA

Geochemical data indicate that the Springfield Plateau aquifer, a carbonate aquifer of the Ozark Plateaus Province in central USA, has two distinct hydrochemical zones. Within each hydrochemical zone, water from springs is geochemically and isotopically different than water from wells. Geochemical data indicate that spring water generally interacts less with the surrounding rock and has a shorter residence time, probably as a result of flowing along discrete fractures and solution openings, than water from wells. Water type throughout most of the aquifer was calcium bicarbonate, indicating that carbonate‐rock dissolution is the primary geochemical process occurring in the aquifer. Concentrations of calcium, bicarbonate, dissolved oxygen and tritium indicate that most ground water in the aquifer recharged rapidly and is relatively young (less than 40 years). In general, field‐measured properties, concentrations of many chemical constituents, and calcite saturation indices were greater in samples from the northern part of the aquifer (hydrochemical zone A) than in samples from the southern part of the aquifer (hydrochemical zone B). Factors affecting differences in the geochemical composition of ground water between the two zones are difficult to identify, but could be related to differences in chert content and possibly primary porosity, solubility of the limestone, and amount and type of cementation between zone A than in zone B. In addition, specific conductance, pH, alkalinity, concentrations of many chemical constituents and calcite saturation indices were greater in samples from wells than in samples from springs in each hydrochemical zone. In contrast, concentrations of dissolved oxygen, nitrite plus nitrate, and chloride generally were greater in samples from springs than in samples from wells. Water from springs generally flows rapidly through large conduits with minimum water–rock interactions. Water from wells flow through small fractures, which restrict flow and increase water–rock interactions. As a result, springs tend to be more susceptible to surface contamination than wells. The results of this study have important implications for the geochemical and hydrogeological processes of similar carbonate aquifers in other geographical locations. Copyright © 2000 John Wiley & Sons, Ltd.     

Determining anisotropic transmissivity using a simplified Papadopulos method

The straight-line method presented by Papadopulos requires a minimum of three observation wells for determining the transmissivity tensor of a homogeneous and anisotropic aquifer. A simplification of this method was developed for fractured aquifers where the principal directions of the transmissivity tensor are known prior to implementation, such as when fracture patterns on outcropping portions of the aquifer may be used to infer the principal directions. This new method assumes that observation wells are drilled along the two principal directions from the pumped well, thus reducing the required number of observation wells to two. This method was applied for an aquifer test in the fractured Navajo Sandstone of southwestern Utah and yielded minimum and maximum principal transmissivity values of 70 and 1800 m(2)/d, respectively, indicating an anisotropy ratio of approximately 24 to 1.     

Monte Carlo Studies of Relations between Fractal Dimensions in Monofractal Data Sets

—Within the fractal approach to studying the distribution of seismic event locations, different fractal dimension definitions and estimation algorithms are in use. Although one expects that for the same data set, values of different dimensions will be different, it is usually anticipated that the direction of fractal dimension changes among different data sets will be the same for every fractal dimension.¶Mutual relations between the three most popular fractal dimensions, namely the capacity, cluster and correlation dimensions, have been investigated in the present work. The studies were performed on the Monte Carlo generated data sets. The analysis has shown that dependence of the fractal dimensions on epicenter distribution, and relations among the fractal dimensions, are complex and variable. Neither values nor even inequalities among dimension estimates are preserved when different fractal dimensions are used. The correlation and the capacity dimensions seem to be good tools to trace collinear tendencies of eipicenters while the cluster dimension is more appropriate to studying uniform clustering of points.     

分形维数计算结果的可靠性

本文以大量分形维数计算结果的实例为基础，着重讨论了影响分维值可靠性的因素，特别指出对于未知动力学系统的自然界中的分形，很难获得真正的分形维数，若研究统计分形维数的变化特征，在采取了某些措施之后，其分维值可比，可用。     

Controls on Methane Occurrences in Aquifers Overlying the Eagle Ford Shale Play,South Texas

Assessing natural vs. anthropogenic sources of methane in drinking water aquifers is a critical issue in areas of shale oil and gas production. The objective of this study was to determine controls on methane occurrences in aquifers in the Eagle Ford Shale play footprint. A total of 110 water wells were tested for dissolved light alkanes, isotopes of methane, and major ions, mostly in the eastern section of the play. Multiple aquifers were sampled with approximately 47 samples from the Carrizo‐Wilcox Aquifer (250‐1200 m depth range) and Queen City‐Sparta Aquifer (150‐900 m depth range) and 63 samples from other shallow aquifers but mostly from the Catahoula Formation (depth <150 m). Besides three shallow wells with unambiguously microbial methane, only deeper wells show significant dissolved methane (22 samples >1 mg/L, 10 samples >10 mg/L). No dissolved methane samples exhibit thermogenic characteristics that would link them unequivocally to oil and gas sourced from the Eagle Ford Shale. In particular, the well water samples contain very little or no ethane and propane (C1/C2+C3 molar ratio >453), unlike what would be expected in an oil province, but they also display relatively heavier δ¹³C_methane (>?55‰) and δD_methane (>?180‰). Samples from the deeper Carrizo and Queen City aquifers are consistent with microbial methane sourced from syndepositional organic matter mixed with thermogenic methane input, most likely originating from deeper oil reservoirs and migrating through fault zones. Active oxidation of methane pushes δ¹³C_methane and δD_methane toward heavier values, whereas the thermogenic gas component is enriched with methane owing to a long migration path resulting in a higher C1/C2+C3 ratio than in the local reservoirs.     

Finite layer method for flow in layered radial two-zone aquifer systems

A new finite layer method (FLM) is presented in this paper for transient flow analysis in layered radial two-zone aquifer systems. A radial two-zone system is an aquifer configuration in which a circular aquifer with finite radius is surrounded by a matrix possessing different permeability and storage properties. The aquifers can be pumped from fully or partially penetrating wells of infinitesimal radius. The trial function for drawdown is obtained through the use of piecewise linear correction functions in the present method. The trial function can satisfy the continuity conditions of flow and possess an appropriate continuity of C(0) at the two-zone interface. On the basis of Galerkin's method and the continuity condition of flow, the finite layer formulation is derived. The proposed method can cope with the anisotropy and layered heterogeneity in radial two-zone aquifer systems. Several numerical examples are presented to verify the validity of the present method through comparison with the analytical solution and the numerical results based on the finite difference method, in which a test of three-dimensional (3D) flow to a partially penetrating well in anisotropic two-zone aquifers is included. Furthermore, an additional application in simulating the two-zone flow in aquitard-aquifer systems is presented to demonstrate the applicability of FLM in modeling flow in more complex aquifer systems.     

Nonlinear Flow Process: A New Package to Compute Nonlinear Flow in MODFLOW

A new MODFLOW package (Nonlinear Flow Process; NLFP) simulating nonlinear flow following the Forchheimer equation was developed and implemented in MODLFOW‐2005. The method is based on an iterative modification of the conductance calculated and used by MODFLOW to obtain an effective Forchheimer conductance. The package is compatible with the different layer types, boundary conditions, and solvers as well as the wetting capability of MODFLOW. The correct implementation is demonstrated using four different benchmark scenarios for which analytical solutions are available. A scenario considering transient flow in a more realistic setting and a larger model domain with a higher number of cells demonstrates that NLFP performs well under more complex conditions, although it converges moderately slower than the standard MODFLOW depending on the nonlinearity of flow. Thus, this new tool opens a field of opportunities to groundwater flow simulation with MODFLOW, especially for core sample simulation or vuggy karstified aquifers as well as for nonlinear flow in the vicinity of pumping wells.     

The influence of well hydraulics on the spatial distribution of well incrustations

In many cases, the operation of wells is hampered by the formation of mineral incrustations. From field studies, it is known that the distribution of incrustations in wells is quite inhomogeneous. Flow models were calculated to assess the hydraulic background of this phenomenon. For horizontal flow, the screen section facing the natural flow direction receives the majority of the total inflow. This phenomenon increases with increasing natural gradients of flow. The vertical distribution of water intake is also quite inhomogeneous. In partially penetrating wells, the uppermost part of the screen receives much more inflow than the deeper screen sections. These flow inhomogenities involve elevated flow velocities and may cause increased influx of shallow, oxygenated water, all conditions favorable for incrustation growth. Field investigations on incrusting wells clearly show that the identified screen areas of elevated flow are indeed much more prone to incrustation deposition. Such sections require more attention during rehabilitations. A suction flow control device can help to homogenize the inflow but can cause elevated entrance loss when affected by incrustation buildup itself.