Characterizing hydraulic properties of the Upper Jurassic aquifer in Southeast Germany using simulated pumping tests of a complex multiwell geothermal site

This paper reports the results of ground penetrating radar (GPR) methods to monitor the groundwater migration caused by the pumping operation and GPR potential to estimate hydlraulic properties 'of the aquifer. We observed the amplitude andl waveform variations of radar signals and applied the quamtitative analysis of radar signal response to changes in the water level. Residual processing showed the groundwater level change very clearly and gave the information on drawdown. Common midpoint (CMP) and velocity analysis indicated the depth of the water table. Combining hydrogeologic data with quantitative information yielded by GPR data, hydraulic properties of an unconfined aquifer could be estimated by assuming a hydraulic model. It gave results similar to a previous study by other techniques. It was concluded that GPR can be successfully employed to monitor groundwater migration and to estimate hydraulic properties of an unconfined aquifer.

The German Continental Deep Drilling Program comprising a pilot borehole down to 4000 m and a main borehole down to 9101 m in southeast Germany (KTB) is continuing to provide a unique opportunity for the identification of important factors and processes in deep‐seated fluid and energy transfer. In situ stress conditions significantly impact flow, transport and exchange characteristics of fracture networks that dominate the permeability of crystalline reservoir rocks. In this paper, several scales of information are combined to present a fully three‐dimensional hydraulic finite element model of the principal KTB fault zones, and linked to a geomechanical model describing the alteration of the hydraulic parameters with stress changes caused by fluid extraction. The concept of geomechanical facies is introduced to define and characterize architectural elements in the subsurface system. Evaluation of a long‐term pump test in the KTB pilot hole, June 2002–July 2003, coupled with a geomechanical model gives an insight into some of the elastic and nonelastic processes controlling hydraulic transport in the basement rocks. Trends in the decline of the permeability and the degree of storage in the system could only partially be explained by elastic processes, clearly indicating the importance of nonelastic processes. A number of inelastic processes are suggested as areas for further research.

Fractured aquifers are widely distributed on the earth’s surface and are frequently encountered in many underground projects, such as groundwater resource evaluation, contaminants remediation, and geothermal energy exploration. Hydraulic properties such as fracture locations, fracture permeability, and hydraulic connectivity, generally play essential roles in these projects, which dominate the fluid flow, solute migration, and heat transport processes in fractured aquifers. Compared to porous media, these processes in fractured aquifers are more complicated due to the complex fracture geometry, highly hydraulic contrast between fractures and rock matrix, and complex hydraulic connectivity. Characterizing hydraulic properties of fractured aquifers is therefore quite challenging. Over the past decades, numerous studies have been devoted to the development of the relevant theory, testing techniques, modeling methods, and characterization methods at laboratory and field scales. In this thesis, the purpose is to characterize the hydraulic properties of fractured aquifers at field scales by using slug test-based hydraulic tomography (HTs) and thermal tracer tomography (TT). These two tomographic methods are firstly modified considering the features of the fractured aquifer, validated in synthetic cases, and further applied in a fractured rock experimental site located at Göttingen, Germany. 
\nFor the HTs method, wellbore effects including inertial effects and wellbore storage can have considerable impacts on slug test responses, especially in deep wells or highly permeable fractured aquifers. To take into account wellbore effects and even the possible skin effects, a three-dimensional forward slug test model (3DHIM) was first developed, considering the inertial effects in a heterogeneous aquifer. Groundwater flow in the wellbore is described by the Navier-Stokes equation and coupled with the Darcian flow in the heterogeneous aquifer by using some specific boundary conditions on the screen interface. To trace the water level movement in the wellbore, a moving boundary defined by the arbitrary Lagrangian-Eulerian method is assigned. After verified by some analytical methods, the proposed slug test model is applied to simulate a series of multilevel cross-well slug tests in a highly heterogeneous aquifer analogue to investigate the influence of wellbore effects on slug test responses. Results indicate that the influence of wellbore effects on hydraulic travel time can be linearly related to the water column height, and the influence on the head attenuation in the observation well is not obvious. 
\nTo accurately characterize the hydraulic parameters, the influence of wellbore effects on slug test responses needs to be removed, otherwise, estimation errors will be introduced. Therefore, two correction methods with respect to the hydraulic travel time and head attenuation in the test well are then proposed. Hydraulic travel time delay caused by wellbore effects is assumed to be linear with the water column length, and head attenuation in test well caused by wellbore effects can be analytically derived by the measured water level of test well. These methods are then verified by successfully reconstructing the hydraulic parameters (i.e., hydraulic diffusivity and specific storage) of an aquifer analogue using the travel time inversion and attenuation inversion.
\nThermal tracer tomography equipped with distributed temperature sensing (DTS) has been shown to improve the accuracy and resolution in characterizing hydraulic properties of porous media. To apply this method in fractured aquifers, some modifications to the TT inversion framework are made considering the hydraulic properties of fractured aquifers. Considering the spatially sparse temperature response induced by the complex fracture geometry and highly hydraulic contrast between fractures and rock matrix, a regularization term and an irregular triangular mesh are introduced. Regarding the possible annular wall flow at an observation well, a specific well zone in the inversion model is assigned to eliminate the distortion of thermal travel times. The performance of the modified TT inversion framework in characterizing hydraulic properties of fractured aquifers was firstly tested through numerical experiments. Features of fractured aquifers, such as different hydraulic connectivity patterns, highly hydraulic contrast between fracture and rock matrix, and some practical issues were investigated. Inversion results indicate that the TT method can efficiently identify directly connected or interconnected fractures, even with the presence of some practical issues. 
\nThe HTs and TT methods were finally applied at the fractured rock experimental site to investigate the hydraulic properties. A total of 96 cross-well slug tests and 96 cross-well thermal tracer tests were conducted. Using the HTs and TT inversion methods, both results revealed three connected fractures at depths about 19 m, 28 m, and 35 m. By combining the results revealed by the two tomographic methods, the uncertainty and non-uniqueness issues of the single inversion method are reduced. By comparing these two tomographic methods, results indicate that the TT method can provide a more accurate and higher-resolution characterization of high-conductive fractures due to the DTS device, and the HTs method based on the fast hydraulic diffusion process can offer more hydraulic information about the medium-k fractures and rock matrix. Both of the proposed inversion frameworks are proved to be efficient and robust and show broad application prospects in the hydraulic characterization of fractured aquifers.

Development of an efficient data-driven method to estimate the hydraulic properties of aquifers from groundwater level fluctuation pattern features

Global curve-fitting method (GCFM) is regarded as an effective approach in hydrogeological parameter estimation, as it integrates and uses pumping data and water recovery data of a transient pumping test for parameter estimation. The impacts of pumping duration on hydrogeological parameter estimation by GCFM were investigated in the present study using 2 in situ pumping tests and 24 simulated transient pumping tests. Empirical formulas for determining the optimal pumping duration were derived. The study results suggest that pumping duration will have impacts on the accuracy of hydrogeological parameter estimation. When pumping duration is longer than a certain period, relative errors of hydrogeological parameter estimation keep relatively stable within an acceptable limit. Therefore, it is unnecessary to continue the pumping for a very long time after the groundwater level has become stable. When the change rate of drawdown over time (γ) in an observation well located within a distance of 100 m to the pumping well reaches 0.134, the pumping can be stopped. If there are more than one observation wells in a pumping test, the smallest γ value should be selected to determine the optimal pumping duration. This research is meaningful in the instruction of pumping tests, and will reduce test costs greatly.

Temporal changes of hydraulic properties of overburden aquifer induced by longwall mining in Ningtiaota coalfield, northwest China

Aquifer properties data from 2100 pumping tests carried out in the Chalk aquifer have been collated as part of a joint British Geological Survey/Environment Agency project. The dataset is highly biased: most pumping tests have been undertaken in valley areas where the yield of the Chalk is highest. Transmissivity values from measured sites give the appearance of log-normality, but are not truly log-normal. The median of available data is 540 m 2 /d and the 25th and 75th percentiles 190 m 2 /d and 1500 m 2 /d respectively. Estimates of storage coefficient from unconfined tests have a median of 0.008 and from confined tests, 0.0006. The data indicate several trends and relationships in Chalk aquifer properties. Transmissivity is highest in the harder Chalk of Yorkshire and Lincolnshire (median 1800 m 2 /d). Throughout much of the Chalk aquifer a direct relation is observed between transmissivity and storage coefficient, reflecting the importance of fractures in governing both storage and transmissivity. Pumping tests undertaken in unconfined conditions give consistently higher measurements of transmissivity than in confined areas, probably as a result of increased dissolution enhancement of fractures in unconfined areas. At a catchment scale the data illustrate a relation between transmissivity and winter flowing streams.

Some analytical solutions for sensitivity of well tests to variations in storativity and transmissivity

Hydraulic connections between aquifers is usually studied through hydrochemical analysis or by pumping tests. However, hydrochemical analyses are usually conducted in areas of variable lithology. In addition, the hydrogeological data obtained by drilling and pumping tests are typically insufficient to get 3D distributions of hydraulic head. In this paper, the time-lapse transient electromagnetic method (TEM) is used to image groundwater migration between aquifers in Inner Mongolia, China. First, 1D geophysical models of aquifers are generalized according to the hydrogeological conditions of the region, and the feasibility of detecting the multiple aquifers by TEM is analyzed and discussed. Then, the 2D models of aquifers pre- and post- pumping test are established based on the distribution of groundwater in the aquifers, and the variation law of induced electromotive force measured on the surface is analyzed. The simulation results show that significant time-lapse electromagnetic anomalies can be observed between pre- and post- pumping test and the variation in the induced electromotive force reaches a distinguishable level between 0.7 ms and 100 ms due to the vertical change in the aquifer properties. Furthermore, the electromagnetic variation generated by hydraulic connection between aquifers is greater than 30% within the range of 3/4 of the transmitting loop. Finally, a successful case history to map hydraulic connections between aquifers is conducted using a time-lapse TEM pre- and post- a pumping experiment. This simulation and field experiment shows that time-lapse TEM could characterize and monitor the groundwater migration more effectively than pump tests or hydrogeochemical methods alone.

A lakebank filtration assessment was carried out on the shallow groundwater surrounding the ISTN lake to evaluate of the shallow groundwater resources in the area. The objective of this research is to describe the shallow groundwater characteristics based on aquifer properties, pH, TDS and microbiological analysis. This research was conducted by making boreholes and observation holes at the bank of the ISTN area for 3 points in a single line perpendicular to the Lakebank together with 3 points in a single line perpendicular to the canalbank for doing the experiments and taking samples for pH, TDS, and microbiology analysis. Based on aquifer properties using boring and pumping test results, the aquifer layer with a thikness around 4 m show the normal storage coefficients between 0.00026 and 0.0316. From the pH, TDS, and microbiological analysis for sampling taken from boring 2.1, 2.2., and 2.3 with the distance around 10, 20, 30 m from the lake boundary were found in range of fresh water with zero patogent microbial population but the pH of some samples was lower than the pH of drinking water requirement in which that should be improved by using simple treatment before consumption.

Investigation of hydraulic properties in fractured aquifers using cross-well travel-time based thermal tracer tomography: Numerical and field experiments

Study RegionRocky Mountains, United States Study FocusGroundwater-flow modeling requires estimates of hydraulic properties, namely hydraulic conductivity. Hydraulic conductivity values commonly vary over orders of magnitudes however and estimation may require extensive field campaigns applying slug or pumping tests. As an alternative, specific-capacity tests can be used to estimate hydraulic properties for large areas when benchmarked with slug or pumping tests. This study combined aquifer testing with specific capacity data to estimate hydraulic properties in a large alluvial aquifer. New hydrological insights for regionIn the Wet Mountain Valley, Colorado, both slug tests and pumping tests were conducted, resulting in a likely range of hydraulic-conductivity values. Aquifer-testing results were related to specific-capacity data, a more spatially distributed dataset, to expand the area of aquifer characterization beyond the distribution of wells included in aquifer testing. Specific-capacity data were used in two ways: (1) a regression was built between specific-capacity values and transmissivity derived from aquifer testing; and (2) an iterative method was utilized to estimate transmissivity from specific capacity at all sites (including sites lacking aquifer tests). Study results indicate that there is a statistically significant difference between hydraulic-conductivity values estimated using the two approaches and that the regression method yields systematically greater values. These results indicate that careful consideration of methods that use specific capacity for extrapolating aquifer properties is warranted as bias could be introduced depending on the applied methodology.

Evaluating the permeability of a caprock overlying a potential gas storage reservoir is a very critical problem. Pumping water from the reservoir can be used as an evaluation tool in analyzing this problem. Fluid level changes that occur in the aquifer as well as in the caprock can be measured with appropriately placed wells. If the leakage of water from the caprock into the aquifer is considerable, the effects will be apparent in the aquifer. If the leakage is slight, however, it will not be possible to detect it with certainty from observations in the aquifer alone. Fluid level measurements in the caprock must be relied upon, and improved methods of analyzing such effects have been developed which are based on a theoretical analysis of fluid flow through a caprock of infinite thickness. An example applying these methods to field data is discussed. Introduction One of the most critical problems in evaluating an aquifer gas storage project is determining the tightness of the caprock overlying the formation to be used as the storage reservoir. A formation that has previously held oil or gas obviously has a suitable caprock, but an aquifer that contains only water gives no such assurance. A number of aquifer projects in the United States have been troubled by gas leaking out of the intended storage zone, and the ensuing difficulties have led to the development of new evaluation methods. One of these new methods is pump testing wherein water is removed from the aquifer at some controlled rate prior to injection of gas. This fluid withdrawal causes a pressure drop to move out through the aquifer for considerable distances in a matter of days or weeks. Depending on the properties of the caprock, a pressure transient can also pass upward (as well as downward) through the caprock layers adjacent to the aquifer. Thus, if the operator has placed observation wells at appropriate distances from the pumping well, the rapidity with which the pressure transients reach different points in the system can be used to investigate the fluid transport properties of both the aquifer and its caprock. The usefulness of pump testing has been recognized by groundwater hydrologists for many years as a means of determining the potential yield and properties of aquifers used in water supply. They have introduced the term "leaky aquifer" for a system in which an aquifer is overlain (or underlain) by semipermeable caprock layers. The ease with which water leaks into the aquifer during pumping can, of course, be very beneficial in bringing additional water to the pumped well. Hydrologists have therefore devoted considerable attention to this problem. From the gas storage standpoint, however, the tighter the caprock layers that overlie the intended storage reservoir, the better are the conditions for minimizing or eliminating any vertical migration of gas. Thus, after a suitable geologic structure has been found, the emphasis in aquifer storage projects is in determining that the caprock is tight. Attention has recently been focused on the use of pump testing as one approach to solving this problem. paper presents a further development on evaluating the permeability of a slightly leaky caprock when the caprock is of infinite thickness. From the practical standpoint, this means that the caprock layers are thick enough that pressure transients do not reach the outer boundaries of the system during the pumping test. In a subsequent paper, an analysis of the case where the caprock is of finite thickness will be presented. PREVIOUS WORK ON LEAKY AQUIFERS Jacob developed a partial differential equation describing the flow of water in an aquifer of permeability k that is overlain by a leaky caprock of permeability k'. Fig. 1 shows a schematic cross-section of the system under consideration. One of his principle assumptions was that if k »k', the direction of flow is essentially vertical in the caprock and horizontal in the aquifer. Neuman confirmed the validity of Jacob's assumption using a mathematical model. Another assumption was that a permeable source layer overlies the caprock (Fig. 1) and is able to maintain a constant hydraulic head at the upper boundary of the caprock. By neglecting the effects of compressibility within the caprock, Jacob developed a solution for bounded circular aquifer. Later, Hantush and Jacob used the same assumptions to solve the case of an infinite radial aquifer that is pumped at a constant rate. JPT P. 949ˆ

Processing of aquifer test drawdowns to obtain estimates of transmissivity, and sometimes storativity, is an integral part of hydrogeological site investigations. Analysis of these data often relies on an assumption of hydraulic property uniformity. Aquifer properties are often estimated by fitting a Theis curve to measured drawdowns. Where an aquifer exhibits heterogeneity, quantities that are forthcoming from such analyses are assumed to represent spatially-averaged properties. However the nature of the averaging process, and the area over which averaging takes place are unknown. In this study we derive spatial averaging functions that link inferred hydraulic properties to real-world hydraulic properties. These functions employ Fréchet integrals derived by previous investigators that link observation well drawdowns to aquifer properties under an assumption of mild aquifer heterogeneity. It is shown that these hydraulic property spatial averaging functions are complex, especially at times that immediately follow the commencement of pumping. Furthermore, they cross hydraulic property boundaries, so that estimates of storativity can be contaminated by heterogeneities in real-world transmissivity, and vice versa. Because of its greater averaging area at later times, estimates of transmissivity are generally more immune to the effects of local hydraulic property heterogeneity than are those of storativity. They are therefore more reflective of broadscale real-world hydraulic properties, particularly those that prevail in areas that are removed from the immediate vicinity of the pumping and observation wells.

Inversion simulations have been applied to reconstruct the spatial variability of the hydraulic characteristics of fractured aquifers. Discrete fracture network (DFN) models exhibit good performances for complex fractured systems, whereas few hydraulic tomography (HT)-based inversion studies of DFN models provide new insights into data selection. The study objective is to find reasonable data selection strategies that accurately estimate the hydraulic properties in a DFN model. A two-dimensional DFN model was constructed, and nine pumping tests were designed for forward simulations. Three cases were designed considering different conditions during inversion simulations. Two hypotheses were verified. First, the feasibility of using HT to identify the hydraulic properties in DFN models with high heterogeneity was demonstrated. Second, the flow behaves as quasi-steady flow because of the minimal effects of storage, and the inverse model can use steady-state conditions to estimate hydraulic conductivity. This study promotes the use of HT surveys for mapping fracture zones.