| dc.description.abstracteng | 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.
For 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.
To 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.
Thermal 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.
The 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. | de |