R. Brauchler

Comparison of Early and Late Travel Times of Pressure Pulses Induced by Multilevel Slug Tests

Abstract During a slug test, injection or withdrawal of a small volume of water through an open borehole in an aquifer induces a pressure pulse propagation process in the aquifer. Hydraulic data sets that are collected during this procedure can be manipulated by mathematical inversion to estimate the hydraulic properties near the borehole. In this paper, we report our finite element forward modeling of slug tests in an aquifer with stratigraphical heterogeneities. Our numerical studies, which resulted in contour plots of head perturbation and travel time, show that the early hydraulic travel time contour associated with partially penetrating slug tests in a heterogeneous aquifer could display a spatial distribution that is quite different from the late travel time contour. While the late travel time contour primarily reflects the integrated effect of widely propagated pressure distribution, the early travel times reflect the preferential flow and therefore can be exploited for hydrostratigraphical imaging of the aquifer. We argue that the early portion of hydraulic signals contains information that is complementary to the conventional peak travel time inversion and plays an indispensable role in gaining enhanced characterization of a geological unit’s hydraulic properties. Furthermore, our modeling results demonstrate that early hydraulic travel times are not always associated with the shortest flow path.

Detection of carbon dioxide leakage during injection in deep saline formations by pressure tomography

CO2 injected into storage formations may escape to the overlying permeable layers. Mixed-phase diffusivity, namely the ratio of hydraulic conductivity and specific storage of the phase mixture, declines with increasing CO2 saturation. Thus, it can be an indicator of CO2 leakage. In this study, we perform interference brine or CO2 injection tests in a synthetic model, including a storage reservoir, an above aquifer, and a caprock. Pressure transients derived from an observation well are utilized for a travel-time based inversion technique. Variations of diffusivity are resolved by inverting early travel time diagnostics, providing an insight of plume development. Results demonstrate that the evolution of CO2 leakage in the above aquifer can be inferred by interpreting and comparing the pressure observations, travel times, and diffusivity tomograms from different times. The extent of the plume in reservoir and upper aquifer are reconstructed by clustering the time-lapse data sets of the mixed-phase diffusivity, as the diffusivity cannot be exactly reproduced by the inversion. Furthermore, this approach can be used to address different leaky cases, especially for leakage occurring during the injection.

A field assessment of site-specific correlations between hydraulic and geophysical parameters

In this study the potential of combining high-resolution hydraulic tomographic and geophysical tomographic measurements to define site-specific relationships between geophysical and hydraulic parameters was investigated. We exploit the high-spatial resolution of hydraulic and geophysical tomographic images to define a representative and reliable site-specific relationship, if it exists, over an area, where geophysical and hydraulic tests are performed. The parameters involved in this study were: seismic P-wave velocity derived from seismic tomography; resistivity and electrical conductivity derived from electrical tomography (ERT); diffusivity, hydraulic conductivity and specific storage derived from hydraulic tomography. We derived a site-specific correlation function between the parameters P-wave velocity and diffusivity that shows the highest correlation of all hydraulic and geophysical parameter combinations. The transformation of the P-wave velocity field into a diffusivity field using the estimated site-specific correlation function allowed us to increase the significance of hydraulic tomographic as well as seismic tomographic measurements with respect to the spatial diffusivity distribution in the near subsurface.

An Aquifer Analogue Study of High Resolution Aquifer Characterization Based on Hydraulic Tomography

Hydraulic tomography is an efficient method for characterizing the subsurface heterogeneity of hydraulic parameters. In this paper the authors introduce a hydraulic tomographic travel time based inversion procedure showing the advantageous of the inversion of data subsets. The results demonstrate that the proposed inversion scheme is competent for the reconstruction of individual architectural elements as well as their hydraulic properties with a higher resolution in contrast to conventional hydraulic and geological investigation methods.

Identification of fracture zones by a travel time-based diffusivity approach

Recently the irnportance of understanding fractured aquifers is increasing particular with the relevance to the question of water resources development, groundwater contamination and the search of suitable nuclear waste deposits. For the analysis of engineering, geotechnical and hydrogeological questions is the knowledge about the spatial position of fractures of particular irnportance. Conventional aquifer investigation methods like pumping or slug tests lead to integral information averaged over a large volume. This kind of information is insufficient to develop groundwater models requiring detailed information about the spatial distribution of fractures. To circumvent this problem new investigation methods have to be developed.