Differentiated influence of the double porosity of the chalk on solute and heat transport

Abstract

The chalk porosity plays a decisive role in the transport of solutes and heat in saturated chalk. From a geological point of view, there are at least two types of porosities: the porosity of pores corresponding to the micro-spaces between the fossil coccoliths that form the chalk matrix, and the porosity due to the micro- and macro-fractures (i.e., secondary porosity). For groundwater flow, the fracture porosity is a determining factor at the macroscopic scale. The multi-scale heterogeneity of the porous/fractured chalk is inducing different effects on solute and heat transport. For solute transport considered at the macroscopic scale, tracer tests have shown that the ‘effective transport porosity’ is substantially lower than the ‘effective drainable porosity’. Moreover, breakthrough curves of tracer tests are showing an important influence of diffusion in a large portion of ‘immobile water’ (‘matrix diffusion’) together with quick preferential advection through the fractures. For heat transport, the matrix diffusion in the ‘immobile water’ of the chalk is hard to distinguish from conduction within the saturated chalk.

Heat Tracing in a Fractured Aquifer with Injection of Hot and Cold Water

“…The fractures are usually embedded in a rock matrix, which can be considered impervious or as a porous medium with a very low permeability, where diffusive processes can dominate (Bear and Verruijt 1987 ; Maloszewski and Zuber 1993 ; Carrera et al 1998 ; Kang et al 2015 ). Robust decisions for sustainable management of fractured aquifers require both a realistic assessment of preferential flow paths in the fractures (Berkowitz et al 1988 ; Tsang et al 1988 ; Bear et al 1993 ; Dassargues 2018 ), as well as a quantification of matrix diffusion and interactions between the rock matrix and the fractures (Kang et al 2015 ; Hyman et al 2019 ; Hoffmann et al 2020 ; Hoffmann et al 2021 ). For example, several authors interpret asymmetric tailing of observed tracer breakthrough curves in fractured rocks with matrix diffusion (Neretnieks et al 1982 ; Maloszewski and Zuber 1993 ; Meigs and Beauheim 2001 ; Bodin et al 2003 ; Reimus et al 2003 ; Hoffmann et al 2021 ).…”

“…Robust decisions for sustainable management of fractured aquifers require both a realistic assessment of preferential flow paths in the fractures (Berkowitz et al 1988 ; Tsang et al 1988 ; Bear et al 1993 ; Dassargues 2018 ), as well as a quantification of matrix diffusion and interactions between the rock matrix and the fractures (Kang et al 2015 ; Hyman et al 2019 ; Hoffmann et al 2020 ; Hoffmann et al 2021 ). For example, several authors interpret asymmetric tailing of observed tracer breakthrough curves in fractured rocks with matrix diffusion (Neretnieks et al 1982 ; Maloszewski and Zuber 1993 ; Meigs and Beauheim 2001 ; Bodin et al 2003 ; Reimus et al 2003 ; Hoffmann et al 2021 ). These interpretations show first that assuming the rock matrix as impervious is not very realistic at large time scales.…”

“…Consequently, solutions based only on the advection–dispersion equation can have limited prediction capabilities for transport in fractured rocks (e.g., Bodin et al 2003 ). Recent studies using stronger diffusing tracers such as dissolved gases (Hoffmann et al 2020 ) or heat (Read et al 2013 ; Klepikova et al 2016a ; de la Bernardie et al 2018 ; de la Bernardie et al 2019 ; Hoffmann et al 2021 ) have shown a potential to better constrain fractured aquifer conceptualizations with matrix diffusion information. Although reliable temperature tracer experiments have been performed in porous alluvial aquifers (Wagner et al 2014 ; Wildemeersch et al 2014 ; Klepikova et al 2016b ; Sarris et al 2018 ) and fractured porous media (Read et al 2013 ; Klepikova et al 2016a ; de la Bernardie et al 2018 ; de la Bernardie et al 2019 ; Hoffmann et al 2021 ), temperature information is still rarely analyzed by hydrogeologists, because dilution is high (Kurylyk and Irvine 2019 ) and the signal may be difficult to detect.…”

“…Recent studies using stronger diffusing tracers such as dissolved gases (Hoffmann et al 2020 ) or heat (Read et al 2013 ; Klepikova et al 2016a ; de la Bernardie et al 2018 ; de la Bernardie et al 2019 ; Hoffmann et al 2021 ) have shown a potential to better constrain fractured aquifer conceptualizations with matrix diffusion information. Although reliable temperature tracer experiments have been performed in porous alluvial aquifers (Wagner et al 2014 ; Wildemeersch et al 2014 ; Klepikova et al 2016b ; Sarris et al 2018 ) and fractured porous media (Read et al 2013 ; Klepikova et al 2016a ; de la Bernardie et al 2018 ; de la Bernardie et al 2019 ; Hoffmann et al 2021 ), temperature information is still rarely analyzed by hydrogeologists, because dilution is high (Kurylyk and Irvine 2019 ) and the signal may be difficult to detect. Nevertheless, to understand and quantify the local matrix diffusion or mobile–immobile water interactions, the use of temperature information collected with high‐resolution sensors is very promising (Anderson 2005 ; Pehme et al 2014 ; Irvine et al 2015 ; Kurylyk and Irvine 2019 ).…”

“…Nevertheless, to understand and quantify the local matrix diffusion or mobile–immobile water interactions, the use of temperature information collected with high‐resolution sensors is very promising (Anderson 2005 ; Pehme et al 2014 ; Irvine et al 2015 ; Kurylyk and Irvine 2019 ). Temperature tracing can also enhance the characterization of the local fracture geometry (de la Bernardie et al 2018 ) and help to interpret retardation on transport transfer times due to matrix diffusion (Hoffmann et al 2020 ; Hoffmann et al 2021 ).…”