Experimental characterization and performance evaluation of geothermal grouting materials subjected to heating–cooling cycles

Abstract

In recent years, the increasing rise in environmental awareness among energy consumers has led to an increasing use of renewable energies such as the geothermal energy. An important role in the efficient exploitation of the geothermal resource is played by the grouting material placed in the borehole between the pipes and the ground. Actually, the use of proper grouts is essential to provide an effective heat transfer between the ground and the heat carrier fluid in the pipes, and also to comply with the mechanical and environmental demands. However, when it comes to the construction of the GHP installations, the grout is especially required to be easy to work with (workable) and for this reason more water than required is sometimes added. In order to assess the suitability of grouting materials with significant water/solid ratios, the thermal conductivity, mechanical strength and permeability of five different grouts and grout-pipe specimens were measured for their laboratory characterization. In addition, the grouts were subjected to heating and cooling cycles to evaluate their durability with time in terms of the potential degradation of the materials and the loss of quality of the grout-pipe interface. According to the results obtained, the grouts here tested are appropriate for most of the geothermal heat pump installations, especially for those with low to medium ground thermal properties.

Assessment and Minimization of Potential Environmental Impacts of Ground Source Heat Pump (GSHP) Systems

“…Tests on cement-sand grout samples conducted by Allan and Philippacopoulos (1998, [21]) reported hydraulic conductivities in the order of 10 −12 m/s, while higher values in the order of 10 −10 m/s were measured for grout-pipe specimens where the interface between the pipe and the grout is a weak point for the hydraulic sealing [22,23]. Geothermal grouts are thus less permeable than most of clayey and silty aquicludes, as shown in Figure 3, but increases in hydraulic conductivity are possible in the case of grouts injected with a very high water/cement ratio subjected to extreme thermal (freeze-thaw) or hydraulic (wet-dry) alternated stresses [21,23]. The results of these studies highlight that geothermal grouts available in the market can ensure suitable protection against aquifer cross-contaminations, but they need to be injected properly, in particular by avoiding the use of excess water for the sake of workability since this would impair the hydraulic sealing properties of the injected mixture.…”

“…Preferential flow through BHEs [20][21][22][23][24] Use specifically developed, well-mixed and properly installed geothermal grouts. Further modelling and field studies needed for preferential flow.…”

“…The drilling of BHEs is considered as a possible trigger of cross-contamination between aquifers through the formation of preferential migration pathways in defective grouts. A few studies addressed this issue from a theoretical point of view, including Bonte et al (2015, [20]), while the theme of the quality and durability of borehole filling materials has been addressed by several studies, including those reported in the works of [21][22][23][24].…”

Doped bentonitic grouts for implementing performances of low-enthalpy geothermal systems

“…In this regard, some authors have already investigated the possibility to improve the thermal exchange by enhancing the thermal conductivity of the sealing grout in the boreholes by adding components (e.g., quartz, sands, carbon-based components, etc.) at high thermal conductivity in the mixture (Delaleux et al 2012;Lee et al 2012;Erol and Francois 2014;Indacoechea-Vega et al 2015). In these instances, however, the mechanical properties of the obtained grouts were not verified or the experimentally determined thermal conductivity is too high (> 6 W/m K) and far from common values observable on Earth (average thermal conductivity of rocks in the subsurface is 3.2 W/m K; Eppelbaum et al 2014 and references therein).…”

“…Beside the chemical-physical characteristics of rocks in the subsurface (which anyhow remain a paramount factor), the total efficiency of the installation is determined by the following: (1) efficiency of the Ground Source Heat Pump, which allows the transfer of heat from the subsoil upward to the surface; (2) efficiency of thermal exchange between probes installed in boreholes and rocks in the subsurface; (3) chemical-physical characteristics of the bentonitic grout to seal probes into boreholes; (4) efficiency of the heat distribution system into the environment at the surface. During the past years, technological development has been pointing with increasing interest towards new materials and solutions able to enhance the performances of one or more of these components, with particular reference to the composition of the grout used as sealing material in the boreholes (e.g., Smith and Perry 1999;Alrtimi et al 2009;Wang et al 2011;Delaleux et al 2012;Desmedt et al 2012;Lee et al 2012;Borinaga-Treviño et al 2013;Kim et al 2013;Erol and Francois 2014;Indacoechea-Vega et al 2015;Blazquez et al 2017). Some innovative solutions have also found applications in other related fields of interest of bentonitic grouts, such as thermal storage (e.g., Sari 2016) or sealing of radioactive wastes (Tang et al 2008;Jobmann and Buntebarth 2009).…”