Influence on thermal response test by groundwater flow in vertical fractures in hard rock

doi:10.1016/S0960-1481(03)00128-9

Renewable Energy

Volume 28, Issue 14, November 2003, Pages 2221-2238

https://doi.org/10.1016/S0960-1481(03)00128-9 Get rights and content

Abstract

In this paper different approaches to groundwater flow and its effect in the vicinity of a borehole ground heat exchanger are discussed. The common assumption that groundwater flow in hard rock may be modelled as a homogeneous flow in a medium with an effective porosity is confronted and models for heat transfer due to groundwater flow in fractures and fracture zones are presented especially from a thermal response test point of view. The results indicate that groundwater flow in fractures even at relatively low specific flow rates may cause significantly enhanced heat transfer, although a continuum approach with the same basic assumptions would suggest otherwise.

Introduction

The influence of groundwater flow on the performance of borehole heat exchangers has been a topic of discussion. Field observations indicate that groundwater influences the borehole performance by increasing the heat transport significantly [1], [2], [3], [4], [5]. Some theoretical studies have been published on the subject. [6], [7] and [3] present models for the influence of regional groundwater flow based on the assumption that the natural groundwater movement is reasonably homogeneously spread over the ground volume. This applies well on homogeneous and porous ground material. Eskilson [6] and Claesson and Hellström [7] use the line source theory for modelling the groundwater effect on a single vertical borehole. They conclude that under normal conditions, the influence of regional groundwater flow is negligible. Chiasson et al. [3] use a two-dimensional finite element groundwater flow and mass/heat transport model and come to the conclusion that it is only in geologic materials with high hydraulic conductivity (sand, gravel) and in rocks with secondary porosities (fractures and solution channels in e.g. karst limestone), that groundwater flow has a significant effect on the borehole performance. Simulations of the effect on thermal response tests give artificially high thermal conductivity values. Witte [5] performed a thermal response test where groundwater flow was induced by pumping in an extraction well located 5 m from the thermal well. Clear indications of enhanced heat transfer due to the induced groundwater flow were observed.

The thermal response test is a method to determine effective ground thermal conductivity in-situ borehole ground heat exchangers and is used in most countries where ground source heat systems are used on a larger scale [1], [3], [8], [9], [10]. The principle of the test is a constant heat pulse injected into the borehole heat exchanger by heating a fluid that is pumped through the heat exchanger pipes, while inlet and outlet temperature is measured and recorded. The test typically takes 50 h to perform.

The influence of single or multiple fractures and fracture zones on the performance of the ground heat exchanger has not been thoroughly studied, and may explain field observations where groundwater effects have resulted in artificially high ground thermal conductivity estimations.

Section snippets

Groundwater flow in fractured rock

Groundwater flow rate is proportional to the hydraulic conductivity, K, and the hydraulic gradient, I, in the ground. The hydraulic gradient is usually of the same order or smaller than the ground surface slope [11]. It is calculated as the change in hydraulic head as we move along the ground surface. Common hydraulic gradients are 0.01–0.001 m/m or less [12].

In fractured crystalline rock, the interconnected fractures are the main passages for groundwater flow, and the solid rock may be

Groundwater flow modelling

Three models for groundwater flow around a ground heat exchanger in fractured rock are discussed. The first model regards the fractured rock volume as a homogeneous medium equal to a porous medium with a certain (small) porosity. The groundwater flow is evenly spread over the rock volume and water flows through the pore openings between the mineral grains. The second model assumes the rock to be completely impermeable, and all groundwater flows through a fracture zone of a certain width and at

Results and discussion

In Fig. 4 the temperature fields around the borehole after 100 h are calculated with the three flow models and the case of a specific flow rate of 10⁻⁶ ms⁻¹. These models are compared with the case of only conductive heat transfer. The bilaterally symmetric temperature pattern around the borehole affected only by conduction is transformed into a considerably cooler borehole, with a laterally symmetric temperature field for the case of a continuum with a specific flow rate. The heat transport

Conclusions

In this paper three different models for estimating the heat transfer effect of groundwater flow have been compared and related to the case of no groundwater flow. The simulations and comparisons result in the following findings:

•
The three flow models cause significantly different temperature field patterns around the borehole and all three cause lower borehole temperatures.
•
A continuum approach gives no effect at specific flow rates below 5·10⁻⁸ ms⁻¹ and small effect of the same magnitude as for

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Citation Excerpt :
In most cases, the geological parameters are fitted to the measured data assuming that the TRT model can fully reproduce the actual thermophysical response. For traditional grouted boreholes with buried U-tube GHEs, one of the important tasks is to determine the effects of flowing groundwater on the subsurface thermal properties to evaluate the model accuracy [86,110]. Therefore, numerical models are chosen for their flexibility and ability to simulate complex environments.
Despite the intensive studies and advances have been made, the penetration of geothermal heat exchangers (GHEs) and ground coupled heat pump (GCHP) systems has been constrained due to high-upfront capital cost. On the other hand, the recent research has revealed the enormous influence that complex geologic and hydraulic systems have on the GCHP efficiency, especially the impact of groundwater. Without this knowledge, their design and operation may lead to lower economic competitiveness or potential systematic degradation. This paper reviews the challenges of integrating groundwater impacts on the performance of GHEs and the strategies through industry standard models. Three principal methodologies consisting of analytical, numerical, and experimental approaches are identified. Multiple topics that have received much debate, such as soil freezing and thawing, are addressed with their latest developments. Furthermore, the environmental impacts associated with GHEs’ thermal response that consider groundwater flow are discussed. It is found that in most situations the presence of flowing groundwater enhances the GHE performance, although potential adverse effects exist when the operation is not monitored and mitigated. However, the natural uncertainties in the geology and the advanced hybrid GCHP implementations needs further exploration to facilitate the on-going groundwater-associated research including borehole thermal storage.
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Thermal properties of rock-soil are of great importance for the design of borehole heat exchangers (BHEs). In the process of testing thermal properties of rock-soil through an in-situ thermal response test (TRT), groundwater seepage directly affects the evaluation of thermal properties in TRTs and leads to a varied arrangement and engineering costs of BHEs. Therefore, comprehensive hydrogeological surveys of groundwater and TRTs to determine geothermal properties are necessary. In this study, we made a preliminary attempt to use hydrogeological surveys and correlated TRTs to investigate the heat exchange efficiency of a single U-tube BHE. The geological conditions were used to predict and analyze the groundwater form, groundwater depth, and flow trajectory in the test area based on the natural electric field frequency selection method. Using a comprehensive dataset, the groundwater velocity was determined with ANSYS. The results showed that the initial temperature of rock-soil was 21.8 °C and that the comprehensive thermal conductivity of rock-soil was 1.942 W/(m·°C). Confined and phreatic aquifers existed in the test area, and the depth of the confined aquifer was approximately 70 m. The groundwater velocity in the test area was considered to be approximately 2–2.5 m/d. The results of this study confirm that TRTs combined with groundwater surveys can improve the test accuracy and consequently provide guidance for engineering practices in ground-coupled heat pump systems.

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