Influence on thermal response test by groundwater flow in vertical fractures in hard rock
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:
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The three flow models cause significantly different temperature field patterns around the borehole and all three cause lower borehole temperatures.
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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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