| dc.description.abstract | In recent years, Aquifer Thermal Energy Storage (ATES) and other subsurface energy systems have gained a lot of popularity as a way to conserve energy and meet CO2-reduction criteria (Lee 2010). In its most basic form, an ATES system uses groundwater for the seasonal storage of warm and cold water that is produced during summer and winter respectively. Most issues with reduced productivity of ATES wells are caused by clogging of the screen slots by suspended particles or chemical and organic precipitates which often requires costly remediation procedures. The research described in this report is part of the FOME-BES project that aims to gain insight in the subsurface energy balance of ATES systems by means of Glass-fiber DTS temperature monitoring. The project specifically focusses on exploring the possibilities of using both passive and active glass-fiber DTS technology for monitoring fluid flow and heat transport in subsurface thermal energy storage systems. The objective is to present a theoretical and empirical foundation that contributes to the development of an algorithm that could be used to monitor well screen quality and predict the maintenance requirements of ATES systems. This would allow for timely interference when a problem is detected, while improving the energetic efficiency and reducing maintenance costs to a minimum.
The format of the research project is roughly reflected in the thesis outline and is structured as follows:
The ‘Literature review and background’ chapter provides the necessary background information on the different types of subsurface energy storage systems and issues associated with ATES and their impact on the energetic and economic efficiency of the system. Additionally, it presents the theory and application of glass-fiber DTS and describes the latest developments in both passive and active DTS monitoring applied in hydrological systems. The final part of this section includes a brief review on the theory of heat transfer and fluid transport applied to a well geometry.
In the ‘Methodology and materials’ chapter, the materials, design and execution of the different experiments are described in detail. Prior to the field survey, a laboratory experiment was conducted to quantify and compare the thermal response times and noise characteristics of two types of fiber-optic cable that was used as a reference during the field monitoring and data processing stages. The monitoring setup used in the filed survey comprises two fiber-optic cables, one passive cable registering the ambient water temperature and one active cable containing a resistive heating wire that is used to generate a temperature offset that is dependent on the flow velocity of the surrounding water. In addition to the DTS monitoring experiments, a propeller flow measurement and multi-tool well test were conducted that served as a reference for the passive and active DTS monitoring experiments.
Before the monitoring data was used for analysis, the raw temperature data was processed and filtered to eliminate signal noise. The findings of the individual experiments are presented in the ‘Results Chapter’. The laboratory experiment offered great insight in the recording characteristics of different types of fiber-optic cables and illustrated the effect of a cables composition on the response characteristics and quality of the temperature data. The passive monitoring experiment focused on the detection of naturally occurring temperature fluctuations which ultimately resulted in the construction of a number of flow profiles of the monitored well. The flow profiles match the reference measurement for the upper section of the well but show a significant reduction in accuracy and resolution for the lower sections due to low velocity conditions and the presence of residual signal noise. The active temperature profiles do show an increase in temperature offset with depth, indicating a reduction in flow velocity, but are of insufficient quality to allow for the derivation of flow data.
The main conclusion brought forward in the final ‘Conclusions’ chapter is that, although the monitoring setup (in its current form) might not provide the resolution necessary for monitoring well screen quality and predicting the maintenance requirements, the passive monitoring technique has proven to be suitable for deriving flow data of medium and high velocity environments and might eventually be used for monitoring well screen quality when long term monitoring data is available and signal processing and analysis methods have been improved. The active DTS experiment has unfortunately not produced any conclusive results. | |
