Borehole thermal energy storage (BTES) exploits the high volumetric heat capacity of rock-forming minerals and pore water to store large quantities of heat (or cold) on a seasonal basis in the geological environment. The BTES is a volume of rock or sediment accessed via an array of borehole heat exchangers (BHE). Even well-designed BTES arrays will lose a significant quantity of heat to the adjacent and subjacent rocks/sediments and to the surface; both theoretical calculations and empirical observations suggest that seasonal thermal recovery factors in excess of 50% are difficult to obtain. Storage efficiency may be dramatically reduced in cases where (i) natural groundwater advection through the BTES removes stored heat, (ii) extensive free convection cells (thermosiphons) are allowed to form, and (iii) poor BTES design results in a high surface area/volume ratio of the array shape, allowing high conductive heat losses. The most efficient array shape will typically be a cylinder with similar dimensions of diameter and depth, preferably with an insulated top surface. Despite the potential for moderate thermal recovery, the sheer volume of thermal storage that the natural geological environment offers can still make BTES a very attractive strategy for seasonal thermal energy storage within a “smart” district heat network, especially when coupled with more efficient surficial engineered dynamic thermal energy stores (DTES).

The paper describes application of the cross-impact method in the process of selecting locations and technologies used in a geothermal system based on energy accumulated in a dry rock formation, where CO₂ is used as the working medium. The survey is based on the opinion of a group of 20 experts representing different fields of earth and technical sciences. They represent Norway and Poland, where the location of such a system is considered. Based on experts’ experience and opinions, all factors that seem to be significant were classified into the following groups: targets, key factors, results, determiners, motor and brakes, regulating factors, external factors, auxiliary factors, and autonomous factors. Direct influences between variables were indicated. Due to major differences in geological conditions in Poland and Norway, the factor of on- or offshore technology was pointed out as the primary determiner. Among key factors, the system operation’s long-term safety and level of technological readiness were indicated. As a target factor, an interest of local authority was pointed out. Among the variables that are important when selecting locations for this type of system, nine are essential: (1) Formal constraints related to local nature protection areas—this variable is essential in the case of an onshore system; (2) Availability of CO₂ sources; (3) Level of geological recognition; (4) The distance of the CO₂-EGS from a thermal energy user and electricity grid; (5) Existing wells and other infrastructure; (6) Depth of the EGS system; (7) Water depth if offshore, this variable is only important when offshore systems are involved; (8) Physical parameters of reservoir rocks; (9) Reservoir temperature.

The utilization of groundwater heat pump systems is increasing in Norway, which are currently widely employed for heating and cooling applications in the town center of Melhus. The investigations of the Melhus installations are detecting gas exsolution as a possible trigger for precipitation reaction that causes incrustation of iron and manganese compounds in the systems. This paper discusses risks associated with gas exsolution and considers gas exsolution triggers in a typical Norwegian groundwater heat pump (GWHP) system configuration. The concept of the solubility grade line (SGL) is developed and suggested as a tool for optimizing the design. Based on SGL analysis and the intention of avoiding gas exsolution during heat production, an alternative system design in the same aquifer is presented and compared. The analyses show that the traditional system design is predisposed to gas clogging risks and prone to vacuum pressures in parts of the system. The alternative design mediates the risks by adjusting the well and piping configuration and by applying a backpressure technique. The results demonstrate how the groundwater heat pump system design can be customized according to local aquifer conditions to avoid gas exsolution during operation. It is recommended that the presented method of analysis should be utilized in dimensioning of systems and included in the monitoring scheme of the systems.

This paper presents results from video inspections of groundwater wells in Melhus and Elverum, in Norway. The method has identified iron hydroxides, bacterial growth and sand production as causes of clogging in different wells. Video inspection has proven itself a reliable, inexpensive and quick method for such investigations. The videos supply documentation for the building owner about the well condition. A video inspection should be incorporated as a standard part of the tender document and an integrated part of the maintenance routine. Finally, open loop GSHP and ATES system wells should be designed and manufactured with integrated video inspection options. This will ease the fault detection process and reduce maintenance costs of the system through the lifetime of the wells.

The principle of Enhanced Geothermal System (EGS) technology is that water injected at a sufficiently high pressure will lead to the fracturing of naturally impermeable rocks, and as a result, this will create hydraulic communication between wells. In this way, reservoirs not previously considered to be perspective can provide geothermal heat to the surface. Since nearly two decades, CO2 is considered, mostly theoretically, as a working fluid that can potentially provide higher net power output than water in EGS’s installation. In this respect, the possibility of accessing high-temperature heat from the Åre and Tilje formations located on the shelf of the Norwegian Sea was analysed. The estimated temperature at the reservoir depth of 4,500–5,000 m is not less than 165°C. For this, a 3D numerical modelling was performed in order to analyse 10 different scenarios for heat extraction using supercritical CO2 (sCO2) as a working fluid. Results indicate that appropriate matching of the mass flow and temperature of the injected CO2 allows to avoid premature temperature decline in the reservoir. However, as Åre and Tilje formations are built from highly porous and relatively highly permeable rocks, the fluid entering the production well will always be a mixture of CO2 and water. This is advantageous from the point of view that a significant part of the injected CO2 is trapped in the reservoir, while the higher water content in the production well allows a significant temperature drop during fluid extraction to the surface to be avoided.