Geothermal Boreholes in Poland—Overview of the Current State of Knowledge

: Geothermal energy can be useful after extraction from geothermal wells, borehole heat exchangers and/or natural sources. Types of geothermal boreholes are geothermal wells (for geothermal water production and injection) and borehole heat exchangers (for heat exchange with the ground without mass transfer). The purpose of geothermal production wells is to harvest the geothermal water present in the aquifer. They often involve a pumping chamber. Geothermal injection wells are used for injecting back the produced geothermal water into the aquifer, having harvested the energy contained within. The paper presents the parameters of geothermal boreholes in Poland (geothermal wells and borehole heat exchangers). The deﬁnitions of geothermal boreholes, geothermal wells and borehole heat exchangers were ordered. The dates of construction, depth, purposes, spatial orientation, materials used in the construction of geothermal boreholes for casing pipes, method of water production and type of closure for the boreholes are presented. Additionally, production boreholes are presented along with their efﬁciency and the temperature of produced water measured at the head. Borehole heat exchangers of different designs are presented in the paper. Only 19 boreholes were created at the Laboratory of Geoenergetics at the Faculty of Drilling, Oil and Gas, AGH University of Science and Technology in Krakow; however, it is a globally unique collection of borehole heat exchangers, each of which has a different design for identical geological conditions: heat exchanger pipe conﬁguration, seal/ﬁlling and shank spacing are variable. Using these boreholes, the operating parameters for different designs are tested. The laboratory system is also used to provide heat and cold for two university buildings. Two coefﬁcients, which separately characterize geothermal boreholes (wells and borehole heat exchangers) are described in the paper.


Introduction
Renewable energy sources are increasingly used around the world. These include geothermal energy, which is exploited by geothermal boreholes. Two types of boreholes are used: geothermal wells (production and injection) and borehole heat exchangers (BHE).
A geothermal well is a borehole that allows production or injection of geothermal waters from both deep and shallow aquifers. The deep layers are used for production and injection of geothermal waters, whereas the shallow layers are mostly used as lowtemperature waters for geothermal heat pumps.
Geothermal boreholes may be vertical, inclined or directional. They can also (earlier) fulfill an exploratory role. As a rule, the construction of the first geothermal borehole must take into account detailed specialist tests, including geophysical and hydrogeological research of the aquifer with geothermal water or thermal response tests in the case of BHEs [1]. In addition, the heat accumulated in the greater depths of the rock mass (mostly between 3000 and 6000 m [2]) can be exploited using HDR and EGS systems [3,4]. Hydraulic frac-

Materials and Methods
Effective exploration and sharing of geothermal water resources is possible across modern technology for drilling. At present, the rotary method with the right circulation of the mud is used [1].

Materials Used in Geothermal Wells
The casing pipes are usually made of steel, and therefore are susceptible to corrosion. While selecting the type of steel used for casing pipes, one should avoid carbon steel and low-alloy steel, because they are highly vulnerable to corrosion. In many cases non-alloyed steel with high strength, such as J-55 (Pyrzyce  and N-80 (LidzbarkWarmiński GT-1), is used [35].
In recent years, lining the inside of steel pipes with plastic has found wide application. Fiberglass pipes are also used. An example of steel pipes with an inner coating are Pyrzyce GT-2 and Pyrzyce GT-4 boreholes [36], as well as Toruń TG-1 where fiberglass pipes were used in the construction [37]. Such applications are used in order to reduce unfavorable processes in geothermal wells, such as corrosion [35,38].
There are many methods limiting processes and results of corrosion, as well as precipitation of secondary mineral substances in geothermal installations. They aim to recover production parameters in geothermal systems. These methods include: application of inhibitors, soft acidizing treatments and processes using non-organic and organic acid solutions [39].
In geothermal wells, Class G cement slurries with various additives and admixtures are most commonly used [40][41][42]. Most often silica flour is added in a quantity varying from 20 to even 100% BWOC (by weight of cement) together with additives and admixtures depending on the need to achieve the appropriate parameters of fresh cement slurry. Additives and admixtures include bentonite, carboxymethylcellulose or lime [40,41]. Another type of cement used in geothermal drilling is Class A cement [43]. The literature also includes the use of Class F and Class J cements in geothermal systems [41].

List of Geothermal Wells with Theirparameters in Poland
This subsection presents a list of geothermal borehole parameters in Poland (Table 1). Presented parameters such as: borehole name, year of construction, depth, production or injection rate, aquifer opening, geothermal water temperature at the head, borehole type, borehole purpose, spatial orientation, construction material, borehole bottom.
Energies 2021, 14, x FOR PEER REVIEW 12 of 26 Table 1 does not include boreholes in: Dębica GT-1, Lądek Zdrój LZT-1, Sękowa GT-1 and others due to the lack of data in available publications.
Geothermal waters in Poland are most often used for recreational, heating and health purposes. Geothermal waters are used in heating plants in Stargard, Pyrzyce, Uniejów, Mszczonów, Poddębice and in Podhale [87]. Often, boreholes in Poland, as well as in other countries, are located in poorly urbanized areas. The distribution of geothermal installations in Poland is shown in Figure 1. Indicators of the ratio between the depth of geothermal boreholes and their power were proposed. The depth/efficiency ratio indicator was first proposed, according to the formula: where ̇ is the flowrate of possible geothermal water and is the depth of the borehole. Qualification of boreholes to be included in the indicator is a debatable issue (1). The issue of selection is difficult because irrespective of the end use of the borehole (whether exploitation or injection), pumping tests are performed to determine the serviceability of the boreholes. Hence, the depth/efficiency ratio can be defined for different borehole configurations. Among the geothermal boreholes in Poland are: exploited production and injection boreholes, negative boreholes (boreholes planned and drilled in order to exploit geothermal water, in which the water was not found), boreholes not in operation. Efficiency is also debatable due to differences in values between multiple sources (cf. Table 1). Geothermal boreholes have approved resources of efficiency (productivity) and absorbency. Taking into account all geothermal boreholes for which data are available, Indicators of the ratio between the depth of geothermal boreholes and their power were proposed. The depth/efficiency ratio indicator was first proposed, according to the formula: where . V is the flowrate of possible geothermal water and H is the depth of the borehole. Qualification of boreholes to be included in the indicator is a debatable issue (1). The issue of selection is difficult because irrespective of the end use of the borehole (whether exploitation or injection), pumping tests are performed to determine the serviceability of the boreholes. Hence, the depth/efficiency ratio can be defined for different borehole configurations. Among the geothermal boreholes in Poland are: exploited production and injection boreholes, negative boreholes (boreholes planned and drilled in order to exploit geothermal water, in which the water was not found), boreholes not in operation. Efficiency is also debatable due to differences in values between multiple sources (cf. Table 1 value was included in the calculation). Approved productivity means water resources determined by research conducted during pumping tests.
If the number of negative boreholes increases, the value of the indicator decreases. Considering, for example, the best geothermal borehole operating in Poland, its indicator equals 0.154 m 3 /h/m. Another issue is the depth of the boreholes, which previously served as reconnaissance boreholes. For example, the Bańska IG-1 well has a depth of 5261 m, while the aquifer which is being exploited occurs at a much smaller depth. The difference between the depth of the geothermal borehole and the depth of the bottom of the aquifer varies for each borehole. The proposed indicator illustrates the "unitary" effort incurred for drilling for geothermal energy (from geothermal waters) in relation to the flow rate of water available for exploitation.
The second indicator proposed is the ratio of depth/theoretical power, according to the formula: In which P is the theoretical heating power, assuming water cooling from the well head temperature to 0 • C according to: where: T wh /2 is the average temperature of geothermal water. The assumed cooling of water to 0 • C was adopted as a simple rule, easy to calculate and compare. By referencing the final temperature to 0 • C, it is not necessary to know additional parameters, such as the average annual temperature of the atmospheric air, which is used to calculate the available geothermal resources (theoretical resources) under a given surface area [88]. Cooling water down to 0 • C is not feasible and technically impossible. Nevertheless, this value is quite universal and enables comparison of the energy resources (heating power) between wells. This is possible for both the high-temperature waters and the shallow water wells and natural hot springs. However, when providing such resources, it is good to also specify the water temperature, which, apart from the heating power, also indicates the quality of the obtained energy. Material parameters (density and specific heat) for calculating the amount of energy were assumed for the average temperature T wh /2, which is a simplification of the method and facilitates the comparison of geothermal wells.
Similar observations regarding depth and heating power relate to this indicator. Its value with the same assumptions as for the N . V equals N P = 3523 W/m. Respectively, for the best Polish borehole (Bańska PGP-1), this indicator equals 16,589 W/m. Figure 2 depicts the heads of selected geothermal boreholes in Poland. the value of the indicator is ̇ = 0.04879 m 3 /h/m. The indicator takes into account all efficiency values, including injection boreholes (e.g., the Biały Dunajec PGP-2 has the approved productivity of 175 m 3 /h and an absorbance of 400 m 3 /h, so only the first value was included in the calculation). Approved productivity means water resources determined by research conducted during pumping tests.
If the number of negative boreholes increases, the value of the indicator decreases. Considering, for example, the best geothermal borehole operating in Poland, its indicator equals 0.154 m 3 /h/m. Another issue is the depth of the boreholes, which previously served as reconnaissance boreholes. For example, the Bańska IG-1 well has a depth of 5261 m, while the aquifer which is being exploited occurs at a much smaller depth. The difference between the depth of the geothermal borehole and the depth of the bottom of the aquifer varies for each borehole. The proposed indicator illustrates the "unitary" effort incurred for drilling for geothermal energy (from geothermal waters) in relation to the flow rate of water available for exploitation.
The second indicator proposed is the ratio of depth/theoretical power, according to the formula: In which P is the theoretical heating power, assuming water cooling from the well head temperature to 0 °C according to: where: /2 is the average temperature of geothermal water. The assumed cooling of water to 0 °C was adopted as a simple rule, easy to calculate and compare. By referencing the final temperature to 0 °C, it is not necessary to know additional parameters, such as the average annual temperature of the atmospheric air, which is used to calculate the available geothermal resources (theoretical resources) under a given surface area [88]. Cooling water down to 0 °C is not feasible and technically impossible. Nevertheless, this value is quite universal and enables comparison of the energy resources (heating power) between wells. This is possible for both the high-temperature waters and the shallow water wells and natural hot springs. However, when providing such resources, it is good to also specify the water temperature, which, apart from the heating power, also indicates the quality of the obtained energy. Material parameters (density and specific heat) for calculating the amount of energy were assumed for the average temperature Twh/2, which is a simplification of the method and facilitates the comparison of geothermal wells.
Similar observations regarding depth and heating power relate to this indicator. Its value with the same assumptions as for the ̇ equals = 3523 W/m. Respectively, for the best Polish borehole (Bańska PGP-1), this indicator equals 16,589 W/m.

Borehole Heat Exchangers
The advantages of the collection of the Earth's heat with borehole heat exchangers include the lack of risk connected with prospecting drilling, very high durability (lifetime) and minimal impact on the environment [89]. This chapter presents the materials most commonly used in borehole heat exchangers, as well as the innovative constructions of BHE at AGH UST in Krakow.

Materials Used in Geothermal BHEs
Borehole heat exchangers have basic construction [89]: Types of plastics are most often used as the material for borehole heat exchangers. Their main advantage is the lack of corrosion on contact with water. The most commonly used materials are [88]:  The most appropriate materials, according to the authors [89,90], for the production of borehole heat exchangers' tubes are polypropylene and polyethylene.
For the grouting of borehole heat exchangers, most commonly used are mixtures with trade names Calidutherm by Terra Calidus, Hekoterm by Hekobentonity, RaugeoTherm by Rehau, StüwaTherm by Stüwa and Thermocem Plus by Górażdże. Hekoterm is also known under brands such as TermorotaS or MuoviTerm [91]. The key parameter that should be specified for grout is increased thermal conductivity.
Grout with increased thermal conductivity is a constantly evolving research topic. The use of graphite as an additive to grout was considered by many authors, such as Lee et al., Sliwa et al.,Delaleux et al.,.
Studies of the heat flow through BHE can be found in the literature. One of the methods is the use of the laboratory model described by Shirazi and Bernier to simulate the well conditions. Moreover, they compared the numerical and experimental results [98]. In classic methods of analyzing a ground heat exchanger, the heat capacity of boreholes is often neglected. Analytical solutions to this issue are presented in the works of Lemarche [99,100]. Taking into account the influence of the thermal capacity of the borehole on the thermal response of the ground was also described by Nian and Cheng [101]. in Krakow has two research stations equipped with borehole heat exchangers of various constructions ( Table 3). The first installation includes five BHEs made in January and February 2008 [102]. The second geothermal field station was constructed in the summer of 2017 on the occasion of the 10th anniversary of the Geoenergetics Laboratory. This installation consists of 14 borehole heat exchangers made using the rotary method [10].

Borehole Heat Exchangers at AGH UST in Krakow
LG-13b double U-pipe cement slurry with increased value of thermal conductivity (TermorotaS) -dz = 32 mm, turbocollector, b = 3 dz = 32 mm, turbocollector, b = 3 Second U-pipe: dz = 40 mm, laminar collector, b = dz = 40 mm, laminar collector, b = LG-14b single U-pipe cement slurry with increased value of thermal conductivity (TermorotaS) with graphite 32 2.9 PE, i pipe For borehole heat exchangers, there is no reason for the ̇i ndicator. geothermal boreholes, one can be tempted to determine the value of the BHEs work with varying loads. The way to determine BHE's energy effici form a Thermal Response Test [19]. TRT allows for the determination o thermal conductivity. Thermal conductivity can also be determined by undisturbed temperature profile in the borehole [104]. The natural temp can be examined with the NIMO-T probe. Many of the temperature-dep some correctness. In general, the temperature in the near-surface layers var on the season. In some profiles, a decrease in the rocks' temperature to a g be observed. High heat penetration from the surface is related to the city not only solar radiation. The main factor influencing the soil environment i urban infrastructure, e.g., the presence of pipelines (water supply, sewa  Table 3). Table 3. Constructions of borehole heat exchangers (Laboratory of Geoenergetics at the Faculty of Drilling, Oil and Gas at AGH UST in Krakow) [10,103].

Type of Grout Outer Diameter of Inner Pipes, D z (d z ), mm
Wall thickness of Pipes, b, mm

Type of Pipes Material
LG LG-5b single U-pipe cement slurry with increased value of thermal conductivity (TermorotaS) 40 3.0 PE, internally smooth pipe (laminar collector) LG-6b single U-pipe cement slurry with increased value of thermal conductivity (TermorotaS) 40 3.0 PE, internally rough pipe (turbocollector) LG-10b innovative system ( LG-12b single U-pipe cement slurry 32 2.9 PE, internally rough pipe (turbocollector) LG V indicator. Similar to the geothermal boreholes, one can be tempted to determine the value of the indicator N p . BHEs work with varying loads. The way to determine BHE's energy efficiency is to perform a Thermal Response Test [19]. TRT allows for the determination of the effective thermal conductivity. Thermal conductivity can also be determined by analyzing the undisturbed temperature profile in the borehole [104]. The natural temperature profile can be examined with the NIMO-T probe. Many of the temperature-depth plots show some correctness. In general, the temperature in the near-surface layers varies depending on the season. In some profiles, a decrease in the rocks' temperature to a great depth can be observed. High heat penetration from the surface is related to the city infrastructure, not only solar radiation. The main factor influencing the soil environment is the extensive urban infrastructure, e.g., the presence of pipelines (water supply, sewage, heat pipelines), asphalt, and black road surfaces, which cause the absorption of additional amounts of solar heat from the surface. The foundations of heated buildings also cause heat transfer to the subsurface rocks. In cities, the depth of periodic heat penetration is usually greater than in non-urban areas [103]. The easiest, but least accurate approach is to determine the thermal conductivity of the ground, based on lithology and literature data [89,105].
Since the proper operation of the plate of geothermal systems is planned for decades, an important issue is to show the long-term behavior of exchangers. The thermal response of slender geothermal boreholes to subannual harmonic excitations is described by Hermanns and Ibanez [106]. Simple empirical formulas correlate the effective thermal conductivity with the unitary heating power of BHEs [107]: and q 2 = 13·λ e f f + 10 (5) However, it is not possible to determine the global (national) value of the indicator N P for BHEs, due to the lack of data on the number and depth of BHEs made in Poland, and the small percentage of TRTs conducted. The collection of data on the created heat pump installations with borehole exchangers is not required, hence it is impossible to identify and collect all information about the created systems. Moreover, there is no legal regulation in Poland regarding the obligation to perform TRT, therefore these tests are performed sporadically and only on large investments. Specification of the individual values for local geology and a given depth is very much possible. For example, for boreholes located in the Laboratory of Geoenergetics AGH UST, the thermal conductivity value of rocks based on literature data (for BHEs LG1a-LG5a) equals 2.039 W/(mK) [89]. The N P value as the mean of q 1 and q 2 from Equations (4) and (5) is 38.64 W/m. It is many times less than the value N P = 3523 W/m for boreholes that exploit geothermal water. As opposed to geothermal waters, which do not occur everywhere, BHEs can be created regardless of geological conditions, and using increasingly affordable methods [108].
TRT tests are currently underway for BHEs belonging to the Laboratory of Geoenergetics AGH UST. Their results will determine the impact of various design parameters on the effective heat conductivity, borehole thermal resistance [109,110] and operational parameters [101].
A not very common variant of BHE is the deep borehole heat exchanger (DBHE). Until now, they have been studied and used only in the USA, Germany, Switzerland and Poland [111], and most recently also in China.
In 1999, one of the world's deepest borehole heat exchangers (2780 m) was made in Poland. It has been used for research purposes only. Due to the use of an inadequate centric tube column, satisfactory results were not obtained [112]. A key structural element in DBHEs is the internal insulating pipe column [99]. The longest-running DBHE is now an exchanger in Prenzlau (Germany), which has been in operation since 1992 [113].
Deep borehole heat exchangers are not currently used for economic reasons. Such installations are unprofitable at current heat prices. They are, however, a forward-looking source of heat when one considers™ hundreds of millions of drilled oil wells around the world.
Research on systems based on exploited and negative oil and gas wells should be carried out, as such installations can be used for heating in the future. Areas with old, decommissioned, or intended-for-decommissioning wells may then become more valuable due to the availability of an independent heat source. Only the energy which drives the heat pump (not always necessary-depending on the borehole depth) and the circulation of the heat carrier in the exchanger would have to be provided.
For instance, in the years 2016-2017, more than 120,000 oil and gas exploration and reconnaissance boreholes with a total depth of over 337.5 million meters [114] were made worldwide. With a careful approach, they could exchange heat with a rock mass reaching the heating power of more than 8 GW. It seems prudent to consider drilling new boreholes with potential future use in the form of deep borehole heat exchangers. For example, appropriately modified sealing slurry (with adjustable thermal conductivity) could be used. Table 4 shows the present deep geothermal district heating plants and other uses for heating. Table 5 summarizes the data on geothermal heat pumps in Poland.

Conclusions
Renewable energy sources are increasingly used around the world. These include geothermal energy, which is exploited by geothermal boreholes and borehole heat exchangers. The authors came to the following conclusions: 1.
In Polish geothermal wells, casing pipes are usually made of steel. 2.
The first geothermal boreholes in Poland were vertical and made of steel pipes. Currently, directional boreholes and fiberglass pipes are present, which reflects the development of techniques and technology. 3.
Borehole heat exchangers (BHEs) are increasingly used. The advantages of collecting Earth's heat with borehole heat exchangers include no risk connected with prospecting drilling, very high durability (lifetime) and minimal impact on the environment.

4.
There are two installations of borehole heat exchangers on the site of the AGH UST in Krakow. The first consists of 5, while the second of 14 borehole heat exchangers with an innovative system. It is the largest installation of BHEs with different designs in the world.

5.
Comparative indicators for drilling efficiency for geothermal boreholes in Poland have been proposed. These indicators can be determined in any country where exploitation boreholes for geothermal heat are made. This applies both to geothermal boreholes (i.e., those related to geothermal water) as well as borehole heat exchangers (i.e., openings which obtain the Earth's heat without hydraulic contact with the rock mass). 6.
Two indicators for the effectiveness of drilling were proposed for geothermal boreholes. The first is the "unitary" cost of obtaining geothermal water's one unit of efficiency N . V , the second is the indicator of theoretical power per one meter of existing and created boreholes N P . For geothermal boreholes in Poland, N . V = 0.04879 m 3 /h/m and N P = 3523 W/m. For borehole heat exchangers, it is impossible to determine the values of these indicators for the entire country due to the reasons described in the article. Local (individual) N P values can be determined based on the rock's heat conductivity. For BHEs located in AGH UST, N P equals 38.64 W/m. The difference is also reflected in the cost. The unitary cost of drilling the BHE is many times less than the unitary cost of drilling a geothermal borehole. 7.
Boreholes drilled in the past (including those already decommissioned) and those which will be drilled in the future can be adapted for geothermal purposes. If there is no aquifer present, they can be used for deep borehole heat exchangers. For this purpose, they can currently be designed taking into consideration future geothermal applications.