Dilemma of Spent Geothermal Water Injection into Rock Masses for Geothermal Potential Development

Abstract

The global shift towards the use of renewable energy is essential to ensure sustainable development, and geothermal energy stands out as a suitable option that can support various cascading projects. Spent geothermal water (SGW) requires proper treatment to ensure that it does not become an environmental burden. Typically, companies often face the dilemma of choosing between discharging spent geothermal water (SGW) into surface waters or injecting it into rock masses, and the economic and environmental impacts of the decision made determines the feasibility of geothermal plant development. In this study, we aimed to comprehensively assess the technical, economic, and environmental feasibility of SGW injection into rock masses. To this end, we employed a comprehensive analytical approach using the Chochołów GT-1 geothermal injection borehole in Poland as a reference case. We also performed drilling and hydrogeological testing, characterized rock samples in the laboratory, and corrected hydrodynamic parameters for thermal lift effects to ensure accurate aquifer characterization. The results obtained highlight the importance of correcting hydrogeological parameters for thermal effects, which if neglected can lead to a significant overestimation of the calculated hydrogeological parameters. Based on our analysis, we developed a framework for assessing SGW injection feasibility that integrates detailed hydrogeological and geotechnical analyses with environmental risk assessment to ensure sustainable geothermal resource exploitation. This framework should be mandatory for planning new geothermal power plants or complexes worldwide. Our results also emphasize the need for adequate SGW management so as to ensure that the benefits of using a renewable and zero-emission resource, such as geothermal energy, are not compromised by the low absorption capacity of rock masses or adverse environmental effects.

1. Introduction

1.1. Background and Motivation

Ensuring economically efficient and environmentally sustainable geothermal resource use remains challenging. Hot water from deep underground rocks is an excellent resource for geothermal complexes, and its applications range from renewable energy generation to recreational balneology and greenhouse heating. Economic feasibility analyses for new geothermal projects typically focus on the costs associated with deep production drilling [1,2,3] as well as technical and construction infrastructure, potential social impacts [4,5], and the life cycle assessment of the environmental effects of geothermal power generation [6,7]. Presently, deep borehole heat exchangers are not widely used owing to their associated costs [8], and their installation is unprofitable at current heat prices.

However, an important aspect that is rarely addressed in the published literature and requires analysis at the planning phase of geothermal projects is the method for handling spent geothermal water (SGW). Each type of geothermal installation produces wastewater that is usually discharged into surface waters or injected into rock masses. The need to balance the environmental benefits and risks associated these projects and explicitly link them to potential policy changes or recommendations is essential for ensuring the sustainable management of geothermal complexes. Menberg et al. [9] revealed that using electricity from a grid line rather than diesel to power drilling rigs can significantly improve the environmental performance of geothermal power plants, provided that the electricity is sourced from environmentally friendly technologies. Furthermore, the anticipated technological improvements in geothermal drilling and plant design suggest that future enhanced geothermal power plants may outperform many other renewable energy technologies from an environmental sustainability perspective. Therefore, it is necessary to extend these ‘technological improvements’ to injection boreholes given that assessments that solely focus on borehole exploitation do not adequately capture the sustainability of such projects.

1.2. Research Scope and Novelty

In this study, we aimed to provide a comprehensive assessment of the feasibility and technical conditions for injecting cooled thermal water into rock masses. Previous geothermal studies have typically focused solely on the exploitation of boreholes and geothermal water (GW) exploitation parameters [10,11,12,13]. However, determining the hydrogeological conditions that allow the injection of SGW into rock masses requires the drilling of an injection borehole (part of a geothermal doublet: production borehole and absorption borehole) and conducting a series of tests and reservoir measurements. Based only on the advanced research findings of this study, the actual possibilities and conditions for injecting cooled thermal water into rock masses can be determined. The challenges associated with such investigations were also discussed in this study.

The approach employed in this paper, which integrates the steps necessary for assessing the parameters that influence SGW injection into rock masses, offers a novel perspective to GW development, provides a direct and effective strategy for enhancing GW injection, and may yield new insights regarding geothermal plants relative to the traditional method. The detailed examination of the localized potential of GW injection is also relatively innovative. The study addresses the critical question of whether SGW injection is geologically and technically feasible, or whether it imposes an undue economic investment burden. The results obtained may have significant implications for investment management in terms of balancing geological, environmental, and economic factors.

1.3. Practical Aplications

This study addresses a critical challenge in geothermal energy development: managing spent geothermal water (SGW) to ensure a balance between environmental sustainability and economic feasibility. This study emphasizes the importance of evaluating rock mass absorption capacity before injecting cooled thermal water, a practice that ensures the long-term viability of geothermal projects while minimizing associated environmental impacts. This study also provides actionable guidance for developers, policymakers, and environmental planners by outlining a clear framework for conducting hydrogeological tests and determining injection feasibility. This framework has real-world application potential in geothermal complexes, such as the Chochołowskie Termy in Poland, where injection into rock masses prevents the adverse effects SGW on surface water systems. Additionally, the proposed framework is globally relevant and supports sustainable geothermal expansion as it addresses challenges such as reservoir stability and environmental compliance, and it not only enhances renewable energy adoption, but also aligns with global goals for reducing carbon emission and safeguarding natural ecosystems. Stakeholders in renewable energy and environmental management can leverage the insights highlighted in this study to optimize geothermal resource utilization and ensure ecological balance and economic efficiency.

2. Proposed Obligatory Course of Assessment, Materials, and Methods

2.1. Drilling of a Geothermal Injection Borehole

Drilling a deep borehole is a complex and time-consuming multi-stage process that involves several technically complicated steps. Initially, the drilling site is prepared via a number of important processes, including hardening the terrain to ensure stability (Figure 1a), allocating a sufficiently large area for technical facilities, including devices, machines, and allocating social and technical spaces for geological, supervisory, and technical drilling services (Figure 1c). Additionally, given that drills are subject to wear and tear, they must be replaced periodically with new and efficient drills (Figure 1b).

2.2. Necessary Hydrogeological Research

The objective of geological studies during the drilling of deep absorption boreholes is to assess the hydrogeological conditions of the thermal water reservoir and establish the conditions for injecting cooled thermal water into rock masses, and it involves the following:

2.2.1. Hydrodynamic Pumping Test

A hydrodynamic pumping test, which is one of the most reliable methods for evaluating the filtration properties of an aquifer, is frequently used to investigate groundwater resources [14,15] as it allows the estimation of key hydrodynamic parameters, such as transmissivity, hydraulic conductivity, and storage coefficient. Classical pumping tests are conducted under steady (or quasi-steady) conditions and typically last from several days to a dozen days. In contrast, hydrodynamic pumping tests can be performed within a much shorter timeframe, typically a few to several dozen hours, even under unsteady inflow conditions, and most often, are executed as single- or multi-stage processes, ending with the observation of groundwater level (or pressure recovery). The fundamental principle of pumping under unsteady inflow conditions is to maintain a constant outflow. The most frequently observed variables during pumping include thermal water level or pressure (drawdown, s) and thermal water temperature. Interpreting drawdown measurements allows the determination of the hydrodynamic type of the aquifer and its filtration parameters. The test scheme and symbols used in the calculations are shown in Figure 2.

2.2.2. Thermal Lift and Head Pressure

In deep boreholes for exploiting geothermal water, the effect of temperature on groundwater level or measured wellhead pressure can be significant, increasing with the increase in reservoir temperature and borehole depth. During well exploitation, wells heat up [16], and the longer the exploitation duration, the closer the temperature inside the casing pipes approaches that at the bottom of the well, thereby reducing the loss of geothermal energy from water during collection. As the borehole heats, the water expands owing to changes in density, resulting in an increase in groundwater level or wellhead pressure relative to that observed for the unexploited well, even though bottom pressure in both cases remains the same. Furthermore, given that it is generally impossible to measure bottom pressure during testing, it is often necessary to correct groundwater level or wellhead pressure measurements for thermal lift by calculating the so-called reduced pressure [10,17]. Thus, the absolute drawdown in the borehole, unaffected by the thermal conditions of the medium can be determined. This correction of hydrodynamic test results for the effects of borehole heating leads to a more accurate estimation of filtration in a given aquifer. The hydrostatic pressure, P, exerted by the water column can be calculated according to Equation (1).

$$P=\rho ·L·g$$

(1)

where ρ represents fluid density, L represents the height of the liquid column, and g represents acceleration due to gravity.

Figure 2. Diagram showing the pumping test. s, drawdown; s′, lift; s″, residual drawdown; t, test duration, including groundwater level recovery; and t′, time from the end of pumping.

Figure 2. Diagram showing the pumping test. s, drawdown; s′, lift; s″, residual drawdown; t, test duration, including groundwater level recovery; and t′, time from the end of pumping.

Given that the pressure at the bottom of the borehole is independent of the liquid temperature, the product of the height of the liquid column and the density of the liquid are the same for both unheated and heated boreholes, as shown in Equation (2).

$${H·\rho }_{av}=H_c{·\rho }_c$$

(2)

where H represents the height of the liquid column observed under given conditions, ρ_av represents the average water density for the entire borehole at a given time, H_c represents reduced pressure, and ρ_c represents the specific density of water under normal conditions (unheated borehole).

Furthermore, the average density of water, ${\rho }_{av}$, in the borehole can be calculated according to Equation (3), as follows:

$${\rho }_{av}={\displaystyle \frac{1}{H}}{\int }_0^H\rho ·dz$$

(3)

where z represents the reference level determined at the base of the borehole.

Even though the water density is generally a nonlinear function of the temperature, it can be approximated as a linear function within the temperature range associated with low-temperature geothermal energy. By omitting a few simple transformations, we obtained Equation (4), according to which the average density of the water column is approximately equal to the density of water determined at the average temperature of the water column.

$${\rho }_{av}=\rho \left(T_{av}\right)$$

(4)

By substituting Equation (4) into Equation (2), we obtained dependence as shown in Equation (5):

$$H\rho \left(T_{av}\right)=H_c\rho \left(T_{av.c}\right)$$

(5)

where T_av.c represents the average temperature of the water column in the unheated borehole.

If the drawdown is calculated relative to the observed static level in the borehole after standing, then Equation (6) would be true.

$$H=D-s\left(T_{av}\right)$$

(6)

where D represents the height of the water column under static conditions and s(T_av) represents the drawdown observed in the borehole at the average water column temperature, T_av.

Additionally, by substituting Equation (6) into Equation (5) followed by transformation, we obtained the actual drawdown value for the borehole, as shown in Equation (7).

$$s\left(T_{av.c}\right)=D+\left[s\left(T_{av}\right)-D\right]{\displaystyle \frac{\rho \left(T_{av}\right)}{\rho \left(T_{av.c}\right)}}$$

(7)

Calculated without taking into account the thermal effect at the average temperature of the water column, T_av.c corresponds to the measured drawdown and s(T_av) to the heated borehole.

The bottom borehole pressure was determined according to Equation (8).

$$P_{a0}=\rho \left(T_{av.c}\right)Lg+P_{s0}$$

(8)

where P_s0 represents the pressure measured at the borehole head (or height of the water column in the borehole) and L represents the depth of the borehole from the aquifer level. During operation, the bottom pressure at a given moment, P_a, was determined according to Equation (9), which is similar to Equation (8).

$$P_a=\rho \left(T_{av}\right)Lg+P_s$$

(9)

where P_s represents the pressure measured at the wellhead (or height of the water column in the borehole) at a given moment.

Thus, the drawdown in the borehole was determined according to Equation (10):

$$s\left(T_{av.c}\right)={\displaystyle \frac{P_{a0}-P_a}{\rho \left(T_{av.c}\right)g}}$$

(10)

By substituting Equations (8) and (9) into Equation (10), we obtained Equation (11), which allowed the calculation of the actual drawdown while ignoring the influence of the thermal effect. The calculation only required the static wellhead pressure (or static water level in the borehole), measured wellhead pressure (or height of the water level in the borehole), and average temperature of the water column at a given moment.

$$s\left(T_{av.c}\right)={\displaystyle \frac{P_{s0}-P_s}{\rho \left(T_{av.c}\right)g}}+\left[1-{\displaystyle \frac{\rho \left(T_{av}\right)}{\rho \left(T_{av.c}\right)}}\right]L$$

(11)

Considering the real value of the drawdown, s(T_av._c), the reduced wellhead pressure or height of the water column in the borehole (P_{s_red}) at a given moment was determined according to Equation (12).

$$P_{s_{red.}}=P_s-\left[1-{\displaystyle \frac{\rho \left(T_{av}\right)}{\rho \left(T_{av.c}\right)}}\right]\rho \left(T_{av.c}\right)Lg·{10}^{-6}$$

(12)

where the wellhead pressure (or height of the water column in the borehole) $P_{s_{red.}}$ and P_s are expressed in MPa.

The formula assumes that the temperature dependence of fluid property is known and can be described mathematically (e.g., via linear expansion coefficients, temperature–pressure correlation functions, or empirical relationships). The correction is most accurate under steady or slowly varying thermal conditions. Rapid temperature fluctuations may not be captured accurately unless a time-dependent model is used. Most correction formulas are calibrated for a certain range of temperatures. Applying them outside that range can introduce significant error. Thermal correction formulas often simplify real-world behavior (e.g., assuming linear expansion or ideal gas behavior), which can lead to inaccuracies in complex systems. At very high or low temperatures, material behavior may become nonlinear or unpredictable, reducing the formula’s accuracy. Corrections may not account for coupled thermo-mechanical or thermo-chemical processes, such as pressure changes due to both thermal expansion and fluid migration. Many correction formulas rely on empirically derived coefficients (e.g., thermal expansion coefficient), which must be accurately known or measured for the specific fluid.

2.2.3. Rock Massive Absorption Capacity Test

It is generally accepted that formations into which water is injected should have the following hydrogeological features: porosity > 20%; permeability > 100 mD; no clay fraction that can swell; and significant distribution of absorbent layers.

In the water injection strategies used to date, water is injected into formations with lower porosities and permeabilities [18,19]. Formations characterized by permeability > 30 mD and porosity > 10% are considered absorbents. It has also been shown that the changes in the porosity and permeability of rocks near the injection area can significantly affect the injection capacity [20].

2.2.4. Determination of Skeletal Density, Bulk Density, and Total Porosity

Skeletal density (ρm) can be determined using the AccuPyc II 1340 helium pycnometer (Micromeritics, Norcross, GA, USA), which effectively utilizes the properties of helium, i.e., its ability to penetrate even the smallest sub-micropores, to determine the skeletal density. A test sample with a diameter of 2.54 cm (1 inch) and length in the range of 3–4 cm was weighed and placed in a calibrated chamber, into which a specified amount of helium was introduced. Next, to determine the apparent density (ρb), the GeoPyc 1360 analyzer (Micromeritics, Norcross, GA, USA), which employs the quasi-liquid pycnometry method as well as a so-called ‘dry liquid’ (DryFlo^® powder), was used to determine the density of the tested samples based on the quasi-liquid displacement phenomenon. As recommended, measurements were performed on the same samples that were used for the skeletal density measurements. The obtained densities were then used to calculate total helium porosity (ϕ_He) according to Equation (13).

$${\varphi }_{He}={\displaystyle \frac{({\rho }_m-{\rho }_{b)}}{{\rho }_m}}100$$

(13)

2.2.5. Permeability Marking

Effective permeability is typically determined using nitrogen as the working gas. The measurement process involves establishing a steady laminar gas flow through the test sample and calculating the permeability coefficient of the sample using Darcy’s equation [21,22]. In brief, permeability coefficient measurements were performed considering a linear geometry, that is, the gas was allowed to flow through a sample with a constant cross-section and length, and cut in the form of a cylinder with a diameter of 2.54 cm (1 inch) and length in the range of 3–4 cm, while the side surfaces were sealed. The permeability coefficient was calculated according to Equation (14), as follows:

$$q={\displaystyle \frac{c·k·A·(P_1^2-P_2^2)}{T·L·\mu ·Z}}$$

(14)

where k represents the permeability coefficient (mD), µ represents gas viscosity (cPu), c is a constant depending on the units used, T represents temperature, Z is the deviation coefficient, L is cylinder length (cm), A represents the cross-section of the cylindrical test sample (cm), and P₁ and P₂ represent inlet and outlet pressures (atm), respectively.

2.2.6. Micro- and Macro-Fracture Designations

Under laboratory conditions, micro-fracture tests were conducted on thin plates using replicate cube-shaped rock samples with an edge length of 4 cm. On the thin plates, fractures with openings below 0.1 mm were marked, and the porosity and permeability of fractures with openings greater than 0.1 mm on the replicas were determined. The numerical determination of fracture porosity and permeability is inherently statistical. Therefore, for statistical calculations, a preliminary characterization (directional, vertical, horizontal, or random) of the fracture system was necessary, allowing the adoption of appropriate constants in the calculations. For the replicas, the fracture porosity and permeability were calculated by treating the observed fractures as smooth fractures with measured openings and lengths much greater than that of the opening on the thin plate. The size of laminar flow through such a fracture has been described by Boussinesq [23], as shown in Equation (15).

$$q=-{\displaystyle \frac{l·b^3·∆P}{12·\mu ·∆L}}$$

(15)

where q represents the liquid flow rate, µ represents the dynamic viscosity, l represents the slot length, b represents the gap width, ΔL represents the flow path length, and ΔP represents the pressure change.

By equating Equation (15) to Darcy’s equation and introducing the average linear density of cracks, τ, equal to n/L, we obtained the expression for permeability, k (Equation (16)).

$$k=\left(a·b^3·\tau \right)÷12$$

(16)

where a is an indicator characteristic of the adopted units and n represents the number of cracks observed along the section with length, L, for a quasi-isotropic homogeneous crack system.

Fissure and cavern porosity, P, was calculated according to Equation (17), as follows:

$$P={\displaystyle \frac{c·b·l}{s}}$$

(17)

where s represents the tested replica surface and c is a constant resulting from the units used.

Tests for smaller cracks were conducted on thin plates using the random traverse method, which involves randomly applying a section of length L to the tested thin plate and examining the number of intersections between this section and microcracks. Based on these assumptions, the coefficient of the volumetric density of the cracks, Г, was determined according to Equation (18):

$$Г={\displaystyle \frac{{\pi }^2}{4·L}}·{\displaystyle \frac{1}{m_l}}·{\sum }_{i=1}^{m_l}{n}_i$$

(18)

where m_l represents the number of fields of view applied to thin plate no. l and n represents the number of intersections between crack traces and section m_l, each with length L. By inserting the value, Г, into Equation (17), we obtained Equation (19).

$$P={\displaystyle \frac{\pi }{2·a}}·{\displaystyle \frac{b_l}{k_l}}·{\sum }_{i=1}^{m_l}{n}_i$$

(19)

For thin plate no. l, the permeability, k, was determined according to Equation (20).

$$k={\displaystyle \frac{c·\pi }{2·L}}·{\displaystyle \frac{b_l^3}{k_l}}·{\sum }_{i=1}^{m_l}{n}_i$$

(20)

where c is a constant depending on the units employed for other parameters.

3. Sample Results in the Reference Borehole and Discussion

3.1. Drilling of the Reference Geothermal Injection Borehole Chochołów GT-1

The Chochołów GT-1geothermal injection borehole, located in Eastern Europe, was selected as a reference borehole. The region where this borehole is located is rich in geothermal water, which is exploited via several deep boreholes and primarily used for heating and recreational purposes [24,25,26]. The Palaeogene Podhale Basin, where the Chochołów GT-1 well was drilled, is situated in the northern part of the Inner Carpathians, flanked by the Tatra Mountains to the south and the Pieniny Klippen Belt to the north (Figure 3). The subflysch aquifers in this area are replenished with rainwater originating from the Tatra Mountains [27,28], and infiltrating rainwater in this region moves northward through a system of cracks and karst voids, following the direction of the subsidence of the Tatra series. Additionally, mainstream surface water penetrates the depths of this region, heats up, and is eventually captured as thermal water by boreholes.

The drilled absorption borehole, the Chochołów GT-1 borehole, together with the pre-existing exploitation Chochołów PIG-1 borehole, form a ‘geothermal doublet’. The Chochołów GT-1 borehole was intended to serve as an alternative to discharging SGW into the Czarny Dunajec surface stream, a practice that has been in place since 2016, following the commencement of the Chochołowskie Termy swimming pool complex, which is based on GW exploited via the Chochołów PIG-1 borehole, designed as a directional type ‘S’ borehole with a final depth of 4.122 m MD (3820 m TVD) and a drilling azimuth of 20°. Furthermore, the Chochołów GT-1 borehole was drilled between April 2022 and February 2023, reaching a final depth of 3795 m MD (3531.8 m TVD) in the Middle Triassic formations, thereby achieving the drilling objective [29] and marking the successful completion of the first stage of the investment despite several complications during drilling, including the interception of the drill string at a depth exceeding 3 km and the necessity for drilling a side-track hole. It should be remembered that, in the Podhale basin area, this is only the fourth absorption hole, next to Biały Dunajec PAN-1 (depth of 2394 m), Biały Dunajec PGP-2 (depth of 2450 m), and Biały Dunajec PGP-5 (depth of 3564 m).

3.2. Interpretation of the Research Pumping Results

For the Chochołów GT-1 borehole, the hydrogeological parameters of the thermal water reservoir were determined based on the results of relevant hydrodynamic tests under conditions of water self-outflow, which is significantly influenced by the pressure in the borehole. The tests were performed in two stages [29], as shown below:

• Step I: The average flow rate during this step was determined to be 24.3 m³/h, with the initial and final pressures at 5.11 and 4.77 bar, respectively, and the maximum recorded temperature at 54.02 °C. The pumping duration was 12 h, and during this period, 292 m³ of water was collected. Following the test, the reservoir pressure was restored over a period of 12 h.
• Step II: This step was conducted at an average flow rate of 23.7 m³/h, and the initial and final pressures were 5.10 and 1.85 bar, respectively, with the maximum recorded temperature being 60.23 °C. Furthermore, the pumping duration was 36 h, and during this period, 850 m³ of water was collected. After the test, a 36-hour reservoir pressure restoration period was observed, resulting in a final pressure of 5.40 bar.

Therefore, a total of 1142 m³ of water was pumped out during the measurement. A graphical record of the changes in pumping parameters is shown in Figure 4.

Reduced Head Pressure Value

Based on the calculation procedure in the Methods Section and the measurement data, the reduced value of the wellhead pressure in the Chochołów GT-1 borehole was calculated. Figure 5 shows the variability in the observed wellhead pressure and reduced pressure, accounting for the thermal effect, as well as changes in absorption efficiency and temperature during the pumping measurements.

Figure 4. Observed parameters during hydrodynamic tests at the reference Chochołów GT-1 borehole.

Figure 4. Observed parameters during hydrodynamic tests at the reference Chochołów GT-1 borehole.

Figure 5. Variability of parameters during pumping at the reference Chochołów GT-1 borehole.

Figure 5. Variability of parameters during pumping at the reference Chochołów GT-1 borehole.

3.3. Determination of Hydrogeological Parameters

The collected data was interpreted using the Theis method [30,31], while the Moench [32] method was employed to characterize flow in the fractured medium. Then, to determine hydraulic jump, known as the ’skin effect,’ the method proposed by Agarwal [33] was used. For the data interpretation, the obtained pumping data was categorized under two steps as follows: Step I involved pumping at an average flow rate of 24.3 m³/h with groundwater table restoration, and Step II involved pumping at an average flow rate of 23.7 m³/h with groundwater table restoration as well. The symbols and graphical representations of the pumping data are shown in Figure 2. Calculations were further performed for the following periods using different methods as follows:

• Step I with restoration: Moench Fracture Flow, Theis, and Agarwal Skin (Figure 6).
• Recovery after Step I: recovery and Agarwal skin recovery (Figure 7).
• Step II with restoration: Moench Fracture flow, Theis skin, and Agarwal Skin (Figure 8)
• Recovery after Step II: Theis and Agarwal Skin Recovery (Figure 9).

The designations in Figure 6, Figure 7, Figure 8 and Figure 9 are consistent with the pumping diagram shown in Figure 2.

Figure 6. Changes in drawdown in the reference Chochołów GT-1 borehole during Step I of pumping and recovery based on: (A) Moench, (B) Theis, and (C) Agarwal Skin.

Figure 6. Changes in drawdown in the reference Chochołów GT-1 borehole during Step I of pumping and recovery based on: (A) Moench, (B) Theis, and (C) Agarwal Skin.

Figure 7. Changes in drawdown in the reference Chochołów GT-1 borehole during restoration after Step I of pumping based on: (A) Theis Recovery and (B) Agarwal Skin Recovery.

Figure 7. Changes in drawdown in the reference Chochołów GT-1 borehole during restoration after Step I of pumping based on: (A) Theis Recovery and (B) Agarwal Skin Recovery.

Figure 8. Changes in drawdown in the reference Chochołów GT-1 borehole during Step II of pumping and recovery based on: (A) Moench, (B) Theis, and (C) Agarwal Skin.

Figure 8. Changes in drawdown in the reference Chochołów GT-1 borehole during Step II of pumping and recovery based on: (A) Moench, (B) Theis, and (C) Agarwal Skin.

Figure 9. Changes in drawdown in the Chochołów GT-1 borehole during restoration after Step II of pumping based on: (A) Theis Recovery and (B) Agarwal Skin Recovery.

Figure 9. Changes in drawdown in the Chochołów GT-1 borehole during restoration after Step II of pumping based on: (A) Theis Recovery and (B) Agarwal Skin Recovery.

Via the analysis, the values of hydraulic conductivity T, which subsequently allowed the determination of the permeability coefficient, k, were calculated. Specifically, to calculate k, the effective thickness of the Middle Triassic dolomites in the Chochołów GT-1 borehole was assumed to be 86.2 m TVD (88 m MD), corresponding to the total thickness of the Middle Triassic dolomite.

Table 1. Interpretation of pumping measurement data obtained for the Chochołów GT-1 borehole.

Analytic Method	Hydraulic Transmissivity T [m2/s]	Permeability Coefficient k [m/s]	Skin Effect
I pumping step with recovery
Moench Fracture Flow	7.30·10−5	8.47·10−7	0.000000195
Theis	7.98·10−5	9.26·10−7	-
Agarwal Skin	7.85·10−5	9.11·10−7	−2.89
Recovery after I pumping step
Theis Recovery	7.84·10−5	9.10·10−7	-
Agarwal Skin Recovery	7.38·10−5	8.56·10−7	−5.00
II pumping step with recovery
Moench Fracture Flow	2.50·10−5	2.90·10−7	0.0132
Theis	4.37·10−5	5.07·10−7	-
Agarwal Skin	1.83·10−5	2.12·10−7	−2.40
Recovery after II pumping step
Theis Recovery	1.90·10−5	2.20·10−7	-
Agarwal Skin Recovery	4.28·10−5	4.97·10−7	−5.00
Medium value	5.32·10−5	6.18·10−7	-

provides a summary of the results of the interpretation of the pumping measurement data for the Chochołów GT-1 borehole corrected for thermal effects.

Both the hydraulic conductivity, T, and permeability coefficient, k, calculated with thermal correction, were lower than the values obtained without thermal correction. This was because the actual drawdown (after thermal correction) was significantly greater than that obtained without thermal correction (Figure 5). Therefore, when interpreting data obtained from test performed in deep boreholes with thermal water, it is essential to consider ‘thermal lift,’ which refers to the elevation of the groundwater table in the borehole owing to the thermal expansion of water. This effect, if not taken into account, distorts the observed results, leading to an overestimation of the measured hydrogeological parameters.

3.4. Density, Porosity, and Permeability Results

The tested samples obtained from the potential absorbent layer in the new Chochołów GT-1 borehole exhibited extremely low total porosity (<1%) and effective permeability (0.004–0.021 mD) values, as summarized in

Table 2. Density, total porosity, and gas permeability measurement results inthe Chochołów GT-1 borehole.

Sample No.	Total Porosity [%]	Skeletal Density [g/cm3]	Bulk Density [g/cm3]	Effective Permeability [mD]
1	0	2.761	2.757	0.008
2	0.8	2.729	2.705	0.021
3	0.5	2.693	2.678	0.004

. These values are significantly lower than those observed for the Lower Cretaceous porous thermal water deposits exploited in Uniejów (sandstones—quartz arenites). The Uniejów reservoir, with a total thickness of approximately 120–150 m, is characterized by a total porosity of approximately 15–20% (close to effective porosity) and permeability ranging from several dozen to 2000–3600 mD [34]. Also the obtained value of porosity in Chochołów GT-1 is similar as typical injection boreholes in Podhale Basin as Biały Dunajec PAN-1 (0.5–1.5%) and Biały Dunajec PGP-2 (3%) [35].

3.5. Micro- and Macro-Fracturing Results

The widths of the observed micro-fractures (thin plates) ranged from 0.0003 to 0.012 mm, with the mean width at 0.008 mm. Furthermore, the widths of the fractures in the microsections ranged from 0.1 to 0.2 mm. Some microfractures were also found to be filled with secondary minerals (

Table 3. Fracture tests results for the Chochołów GT-1 borehole.

Depth of Sampling [m]	Studies on Microsections			Thin Plate Studies			Micro-Fracture Balance
	Gap Width [mm]	Fracture Porosity [%]	Fracture Permeability [mD]	Volumetric Index of Gaps [1/cm]	Fracture Porosity [%]	Fracture Permeability [mD]	Fracture Porosity [%]	Fracture Permeability [mD]
3717.04–3717.08	0.2	0.6	51.94	5.33	3.05	4.23	3.65	56.17
4048.63–4048.68	0.1	0.2	5.2	1.10	0.63	0.88	0.83	6.08
4158.32–4158.38	0.1	0.3	9.8	1.01	0.58	0.81	0.88	10.61
4163.11–4163.18	0	0	0	2.66	1.53	2.12	1.53	2.12

). This table also shows the balance between micro-fracture porosity and permeability based on the analyses conducted on both thin plates and microsections.

In order to obtain an optimal database for interpretation, we suggest introducing sampling standards for microcrack analysis as sampling depth intervals of at least 1 interval for every 100 m of the extracted aquifer.

3.6. Rock Massive Absorption Capacity Test

To determine the conditions suitable for injecting SGW into the rock mass at the absorption borehole, absorption tests were performed for the Chochołów GT-1 borehole using a three-stage system, as shown in Figure 10. First, the static pressure at the head of the borehole was determined to be 6.3 bar, consistent with that reported by Bielec and Operacz [29]. Next, based on the curve fitted to the measurement points, the overpressures for injection flow rates of 120, 140, and 160 m³/h were forecasted. Additionally, at a flow rate of 176 m³/h, the maximum overpressure (56.7 bar), approximately equal to the pressure at the head of the well (63 bar), was obtained. This pressure represents the maximum permissible head pressure at the head of the Chochołów GT-1 borehole.

Based on petrographic studies, the absorbent layers in the Chochołów GT-1 borehole exhibited permeabilities up to 56 mD and porosities up to 3.65%, with secondary porosity reaching 100% owing to the fracturing effect. These parameters, along with the lithological development of the absorbent layer and its identified distribution pattern, support the fulfilment of the geological criteria. Additionally, the results of absorbency tests (based on the extrapolated data) allowed the determination of the amount of water that could be injected into the Chochołów GT-1 borehole at 176 m³/h. The maximum permissible position of the dynamic water table, considering the borehole conditions, was determined to be 1423.24 m a.s.l. (for a borehole elevation of 793.24 m a.s.l.), which corresponds to the permissible pressure at the borehole head (63 bar). The aquifers subjected to SGW injection originated from the Middle Triassic period and the absorptivity test conducted for the borehole showed that the rocks in this borehole are suitable for the intended purpose. The advanced analysis of the absorptive capacity of the Middle Triassic formations also confirmed the good absorptivity of the borehole.

3.7. Rock Massive Absorption Capacity Test

3.7.1. Preliminary Forecast of Hydrogeochemical Changes

In the Chochołów GT-1 borehole, where the water injection rate can reach 176 m³/h, the theoretically determined impact range was 224 m from the point where the absorption borehole enters the deposition zone. Notably, there were no other boreholes within this range of injection impact. Previous exploitations of boreholes in the Podhale Basin have not shown any adverse changes in the hydrogeochemical characteristics of reservoir rocks into which thermal water is injected [8]. In this study, changes, such as colmatation with iron compounds, were not observed in the transmission pipelines or absorption boreholes, implying favorable geological characteristics of the rock mass and deposits in both the thermal water exploitation and injection zones.

3.7.2. Impact of Injection on Usable Aquifers

In the reference area of the Chochołów GT-1 borehole, the usable aquifer was found to be composed of Quaternary formations in the Czarny Dunajec valley and sub-Quaternary outcrops of sandstone–shale layers from the Chochołów and Upper Zakopane layers belonging to the Podhale flysch (Figure 3). The flysch formations primarily exhibited fissure-type characteristics, with fissure–pore-type aquifers occurring less frequently. The groundwater table was predominantly the pressure type, and based on our estimation, prospective aquifers are confined to depths in the range of 60–100 m, corresponding to the active water exchange depth under flysch conditions. In the Czarny Dunajec valley, the thickness, permeability coefficient, and potential efficiency of Quaternary formations were estimated to be 10 m, 43.3 m/d, and less than 10 m³/h, respectively, meanwhile for the flysch aquifers, they were estimated as 15 m, 1 m/d, and 2–5 m³/h, respectively [36]. Generally, the water in this usable aquifer was found to be of good quality, even though it contained elevated levels of iron and manganese relative to their prescribed limits in water, and was typically of the HCO₃–Ca–Mg type, with mineralization levels not exceeding 400 mg/L.

Podhale flysch formations, which constitute the overburden of the deposit zone for the Chochołów GT-1 borehole (shales and sandstones), showed low permeabilities and, at the regional scale, could be considered as isolating formations. The permeability coefficient for these formations was less than 10−8 m/s. Additionally, the thickness of the overburden of the thermal water deposit zone in the Chochołów GT-1 borehole exceeded 3218 m, and this thickness, combined with the borehole, excluded any impact of SGW injection on the usable aquifer. This has been confirmed by several years of thermal water exploitation in the Podhale Basin and indicates that quality monitoring of usable aquifers is not necessary during SGW injection in this region.

However, it is important to acknowledge that injected water can lead to formation damage, including permeability reduction [37]. Studies have shown that injected fluids primarily migrate along fractures, resulting in a rapid decrease in temperature in regions close to fractures [38]. Overall, usable aquifers, located approximately 3 km above the injection site, were unaffected by SGW injection.

3.8. Dilemma Between Injection into Rock Mass and Discharge to the River (Environment–Economy Balance)

SGW, constituting a type of industrial wastewater, can be discharged into the environment either via injection into rock masses (when technically and geologically feasible) or discharged directly into nearby rivers (provided this method does not adversely affect the conditions of the surface water environment). For brackish water, saltwater, and brine (mineralization > ~5 g/L), the obligation to discharge the wastewater via rock mass injection is generally unquestionable. In such cases, investors are required to conduct a thorough analysis following the procedure outlined in this study and include the cost associated with drilling an absorption borehole in their investment plans.

For low-salinity freshwater, it is possible to choose between discharging the thermal wastewater into surface-flowing waters, and in such a case, it is necessary to monitor the conditions of the receiving water body. It appears that only with a sufficiently long observation period (a minimum of 2 years), demonstrating a clear absence of negative impacts on the receiving surface water, should the obligation to drill an absorption borehole be waived.

Regardless, SGW discharge via an absorption well has a positive effect the preservation of thermal water reservoir resources as it replenishes exploited water, thereby improving balance. It is also important to note that, when choosing a method for managing used thermal water, it is crucial to accurately assess the stability of the fundamental parameters of the raw water, considering natural variability. Analyzing this variability and the expected stability of GW parameters is vital for the environmentally safe SGW management. Whatever the case, maintaining a low and well-defined variability for key parameters is extremely important for protecting the natural environment of the receiving surface water and ensuring the sustainability of the injection process [39].

In terms of environmental impact, it is evident that injecting water into rock masses via a deep absorption borehole is a safer option. This method does not pose the risk of surface water pollution or bring about changes in the aquatic environment. However, investments driven by environmental concerns must be both technically and economically feasible.

The most commonly used static criterion for assessing the economic efficiency of an investment is the Simple Payback Time (SPBT), which is defined as the time required to recover the investment associated with a specific project. In its classic form, the SPBT is calculated according to Equation (21), considering period from when the investment was commenced to the point when the total gross benefits resulting from the investment implementation equal the incurred outlay. In this case, SGW discharge/injection installations did not generate any benefits. Therefore, the SPBT was estimated using a modified approach in which the benefits were defined as investors’ benefits from conducting the activity using the SGW.

$$SPBT={\displaystyle \frac{-INV}{Z_i}}$$

(21)

where INV represents the investment cost associated with SGW discharge/injection installation (EUR], and the negative sign results from the adopted convention that expenses are represented as negative values. Furthermore, Z_i represents the net profit (EUR) resulting from the use of GW in subsequent years.

For the Chochołowskie Termy complex, the actual investment costs (INV) incurred for the SGW injection installation, i.e., the construction of the GT-1 absorption borehole, amounted to EUR 8,820,000. The installation of SGW pre-treatment, aeration, and cooling facilities as well as the construction of the outlet to the surface stream amounted to EUR 350,000. Then, the annual financial surplus from the operation of the Chochołowskie Termy recreational complex was estimated to be EUR 5,300,000, implying a strong financial condition enabling the implementation of both variants. However, a comparison of the SPBT values for both variants indicated that discharge into rivers is significantly more economical, with an almost 30-fold shorter cash payback time (

Table 4. Simple payback time for SGW development options.

Variant	SPBT [Years]
injection into rock mass	1.66
discharge to the river	0.07

).

In this context, the decision on how to manage used thermal waters should be based on a reliable assessment of the impact of introducing SGW, which is considered as wastewater, into receiving surface waters. Therefore, an exclusive economic analysis cannot serve as the sole basis for selecting a given SGW management strategy.

To assess the economic robustness of the project under changing market conditions, a sensitivity analysis was conducted to examine the effect of electricity price fluctuations on the Simple Payback Time (SPBT). Electricity costs influence operational savings and, indirectly, the annual financial surplus, especially when geothermal systems offset conventional energy usage. In this analysis, a ±20% variation in electricity price was considered around the assumed base-case value. It was assumed that a 10% change in electricity prices corresponds approximately to a 6% change in annual financial surplus from geothermal use, based on internal energy consumption patterns at the Chochołowskie Termy complex (

Table 5. Sensitivity of SPBT to electricity price variations (injection variant).

Electricity Price Change	Estimated Annual Surplus [EUR]	SPBT [Years]
−20%	4,784,000	1.84
−10%	5,042,000	1.75
Base case	5,300,000	1.66
10%	5,558,000	1.59
20%	5,816,000	1.52

).

A 20% increase in electricity prices shortens the SPBT by approximately 0.14 years due to higher cost avoidance and operational benefits. Conversely, a 20% price drop extends the payback period by around 0.18 years. Despite these shifts, the SPBT for the injection variant remains under 2 years in all cases, confirming the economic resilience of the project. The sensitivity analysis highlights that, while electricity price fluctuations do affect the SPBT, their impact is moderate and does not undermine the overall financial viability of the investment.

4. Conclusions

The objective of this study was to provide a comprehensive assessment of the feasibility of injecting SGW into a rock mass, considering geological, economic, and environmental factors. Thus, we proposed a detailed assessment framework for evaluating the actual possibilities and conditions for injecting cooled thermal water into rock masses in relation to discharge into surface waters. Our key conclusions were as follows:

• The proposed assessment framework for SGW injection should be made mandatory during the planning of new geothermal power plants or complexes worldwide. Thus, the benefits of using renewable geothermal resources would not undermined by an insufficient rock mass absorption capacity or adverse impacts on river environments.
• Our method offers a direct and reasonable approach for evaluating geothermal water injection and provides new insights relative to traditional geothermal water exploitation assessment methods.
• The dilemma of choosing between discharging SGW into surface waters or injecting it into rock masses should not be based solely on the results of economic analysis.
• The long-term exploitation of thermal water intake in the Podhale Basin demonstrates that the exploitation and injection of thermal water do not induce any changes in rock mass or lead to any deformation or subsidence of the terrain.
• SGW management via the absorption well has a positive effect on thermal water reservoir resources as it allows the replenishment of exploited water, ensuring balance.

Taken together, our findings address the critical question of whether SGW injection is geologically and technically feasible or whether it imposes an undue economic burden. These findings have significant implications for managing investments considering the balance between geological, environmental, and economic factors. Additionally, the application of this comprehensive and detailed approach for assessing rock mass absorption capacity, considering economic and environmental factors, is highly recommended to obtain a thorough estimate of geothermal potential without compromising the natural environment.