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The Podhale geothermal reservoir simulation for long-term sustainable production
Renewable Energy 99 (2016) 420e430
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The Podhale geothermal reservoir simulation for long-term sustainable production Wiesław Bujakowski a, Barbara Tomaszewska a, b, *, Maciej Miecznik a w, Poland Mineral and Energy Economy Research Institute of the Polish Academy of Sciences, Wybickiego 7, 31-261, Krako AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Department of Fossil Fuels, al. Mickiewicza 30, w, Poland 30-059 Krako a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2015 Received in revised form 25 June 2016 Accepted 13 July 2016 Available online 27 July 2016
The Podhale geothermal system, located in the southern, mountainous part of Poland, is the most valuable reservoir of geothermal waters discovered in the country to date and the one with the highest capacities in Central and Eastern Europe. Over 20 years of continuous operation has proved its stable operating parameters e a small drop in pressure and an unnoticeable temperature change. Production of over 500 m3/h of geothermal water with an 86 C wellhead temperature is current practise, while drilling a new production well and reconstruction of an injection well allows for production that may significantly exceed 600 m3/h. To utilize these vast resources, a binary power cycle for electricity and heat production is considered by group of researchers. The results of numerical modelling of heat extraction from the Podhale reservoir are presented in the article as a preliminary step to the detailed analysis of combined heat and power production through a binary power cycle. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Geothermal water Geothermal system modelling Conceptual model Podhale geothermal system
1. Introduction Located in the southern part of the country, the Podhale Basin is among the most promising and interesting areas in Poland from the point of view of extracting geothermal energy (Fig. 1). It has a very favourable reservoir and operating conditions (Fig. 2), which provide an appropriate basis for district heating, recreation, balneotherapy and other applications. It was precisely in Podhale that the use of geothermal water for generating energy commenced in Poland in the mid-1980s [1,2]. The Podhale heating system is currently the largest geothermal ska installation in Poland. It is operated by PEC Geotermia Podhalan S.A. and utilises three production wells with a total production capacity of 960 m3/h and two injection wells through which a maximum of 600 m3/h of water can be injected as of 2013 [3e5]. Wellhead pressure in the production wells is artesian pressure and amounts to ca. 2.7 MPa in static conditions; when operated at maximum capacity, wellhead pressure is ca. 1.2 MPa. Surplus water
* Corresponding author. Mineral and Energy Economy Research Institute of the w, Poland. Polish Academy of Sciences, Wybickiego 7, 31-261, Krako E-mail addresses: [email protected] (W. Bujakowski), tomaszewska@meeri. pl, [email protected] (B. Tomaszewska), [email protected] (M. Miecznik). http://dx.doi.org/10.1016/j.renene.2016.07.028 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
is discharged into the nearby Biały Dunajec river (the maximum admissible discharge is 200 m3/h) [6]. The mineral content of the water extracted here ranges from 2.5 to ca. 3 g/dm3, and wellhead temperature during steady operation is 86 C [7e9]. The total capacity of the geothermal heating system is ca. 40.8 MWt, while the total installed capacity of the heating plant is 80.5 MWt (geothermal heat plus the capacity of the peak-load gas/oil boiler and combined heat and power units) [10]. The geothermal heating system analysed is among the largest in Europe and is being steadily upgraded. In 2009, its total energy production came to 324 TJ including ca. 226 TJ from geothermal energy [10]; in 2013, the figures were respectively 393 TJ and ca. 300 TJ [11]. By the end of 2013, 1483 customers were connected to the geothermal heating grid (individual houses, multi-family buildings, hotels and boarding houses, schools and other buildings) [12]. The pipeline between the heating plant and the peak-load boiler room in Zakopane is around 14 km long (Fig. 1). The current total length of the heating network for the system in question is ca. 94,982 m [3e5]. In addition to using geothermal water for heating and recreation purposes, studies have also been undertaken with a view towards assessing the feasibility of building a binary cycle power plant at the site being investigated [13,14]. Calculations of power and energy potential were preceded by model studies aimed
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Fig. 1. Location of the study area (after [15]).
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Fig. 2. The depth coverage of the proposed numerical model against the geological cross section (on the basis of [2], after [15]).
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at evaluating the operation of the reservoir under the anticipated hydrogeothermal and economic conditions. In many geothermal systems, the key factor in determining the operating conditions of the geothermal reservoir, and in particular the amount of energy that can be extracted, is the manner of utilisation (or disposal) of geothermal waters [7e9]. International and domestic experience indicates that the systems used for extracting these waters may operate in one of the following arrangements: closed (with the water being injected back into the rock formation after the energy has been extracted), open (with the water being discharged into surface watercourses or drains after the energy has been extracted) and mixed (after the energy has been extracted, some water is returned to the reservoir and the rest is discharged). In the latter two cases, the extraction of water is often the reason for critical assessments related to reservoir engineering, the viability of the system and ensuring its appropriate long-term parameters, but also for environmental concerns resulting from the potential negative impact of the water discharged on the surface waters which receive it. A model procedure for disposing of the cooled geothermal water after the heat has been recovered should aim at optimising the operation of the reservoir in the long term. It should also take into account the renewal of water resources and the need to care for the environment. The Paris Basin area is a good example of long-term exploitation of the energy resources in geothermal water while maintaining a stable situation in the reservoir. The Middle Jurassic carbonate reservoir started to be exploited there in the early 1970s. Of the 55 geothermal doublets working in the 1980s, 34 are still in operation, of which 29 doublets produce thermal energy for district heating purposes [16]. More than 40 years of operation during which geothermal waters were sourced from numerous wells, the majority of which have capacities of more than 200 m3/h, have not depleted the thermal energy available in the reservoir. In none of the existing production wells has the cold front been reached. The decommissioning of 42 wells was caused solely by issues related to scaling and clogging or price competition from fossil fuels [16]. Similarities between the Podhale geothermal system and the Paris Basin include, among other matters, the type of rocks, their permeability and the well flow rates achieved. This gives cause for optimism about the continued operation of the most valuable geothermal area in Poland. The numerical modelling conducted confirms this belief. For many years, numerical modelling has been considered as an appropriate method and effective tool for forecasting conditions for the stable and safe operation of geothermal energy resources in the long term. The first computational codes for reservoir engineering purposes were developed in the early 1970s. By 2000, numerical modelling had been applied in more than 100 areas that showed significant potential for geothermal energy extraction [17]. The TOUGH simulation software package is widely used for the numerical modelling of combined mass and heat transport in porous and fractured media for multiphase and multicomponent fluids [18e27]. For simulating the processes that occur in geothermal systems, other simulators are used as well, e.g. TETRAD Reservoir Simulation [28], STAR [29] and SHEMAT [30]. In forecasting the operating conditions of the geothermal system, realistic capacities related to the extraction and disposal of water during the period in question were taken into account. This paper presents a conceptual and numerical model for the operation of the Podhale geothermal system, a part of which, including the concession area, has been ska S.A. company for more operated by the PEC Geotermia Podhalan than 20 years. The study evaluates the stability of operating conditions and forecasts of reservoir operation during a long-term (50year) horizon. The basic thesis to be verified was whether the disposal of some geothermal water to surface waters would not
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threaten the stable extraction of renewable energy resources in the region concerned.
2. Materials and methods A conceptual geological model was first created presenting the location and situation profiles of the hydro-geothermal reservoir as well as the geological structures situated above and below them. Perspective maps of the geological structures as well as geological cross-sections involving evaluation of the thickness of the rock structures [1,2,31e35] were created based on a detailed analysis of the publications and archive material available (mainly documentation relating to deep drilling) as well as on the mineralogical, petrographic and petrophysical evaluation of the rock strata conducted as part of the current study accompanied by the reinterpretation of archive seismic data. The hydro-geothermal parameters and lithostratigraphic data identified constituted the basis on which to develop spatial numerical models. For the numerical modelling of the thermal characteristics of the rock mass as well as the operation of the reservoir in a long-term (50-year) perspective, the PetraSim software e a pre- and post-processor for the TOUGH2 code e was used. The TOUGH2 (Transport of Unsaturated Groundwater and Heat) simulator was developed at the Lawrence Berkeley National Laboratory (LBNL). It is a software package for the numerical computation of mass and heat transport in porous and fractured media for multi-component and multi-phase fluids [18]. The uses of TOUGH2 include geothermal reservoir engineering, radioactive waste disposal, hydrogeology and carbon dioxide storage in geological strata. The basic mathematical equations that define the functionality of the code are as follows: mass and energy balance:
d ∭ M k dVn ¼ ∭ F k $ndGn þ ∭ qk dVn dt Vn Vn Vn
(1)
where:
Vn e subdomain of the flow system [m3], M e mass or energy per subdomain volume [kg/m3, J/m3], k e mass (water, air, CO2) or heat component indicator [], F e mass or heat flux [kg/(s,m2), J/(s,m2)], Gn e bounding surface [m2], q e sources or sinks (mass or heat) [kg/(s,m3), J/(s,m3)], n e normal vector on surface element dGn, pointing inward into Vn [], fluid transport in porous media is governed by Darcy's law:
k u ¼ ðVp rgÞ
m
(2)
where:
u e Darcy velocity of fluid flowing through a porous formation [m/s], k e absolute permeability [m2], m e dynamic viscosity [Pa$s], Vp e pressure gradient [Pa/m], r e fluid density [kg/m3], g e vector of gravitational acceleration [m/s2], mass accumulation:
Mk ¼ 4 where:
X b
sb rb Xbk
(3)
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F e porosity [],
Sb e saturation of phase b [], rb e density of phase b [kg/m3], b e phase indicator [], X e mass fraction [], k e mass (water, air, CO2) or heat component indicator []. heat accumulation:
M k ¼ ð1 4ÞrR cR T þ 4 where:
X b
sb rb ub
(4)
F e porosity [],
Sb e saturation of phase b [], rb e density of phase b [kg/m3], rR e rock density [kg/m3], b e phase indicator [],
cR e specific heat of rock [J/(kg,K)], ub e specific internal energy of phase b [J/kg], T e temperature [K, C]. TOUGH2 uses the integral finite differences method for spatial and temporal discretization. A system of 2N (N e number of elements) linear differential equations is solved using direct or iterative methods. The first type of method is more reliable, but requires a large amount of memory and a computation time that grows at a rate of N3, where N is the number of equations to be solved. The opposite of this are iterative methods (eg. the conjugate gradient method) which are faster and in which the calculation time is less dependent on the size of the computing grid (Nw, where w z 1.4e1.6). Increased computational efficiency comes at the expense of reduced accuracy and reliability in comparison with direct methods [18]. The numerical model domain is a 7.5 km 7 km space with a total surface area of 52.5 km2 (Fig. 3). The reservoir in question is located within the boundaries of the study area. It includes five ska IG-1, Ban ska PGP-1, geothermal wells: three production (Ban ska PGP-3) and two injection: Biały Dunajec PAN-1 and Biały Ban ska PGP-1 well Dunajec PGP-2. The operating parameters of the Ban
place it among the best in Europe. It is a self-outflow well with an approved capacity of 550 m3/h. The Pieniny Klippen Belt presents a barrier to the movement of geothermal waters towards the north (Fig. 2). The southern boundary of the model is situated 3.1 km south of the Biały Dunajec ska PGP-1 proPGP-2 injection well, i.e. 4.9 km south of the Ban duction well. Since there is no significant lithological or tectonic boundary to the east and to the west of the area analysed, the model adopted measures which covered 7 km in the W-E direction with the wells being positioned near its centre. Vertically, the model extends from 500 m a.s.l. (top) to 3500 m a.s.l. (bottom), resulting in a model thickness of 3000 m. The computational grid consists of Voronoi polyhedra whose size increases together with the distance from the wells implemented (Fig. 4). Digitised structural maps of major lithostratigraphic units resulting from the interpretation of a 3D seismic image from 2002 were imported into the numerical model (Fig. 5). The model was constructed from 30 layers, each 100 m thick. The total number of elements was 24,360.
3. Results 3.1. Conceptual modelling The analysis of all available source materials made it possible to isolate 19 groups of geological structures representative of the lithological diversity of the study area (Table 1). In most cases, these are not formal geological units but they have been distinguished in a way that enables the mapping of actual litho-structural diversity. The Podhale geothermal system exhibits complex tectonics, which are expressed as a system of faults resulting from the movement and overthrusting of nappes and vertical movements of the rock formation with its subsequent post-kinematic annealing. As a result, the formations within this system have a block structure. Fault amplitudes reach several dozen (and in some regions e several hundred) metres, which is indicated by surface research [36] and confirmed by the results of drilling and to some extent by 3D seismic surveys [35]. Faults and cracks largely determine the
Fig. 3. The numerical model of the Podhale geothermal reservoir.
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Fig. 4. The numerical model grid for the Podhale geothermal reservoir (based on [14]).
Fig. 5. Lithostratigraphic layer system in the numerical model (based on [14]).
circulation of waters, and their spatial orientation modifies local flow directions. The main collector of geothermal waters is the Triassic Biały Dunajec unit, which is composed of carbonate rocks (dolomites, limestones) together with an overlying layer of Eocene limestones and conglomerates (Figs. 2 and 3). Above the water bearing nummulitic Eocene there are layers of impermeable Podhale Flysch up to 2700 m thick in the production well zone and over 3000 m thick w and in the western part of the basin, in the vicinity of Wito w [2,31,32]. The Biały Dunajec unit dips towards the north, Chochoło thinning out where it comes into contact with the Pieniny Klippen
Belt in a zone where a network of parallel faults is present, intersecting the underlying impermeable layers. These faults guide the flow of water towards the surface (the northern ascension zone). The Pieniny Klippen Belt barrier, which has been interpreted on the ska basis of hydrodynamic tests, is located ca. 1700 m from the Ban PGP-1 well at the depth of the geothermal water collector, i.e. the Biały Dunajec unit [1], and is a contact zone with impermeable formations. Similar conditions are present in the injection well zone in Biały Dunajec, where the interpretation of seismic images has revealed the existence of three parallel local faults located ca. 1.3 km south of the Biały Dunajec PGP-2 well, and one of which acts
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Table 1 List of specified geological structures constituting the model and their parameters.
as a sealing barrier [31,32]. The total thickness of the aquifer around ska and Biały Dunajec wells is up to 700 m, but its effective the Ban ska PGP-1 well thickness is much smaller (up to 187 m for the Ban ska PGP-3). Both the production and injection and 236 m for the Ban ska wells mainly reach the upper part of the aquifer. Only the Ban PGP-3 well spans the entire thickness of the main geothermal collector. The geothermal waters circulate in the direction in which the water-bearing strata dip, i.e. from the south to the north, splitting out before the impermeable barrier of the Pieniny Klippen Belt towards the east and west (Fig. 2). The speed of water flow gradually decreases from the south to the north from several dozen to a few metres per year. These waters are drained by the overlying flysch strata, but only to a minor degree, and this results in slightly elevated temperatures in some springs [2]. In the central part of the area modelled, in the vicinity of the Biały Dunajec wells, a positive thermal anomaly was found, which was reflected by reservoir temperatures of 80e100 C at a depth of 2e3 km, higher than that indicated by the local geothermal gradient, which is of the order of 1.9e2.3 C/100 m [31]. This is explained by the transport of heat through the network of deep ska IG-1 well, the presence of faults. In the thermal profile of the Ban impermeable layers of Podhale Flysch can be discerned which provide insulation against the further migration of heat towards the surface, however in the contact zone with the Podhale system near Pieniny, positive thermal anomalies were discovered in flysch layers (northern ascension zone) [37]. 3.2. Numerical model Boundary conditions, including a zone of constant pressure and
temperature, were adopted for the top layer of the model at a depth of 500 m a.s.l., which is considered to be far enough from the zone where intensive heat and mass transport occurs. A similar boundary was adopted for the bottom layer of the model at a depth of 3500 m a.s.l. The Dirichlet boundary condition was also applied to the southern boundaries of the model, which are at a significant distance from the existing wells. Groundwater flows from the south to the north within the hydrogeothermal reservoir. The northern boundary of the model runs along the contact zone with impermeable Pieniny Klippen Belt formations. The Neumann boundary condition (no flow boundary) was applied to this unit. Since the geothermal waters in the Podhale geothermal system have a low mineral content (up to 3 g/dm3), the EOS1 (Equation of State) of the TOUGH2 code was used to simulate mass and heat transport. Based on the thermal profiles obtained in geothermal wells during the studies that were conducted, the top model layer (z ¼ 500 m a.s.l.) is assigned temperatures in the range 46e58 C (Fig. 6a) and a reservoir pressure of p ¼ 146.3 bar. On the other hand, the bottom layer of the model, with the ordinate z ¼ 3500 m a.s.l., is assigned temperatures in the range 107e120 C (Fig. 6b) and a reservoir pressure p ¼ 425.3 bar. Temperature distribution in the model area was calibrated on the basis of the thermal profiles of the Biały Dunajec PGP-2 and ska IG-1 wells for steady-state conditions. The temperature and Ban pressure values assigned to boundary layers of the model (Dirichlet boundary condition) were adopted on the basis of the documentation of the wells and changes in the petrophysical parameters (thermal conductivity and permeability) of individual rock formations. These resulted in a very good fit between the numerical model and the values obtained during the measurements carried out (Fig. 7). The temperature distribution in the pre-production
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Fig. 6. Forecast temperature in the top layer (z ¼ 500 m a.s.l e fig. a.) and in the bottom layer (z ¼ 3500 m a.s.l. e fig. b) of the model (based on [14]).
ska IG-1 well. Fig. 7. Thermal calibration of the numerical model. Curves of best fit of thermal measurement and modelling for the: a) Biały Dunajec PGP-2 well and b) Ban
Fig. 8. Forecast temperature distribution prior to exploitation along the AA' cross section.
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ska IG-1 well. Fig. 9. Calibration of the exploitation model e Ban
(steady) state along the SeN axis (AA0 section) of the model is shown in Fig. 8. The operating model was calibrated on the basis of the data obtained during the monitoring of the operational parameters. Data showing the operating history are reliable, since they illustrate the operation of the system over a long period of time. Examples of the quality of fit between the model and measurement curves are illustrated in Figs. 9 and 10. The data range presented for the fit between the model calibration results and the operational data corresponds to the period during which water was extracted from each of the individual geothermal wells analysed. The second well did not operate at the time. By adopting such an approach, hydraulic parameters could be more precisely matched to the reservoir structures analysed.
The operational variant modelled assumed that it would be possible to make cascading use of the energy resources in: 1) a binary cycle power plant; 2) a district heating system; and 3) a recreation facility. Therefore it was assumed that water would be extracted from the two wells with the best operating parameters, ska PGP-1 and PGP-3. Total output at a level of 600 m3/h and i.e. Ban a temperature of 86 C was assumed, of which 430 m3/h is dis ska PGP-1 well and 170 m3/h from the Ban ska charged from the Ban PGP-3 well. At the study stage, it was assumed that there would be realistic possibilities for the disposal of the water used, i.e. the injection of cooled water into the formation using the Biały Dunajec PGP-2 well with a capacity of 400 m3/h and a wellhead pressure of 4.2e5.0 MPa, and 200 m3/h of water being discharged to surface watercourses. Taking the manner of its utilisation into account, it
ska PGP-1 well. Fig. 10. Calibration of the exploitation model e Ban
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Fig. 11. Change of the reservoir temperature and pressure versus time in the pro ska PGP-1 (z ¼ 2150 m a.s.l.); b) Ban ska PGP-3 (z ¼ 2350 m duction wells: a) Ban a.s.l.).
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was assumed that the water would be cooled to 30 C. In the ska IG-1 production well reservoir operation model above, the Ban was not included due to its relatively low capacity and lower water temperature (approx. 82 C), which would be less than optimal in the context of using the water for the generation of electricity. The Biały Dunajec PAN-1 injection well, which was not in operation during work on the study, was not included either because of injectivity problems and the need to reconstruct the well. Fig. 11 shows a forecast of changes in reservoir conditions (temperature and pressure) within 50 years from the commencement of operation. It is expected that the decline in reservoir ska PGP-1 well will amount pressure in the surroundings of the Ban to ca. 0.57 MPa and that the reservoir temperature will fall by DT z 0.5 C. The forecast results obtained are in line with the results of tests and continuous monitoring data after 20 years of reservoir operation with a mean capacity of ca. 400 m3/h. The ska PGP-1 production well has been the main source of energy Ban for the district heating system for many years. Its operation has been stable, and in particular no change in the temperature of the water extracted has been observed. As concerns the newly drilled ska PGP-3 well, the projected drop in reservoir pressure after a Ban period of 50 years amounts to ca. 0.4 MPa, but at the same time a slight increase in the temperature of the water produced from this well by about 0.4 C is expected. This increase can be explained as the effect of the observed fault zone and the adoption of increased vertical permeability in the well zone in the model as well as from the presence of deeper reservoir levels in the north-western part of the area modelled, from which water is expected to flow into the well (Fig. 3). In the analytical forecast, the possible extent of the cold front after a period of 50 years from the commencement of operation has also been assessed. The results of the numerical modelling conducted indicate that the production well zone is not threatened by cooling in the variant adopted for the extraction of geothermal water from the reservoir (Fig. 12). The current water discharge to surface watercourses at the permissible rate of 200 m3/h (an annual average of 135 m3/h in the years 2008e2010) has contributed to a drop in wellhead pressure by about 0.4 MPa both in summer and in winter. The minimum ska PGP-1 well during the period of wellhead pressure for the Ban maximum water extraction fell from ca. 1.6 MPa in 2005 to 1.2 MPa in 2010. The drop in wellhead pressure is also associated with the total amount of water produced, which increased from ca. 400 m3/h
Fig. 12. Forecast temperature distribution after 50 years of operation along the AA0 cross section.
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in 2007 to ca. 500 m3/h in 2010. The discharge of part of the cooled water did not have a significant adverse impact on reservoir parameters, and it allowed for significant cost savings that were associated with the need to purchase electricity to drive the pumps that inject cooled water into the Biały Dunajec PGP-2 injection well. The operating system as modelled, which assumes the injection of only two-thirds of the geothermal water extracted and the discharge of one-third to surface waters, shows a slight increase in depression in the production well area. Taking into account the forecast results and the possibility of the future operation of existing wells at a capacity considerably in excess of 600 m3/h, the mining company has reconstructed the out-of-commission Biały Dunajec PAN-1 injection well, which has resulted in a significant improvement in the conditions for injecting water into the formation. Following reconstruction, the injection capacity of the well is estimated at 375 m3/h. 4. Conclusion A model procedure for extracting geothermal energy resources should ensure that the resources are renewable, enable long-term safe reservoir operation, and also ensure that the activities are both cost-effective and environmentally sound. Closed systems, which assume the injection of the entire stream of water extracted into the formation, are the best and safest from the point of view of the geothermal water reservoir. In the case under analysis, it is assumed that only two-thirds of the water extracted will be re-injected and the rest will be discharged into surface waters. The completed study, which assumes that the system would operate for 50 years, has demonstrated that reservoir pressure in the production well zone could drop by ca. 0.57 MPa and the injection well injection pressure could increase by ca. 0.87 MPa. The forecast results are in line with measurement results and tests of the geothermal system, which demonstrate that since the operation of the reservoir started 20 years ago, reservoir pressure has decreased by ca. 0.4e0.5 MPa. Acknowledgements This work was carried out under project number 398/2011/Wn06/FG-hg-tx/D, ordered by the Ministry of the Environment and financed by the National Fund for Environmental Protection and Water Management. References [1] W. Bujakowski, A. Barbacki, Potential for geothermal development in southern Poland, Geothermics 33 (2004) 383e395. [2] J. Chowaniec, Hydrogeology study of the western part of the Polish Carpathians, Biul. PIG 734 (2009) 1e98 (in Polish). [3] B. Tomaszewska, L. Paja˛ k, Geothermal water resources management e economic aspects of their treatment, Gospod. Surowcami Miner. 4 (2012) 59e70. [4] B. Tomaszewska, L. Paja˛ k, Using treated geothermal water to replenish network water losses in a district heating system, Pol. J. Environ. Stud. 22 (1) (2013) 243e250. [5] B. Tomaszewska, L. Paja˛ k, Cooled and desalinated thermal water utilization in the Podhale heating system, Gospod. Surowcami Miner. 29 (1) (2013) 127e139 (in Polish). ski, Possibilities for the efficient utilisation of [6] B. Tomaszewska, A. Szczepan spent geothermal waters, Environ. Sci. Pollut. Res. 21 (2014) 11409e11417. [7] B. Tomaszewska, M. Bodzek, Desalination of geothermal waters using a hybrid UF-RO process. Part I: boron. removal in pilot-scale tests, Desalination 319 (2013) 99e106. [8] B. Tomaszewska, M. Bodzek, The removal of radionuclides during desalination of geothermal waters containing boron using the BWRO system, Desalination 309 (2013) 284e290. [9] B. Tomaszewska, L. Paja˛ k, M. Bodzek, Application of a hybrid UF-RO process to geothermal water desalination. Concentrate disposal and costs analysis, Arch. Environ. Prot. 40 (3) (2014) 137e151.
d termalnych w Polsce (stan na rok 2009), [10] W. Bujakowski, Wykorzystanie wo Prz. Geol. 58 (7) (2010) 580e588 (in Polish). ska SA e Stan Obecny, Perspektywy [11] Cz Slimak, PEC Geotermia Podhalan Rozwoju. Ekologicznie I Ekonomicznie “Na Plusie”, Tech. Poszuk. Geol. Geo wnowazony _ term. Zro Rozw. 1 (2013) 25e33. ska, Geothermal energy country update report from Poland, [12] B. Ke˛ pin 2010e2014, in: Proceedings World Geothermal Congress, 2015. Melbourne. [13] L. Paja˛ k, W. Bujakowski, Geothermal energy in binary systems, Prz. Geol. 61.11 (2) (2013) 699e705 (in Polish). [14] W. Bujakowski, B. Tomaszewska (Eds.), Atlas of the Possible Use of Geothermal Waters for Combined Production of Electricity and Heat Using w, 2014, 307 pp. (in Binary Systems in Poland, Wydawnictwo “Jak”, Krako Polish with English extended abstracts). [15] B. Tomaszewska, B. Bielec, M. Miecznik, The assessment of hydrogeothermal conditions for treatment of part of cooled geothermal waters. The conceptual model of the Podhale geothermal system, Prz. Geol. 63 (2015) 1115e1121. [16] S. Lopez, V. Hamm, M. Le Brun, L. Schaper, F. Boissier, C. Cotiche, E. Giuglaris, 40 years of Dogger aquifer management in Ile-de-France, Paris Basin, France, Geothermics 39 (2010) 339e356. [17] M.J. O'Sullivan, K. Pruess, M.J. Lippmann, Geothermal reservoir simulation: the state-of-practice and emerging trends, in: Proceedings World Geothermal Congress, 2000. Kuyshu-Tohoku. [18] K. Preuss, C. Oldenburg, G. Moridis, TOUGH2 User's Guide, Version 2.0, Lawrence Berkley National Laboratory, California, 1999. [19] A. Arnaldsson, J.C. Berthet, S. Kjaran, S. Sigurðsson, Numerical scheme to simulate flow through an isotropic rocks in TOUGH2, Comput. Geosci. 65 (2014) 37e45. _ zał, _ [20] M. Miecznik, A. Sowizd B. Tomaszewska, L. Paja˛ k, Modelling geothermal conditions in part of the Szczecin Trough e the Chociwel area, Geologos 21 (3) (2015) 187e196, http://dx.doi.org/10.1515/logos-2115-0013. [21] H. Lei, J. Zhu, Numerical modeling of exploitation and reinjection of the Guantao geothermal reservoir in Tanggu District, Tianjin, China, Geothermics 48 (2013) 60e68. [22] S. Finsterle, E.L. Sonnenthal, N. Spycher, Advances in subsurface modeling using the TOUGH suite of Simulator, Comput. Geosci. 65 (2013) 2e12. [23] P. Jeanne, J. Rutqvist, D. Vasco, J. Garcia, P.F. Dobson, M. Walters, C. Hartline, A. Borgia, A 3D hydrogeological and geomechanical model of an enhanced geothermal system at the Geysers, California, Geothermics 51 (2014) 240e252. ska, L. Paja˛ k, M. Pussak, M. Stefaniuk, [24] W. Bujakowski, A. Barbacki, B. Czerwin Z. Trzesniowski, Integrated seismic and magnetotelluric exploration of the Skierniewice, Poland, geothermal test site, Geothermics 39 (2010) 78e93. [25] S. Carlino, A. Troiano, M.G. Di Giuseppe, A. Tramelli, C. Troise, R. Somma, G. De Natale, Exploitation of geothermal energy in active volcanic areas: a numerical modelling applied to high temperature Mofetegeothermal field, at Campi Flegrei caldera (Southern Italy), Renew. Energy 87 (1) (2016) 54e66. [26] L. Magnusdottir, S. Finsterle, An iTOUGH2 equation-of-state module for modeling supercritical conditions in geothermal reservoirs, Geothermics Geothermics 57 (2015) 8e17. _ zał, _ [27] W. Bujakowski, A. Barbacki, M. Miecznik, L. Paja˛ k, R. Skrzypczak, A. Sowizd Modelling geothermal and operating parameters of EGS installations in the lower triassic sedimentary formations of the central Poland area, Renew. Energy 80 (2015) 441e453. [28] Shook M. TETRAD reservoir simulation. Proceedings “Geothermal Energy and the Utility Market e The Opportunities and Challenges for Expanding Geothermal Energy in a Competitive Supply Market. March 24-26, 1992, San Francisco, CA. [29] Pritchett JW. STAR: A geothermal reservoir simulation system. Proc. World Geothermal Congress 1995, Florence, 18e31 May 1995, 2959e2963. [30] C.H. Clauser (Ed.), Numerical Simulation of Reactive Flow in Hot Aquifers: SHEMAT and Processing SHEMAT, Springer Berlin Heidelberg, 2003. ska, Warunki hydrotermalne i termiczne podhalan skiego systemu [31] B. Ke˛ pin geotermalnego w rejonie otworu Biały Dunajec PAN-1, Studia, rozprawy, w, 2001 (in Polish). monografie. Nr 93. Wyd. IGSMiE PAN. Krako ska, Warunki termiczne i paleotermiczne podhalan skiego systemu [32] B. Ke˛ pin geotermalnego, Studia, Rozprawy, Monografie nr 135, Wyd. IGSMiE PAN, w, 2006 (in Polish). Krako _ [33] Nagy S, Such, J, Szott W. Weryfikacja hydrodynamicznego modelu złozowego d termalnych niecki podhalan skiej na podstawie testo w podfliszowych wo hydrodynamicznych e X Mie˛ dzynarodowa konferencja naukowo-techniczna Nowe metody i technologie w geologii naftowej, wiertnictwie, eksploatacji w, 24e25 czerwca 1999. T. 2, 15e21 (in otworowej i gazownictwie: Krako Polish). [34] S. Nagy, P. Długosz, Identification of the low-enthalpy Podhale geothermal reservoir based upon long term interference and pulse hydrodynamic testing, World Geotherm. Congr. (2000). KyushueTohoku, Japan. w badan sejsmicznych; Temat: [35] Ochał J, Ga˛ siorek E. Opracowanie wyniko ska Nizna _ Ban e Biały Dunajec e 3D. Rok: 2001/2002-Rozdz. 9, 10, Geofizyka w (Arch. PEC Geotermia Podhalan ska S.A.) (in Polish). Krako [36] L. Mastella, Tektonika fliszu we wschodniej cze˛ sci Podhala, Roczn. PTG 45 (1975) 3e4 (in Polish). skiego pasa [37] P. Pomianowski, Anomalie termiczne nad strefa˛ kontaktu pienin skiego, Prz. Geol. 2 (1988) 127e128 (in Polish). skałkowego i fliszu podhalan
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The Podhale geothermal reservoir simulation for long-term sustainable production Wiesław Bujakowski a, Barbara Tomaszewska a, b, *, Maciej Miecznik a w, Poland Mineral and Energy Economy Research Institute of the Polish Academy of Sciences, Wybickiego 7, 31-261, Krako AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Department of Fossil Fuels, al. Mickiewicza 30, w, Poland 30-059 Krako a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2015 Received in revised form 25 June 2016 Accepted 13 July 2016 Available online 27 July 2016
The Podhale geothermal system, located in the southern, mountainous part of Poland, is the most valuable reservoir of geothermal waters discovered in the country to date and the one with the highest capacities in Central and Eastern Europe. Over 20 years of continuous operation has proved its stable operating parameters e a small drop in pressure and an unnoticeable temperature change. Production of over 500 m3/h of geothermal water with an 86 C wellhead temperature is current practise, while drilling a new production well and reconstruction of an injection well allows for production that may significantly exceed 600 m3/h. To utilize these vast resources, a binary power cycle for electricity and heat production is considered by group of researchers. The results of numerical modelling of heat extraction from the Podhale reservoir are presented in the article as a preliminary step to the detailed analysis of combined heat and power production through a binary power cycle. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Geothermal water Geothermal system modelling Conceptual model Podhale geothermal system
1. Introduction Located in the southern part of the country, the Podhale Basin is among the most promising and interesting areas in Poland from the point of view of extracting geothermal energy (Fig. 1). It has a very favourable reservoir and operating conditions (Fig. 2), which provide an appropriate basis for district heating, recreation, balneotherapy and other applications. It was precisely in Podhale that the use of geothermal water for generating energy commenced in Poland in the mid-1980s [1,2]. The Podhale heating system is currently the largest geothermal ska installation in Poland. It is operated by PEC Geotermia Podhalan S.A. and utilises three production wells with a total production capacity of 960 m3/h and two injection wells through which a maximum of 600 m3/h of water can be injected as of 2013 [3e5]. Wellhead pressure in the production wells is artesian pressure and amounts to ca. 2.7 MPa in static conditions; when operated at maximum capacity, wellhead pressure is ca. 1.2 MPa. Surplus water
* Corresponding author. Mineral and Energy Economy Research Institute of the w, Poland. Polish Academy of Sciences, Wybickiego 7, 31-261, Krako E-mail addresses: [email protected] (W. Bujakowski), tomaszewska@meeri. pl, [email protected] (B. Tomaszewska), [email protected] (M. Miecznik). http://dx.doi.org/10.1016/j.renene.2016.07.028 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
is discharged into the nearby Biały Dunajec river (the maximum admissible discharge is 200 m3/h) [6]. The mineral content of the water extracted here ranges from 2.5 to ca. 3 g/dm3, and wellhead temperature during steady operation is 86 C [7e9]. The total capacity of the geothermal heating system is ca. 40.8 MWt, while the total installed capacity of the heating plant is 80.5 MWt (geothermal heat plus the capacity of the peak-load gas/oil boiler and combined heat and power units) [10]. The geothermal heating system analysed is among the largest in Europe and is being steadily upgraded. In 2009, its total energy production came to 324 TJ including ca. 226 TJ from geothermal energy [10]; in 2013, the figures were respectively 393 TJ and ca. 300 TJ [11]. By the end of 2013, 1483 customers were connected to the geothermal heating grid (individual houses, multi-family buildings, hotels and boarding houses, schools and other buildings) [12]. The pipeline between the heating plant and the peak-load boiler room in Zakopane is around 14 km long (Fig. 1). The current total length of the heating network for the system in question is ca. 94,982 m [3e5]. In addition to using geothermal water for heating and recreation purposes, studies have also been undertaken with a view towards assessing the feasibility of building a binary cycle power plant at the site being investigated [13,14]. Calculations of power and energy potential were preceded by model studies aimed
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Fig. 1. Location of the study area (after [15]).
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Fig. 2. The depth coverage of the proposed numerical model against the geological cross section (on the basis of [2], after [15]).
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at evaluating the operation of the reservoir under the anticipated hydrogeothermal and economic conditions. In many geothermal systems, the key factor in determining the operating conditions of the geothermal reservoir, and in particular the amount of energy that can be extracted, is the manner of utilisation (or disposal) of geothermal waters [7e9]. International and domestic experience indicates that the systems used for extracting these waters may operate in one of the following arrangements: closed (with the water being injected back into the rock formation after the energy has been extracted), open (with the water being discharged into surface watercourses or drains after the energy has been extracted) and mixed (after the energy has been extracted, some water is returned to the reservoir and the rest is discharged). In the latter two cases, the extraction of water is often the reason for critical assessments related to reservoir engineering, the viability of the system and ensuring its appropriate long-term parameters, but also for environmental concerns resulting from the potential negative impact of the water discharged on the surface waters which receive it. A model procedure for disposing of the cooled geothermal water after the heat has been recovered should aim at optimising the operation of the reservoir in the long term. It should also take into account the renewal of water resources and the need to care for the environment. The Paris Basin area is a good example of long-term exploitation of the energy resources in geothermal water while maintaining a stable situation in the reservoir. The Middle Jurassic carbonate reservoir started to be exploited there in the early 1970s. Of the 55 geothermal doublets working in the 1980s, 34 are still in operation, of which 29 doublets produce thermal energy for district heating purposes [16]. More than 40 years of operation during which geothermal waters were sourced from numerous wells, the majority of which have capacities of more than 200 m3/h, have not depleted the thermal energy available in the reservoir. In none of the existing production wells has the cold front been reached. The decommissioning of 42 wells was caused solely by issues related to scaling and clogging or price competition from fossil fuels [16]. Similarities between the Podhale geothermal system and the Paris Basin include, among other matters, the type of rocks, their permeability and the well flow rates achieved. This gives cause for optimism about the continued operation of the most valuable geothermal area in Poland. The numerical modelling conducted confirms this belief. For many years, numerical modelling has been considered as an appropriate method and effective tool for forecasting conditions for the stable and safe operation of geothermal energy resources in the long term. The first computational codes for reservoir engineering purposes were developed in the early 1970s. By 2000, numerical modelling had been applied in more than 100 areas that showed significant potential for geothermal energy extraction [17]. The TOUGH simulation software package is widely used for the numerical modelling of combined mass and heat transport in porous and fractured media for multiphase and multicomponent fluids [18e27]. For simulating the processes that occur in geothermal systems, other simulators are used as well, e.g. TETRAD Reservoir Simulation [28], STAR [29] and SHEMAT [30]. In forecasting the operating conditions of the geothermal system, realistic capacities related to the extraction and disposal of water during the period in question were taken into account. This paper presents a conceptual and numerical model for the operation of the Podhale geothermal system, a part of which, including the concession area, has been ska S.A. company for more operated by the PEC Geotermia Podhalan than 20 years. The study evaluates the stability of operating conditions and forecasts of reservoir operation during a long-term (50year) horizon. The basic thesis to be verified was whether the disposal of some geothermal water to surface waters would not
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threaten the stable extraction of renewable energy resources in the region concerned.
2. Materials and methods A conceptual geological model was first created presenting the location and situation profiles of the hydro-geothermal reservoir as well as the geological structures situated above and below them. Perspective maps of the geological structures as well as geological cross-sections involving evaluation of the thickness of the rock structures [1,2,31e35] were created based on a detailed analysis of the publications and archive material available (mainly documentation relating to deep drilling) as well as on the mineralogical, petrographic and petrophysical evaluation of the rock strata conducted as part of the current study accompanied by the reinterpretation of archive seismic data. The hydro-geothermal parameters and lithostratigraphic data identified constituted the basis on which to develop spatial numerical models. For the numerical modelling of the thermal characteristics of the rock mass as well as the operation of the reservoir in a long-term (50-year) perspective, the PetraSim software e a pre- and post-processor for the TOUGH2 code e was used. The TOUGH2 (Transport of Unsaturated Groundwater and Heat) simulator was developed at the Lawrence Berkeley National Laboratory (LBNL). It is a software package for the numerical computation of mass and heat transport in porous and fractured media for multi-component and multi-phase fluids [18]. The uses of TOUGH2 include geothermal reservoir engineering, radioactive waste disposal, hydrogeology and carbon dioxide storage in geological strata. The basic mathematical equations that define the functionality of the code are as follows: mass and energy balance:
d ∭ M k dVn ¼ ∭ F k $ndGn þ ∭ qk dVn dt Vn Vn Vn
(1)
where:
Vn e subdomain of the flow system [m3], M e mass or energy per subdomain volume [kg/m3, J/m3], k e mass (water, air, CO2) or heat component indicator [], F e mass or heat flux [kg/(s,m2), J/(s,m2)], Gn e bounding surface [m2], q e sources or sinks (mass or heat) [kg/(s,m3), J/(s,m3)], n e normal vector on surface element dGn, pointing inward into Vn [], fluid transport in porous media is governed by Darcy's law:
k u ¼ ðVp rgÞ
m
(2)
where:
u e Darcy velocity of fluid flowing through a porous formation [m/s], k e absolute permeability [m2], m e dynamic viscosity [Pa$s], Vp e pressure gradient [Pa/m], r e fluid density [kg/m3], g e vector of gravitational acceleration [m/s2], mass accumulation:
Mk ¼ 4 where:
X b
sb rb Xbk
(3)
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F e porosity [],
Sb e saturation of phase b [], rb e density of phase b [kg/m3], b e phase indicator [], X e mass fraction [], k e mass (water, air, CO2) or heat component indicator []. heat accumulation:
M k ¼ ð1 4ÞrR cR T þ 4 where:
X b
sb rb ub
(4)
F e porosity [],
Sb e saturation of phase b [], rb e density of phase b [kg/m3], rR e rock density [kg/m3], b e phase indicator [],
cR e specific heat of rock [J/(kg,K)], ub e specific internal energy of phase b [J/kg], T e temperature [K, C]. TOUGH2 uses the integral finite differences method for spatial and temporal discretization. A system of 2N (N e number of elements) linear differential equations is solved using direct or iterative methods. The first type of method is more reliable, but requires a large amount of memory and a computation time that grows at a rate of N3, where N is the number of equations to be solved. The opposite of this are iterative methods (eg. the conjugate gradient method) which are faster and in which the calculation time is less dependent on the size of the computing grid (Nw, where w z 1.4e1.6). Increased computational efficiency comes at the expense of reduced accuracy and reliability in comparison with direct methods [18]. The numerical model domain is a 7.5 km 7 km space with a total surface area of 52.5 km2 (Fig. 3). The reservoir in question is located within the boundaries of the study area. It includes five ska IG-1, Ban ska PGP-1, geothermal wells: three production (Ban ska PGP-3) and two injection: Biały Dunajec PAN-1 and Biały Ban ska PGP-1 well Dunajec PGP-2. The operating parameters of the Ban
place it among the best in Europe. It is a self-outflow well with an approved capacity of 550 m3/h. The Pieniny Klippen Belt presents a barrier to the movement of geothermal waters towards the north (Fig. 2). The southern boundary of the model is situated 3.1 km south of the Biały Dunajec ska PGP-1 proPGP-2 injection well, i.e. 4.9 km south of the Ban duction well. Since there is no significant lithological or tectonic boundary to the east and to the west of the area analysed, the model adopted measures which covered 7 km in the W-E direction with the wells being positioned near its centre. Vertically, the model extends from 500 m a.s.l. (top) to 3500 m a.s.l. (bottom), resulting in a model thickness of 3000 m. The computational grid consists of Voronoi polyhedra whose size increases together with the distance from the wells implemented (Fig. 4). Digitised structural maps of major lithostratigraphic units resulting from the interpretation of a 3D seismic image from 2002 were imported into the numerical model (Fig. 5). The model was constructed from 30 layers, each 100 m thick. The total number of elements was 24,360.
3. Results 3.1. Conceptual modelling The analysis of all available source materials made it possible to isolate 19 groups of geological structures representative of the lithological diversity of the study area (Table 1). In most cases, these are not formal geological units but they have been distinguished in a way that enables the mapping of actual litho-structural diversity. The Podhale geothermal system exhibits complex tectonics, which are expressed as a system of faults resulting from the movement and overthrusting of nappes and vertical movements of the rock formation with its subsequent post-kinematic annealing. As a result, the formations within this system have a block structure. Fault amplitudes reach several dozen (and in some regions e several hundred) metres, which is indicated by surface research [36] and confirmed by the results of drilling and to some extent by 3D seismic surveys [35]. Faults and cracks largely determine the
Fig. 3. The numerical model of the Podhale geothermal reservoir.
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Fig. 4. The numerical model grid for the Podhale geothermal reservoir (based on [14]).
Fig. 5. Lithostratigraphic layer system in the numerical model (based on [14]).
circulation of waters, and their spatial orientation modifies local flow directions. The main collector of geothermal waters is the Triassic Biały Dunajec unit, which is composed of carbonate rocks (dolomites, limestones) together with an overlying layer of Eocene limestones and conglomerates (Figs. 2 and 3). Above the water bearing nummulitic Eocene there are layers of impermeable Podhale Flysch up to 2700 m thick in the production well zone and over 3000 m thick w and in the western part of the basin, in the vicinity of Wito w [2,31,32]. The Biały Dunajec unit dips towards the north, Chochoło thinning out where it comes into contact with the Pieniny Klippen
Belt in a zone where a network of parallel faults is present, intersecting the underlying impermeable layers. These faults guide the flow of water towards the surface (the northern ascension zone). The Pieniny Klippen Belt barrier, which has been interpreted on the ska basis of hydrodynamic tests, is located ca. 1700 m from the Ban PGP-1 well at the depth of the geothermal water collector, i.e. the Biały Dunajec unit [1], and is a contact zone with impermeable formations. Similar conditions are present in the injection well zone in Biały Dunajec, where the interpretation of seismic images has revealed the existence of three parallel local faults located ca. 1.3 km south of the Biały Dunajec PGP-2 well, and one of which acts
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Table 1 List of specified geological structures constituting the model and their parameters.
as a sealing barrier [31,32]. The total thickness of the aquifer around ska and Biały Dunajec wells is up to 700 m, but its effective the Ban ska PGP-1 well thickness is much smaller (up to 187 m for the Ban ska PGP-3). Both the production and injection and 236 m for the Ban ska wells mainly reach the upper part of the aquifer. Only the Ban PGP-3 well spans the entire thickness of the main geothermal collector. The geothermal waters circulate in the direction in which the water-bearing strata dip, i.e. from the south to the north, splitting out before the impermeable barrier of the Pieniny Klippen Belt towards the east and west (Fig. 2). The speed of water flow gradually decreases from the south to the north from several dozen to a few metres per year. These waters are drained by the overlying flysch strata, but only to a minor degree, and this results in slightly elevated temperatures in some springs [2]. In the central part of the area modelled, in the vicinity of the Biały Dunajec wells, a positive thermal anomaly was found, which was reflected by reservoir temperatures of 80e100 C at a depth of 2e3 km, higher than that indicated by the local geothermal gradient, which is of the order of 1.9e2.3 C/100 m [31]. This is explained by the transport of heat through the network of deep ska IG-1 well, the presence of faults. In the thermal profile of the Ban impermeable layers of Podhale Flysch can be discerned which provide insulation against the further migration of heat towards the surface, however in the contact zone with the Podhale system near Pieniny, positive thermal anomalies were discovered in flysch layers (northern ascension zone) [37]. 3.2. Numerical model Boundary conditions, including a zone of constant pressure and
temperature, were adopted for the top layer of the model at a depth of 500 m a.s.l., which is considered to be far enough from the zone where intensive heat and mass transport occurs. A similar boundary was adopted for the bottom layer of the model at a depth of 3500 m a.s.l. The Dirichlet boundary condition was also applied to the southern boundaries of the model, which are at a significant distance from the existing wells. Groundwater flows from the south to the north within the hydrogeothermal reservoir. The northern boundary of the model runs along the contact zone with impermeable Pieniny Klippen Belt formations. The Neumann boundary condition (no flow boundary) was applied to this unit. Since the geothermal waters in the Podhale geothermal system have a low mineral content (up to 3 g/dm3), the EOS1 (Equation of State) of the TOUGH2 code was used to simulate mass and heat transport. Based on the thermal profiles obtained in geothermal wells during the studies that were conducted, the top model layer (z ¼ 500 m a.s.l.) is assigned temperatures in the range 46e58 C (Fig. 6a) and a reservoir pressure of p ¼ 146.3 bar. On the other hand, the bottom layer of the model, with the ordinate z ¼ 3500 m a.s.l., is assigned temperatures in the range 107e120 C (Fig. 6b) and a reservoir pressure p ¼ 425.3 bar. Temperature distribution in the model area was calibrated on the basis of the thermal profiles of the Biały Dunajec PGP-2 and ska IG-1 wells for steady-state conditions. The temperature and Ban pressure values assigned to boundary layers of the model (Dirichlet boundary condition) were adopted on the basis of the documentation of the wells and changes in the petrophysical parameters (thermal conductivity and permeability) of individual rock formations. These resulted in a very good fit between the numerical model and the values obtained during the measurements carried out (Fig. 7). The temperature distribution in the pre-production
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Fig. 6. Forecast temperature in the top layer (z ¼ 500 m a.s.l e fig. a.) and in the bottom layer (z ¼ 3500 m a.s.l. e fig. b) of the model (based on [14]).
ska IG-1 well. Fig. 7. Thermal calibration of the numerical model. Curves of best fit of thermal measurement and modelling for the: a) Biały Dunajec PGP-2 well and b) Ban
Fig. 8. Forecast temperature distribution prior to exploitation along the AA' cross section.
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ska IG-1 well. Fig. 9. Calibration of the exploitation model e Ban
(steady) state along the SeN axis (AA0 section) of the model is shown in Fig. 8. The operating model was calibrated on the basis of the data obtained during the monitoring of the operational parameters. Data showing the operating history are reliable, since they illustrate the operation of the system over a long period of time. Examples of the quality of fit between the model and measurement curves are illustrated in Figs. 9 and 10. The data range presented for the fit between the model calibration results and the operational data corresponds to the period during which water was extracted from each of the individual geothermal wells analysed. The second well did not operate at the time. By adopting such an approach, hydraulic parameters could be more precisely matched to the reservoir structures analysed.
The operational variant modelled assumed that it would be possible to make cascading use of the energy resources in: 1) a binary cycle power plant; 2) a district heating system; and 3) a recreation facility. Therefore it was assumed that water would be extracted from the two wells with the best operating parameters, ska PGP-1 and PGP-3. Total output at a level of 600 m3/h and i.e. Ban a temperature of 86 C was assumed, of which 430 m3/h is dis ska PGP-1 well and 170 m3/h from the Ban ska charged from the Ban PGP-3 well. At the study stage, it was assumed that there would be realistic possibilities for the disposal of the water used, i.e. the injection of cooled water into the formation using the Biały Dunajec PGP-2 well with a capacity of 400 m3/h and a wellhead pressure of 4.2e5.0 MPa, and 200 m3/h of water being discharged to surface watercourses. Taking the manner of its utilisation into account, it
ska PGP-1 well. Fig. 10. Calibration of the exploitation model e Ban
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Fig. 11. Change of the reservoir temperature and pressure versus time in the pro ska PGP-1 (z ¼ 2150 m a.s.l.); b) Ban ska PGP-3 (z ¼ 2350 m duction wells: a) Ban a.s.l.).
429
was assumed that the water would be cooled to 30 C. In the ska IG-1 production well reservoir operation model above, the Ban was not included due to its relatively low capacity and lower water temperature (approx. 82 C), which would be less than optimal in the context of using the water for the generation of electricity. The Biały Dunajec PAN-1 injection well, which was not in operation during work on the study, was not included either because of injectivity problems and the need to reconstruct the well. Fig. 11 shows a forecast of changes in reservoir conditions (temperature and pressure) within 50 years from the commencement of operation. It is expected that the decline in reservoir ska PGP-1 well will amount pressure in the surroundings of the Ban to ca. 0.57 MPa and that the reservoir temperature will fall by DT z 0.5 C. The forecast results obtained are in line with the results of tests and continuous monitoring data after 20 years of reservoir operation with a mean capacity of ca. 400 m3/h. The ska PGP-1 production well has been the main source of energy Ban for the district heating system for many years. Its operation has been stable, and in particular no change in the temperature of the water extracted has been observed. As concerns the newly drilled ska PGP-3 well, the projected drop in reservoir pressure after a Ban period of 50 years amounts to ca. 0.4 MPa, but at the same time a slight increase in the temperature of the water produced from this well by about 0.4 C is expected. This increase can be explained as the effect of the observed fault zone and the adoption of increased vertical permeability in the well zone in the model as well as from the presence of deeper reservoir levels in the north-western part of the area modelled, from which water is expected to flow into the well (Fig. 3). In the analytical forecast, the possible extent of the cold front after a period of 50 years from the commencement of operation has also been assessed. The results of the numerical modelling conducted indicate that the production well zone is not threatened by cooling in the variant adopted for the extraction of geothermal water from the reservoir (Fig. 12). The current water discharge to surface watercourses at the permissible rate of 200 m3/h (an annual average of 135 m3/h in the years 2008e2010) has contributed to a drop in wellhead pressure by about 0.4 MPa both in summer and in winter. The minimum ska PGP-1 well during the period of wellhead pressure for the Ban maximum water extraction fell from ca. 1.6 MPa in 2005 to 1.2 MPa in 2010. The drop in wellhead pressure is also associated with the total amount of water produced, which increased from ca. 400 m3/h
Fig. 12. Forecast temperature distribution after 50 years of operation along the AA0 cross section.
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in 2007 to ca. 500 m3/h in 2010. The discharge of part of the cooled water did not have a significant adverse impact on reservoir parameters, and it allowed for significant cost savings that were associated with the need to purchase electricity to drive the pumps that inject cooled water into the Biały Dunajec PGP-2 injection well. The operating system as modelled, which assumes the injection of only two-thirds of the geothermal water extracted and the discharge of one-third to surface waters, shows a slight increase in depression in the production well area. Taking into account the forecast results and the possibility of the future operation of existing wells at a capacity considerably in excess of 600 m3/h, the mining company has reconstructed the out-of-commission Biały Dunajec PAN-1 injection well, which has resulted in a significant improvement in the conditions for injecting water into the formation. Following reconstruction, the injection capacity of the well is estimated at 375 m3/h. 4. Conclusion A model procedure for extracting geothermal energy resources should ensure that the resources are renewable, enable long-term safe reservoir operation, and also ensure that the activities are both cost-effective and environmentally sound. Closed systems, which assume the injection of the entire stream of water extracted into the formation, are the best and safest from the point of view of the geothermal water reservoir. In the case under analysis, it is assumed that only two-thirds of the water extracted will be re-injected and the rest will be discharged into surface waters. The completed study, which assumes that the system would operate for 50 years, has demonstrated that reservoir pressure in the production well zone could drop by ca. 0.57 MPa and the injection well injection pressure could increase by ca. 0.87 MPa. The forecast results are in line with measurement results and tests of the geothermal system, which demonstrate that since the operation of the reservoir started 20 years ago, reservoir pressure has decreased by ca. 0.4e0.5 MPa. Acknowledgements This work was carried out under project number 398/2011/Wn06/FG-hg-tx/D, ordered by the Ministry of the Environment and financed by the National Fund for Environmental Protection and Water Management. References [1] W. Bujakowski, A. Barbacki, Potential for geothermal development in southern Poland, Geothermics 33 (2004) 383e395. [2] J. Chowaniec, Hydrogeology study of the western part of the Polish Carpathians, Biul. PIG 734 (2009) 1e98 (in Polish). [3] B. Tomaszewska, L. Paja˛ k, Geothermal water resources management e economic aspects of their treatment, Gospod. Surowcami Miner. 4 (2012) 59e70. [4] B. Tomaszewska, L. Paja˛ k, Using treated geothermal water to replenish network water losses in a district heating system, Pol. J. Environ. Stud. 22 (1) (2013) 243e250. [5] B. Tomaszewska, L. Paja˛ k, Cooled and desalinated thermal water utilization in the Podhale heating system, Gospod. Surowcami Miner. 29 (1) (2013) 127e139 (in Polish). ski, Possibilities for the efficient utilisation of [6] B. Tomaszewska, A. Szczepan spent geothermal waters, Environ. Sci. Pollut. Res. 21 (2014) 11409e11417. [7] B. Tomaszewska, M. Bodzek, Desalination of geothermal waters using a hybrid UF-RO process. Part I: boron. removal in pilot-scale tests, Desalination 319 (2013) 99e106. [8] B. Tomaszewska, M. Bodzek, The removal of radionuclides during desalination of geothermal waters containing boron using the BWRO system, Desalination 309 (2013) 284e290. [9] B. Tomaszewska, L. Paja˛ k, M. Bodzek, Application of a hybrid UF-RO process to geothermal water desalination. Concentrate disposal and costs analysis, Arch. Environ. Prot. 40 (3) (2014) 137e151.
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