Step-drawdown tests in exploitation wells for thermal and mineral water – Case study from Slovenia Črpalni preizkusi v korakih v eksploatacijskih vodnjakih za rabo termalne in mineralne vode – študija primera Slovenije Luka SERIANZ 1,2 , Nina RMAN 1 & Mihael BRENČIČ 3,1 1 Geological Survey of Slovenia, Dimičeva ulica 14, SI-1000 Ljubljana, Slovenia; e-mail: luka.serianz@geo-zs.si 2 Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova c. 2, SI-1000 Ljubljana, Slovenia 3 Faculty of Natural Sciences and Engineering, University of Ljubljana, Aškerčeva c. 12, SI-1000 Ljubljana, Slo venia Prejeto / Received 19. 11. 2019; Sprejeto / Accepted 13. 11. 2020; Objavljeno na spletu / Published online 7. 12. 2020 Key words: step-drawdown test, mineral water, thermal water, well performance, Slovenia Ključne besede: črpalni preizkus v korakih, mineralna voda, termalna voda, učinkovitost vodnjaka, Slovenija Abstract A comparative analysis of step-drawdown tests was performed in order to estimate the well performance in Slovenian thermal and mineral water wells. Tests were performed in 30 wells, each having its own maximum production rate determined in the concession decrees. The main focus of well performance analysis, using graphical analysis of the Jacob approximate equation, was to estimate the adequacy of the wells production rate as well as to identify possible changes in the technical status of the wells over years. 5 of total 30 wells were not included in the analysis due to technical issues during test performance. Well performance analysis includes the calculation of nonlinear well losses related to turbulent flow and linear head loss (aquifer and well) assumed to be related to laminar flow. Results indicate that the ratios between nonlinear well losses and linear head (well and aquifer) losses, in this paper referred as laminar losses, are from 6.9 % to 97.4 %. Laminar losses parameter suggests, all investigated wells were classified with either good (11 wells), medium (7 wells) or poor (7 wells) performance. The addressed analysis represents a very important basis for further thermal and mineral water extraction, e.g. optimizing the maximum allowed production rate as granted in concession decrees and diagnose potential changes in the technical status of each well. Izvleček Za oceno učinkovitosti eksploatacijskih vodnjakov za rabo termalne in mineralne vode je bila izvedena primerjalna analiza črpalnih preizkusov v korakih. Črpalni preizkusi v korakih so bili izvedeni v 30 vodnjakih, pri čemer je bila najvišja količina črpanja v posameznem vodnjaku enaka najvišji količini, ki izhaja iz koncesijskih uredb. Glavni namen analize učinkovitosti vodnjakov, ki je temeljila na Jacobovi grafični metodi obdelave črpalnega preizkusa v korakih, je oceniti in preveriti ustreznost sedaj dovoljenih količin črpanja, hkrati pa tudi določiti morebitne spremembe v tehničnem stanju vodnjakov. Pet od skupno 30 vodnjakov, zaradi tehničnih težav med samo izvedbo črpalnega preizkusa v korakih ni bilo vključenih v analizo. Sama analiza učinkovitosti vodnjakov temelji na izračunu nelinearnih izgub vodnjaka kot posledica turbulentne komponente toka in linearnih tlačnih izgub (vodonosnika in vodnjaka), privzetih kot posledica laminarne komponente toka. Rezultati analize kažejo, da so razmerja med nelinearnimi in linearnimi izgubami, ki so v tem članku opredeljena kot laminarne izgube, med 6,9 % in 97,4 %. S pomočjo parametra laminarnih izgub smo preiskane vodnjake ocenili z dobro (11), srednjo (7) ali slabo (7) učinkovitostjo. Obravnavana analiza predstavlja zelo pomembno podlago za nadaljnje črpanje termalne in mineralne vode, npr. za morebitno optimiziranje najvišje dovoljene količine izkoriščanja, ki izhajajo iz koncesij, in za diagnosticiranje potencialnih sprememb v tehničnem statusu posameznega vodnjaka. GEOLOGIJA 63/2, 281-294, Ljubljana 2020 https://doi.org/10.5474/geologija.2020.021 © Author(s) 2020. CC Atribution 4.0 License 282 Luka SERIANZ, Nina RMAN & Mihael BRENČIČ Introduction In Slovenia, many mineral and thermal wa- ter resources are found (Lapanje & Rman, 2009), however their management was not very efficient in the past (Rman et al., 2011, 2015). Thermal wa - ter is defined in the Water Act (Official Gazette, Nos. 67/02, 2/04 – ZZdrI-A, 41/04 – ZVO-1, 57/08, 57/12, 100/13, 40/14, 56/15 and 62/20) as a ground- water which exceeds the temperature of 20 °C at its outflow to the surface. Three types of low-tem- perature thermal systems occur here: warm spring systems in fissured and karstified car - bonate aquifers, aquifers in fissured carbonate and metamorphic rocks in basement rocks below sedimentary basins, and intergranular aquifers in sedimentary basins (Lapanje & Rman, 2009). Two thermal water regional flow systems are exploited by several users: mineral and thermal water bearing sandy aquifers in the Mura-Zala sedimentary basin in NE Slovenia and thermal water in dolomite aquifers in the basement of the Kriško-Brežice sedimentary basin (Rman et al., 2019). Mineral water is defined in the Water Act (Official Gazette, Nos. 67/02, 2/04 – ZZdrI-A, 41/04 – ZVO-1, 57/08, 57/12, 100/13, 40/14, 56/15 and 62/20) as groundwater which fulfils the writ - ten criteria and originates from a well, spring or capture but the criteria are not listed anywhere. In hydrogeological practice, we usually classify mineral waters as the ones having more than 1 g/l of total dissolved solids or more than 250 mg/l of CO 2 . Confusion is often caused because the term natural mineral water is also used in legislation. It is used for bottled groundwaters according to the Rules on natural mineral water, spring wa- ter and table water (Official Gazzette, Nos. 50/04, 75/05 and 45/08 – ZKme-1), which do not have a unique hydrogeological classification similar to aforementioned. In this paper, we use expression mineral water for a group of wells which produce waters for beverages. most of them are enriched in CO 2 an d th eref o re also ha v e high er min eral - ization. It would be expected that a reliable resource assessment is performed prior to the start of ex- ploitation but, in practice, the approach was rath - er different in the past. At sites with decades-long exploitation of mineral and thermal waters most water-producing objects (mostly wells) were not properly and/or systematically tested on capac- ity, if tested at all. No systematic research has yet been conducted on possible differences in hydraulic properties of production wells tapping intergranular or fissured aquifers. Average age of more than half of producing thermal and mineral water wells is above 30 years (Rman & Lapan- je, 2018). In some cases, operational issues such as mineral precipitation, corrosion, gas erup- tions and silt clogging are also reported. As it is necessary to determine whether the reasons for some noticed changes in well capacity are in de- terioration of the aquifer state or the object itself (Kralj et al., 2009; Rman, 2014; Szőcs et al., 2013), it is necessary to systematically monitor well’s efficiency and to timely implement measures for preventing possible deterioration. Considering the above, a methodology for comparison of well’s performance over a lifetime is reasonable to be applied systematically in or- der to, in the event of a change, identify the need for well revitalization or improvement of the aq- uifer’s status. This approach was identified also by the Slovenian Ministry of the Environment and Spatial Planning which implements decrees on the concession for the use of thermal water according to the Water Act (Official Gazette, Nos. 67/02, 2/04 – ZZdrI-A, 41/04 – ZVO-1, 57/08, 57/12, 100/13, 40/14, 56/15 and 62/20). There is a difference between the ones issued prior to the year 2015 (e.g. Official Gazette, No. 125/04) and afterwards (e.g. Official Gazette, Nos. 103/15 and 14/18). The newest Decrees contain more exten- sive monitoring requirements. Continuous mon- itoring of groundwater level, temperature and production rate, waste water temperature and quantity, plus regular water chemical and iso- topic composition have to be determined annu- ally. When annual abstraction at a site exceeds 200,000 m 3 /year monitoring data have to be on- line, daily transmitted to the database of the Slovenian Environmental Agency. Requirements include also systematic measurements of hydrau- lic characteristics of production wells (efficiency and specific capacity) in the period of every 3 and 6 years. A single-well step-drawdown test, also called step test, is used to quantify well performance criteria, such as well efficiency and its specif- ic capacity, and can provide an estimate of the maximum yield of the well (Abdalla & Mou- bark, 2018). Therefore, the step-drawdown test is one of the most frequently performed types of pumping test, particularly in the case of sin- gle well (Kawecki, 1995). Jacob (1947) was the first to present the conceptual formulation of step-drawdown test. Since that time, a number of articles were published in order to refine in- terpretation (Rorabaugh, 1953; Bierschenk, 1963; Lennox, 1966; Mogg, 1969; Sheahan, 1971; Birsoy and Summers, 1980; Gupta, 1989; Helweg, 1994 283 Step-drawdown tests in exploitation wells for thermal and mineral water– Case study from Slovenia and Kawecki, 1995). Those interpretations are based on graphical procedures, however some published papers on numerical analysis are also published (e.g. Louwyck et al., 2009). In this paper, the summary results of testing of 30 mineral and thermal water wells in Slo- venia are presented, which were performed in years from 2016 to 2018. The Jacob (1947) graph - ical method for step-drawdown test interpreta- tion in controlled and variable abstraction con- ditions was used as described by Kruseman and De Ridder (1990) as it provides an approximation of specific capacity e.g. well capacity versus mea - sured drawdown at different abstraction stages. The difference among mineral and thermal wa- ter wells, and fissured and intergranular aqui- fers was investigated. Appropriateness of the maximum allowed production rate as granted in concession decrees was compared to currently calculated value considering the actual technical status of the well. Methodology Theoretical background Performance Analytical approach In is very likely that in the immediate vicini- ty of the well, due to nature of groundwater flow there may be a deviation from the Darcy law de- scribing linear movement of fluid flow through a porous media. The deviation can be reflected as larger drawdown in producing well as the the- oretical model could predict. It is assumed that the measured drawdown in a pumped well con- sists of two components: aquifer losses (linear) and well losses (linear and non-linear). For an ideally confined system with radial flow to well with constant discharge with no well losses the drawdown s, using Theis (1935) nonequilibrium formula is given by: where Q is the discharge, t is the time and r w is the true radius of the pumped well, T is aqui- fer transmissivity, W(u) is the Theis well function (Theis, 1935) and: where where S is the storage coefficient. It was recog - nized that the terms beyond ln( u) in the expanded series of the well function W(u) can be neglected if u is sufficiently small (i.e. large values of elapsed time). Jacob (1950) suggested an approximation of u < 0.01. When u is small the well function may be approximated by: Substituting (4.) in (1.) gives: Considering equation (5), the total well loss is than given by: where s w is the total well loss and s(t) i s th e observed drawdown in the pumped well at time t. Assuming that total drawdown in the well is a sum of s 1 , s 2 and s 3 as suggested in Figure 1 the proposed model would then be (7): where B 1 is the linear aquifer loss coefficient occurring in the area where the flow is laminar (T/L 2 ), B 2 is the linear well loss coefficient (T/L 2 ), C is the non-linear well loss coefficient in T P /L 3P – 1 and P is an exponent of the well discharge (note: T is unit of time and L unit of length). All three coefficients are derived from the observation of the flow towards well. These are laminar and turbulent flow, or a combination of both. Lami- nar losses usually occur away from the boreholes, where the velocities are low, which is the case for linear aquifer losses. On the other hand the lin- ear well losses occurs relatively close to well bore in the damage zone of the aquifer (e.g. caused by drilling), where the hydraulic conductivity is usually considerably lower than that of the aqui- fer. The larger the hydraulic conductivity differ - ence, the more important is the value of the lin- ear well losses within the parameter B. Although some authors (e.g. Williams, 1985) suggested that head losses through the damage zone are gener- ally laminar, the arguments for such estimation are rather uncertain. In practice if the »independent« aquifer prop- erties are unknown, it is seldom possible to take B 1 and B 2 into account separately (Kruseman and ( ,) () 4 w Q sr t Wu T π = ( ) . ln ... !!! 234 0 5772 22 33 44 uuu Wu u u = − − +− + − + ⋅⋅⋅ 2 4 w rS u Tt = . ( ) ln 2 2 25 w Tt Wu rS ≅ . ( , ) ln 2 2 25 4 w w Q Tt sr t T rS π = () ( ,) ww s st sr t = − () ( ,) 2 12 w s t B r t Q B Q CQ = ++ (1) (2) (3) (4) (5) (6) (7) 284 Luka SERIANZ, Nina RMAN & Mihael BRENČIČ de Ridder, 1990). Therefore, we can determine B (B 1 +B 2 ) as aquifer circulation loss coefficient representing linear losses related as suggested, mainly to laminar flow nature and the drawdown is than (8) (Rorabaugh, 1953): where BQ and CQ P are drawdowns due to lin - ear and nonlinear losses respectively. According to Lennox (1966), the value of P is assumed to be in between 1.5 to 3.5, depending on the value of Q. Originaly Jacob (1947) suggested that the to- tal drawdown in the production well could be expressed as the sum of drawdown due to lam- inar flow ( BQ) and drawdown due to production well turbulence (CQ 2 ). This model was applied for the step-drawdown tests interpretation in this research. According to Jacob the drawdown in pumping well can be defined as (9): In literature, the ratio of the aquifer head loss to the total head losses is expressed as a well ef- ficiency (10): Values of E w ≥ 70 % or more is usually consid- ered acceptable and indicate a properly designed and developed well (Kresic, 1997). The well ef- ficiency can be expressed both with the results of a step–drawdown and aquifer test. The latter is needed in order to determine the value of B 1 . In practice, only the drawdown measurements in a pumping well are usually available, therefore the value of B 1 cannot be determined. The substi- tution of B and C into equation (10.) would over - estimate the well efficiency since B > B 1 . Driscoll (1986) therefore introduced parameter L p repre- senting laminar losses, which are interpreted as a ratio of the laminar head losses to the total head losses (11) (Kruseman and de Ridder, 1990): In case of examined step-drawdown tests the values of B 1 and B 2 cannot be calcula ted, there - fore the sum of linear well and aquifer losses is assumed as a parameter of linear head loss (BQ). Introducing the laminar loss (L p ) in order to eval- uate the ratio between non-linear well loss and linear head loss, leads to assumption that lin- ear well losses are also due to laminar flow. One could argue such simplification, but for the pur- pose of this research the conservative approach should satisfied the previous stated arguments. Fig. 1. Various components of head losses in a produ- ction well (modified from Kruseman and de Ridder, 1990). Sl. 1. Različne komponente tlačnih izgub v črpalnem vodnjaku (prirejeno po Kruseman in de Ridder, 1990). 2 s BQ CQ = + % 1 2 100 w BQ E BQ CQ  = ×  +  % 2 100 p BQ L BQ CQ  = ×  +  P s BQ CQ = + (8) (9) (10) (11) 285 Step-drawdown tests in exploitation wells for thermal and mineral water– Case study from Slovenia Some researchers propose the comparison be- tween wells based on range of C value (Walton, 1962) or C/B ratio (Bierschenk, 1963) in order to approximate well development indicating well deterioration and possible screen clogging. How- ever, such comparison might work in case of large diameter wells, but it is not appropriate in our case remarking the uncertainties explained hereinafter. Mogg (1969) asserted that the mag- nitude of C should not be used as an indicator of whether or not the well is properly designed or effectively developed because the correlation of field data shows that C is inversely proportional to the product of the discharge rate and the spe- cific capacity. In this paper we classify the well performance into three groups, according to L p value: good well performance ( L p > 70 %), medium well performance (30 % < L p ≤ 70 %) and poor well performance (L p ≤ 30 %). The relationship between the drawdown and discharge can be expressed as the specific capacity of a well, Q/s, which describes the productivity of both the aquifer and the well. The specific ca- pacity is not a constant but decreases as produc- tion continues. Several factors affect the specific capacity e.g. aquifer characteristics (hydraulic conductivity and storage coefficient), hydraulic barriers, technical performance of the well (e.g. penetration of well) and effective well screen per - foration. The Q/s ratio is useful also to compare pumping tests at different periods and allows predicting possible changes in well performance due to technical issues or variable hydraulic con- ditions in aquifers. Step-drawdown test performance In step-drawdown tests groundwater is ex- tracted in a number of consecutive time-intervals during which the pumping rate is constant but increases steadily with the number of time-in- tervals (Driscoll, 1986). By plotting s/Q versus Q and fitting the straight line thought the meas- urements points, the well coefficient C is given by the slope of the line and the aquifer loss coeffi- cient B is equal to the intercept, considering P = 2 (Kruseman & de Ridder, 1990). The reliability of the derived value for C increases with the num - ber of steps, since more data points are available to derive the slope of the straight line in the s/Q versus Q plot. The number of pumping steps is de - termined on the basis of known production rate, aquifer characteristics and available time inter- val for test performance, including pre-pump- ing interval and groundwater recovery when the well is not producing. All pumping steps have to be of same duration, usually 30 – 120 min or till drawdown stabilization in order to provide the minimal storativity effect (Kruseman and de Ridder, 1994). The maximum pumping rate has to be determined according to maximum exploita- tion rate of the well, or better should fit maxi- mum pump capacity. The Jacob (1947) graphical method idea is that the drawdowns measured at the end of individual steps should be steady. However, in reality, drawdown in a pumping well seldom stabilises. As a result, the quasi-steady drawdown measured at the end of each step is generally used in the analysis (Louwyck et al., 2009). Well num. Aquifer 1 WT 2 Date N 3 Stab. 4 Well num. Aquifer 1 WT 2 Date N 3 Stab. 4 1 I T 22.02.2018 3 yes 16 F T 28.08.2017 4 yes 2 I T 14.12.2017 4 no 17 F T 05.04.2017 3 yes 3 F T 22.11.2017 4 yes 18 F T 04.04.2017 3 yes 4 F T 15.11.2017 4 yes 19 F T 29.11.2017 3 no 5 F T 30.01.2018 3 yes 20 F T 28.11.2017 3 no 6 F T 30.06.2017 3 yes 21 F M 27.12.2017 3 yes 7 F T 29.06.2017 3 yes 22 I M 14.05.2016 3 yes 8 F T 07.06.2017 3 no 23 I M 13.05.2016 3 yes 9 F T 29.06.2017 3 yes 24 I M 07.05.2016 3 yes 10 F T 15.11.2017 4 yes 25 I M 20.04.2016 3 yes 11 I T 05.12.2017 3 no 26 I M 21.04.2016 3 yes 12 I T 20.12.2017 3 no 27 I M 25.04.2016 3 yes 13 I T 19.12.2017 4 no 28 I M 22.04.2016 3 yes 14 I T 16.11.2017 3 no 29 I M 03.05.2016 3 yes 15 F T 06.07.2017 1 yes 30 I M 26.04.2016 3 yes 1 Aquifer type: I = intergranular, F = fractured 2 WT (Water type): T = thermal, M = mineral 3 Number of pumping steps 4 Water level stabilization before pumping Table 1. Basic information about the performed step-drawdown tests. Tabela 1. Osnovne informacije o izvedbi črpalnih preizkusov v korakih. 286 Luka SERIANZ, Nina RMAN & Mihael BRENČIČ The Slovenian examples Step-drawdown tests were performed in 30 wells in years 2016 (8 wells), 2017 (19 wells) and 2018 (3 wells). At some sites (users), several wells were tested (Fig. 2). In Slovenia, one third of test - ed wells exploit mineral water while the other two thirds exploit thermal water (Table 1). Half of tested wells produce water from intergranular aquifers (mostly sandy layers) and others from fractured aquifers (mostly dolomite). In gener- al, three pumping steps were applied, while in six cases we were able to perform four pumping steps. In one case, the pumping rate was decreas- ing while the drawdown in the well progressed, therefore it was impossible to maintain the stable discharge during pumping. This case was not in- cluded in further analysis. All tested wells have been granted water concession and are active. Age of tested wells at reference year 2017 is between 6-60 years (Table 2). Water temperature is up to 63 °C in exploitation wells for thermal water and up to 30 °C in e xp l o i t a t i o n w e ll s f o r mineral water. Low to high mineralized water can be found in tested wells according to EC range of 391-14300 µS/cm. CO 2 level is highest in exploitation wells for mineral water in inter- granular aquifers. Prevailing Ca-Mg-HCO 3 wa- ter type in exploitation wells for mineral and thermal water in fracture aquifers is related to prevailing dolomite recharge area. Na-Cl water type can be found only in one well in the coastal area. Various water types can be found in inter- granular aquifers from Ca-Mg-HCO 3 to Na-Ca- HCO 3 -Cl in wells exploiting mineral water and from Na-HCO 3 to N a-HCO 3 -Cl in wells exploit- ing thermal water. Category Well age (years) T (°C) EC (µS/cm) CO 2 (g) (mg/l) Water type FM 6 12,5 400 nd Ca-Mg-HCO 3 IM 10-46 10-30 650-6450 176-2420 Ca-Mg-HCO 3 to Na-Ca-HCO 3 -Cl FT 7-49 21-40 391-14300 37-200 Ca-Mg-HCO 3 to Na-Cl IT 12-60 55-63 600-6813 20-50 Na-HCO 3 to Na-HCO 3 -Cl Fig. 2. Locations of tested wells. Sl. 2. Lokacije testiranih vrtin. Table 2. Summary information on well ages and basic physico-chemical composition of water for four aquifer type categories. Tabela 2. Povzetek informacij o starosti vrtin in osnovnih fizikalno-kemijskih značilnostih vode za štiri tipe vodonosnikov. 287 Step-drawdown tests in exploitation wells for thermal and mineral water– Case study from Slovenia All tests were performed taking into account the recommendation from the literature (e.g. Kruseman & De Ridder, 1990) as well as interna- tional standards (ISO 22282-4:2012). Neverthe- less, there were several issues identified during step test performance, mainly due to technical issues or limits depending on each site. Most common issue was inappropriate installment of measurement probes and water meters. Eventu- ally the situation improved or we used our own probes during tests performance. Second issue was the available time for test performance. Al- most all wells are active and the water exploita- tion is constant. Therefore, the time available for test performance (discharge reduction) was short and in some cases the recovery time prior to pumping for the test was insufficient to achieve an equilibrium static head before pumping start- ed. It was evaluated that the water level stabili- zation before pumping was achieved in 70 % of wells, while in other remaining wells the stat- ic head was at least very close to stabilization. During the step test performance, the stabiliza- tion level at each step was achieved only in 44 % of wells, even if the pumping step duration was in between 1.5 – 2 hours in 85 % of tests. That is a consequence of relative limited and slow water flow towards wells which is typical for investi- gated aquifers with a very low recharge rate. The pumping rate at each well was roughly deter- mined preliminary, before step test performance. Still, in 37 % cases, the actual applied pumping rate was different to preliminary proposed, most commonly due to unknown technical character- istics of water pump prior to the tests. Results and discussion An example of a case study To illustrate the applied analysis of step-draw - down test, an example of successfully performed pumping test in a well drilled in the intergranu- lar aquifer is presented. The test was performed with four pumping rates: 6 l/s, 17.1 l/s, 22.3 l/s and 27.9 l/s. Each rate was maintained for 1.5 h (Fig. 3a). Measured drawdown versus elapsed time after pumping began was then plotted on semi-logarithmic graph (Fig. 3b). Each step was extrapolated with a straight line beyond the period of pumping in order to obtain the incre- Fig. 3. Example of a step test performance: a.) field measurements of GWL and pumping rates, b.) drawdown versus time since pumping started, c.) specific drawdown versus pumping rate and d.) graphical interpretation of the step test analysis. Sl. 3. Primer izvedbe črpalnega preizkusa v korakih: a.) terenske meritve gladine podzemne vode in črpane količine, b.) zni - žanje v času od pričetka črpanja, c.) specifično znižanje v odvisnosti od črpane količine in d.) grafična interpretacija analize črpalnega preizkusa v korakih. 288 Luka SERIANZ, Nina RMAN & Mihael BRENČIČ mental drawdown caused by different pumping rates. Then s/Q ( s p ec ifi c dr a w d o wn ) v ers us c o r - responding value of Q was plotted on arithmetic graph (Fig. 3c). This approach is used to deter- mine coefficient B (linear losses) and C (nonlinear losses) and was proposed by Hantush (1964) and Bierschenk (1963). Plotting the s/Q values against the corresponding values of Q gave a straight line with a specific slope representing the C coeffi- cient, while the coefficient B represents the value at Q = 0 l/s. The data falls on a straight line (Fig. 3c). The values of identified coefficients were de - termined as B = 0.9397 s/m 2 in C = 0.0048 m∙s 2 /l 2 in this case respectively. Using those coefficient values, we can write the drawdown approximate equation (12): where hence this is a shape of Jacob equation and represents estimation of drawdown in the well within the time interval of 1.5 h. Fig. 3d represents drawdown measurements, calculated linear losses (BQ), well losses (CQ 2 ) and portion of laminar losses for each pumping rate. In the example a drawdown of 30 m was observed at maximum discharge rate of Q = 27.9 l/s. Apply- ing the Jacob equation, the observed drawdown is a consequence of linear loss (aquifer and well loss), which theoretically is approx. BQ = 26.22 m, and non-linear well loss, which is approx. CQ 2 = 3.74 m. At maximum pumping rate the well performance was determined as good, while the 87.5 % of measured aquifer at maximal pumping rate can be attributed to the aquifer loss. Also, the laminar losses decrease slowly. The average exploitation rate of presented well at normal production is about 3 l/s during summer and 28 l/s during winter. High laminar losses mean that this pumping rate does not sig- nificantly affect well performance neither reach- es the aquifer production capacity. Therefore, the winter exploitation rate of 28 l/s for the tested well does not exceed the well maximum capacity. Still, each well reflects its own characteris- tics, therefore it is almost impossible in practice to consider all wells with the same conceptual- ization. It must be emphasized that step test can only help to determine production capacity and performance of the well and is not intended to determine sustainable production rates of the aquifer. Comprehensive summary analysis Well performance The results of well performance analysis are available for 26 of total 30 tested wells (Table 2). Step-drawdown tests in wells 15, 18 and 21 have been subjected to technical issues, either due to inappropriate equipment installation or reduc- tion of pump efficiency due to large drawdowns which resulted in unstable pumping rate. Figure 4 shows the distribution of well loss coefficient C separately for thermal water wells in fractured and integranular aquifers and for mineral water wells. The values are ranging between 0.37 and 447 min 2 /m 5 (c onve r s ion 1 m i n 2 /m 5 = 0.0036 m∙s 2 /l 2 ). In the first decade C < 1 min 2 /m 5 there are 2 wells, 9 wells in C = 1 – 10 min 2 /m 5 , 9 wells in C = 10 – 100 min 2 /m 5 and 7 wells in C > 100 min 2 /m 5 . Some researchers propose the comparison between wells based on range of C val ue in order to a p - proximate well development indicating well de- terioration and possible screen clogging (Walton, 1962). But well loss coefficient C is empirically de- rived and therefore depending on several factors as for example effective open area of perforation. Each well was designed, constructed, and com- pleted for specific reasons in different areas un- der varying hydrogeological conditions, so direct comparison among wells is not possible. For ex- ample, the wells exploiting mineral water would according to high C value (C = 10 – 100 min 2 /m 5 ) indicate very poor well development, but still in same cases the laminar losses are high. Hence those mineral water wells are usually drilled with small diameter since the production rate is often lower than 5 l/s. As suggested by Mogg (1969) the low discharge rates in poor formations would show high values of C, which is often the case in mineral water wells. Another parameter which might also affect the comparison between wells is the penetration factor (Bierschenk, 1963). The partial penetra- tion increases the drawdown in a well because some of the water that enters the well must per- colate upward or downward from the screen or perforations. Water percolating vertically to a well moves through a greater distance than if it had percolated horizontally and across planes of greater resistance (i.e. horizontal permeability is greater than vertical permeability). Therefore, using C values for evaluation of wells status over time would only work in wells of similar tech- nical properties. Consequently, in the presented case it would not be appropriate. /. . 0 0048 0 9397 sQ Q = + .. 2 0 0048 0 9397 sQ Q = + (12) (13) 289 Step-drawdown tests in exploitation wells for thermal and mineral water– Case study from Slovenia Graphical analysis led to production of curves representing the interpolation of laminar losses in individual wells (Fig. 5). Each figure includes curves for the wells exploiting thermal water, separately for fractured or intergranular aqui- fers and mineral water and each curve represents an individual pumping test. In some cases, ex- tended extrapolation was used in order to com- pare results at the same scale. In general, they all show a decrease of laminar losses with increas- ing pumping rate. At the onset of turbulent flow, the specific capacity ( Q/s) decreases proportion- ally to the increase of pumping rate and, at the same time, the laminar losses are reduced. Anal- ysis of step drawdown tests has shown that sim- plified interpretations and comparison between the wells is not straightforward, but requires a detail knowledge about the system. Each well re- flects specific characteristics and conditions in which the test was performed. Therefore, even a small change of hydraulic boundary condition (e.g. activation of additional fractures, hydraulic barriers, …) significantly affects the test perfor- mance. Hydraulic characterisation of carbonate aquifers with fissured porosity is due to specific conditions, which are determined by pronounced heterogeneity and anisotropy, much more com- plex than the characterisation of aquifers with intergranular porosity. The dual porosity con- ceptualization based on the hydraulic exchange between different fractures dimensions is signif- icantly affecting the well performance, resulting in significantly variable Q/s ratio at each step. The theoretical laminar losses were compared with maximum allowed production rates as de- termined in concession decrees (Table 1). Wells 9 and 20 suggest negative linear head loss coef- ficient (B) and therefore cannot be evaluated by calculating laminar losses. Detailed interpreta- tion of calculated B v al u e s w e r e n o t t ak e n in t o account, since it was impossible to separate the linear well losses and aquifer losses. The negative B value which was calculated in two wells is most likely related to »breakthrough« pressure, which means that a certain pressure difference must be reached to develop a depression cone. Moreover it can also be assumed that negative B value in- dicate significant permeability reduction at the wellbore (e.g. compaction of the material during drilling, clogging from drilling mud,…). It is also assumed that the negative B values is related to significant time dependant aquifer characteris- tics. Those are especially important in fractured aquifers where the heterogeneous fractured me- dia determines the hydraulic boundary condi- tions. Laminar losses (L p ) in table 2 were calculated for maximum production rate as determined in concession decree (Q max CD). In case of 1 4 wells, the laminar losses are higher than 50 %, which means that linear head losses (aquifer and well) are still more important than nonlinear well loss - es. The detailed results are presented in Table 1. Based on step-drawdown test results it was pos- sible to evaluate either a maximum production rate (Q max ) should change (increase, decrease) or stay equal prior to Q max CD. From the total of 25 wells for which it was possible to calculate lam- inar losses, 11 can be addressed with good well performance. In all this wells, except one, the Q max CD can be increased and decree corrected. This is because the well losses represent a relatively small portion in the measured drawdown even after years of thermal water production. Medi- um well performance was identified in case of 7 wells. Also, in these wells, except one, the Q max CD can be increased and decree corrected. Five of those wells are producing mineral water with relatively low production rate. Therefore, the in- creased maximum production rate will not affect the aquifer capacity as allowed annual produc- tion quantity will not be changed in decrees. The other two wells are drilled in fractured aquifer where calculation of laminar losses can be un- certain. In 7 wells where their performance was evaluated as poor, the Q max CD not improved but on the contrary, in 3 wells the Q max CD should be decrease. Fig. 4. Distribution of C coefficient for different water and aquifer types. Sl. 4. Porazdelitev koeficienta C za različne tipe vod in vodonosnikov. 290 Luka SERIANZ, Nina RMAN & Mihael BRENČIČ *Black points represent tested pumping rates. Fig. 5. Laminar losses (L p ) in investigated wells according to water and aquifer type (a. thermal water – intergranular aquifer, b. thermal water – fractured aquifer and c. mineral water – intergranular aquifer). Sl. 5. Laminarne izgube v preiskovanih vodnjakih glede na tip vode in vodonosnika (a. termalna voda – medzrnski vodonos- nik, b. termalna voda – razpoklinski vodonosnik in c. mineralna voda – medzrnski vodonosnik). 291 Step-drawdown tests in exploitation wells for thermal and mineral water– Case study from Slovenia Table 3. Results of step-drawdown test analysis for 30 thermal and mineral water wells in Slovenia. Tabela 3. Rezultati črpalnih preizkusov v korakih za 30 testiranih termalnih vrtin in vrtin z mineralno vodo v Sloveniji. Well number Q 1 Q 2 Q 3 Q 4 t 1 t 2 t 3 t 4 s 1 s 2 s 3 s 4 C B Q max CD Q max Lp l/s min m m∙s 2 /l 2 m∙s/l l/s % l/s 1 3.6 3.8 4.1 / 109 115 116 / 24.8 29.1 33.0 / 1.609 1.388 7 4.0 11.0 2 10.6 12.5 15.3 17.8 96 95 95 91.0 22.0 20.5 25.3 30.9 0.018 1.405 12 21.0 86.8 3 4.8 10.3 13.1 17.9 118 130 110 140.0 3.8 9.5 14.5 21.1 0.032 0.635 4.5 19.0 81.7 4 3.5 5.4 7.4 9.1 125 130 105 125.0 3.8 6.6 12.1 17.8 0.161 0.469 2 9.3 59.3 5 2.2 3.4 3.7 / 95 95 100 / 0.4 0.8 0.9 / 0.034 0.128 15 5.5 20.0 6 5.3 11.0 17.3 / 120 120 122 / 0.2 0.4 0.9 / 0.002 0.015 15.8 17.3 30.3 7 4.5 8.5 9.5 / 134 138 138 / 0.3 0.6 0.6 / 0.001 0.053 7 9.5 85.0 8 12.2 16.7 27.9 / 202 204 202 / 4.4 6.4 12.3 / 0.005 0.299 15 28.0 80.0 9 4.4 7.0 10.3 / 130 134 202 / 0.7 2.3 4.4 / 0.045 -0.022 6.7 11.0 / 10 2.3 4.3 6.0 8.9 21 19 20 20.0 0.3 1.2 2.3 4.2 0.051 0.039 10 10.0 7.1 11 2.0 3.8 7.7 / 28 30 30 / 0.2 1.0 3.0 / 0.048 0.028 8 8.0 6.9 12 17.1 26.9 28.5 / 90 90 91 / 10.2 15.8 19.2 / 0.004 0.524 14.9 28.0 89.9 13 6.0 17.1 22.3 27.9 90 90 90 90.0 5.8 17.6 23.2 30.0 0.005 0.940 9.5 28.0 95.4 14 7.5 13.9 23.1 / 124 120 121 / 7.0 15.1 25.5 / 0.010 0.888 6 28.0 93.5 15 0.2 / / / 2901 / / / 133.0 / / / / / 0.79 0.4 / 16 2.8 5.0 7.2 10.5 125 120 120 120.0 0.5 1.4 3.0 4.7 0.038 0.081 11.1 11.1 15.9 17 4.2 6.9 10.4 / 120 120 120 / 2.0 4.0 7.0 / 0.031 0.355 28 28.0 29.0 18 4.5 6.3 7.3 / 120 120 120 / 5.0 7.0 3.0 / / / 11 / / 19 8.0 9.3 10.0 / 30 30 30 / 1.0 1.3 1.5 / 0.010 0.046 8.5 8.5 29.3 20 2.2 3.6 6.0 / 17 23 24 / 0.4 1.3 5.0 / 0.180 -0.253 11.3 5.0 / 21 0.3 0.5 0.8 / 120 138 102 / 40.0 68.0 113.0 / / / 2 0.8 / 22 0.6 0.7 1.5 / 91 197 200 / 4.0 5.9 11.9 / 0.648 6.976 1.1 1.6 90.7 23 0.3 1.0 1.6 / 139 185 182 / 1.1 3.4 6.0 / 0.069 3.602 1.39 3.5 97.4 24 0.5 1.5 2.5 / 90 231 188 / 1.8 6.2 11.9 / 0.779 2.926 2.35 2.6 61.5 25 0.6 0.9 1.3 / 183 181 185 / 1.1 2.0 3.3 / 1.424 0.891 1 1.4 38.5 26 0.9 1.9 3.1 / 179 178 181 / 0.4 1.0 2.1 / 0.119 0.302 3 4.8 45.8 27 0.5 1.2 1.8 / 85 188 321 / 1.7 4.7 6.9 / 0.452 3.127 1.42 2.0 83.0 28 1.2 2.2 5.3 / 166 188 981 / 2.3 4.9 14.8 / 0.215 1.661 5 5.0 60.7 29 1.4 2.0 2.7 / 191 181 181 / 8.6 13.6 18.7 / 0.709 5.162 2.4 2.4 75.2 30 1.0 1.8 2.5 / 183 179 247 / 2.6 5.5 8.9 / 1.609 1.388 2.36 2.5 49.8 292 Luka SERIANZ, Nina RMAN & Mihael BRENČIČ Conclusion In the present study, an attempt has been made to evaluate and compare the well perfor- mance using Jacob empirical method for calcu- lation of well losses and aquifer losses. This pa- rameter was recognized as very useful in order to quantify technical characteristics of production mineral and thermal water wells also referred to well efficiency. On the contrary, well loss coeffi - cient magnitude was recognized as a very inap- propriate indicator whether the well is properly designed or effectively developed. The presented approximate method of step-drawdown test in- terpretation calculates only the nonlinear com- ponent of well loss, while the linear component, which is generated due to partial penetration, skin effect or hydrogeological boundary effect, is included within the linear head loss. Most of the tests (23 of 30) were carried out in three steps, averaging up to 1.5 hour for each. The drawdown stabilization during pumping was reached in approximately 70 % of wells. The main problems for the successful implementa- tion were: insufficient observation equipment, a non-optimal system for regulating the pump- ing discharge rate, rare observation wells and a constant need for water production. The latter prevented either the establishment of constant pumping rate or complete suspension of produc- tion for several hours. There are many potential improvements in test implementation, but they are to a certain extent related to investments in the technology system for the use and control of the use of groundwater by the concessionaires. Although a lot of investigated wells are a few decades old, surprisingly the calculated laminar losses were higher than 70 % in 44 % of these. Moreover, the analysis showed that for a large number of wells (at least 7), their performance is relatively poor, which means that the nonlin- ear losses in the well are significantly higher than the linear losses in the aquifer. It is expect- ed that such situation may be attributed to the inappropriate technical condition of the well in some places (e.g. well deterioration). A special consideration was given to 28 % of wells where the calculated laminar losses were between 30 and 70 %. In such cases it is very difficult to ob- tain an appropriate conclusion, since in most cases a technically appropriate step drawdown test was performed for the first time. Therefore, it was impossible to compare the acquired data with previous tests and consequently, impossible to evaluate time-dependant changes in well per - formance. The results indicate possible changes in the technical condition of some wells. However this will be verified only when at least two comparable step-drawdown tests will be performed in each well. In order to timely implement measures for preventing further deterioration, it is necessary to constantly monitor the well performance. Fur - ther investigation will include also constant-rate pumping test in order to determine hydraulic properties of the aquifer along with the consider - ation of the total well losses and consequently the significance of the linear well loss component. Acknowledgment This paper was prepared under the PhD Grant 1000-19-0215 at the Geological Survey of Slovenia, financed by the Slovenian Research Agency (ARRS) through research program P1-0020 Groundwater and Geochemistry in the frame of the Young Researchers programme. Part of the research was conducted under the activities financed by the Environmental Agency of Slovenia by grant agreement No. 2551-18-700022. Nomenclature Symbol Parameter Unit s Drawdown m s 1,2,3,4 Drawdown for each step respectively m s w Well loss drawdown m s(t) Measured drawdown m s(r w ,t) Drawdown with no well loss m t Time min t 1,2,3,4 Duration of each step respectively min r w Well radious m T Transmisivity m 2 /s Q Discharge (pumping rate) l/s Q 1,2,3,4 Pumping rate for each step respectively l/s W(u) Theis well function / S Storage coefficient / B 1 Linear aquifer loss coefficient m∙s/l B 2 Linear well loss coefficient m∙s/l P Exponent of the well discharge / B Aquifer circulation loss coefficient m∙s/l C Non-linear well loss coefficient m∙s 2 /l 2 E w well efficiency % L p Laminar losses % Q max CD Maximum production rate determined in concession decree l/s Q max Maximum production rate suggested by step-drawdown test l/s 293 Step-drawdown tests in exploitation wells for thermal and mineral water– Case study from Slovenia Literature Abdalla, F. & Moubark, K. 2018: Assessment of well performance criteria and aquifer char- acteristics using step-drawdown tests and hydrogeochemical data, west of Qena area, Egypt. 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