Hydrogeol J DOI 10.1007/s10040-016-1405-z

PAPER

Hydrologic testing during drilling: application of the flowing fluid electrical conductivity (FFEC) logging method to drilling of a deep borehole Chin-Fu Tsang 1,2 & Jan-Erik Rosberg 3 & Prabhakar Sharma 4 & Theo Berthet 2 & Christopher Juhlin 2 & Auli Niemi 2

Received: 16 September 2015 / Accepted: 16 March 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract Drilling of a deep borehole does not normally allow for hydrologic testing during the drilling period. It is only done when drilling experiences a large loss (or high return) of drilling fluid due to penetration of a large-transmissivity zone. The paper proposes the possibility of conducting flowing fluid electrical conductivity (FFEC) logging during the drilling period, with negligible impact on the drilling schedule, yet providing important information on depth locations of both high- and low-transmissivity zones and their hydraulic properties. The information can be used to guide downhole fluid sampling and post-drilling detailed testing of the borehole. The method has been applied to the drilling of a 2,500-m borehole at Åre, central Sweden, firstly when the drilling reached 1,600 m, and then when the drilling reached the target depth of 2,500 m. Results unveil eight hydraulically active zones from 300 m down to borehole bottom, with depths determined to within the order of a meter. Further, the first set of data allows the estimation of hydraulic transmissivity values of the six hydraulically conductive zones found from 300 to 1,600 m, which are very low and range over one order of magnitude.

Keywords Hydraulic testing . Fractured rocks . Heterogeneity . Well logging . Drilling * Chin-Fu Tsang [email protected]

1

Lawrence Berkeley National Laboratory, Berkeley, CA, USA

2

Uppsala University, Uppsala, Sweden

3

Lund University, Lund, Sweden

4

Nalanda University, Nalanda, Bihar, India

Introduction To understand the hydrogeology of the deep subsurface, information is required on the spatial distribution (locations and extent) of hydraulically conductive zones, their hydraulic transmissivities, as well as their hydraulic pressure heads, temperature, and water salinity or chemical characteristics. Both highly or not-so-highly conductive flow zones are of interest. At depth, these flow zones often correspond to hydraulically conductive fractures or faults. Direct data on these conductive fractures can be obtained through downhole tests in deep boreholes—for example, fluid production or injection tests can be conducted at selected depths in the borehole bracketed by two packers and in these bracketed intervals fluid sampling can also be made. Such tests are mainly carried out after drilling is completed and are quite time-consuming, especially for a deep borehole. Also, the depth locations of potential hydraulically conductive zones, especially for ones with low transmissivities, are often not known. Borehole televiewer logging of a borehole can show many fractures intercepted by the borehole; however, the majority of these fractures will not be hydraulically active—for example, at the Laxemar and Forsmark sites in Sweden, detailed fracture investigations were conducted on cores from several tens of boreholes down to a depth of greater than 1,000 m, and it was found that only about 10 % of the nearly 100,000 fractures inspected were characterized as open or partly open and then only 2–3 % of all fractures had measurable transmissivity (Rhén et al. 2008; Follin 2008; Follin et al. 2014). Direct downhole measurements of flowing features can be made by heat-pulse flowmeter (Hess and Paillet 1990) or electromagnetic flowmeter (Molz and Young 1993; Young and Pearson 1995), but these require specialized probes not usually available on a drill site and they may not have sufficient sensitivity for low flowing fractures.

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During the drilling period of a deep borehole, hydrologic testing is normally not performed. If the drilling encounters a zone with a large loss or high return of drilling fluid (drill mud), then the drilling operation is halted and the zone may either be plugged or cased-off to enable drilling to continue, or is subject to a drill stem test (Earlougher 1977) to investigate if the zone is potentially a productive zone of petroleum or ground water; both these alternatives are major interruptions to the drilling operation. In the first case of plugging or casing off the flow zone, all hydraulic information related to this zone will be lost from subsequent tests in the borehole. In the case of drill stem testing, the properties of the zone will be obtained with significant loss of time and only major hydraulic zones will be identified and characterized in this way. Minor hydraulically conductive zones will usually be neglected. In this paper, a way to conduct hydrologic testing during drilling without the need for specialized probes is proposed. The method takes advantage of 1-day breaks often scheduled during drilling, Such breaks may occur with the drilling crew taking 1-day off for each 1-week of drilling or with the need of changing drill bits and other necessary operational steps; thus, the hydrologic testing proposed will have minimum impact on the drilling schedule. As shown below, the 1-day test is able to identify both high and low flow zones all along the borehole down to the drilling depth at the time, and to provide data that can be analyzed to yield initial estimates of transmissivity and water salinity of all these flow zones. The next section presents the method used, flowing fluid electrical conductivity (FFEC) logging, which, until now, has been applied successfully in post-drilling hydraulic tests that last for a few days to a week. Then the following sections will present the application of this method in a 1-day test during the 4-month drilling period of a 2.5-km borehole at Åre, Sweden, as part of the Swedish Scientific Drilling Program (Gee et al. 2010; Lorenz et al. 2015). After presenting the results of the test, some discussion and remarks conclude the paper.

Flowing fluid electrical conductivity (FFEC) logging method In the FFEC logging method proposed by Tsang et al. (1990), the wellbore water is first replaced by water of a constant salinity (or electrical conductivity) significantly different from that of the formation water. This may be accomplished by, for example, injecting water from municipal tap water or a water de-ionization unit, through a tube to the bottom of the wellbore at a constant rate, while simultaneously pumping from near the top of the well at the same rate. In this way, the wellbore water is replaced by injected water without a large change in wellbore hydraulic head, so that neither the injected water is pushed out into the formation nor the formation water is drawn into the well. The fluid electrical

conductivity (EC) of the effluent is monitored at the wellhead until a low, stable value is reached, which typically takes overnight or 1 day for a deep (1–2 km) well. If the final stable effluent EC is substantially different from the EC of the injected replacement water, it indicates that native fluid has entered the wellbore during the recirculation phase. This may occur because wellbore hydraulic head could have unintentionally dropped during recirculation, or if natural regional groundwater flow is intercepted by the well. It can also occur if different permeable features intercepted by the wellbore have different hydraulic heads, resulting in internal wellbore flow. Formation water will enter the wellbore through the features with higher hydraulic head and borehole water exit to the formation through features with lower hydraulic head. Nevertheless, the method is quite robust and can still work even in such cases as long as a carefully measured log of EC versus depth profile is recorded after the water replacement phase and before the start of FFEC logging. After the borehole water is replaced by water of salinity significantly different from that of the formation water, the well is pumped at a constant rate and formation water will enter into the borehole and mix with borehole water at the inflow depths. Then FFEC profiles are measured along the wellbore at a series of times after the start of pumping (Fig. 1). This is done by moving an electrical conductivitytemperature (EC/T) probe up and down the wellbore, with the pump and a pressure sensor emplaced in the shallow part of the well, below the anticipated drawdown depth of the water table. The FFEC profiles thus obtained will display peaks in EC values at depths where water enters into the well. The peaks will be smeared around the inflow points in the borehole because of the moving probe and solute diffusion; however, such smearing will be of the order of the borehole diameter and hence the position of the inflow zones can be determined with accuracy of the order of decimeter. This has been confirmed by field data (Doughty and Tsang 2005, 2013). The area A under each peak at any time is given by A ¼ q  C  Δt where q is the inflow rate, Δt is the time lapse from the starting time to of formation water flowing out of the conductive fractures into the borehole, and C is the salinity of the formation water from the particular flow zone. Here C can be expressed in NaCl ionic concentration in g/L, or in terms of its fluid electrical conductivity (EC) with units of μS/cm. In the EC profile, the peaks are skewed in the direction of water flow at their locations in the borehole. The degree of skewness is dependent on the local flow rate along the borehole. A simple fitting code BORE-II (Doughty and Tsang 2000) has been developed to analyze such data to yield values of q and C of each flow zone intercepted by the well. In this code, there is also a parameter to describe vertical Bdispersion^ along the

Hydrogeol J Fig. 1 Schematic picture to illustrate the principle of the flowing fluid electrical conductivity (FFEC) logging method

borehole, which is adjusted to obtain the best fit. It is noted that alternative analysis methods for FFEC logging data have been developed (Moir et al. 2014; Maurice et al. 2010; West and Odling 2007); however, for this paper it is found that the simple BORE-II code is adequate for our purpose. From the inflow rates q and the pressure drawdown in the borehole due to the constant-rate pumping, transmissivity values of all inflow zones can be calculated if it can be assumed that they have the same initial pressure heads. Note that the method does not require a specialized probe, just a typical EC/T probe, a pressure sensor, and a downhole pump, which are all normally available at a drill site for monitoring properties of the drilling fluid. As a useful and important extension of the method, if the procedure is repeated using one (or two) more higher or lower pumping rate at the top of the well, analysis of the data would yield the initial hydraulic heads of the flow zones at the different depths, which could be different from each other (Tsang and Doughty 2003). The occurrence of different initial or inherent pressure heads is to be expected for a fractured rock or a heterogeneous permeable medium, which under a pressure gradient, will present a heterogeneous pressure field; thus, a borehole penetrating the medium will have pressure heads at different depths deviating from those of a normal hydrostatic pressure-depth relationship. Knowing these pressure heads will add to our knowledge of the heterogeneity structure of the permeable medium. Now, let it be assumed that, for an inflow zone at a particular depth with an inherent hydraulic head h, the two pumping rates, with two water level drawdown values in the boreholes hD1 and hD2 (positive numbers), yield inflow rates q1 and q2 respectively from separate analyses of the FFEC logs. Then, q1 =q2 ¼ ðh þ hD1 Þ=ðh þ hD2 Þ h=ΔhD ¼ qm =Δq−hm =ΔhD where qm = (q1+ q2) / 2, hm = (hD1 + hD2) / 2, Δq = q2 – q1, ΔhD = hD2 – hD1 and h is measured positive upwards with reference to the original water level in the borehole without pumping. The afore-mentioned equations work when both q1

and q2 are measurable in the FFEC logging. If the inherent head of a conductive fracture is negative and larger in magnitude than the pumping drawdown, then the inflow rate will be negative and no EC peak will be seen; however, in principle, additional FFEC logging can be made with a higher pumping rate and larger borehole drawdown, so that there can be a positive inflow at this fracture. The afore-given procedure has been applied to existing (postdrilling) boreholes and typically lasts for a few days to a week and has proven to be an effective method to yield estimates of hydraulic conductivity values, water salinity and in some cases, inherent pressure heads of the inflow zones all along the borehole in just one test (Doughty et al. 2005; 2013). The present paper, however, is concerned with FFEC logging conducted during drilling of the COSC-1 borehole within the Swedish Scientific Drilling Program (Lorenz et al. 2015), with data obtained in just 1 day with one pumping rate and one pressure drawdown.

The COSC Scientific Drilling Project The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project (Gee et al. 2010; Hedin et al. 2012; 2014) is part of the Swedish Scientific Drilling Program (formerly the Swedish Deep Drilling Program, Lorenz 2010). It focuses on a study of mountain building processes in a major mid-Paleozoic orogen in western Scandinavia and its comparison with modern analogs. The transport and emplacement of subduction-related high-grade continent-ocean transition complexes onto the Baltoscandian platform and their influence on the underlying allochthons and basement will be studied in a section provided by two fully cored 2.5-km deep drill holes. The first drill hole, COSC-1, located in the vicinity of the closed Fröå mine, close to the town of Åre in Jämtland, Sweden, was drilled over a 4-month period, from May 1 to August 26, 2014, and reached 2,495.8m driller’s depth with nearly 100 % core recovery (Lorenz et al. 2015). The borehole diameter is 101.6 mm down to

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103 m, 96 mm from 103 m down to 1,600 m, and 76 mm from 1,600 m to hole bottom at 2,496 m. The scientific objectives of the COSC scientific drilling project have been developed by topical working groups. Major objectives are: (1) to establish a coherent model of mid Paleozoic (Scandian) mountain building and to apply these new insights to the interpretation of modern analogs, in particular the Himalaya-Tibet mountain belt; (2) to determine the origin of observed seismic reflections and constrain geophysical interpretations in order to use this information to further our understanding of the geological structure of the mountain belt and the Fennoscandian basement; (3) to refine knowledge on climate change at high latitudes (i.e. Scandinavia), including historical global changes, recent paleo-climate development (since the last ice age); (4) to understand the hydrological characteristics of the geological units and research present groundwater circulation patterns of the mountain belt; (5) to analyze the extent, functions and diversity of microorganisms in the drill hole as a function of the different penetrated geological strata and their depth. This paper is concerned with the fourth objective, the hydrogeological characteristics of the deep subsurface.

Application and results of FFEC logging during drilling of COSC-1 borehole During the drilling of the 2.5-km COSC-1 borehole, the drillers worked on a 6-day schedule with a break of 1 day on Sundays before resuming drilling the following week. Before

Fig. 2 Measured fluid electrical conductivity (EC) as a function of depth down to 1,600 m. Profile P0 gives the data before the start of pumping, and P1 and P2 are profiles obtained at about 3 and 11 h after pumping start. a Shows the measured fluid electrical conductivity values, b shows the differences when profile P0 before pumping start is subtracted from the profiles P1 and P2 obtained during pumping

the break, the drill string was pulled and the borehole flushed out with water from a nearby river. The resulting borehole water, and actually also the drilling fluid turned out to have a low salinity value corresponding to an EC value of about 200 μS/cm. This implies that at the beginning of the 1-day break the borehole was already in a condition corresponding to the point between the first and second of the three-step process for FFEC logging shown in Fig. 1, and thus no replacement of borehole water was needed for an FFEC logging operation. Once this was recognized, a decision was made to conduct an FFEC logging test during this 1-day period, using a downhole pump, a pressure sensor, and an EC/T probe, which were standard equipment already available on the drill site. Such a test was performed when the drilling of COSC-1 borehole had reached a depth of 1,600 m, with borehole diameter of 101.6 mm down to 103 m and 96 mm from 103 m down to 1,600 m. The pump and pressure sensor were emplaced 70 m below the water table which was close to the land surface. The EC/T probe was initially set below the pump depth with its cable guided through a tubing attached to the side of the pump. In this way the EC/T probe could be lowered to scan the borehole from about 100 m (which was the cased part of the borehole below which the borehole is uncased) to the borehole bottom, which was at that time, 1,600 m. Because of the design of the EC/T probe, only the EC data recorded during the downward scan of the probe were used in subsequent data analysis. The downward speed of the probe was 10 m/min, and the return of the probe back to the top of borehole was at a higher speed of 20 m/min, which means that

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it took about 4 h to complete each logging scan from 100 to 1, 600 m and back. In the 1-day FFEC logging operation carried out at COSC-1 borehole, three FFEC versus depth profiles were obtained. The first profile was obtained before the start of pumping and the other two at two times after pumping start. Since it is the first attempt of such a test, a number of field problems were encountered which were unrelated to the proposed method, such as accidental sliding of the pump in the borehole by 2 m, entangling of the pump and logging cable, and an interruption of the electric power supply at the drill site. In addition, the initial estimate of the required pump rate was too high so that the water level drawdown reached the depth of the pump, resulting in a fluctuating pumping rate; nevertheless, an average flow rate of 2 L/min out of the borehole was obtained at a drawdown of 70 m. Prior to pumping, an FFEC logging profile was obtained, P0, and then two more profiles during pumping, P1 and P2, were obtained at about 3 and 11 h, respectively, after the start of pumping. Figure 2 presents the results, with Fig. 2a showing the FFEC data and Fig. 2b showing the differences (P1–P0) and (P2–P0). In these plots the EC values have been corrected to 20 °C-equivalent values (Tsang et al. 1990) using the measured temperature along the borehole at the time of running the EC/T probe. The EC values in these plots can be related to salinity or NaCl concentration, C (g/L), through an approximate formula (Tsang et al. 1990), valid for the range of EC values (μS/cm) encountered in this study: EC≈1; 870C In this paper, C and EC have been used interchangeably with this conversion in mind. Figure 2a shows that the EC data are erratic at depths shallower than 300 m, which correspond to the part affected by the afore-mentioned unrelated operational problems. From 300 m down to 1,600 m, the P1–P0 and P2–P0 results show distinct peaks in EC values at six locations, indicating quite definitely inflows at depths of 339, 507, 554, 696, 1214, and Fig. 3 Areas under the six peaks identified in the FFEC logging profiles (P1–P0) and (P2–P0) as a function of time. The unit in the vertical axis is the salinity as measured in μS/cm times the distance in cm along the borehole covered by the peak

Table 1 Estimated concentration and inflow rates at inflow points from 300 to 1,600 m in COSC-1 borehole (with 70-m drawdown) Peak

Peak depth (m)

Time delay, t0 (h)

C (g/L)

q (ml/min)

1

339

0.67

0.4

3

2 3

507 554

1.1 1.16

0.8 0.6

5 19

4

696

0

0.7

22

5 6

1,214 1,245

1.67 0

0.5 1.2

28 10

1,245 m (see Table 1); thus, this simple 1-day test already yielded very useful information; i.e., the identification of the depths, at a high resolution, of hydraulically active zones with both large and small flow rates. It is at these depths where post-drilling double packer tests and water sampling should be done. For each of the six inflow zones shown in the (P2–P0) and (P1–P0) plots, the area, A, under each peak can be calculated. Figure 3 shows the growth with time of the area, which is the rate of salinity (or EC) increase around each inflow depth. This is given by A = qCΔt = qC(t–t0), where t is the time of the EC at the peak being measured, which is not the same for all peaks since the EC/T probe was lowered down the borehole at a finite speed, and thus reached the different peaks at different times. The time t0 is the time after pumping started when the formation water began to enter into the borehole. A non-zero positive t0 value means that the borehole water had entered into the hydraulic conductive zone prior to the test (e.g., during the drilling operation); in other words, t0 is the time required to pumping it back out before formation water emerged. In Fig. 3, t0 is the intercept of the line representing each EC peak on the time axis. Two of the lines in Fig. 3 are found to tend to a to-value less than zero, which may suggest that formation water already began to flow into the borehole before the start of pumping in these two depths, for example because the flowing fractures at these depths have an higher

Hydrogeol J Fig. 4 Fit of BORE-II results to measured FFEC logging data. Profile 1 (a–b) is same as PI and profile 2 (c–d) is same as P2. Sensitivity to parameters is also displayed

initial pressure heads than the average head of all flowing fractures intercepted by the borehole. In this case, the P0 profile obtained before pumping start would show sharp peaks at these depths; however, in our results no sharp peaks are found in the P0 profile. For a first-try effort, without getting into

details, the t0 values are set to 0 as representing inaccuracies in the data. The t0 values for all the peaks are shown in Table 1. Given the t0 values and the initial FFEC profile before pumping, P0, the code BORE II (Doughty and Tsang 2000) was used to obtain q and C values for each peak by fitting the

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calculated and measured EC profiles P1 and P2 given in Fig. 2a. The fit is not expected to be perfect because of the encountered operational problems. Further, since there are only two FFEC profiles separated by a short time period during pumping, the skewness of the peaks in the direction of flow in the borehole is not pronounced, which makes it hard to separate the q and C parameter values. Nevertheless, reasonable results have been obtained as shown in Table 1 and Fig. 4. Figure 4 shows the optimal fit of data with the calculated results based on the q and C values shown in Table 1. The fitting sensitivity using half and then double the fitting q values (with corresponding double and half C values) is also shown in the figure. In Table 1, C is given in g/L, with 1 g/L being equivalent to 1,870 μS/cm. They show an abrupt increase of salinity of 0.5–1.2 g/L for the depth level of the inflow zone at 1,214 m to that at 1,245 m. Such an abrupt change in salinity between two adjacent inflow points has also been observed at other sites (Tsang, et al. 1990); however, the present set of data has enough uncertainties that the confidence level of this result is not high and further studies on these inflow zones is needed. Generally, the values indicate that the water at these depths are relatively fresh, suggesting that meteoric water circulation is deep, as anticipated based on other studies (Juhlin et al. 1998). The evaluated values of q range from 3 to 28 ml/min, a range of one order of magnitude. A detailed examination of the cores obtained from the borehole at the depth locations of these inflow zones has been made and potential single fractures have been found that may be the ones responsible for flow shown as peaks in the FFEC logs. Figure 5 shows, as examples, 360°-unfolded photos of two cores near the depths of the first two inflow peaks, where it can be seen that each inflow peak may be associated with one single hydraulically conducting fracture; thus, assuming each inflow zone is due to the presence of one fracture with an initial pressure head the same as the initial pressure head in the borehole, and using the observed drawdown of ΔhD = 70 m, the fracture hydraulic aperture b can be calculated from the cubic law: q = Δ hD ¼ α b3 = 12 α ¼ 2π ðρg=μÞ=lnðre =rb Þ where ρ and μ are the density and viscosity of water, respectively, and g is the gravitational constant. The factor ln(re/rb) is the natural logarithm of the ratio of the radial distance of pressure influence to the borehole radius, which is not sensitive to the value of re and a value of 1,000 was assumed for re/ rb. Using these formulas, it can be calculated that a flow rate of 10 ml/min would correspond to a fracture hydraulic aperture of 15 μm or fracture hydraulic conductivity of 2 × 10−4 m/s. The equivalent hydraulic transmissivity of the fracture would be the product of its conductivity and its aperture, which is calculated to be 3 × 10−9 m2/s. It has to be emphasized that the

Fig. 5 Photos of two cores (in 360° unfolded picture) at depths around the first two FFEC peaks. A fracture is identified in each case to be possibly associated with the two inflow zones

results in Table 1 are highly preliminary, because of the assumption of equal inherent pressure heads in all flowing fractures and uncertainties due to a number of unrelated operational problems reflected in the field data. The preceding results were obtained based on data from a 1-day test on July 10, 2014, when the drilling reached a depth of 1,600 m. The drilling reached the target depth of 2,500 m on August 26, 2014. Then, there was an opportunity of doing another very short-term FFEC logging on October 11, 2014, without replacement of borehole water (see Fig. 1). The purpose of this shortterm logging was to cover the depth interval from 1,600 Table 2 Characteristics of the 10 July 2014 FFEC logging as compared with those of 11 October 2014 10 July 2014 11 October 2014 Duration of test Log interval

1 day 100– 1,610 m Maximum drawdown during pumping About 70 m Time of pumping to P1 About 3 h Time of pumping to P2 About 11 h

About 8 h 1,600–2,500 m About 33 m About 2 h –

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to 2,500 m. The characteristic of the second FFEC logging as compared with the July 10 FFEC logging are shown in Table 2. Only one EC profile before pumping and one profile after start of pumping were obtained. The results are shown in Fig. 6, showing that there are two inflow zones at 2,300 and 2,380 m respectively. Since there is only one EC profile obtained after pumping, further analysis was not done. In summary, the FFEC logging of the COSC-1 borehole has identified eight hydraulically active zones at the depths of 339, 507, 554, 696, 1,214, 1,245, 2,300 and 2,380 m. It is to be emphasized that these are the inflow zones whose water salinity is above the Bbackground^ EC value, as shown by the EC log before pumping (Figs. 2a and 6a). For borehole section between 300 and 1,600 m, the six inflow zones are the ones whose formation water has an EC value above about 200 μS/ cm, and for the section between 1,600 and 2,500 m, the two inflow zones are the ones having water EC above 200–600 μS/cm. Further studies are currently underway to confirm these results along two lines. First, the core sections containing potential fractures corresponding to the FFEC peaks have been taken for laboratory studies both to measure directly the fracture permeability under appropriate stresses and to verify if the mineralogy of the fracture surfaces shows water flow characteristics. Secondly, a series of post-drilling FFEC logging runs with borehole water replacement, as shown in Fig. 1, are being planned in order to obtain five or more FFEC profiles over an extended time. These will hopefully provide EC profiles with a clear upward skewness of the peaks to enable a more definite separation of q and C in the data analysis. Fig. 6 Measured fluid electrical conductivity (EC) as a function of depth from 1,600 m to 2,500 m. a An EC profile obtained before the start of pumping and another obtained 2 h after pumping started; b the difference between the two profiles

Concluding remarks The FFEC logging presented in this paper was a test performed during the drilling period of a deep borehole. Understandably, a number of Bfirst-try^ operational problems unrelated to the method were encountered. The fact that, in spite of these problems, the method was able to identify all large and small inflow locations along the borehole and provide a first estimate of their salinity and hydraulic transmissivity even before drilling was completed is highly encouraging. Furthermore, this activity was conducted with no or minimal impact on the drilling schedule (since drillers had 1 day off per week) and without the need of any specialized downhole probes or equipment. For this reason, it is recommended that the FFEC logging to be conducted during drilling of a deep hole whenever a day (or several separate 1-day periods) becomes available during the drilling period. Many of the Bfirst-try^ operational problems can be easily avoided based on our experience. In addition, it is useful to conduct a preliminary pumping test in the early part of drilling to allow a better estimate of the pumping rate to be used so as to avoid water drawdown to the depth of the pump. An alternative is using a pump with a pressure sensor. The design of bypass tubing around the downhole pump for passing the cable of the EC/T probe can be easily improved to avoid entangling the cable with the pump. It is also strongly recommended to make use of two or three separate 1-day breaks during the drilling period to conduct FFEC logging with two or more pressure drawdowns, which will allow the determination of the inherent pressure heads of all conducting fractures and more accurate determination of their conductivity

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values. No significant problems are anticipated to conduct FFEC logging during the drilling of the next 2.5-km borehole, being planned for 2017 within the Swedish Scientific Drilling Program. Acknowledgements The authors cordially acknowledge the support of Swedish Geological Survey (SGU), grant number 1724, for the research reported in this paper. The first author would also like to acknowledge partial support for preparation of this paper by the Used Fuel Disposition Campaign, Office of Nuclear Energy of the U.S. Department of Energy, under contract number DE-AC02-05CH11231 with Lawrence Berkeley National Laboratory. The drilling of the COSC-1 borehole was financed by the International Continental Scientific Drilling Program (ICDP) and the Swedish Research Council (VR: grant 2013–94). Special thanks to Per-Gunnar Alm and the logging crew from Lund University in conducting the field operation for the FFEC logging. We are also grateful to ICDP-OSG logging teams for collecting the logging data shown in Fig. 6.

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