Proceedings World Geothermal Congress 2015

Melbourne, Australia, 19-25 April 2015

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Hydraulic Testing and Reservoir Characterization of the Taufkirchen Site in the Bavarian

Molasse Basin, Germany

Hansruedi Fisch1, Jörg Uhde

1, Curd Bems

2, Philipp Lang

2 and Jörn Bartels

3

1Axpo Power AG (Switzerland), 2Exorka GmbH (Germany), 3Geothermie Neubrandenburg GmbH (Germany)

Axpo Power AG, Parkstrasse 23, CH-5400 Baden, Switzerland

hansruedi.fisch@axpo.com

Keywords: reservoir characterization, hydraulic testing, injectivity, productivity, low enthalpy, Bavarian Molasse Basin, Malm

ABSTRACT

The geothermal project Taufkirchen in Southern Bavaria (Germany, south of Munich) is expected to produce energy equivalents of

40 MWth and 4 MWel with an anticipated thermal water production rate of 120 l/s at a temperature of 133 °C. Extensive hydraulic

well testing in various configurations was carried out to hydraulically characterize the thermal aquifer situated in porous-fractured

limestones of the Malm formation.

The success of a geothermal well is usually rated in productivity and injectivity indices, respectively, specifying the

production/injection rate as function of the differential pressure applied (e.g. liters per seconds per bar). The instant short-term well

productivity can be assessed relatively quickly with airlift or pump tests. The results obtained at that early stage are preliminary and

may not reveal the full sustainable potential of the reservoir. Additional hydraulic testing is designed to obtain a more

comprehensive data set which can be interpreted towards a profound understanding of the reservoir characteristics. A careful test

layout is essential with regard to the test quality and prerequisite for a sound test interpretation. A good understanding of the

thermal reservoir improves predicting the long-term thermal water extraction rates and the dimensioning of the power plant. It may

also provide the necessary information to undertake well improvement measures or to plan additional wells.

The influence of thermal effects, borehole skin and rate dependent friction loss on the test interpretation is demonstrated in the two

circulation tests conducted at the Taufkirchen site. Sensitive issues of the test design and test-setup are identified and proposals for

an optimized test configuration in comparable aquifers are presented.

1 INTRODUCTION

1.1 Project Description and Wells

The geothermal project Taufkirchen includes two deep boreholes with open hole sections connecting to the regional Malm aquifer

(Figure 1). Electric energy will be generated by pumping hot thermal water, converting the heat and driving a turbine using a

Kalina binary cycle system. The cooled thermal water is re-injected into the deep aquifer via the second borehole. The power plant

will generate energy equivalents of 40 MWth and 4 MWel with an anticipated thermal water production rate of 120 l/s at a

temperature of 133°C.

The first activities in the Taufkirchen project date back to 2006 when a feasibility study including reprocessing of old seismic lines

was commissioned by the former project owner. Additional 2D and 3D seismic surveys (2007 and 2009), risk assessments, the

establishment of a new project company and approval procedures took several years before the preparatory drilling works started in

December 2010. The project plan calculated with 2 to 4 boreholes depending on the yield of thermal water.

Subsurface works started with the drilling of the first well, GT3 of the current doublet, in July 2011. The 3rd borehole section in

molassic (tertiary) rock had to be abandoned after severe blockage of the drill string. The subsequent side track GT3a reached its

target by end of 2011 with a total depth of 4 259 m (MD). Subsequent well logging and testing was carried out to investigate well

conditions, well productivity and reservoir properties. The initial expectations were not fully met. A second well of 3 933 m (MD)

was drilled in 2012. Again, the 3rd section of the well had to be abandoned after a series of pitfalls initiated by a full loss of drilling

fluid in the Gault sandstone, 42 m above the target horizon. Initial hydraulic tests indicated the productivity of this well was twice

as high compared to the first well. Substantial well logging programs were executed in both wells during various phases of drilling

and after completion of each well. In addition, hydraulic well testing in various configurations was carried out to hydraulically

characterize the thermal aquifer situated in porous-fractured limestones of the Malm formation.

The trajectories of the Taufkirchen wells start from outside of the area of the 3D seismic survey. Both wells are deviated with kick-

off points within the Molasse formation between 1500 and 2240 m MD, respectively. The drill path of GT3a deviates towards

southeast and then gradually changes direction towards east-north-east to reach the Malm target at a relatively deep position

between two fault systems. (Figure 2, Figure 4). The casing schemes, length of open hole sections, diameter and depth information

is given in Figure 3. The distance between the wells is 940 m at top Malm, and 1155 m at well bottom (near base Malm).

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Figure 1: Location map with estimated geothermal resources (Thomas, 2010). The Taufkirchen site, marked with a yellow

rectangular box, is neighboring the Unterhaching geothermal claim.

Figure 2: Taufkirchen well paths shown in relation to geological surfaces. Left: top view, right: 3D view direction E-NE.

Trajectory of GT1a shown as blue line, GT3a as green line. Space between sectors along drill paths is 200 meters.

Top of Purbeck layer shown in rainbow colors (relative low elevation = blue, high elevation = orange). Major fault

zones are shown as grey or violet surfaces. Distance between grid lines (blue) is 500 m.

Figure 3: Casing schemes and diameters for GT3a (left) and GT1a (right)

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1.2 Geological and structural setting

The subsurface Upper Jurassic Malm carbonates form the most important reservoir rocks for hydrothermal exploration in Southern

Germany. Overall, they consist of up to 600 m marls and limestones which can locally be dolomitized. In the greater Munich area,

the Mesozoic layers including the geothermal target aquifer dip towards SE. The temperatures in the Malm aquifer increase across

the agglomeration of Munich from N to S, from around 65 °C at Erding to 140 °C at Sauerlach. Hydraulic conductivity (expressed

as T/L over longer borehole sections) in the Malm varies over several order of magnitudes from 1E-7 m/s to 5E-4 m/s (Birner et al.

2012). Besides local variations, a general decrease of permeability from N towards S is reported.

Lithological and facial aspects of the Malm in the greater Munich area are described in numerous (congress) publications and PhD

reports: Birner (2012, 2013), Böhm (2010, 2012), Böhm et al. (2010, 2013), Wolfgramm et al. (2012). Parts of this work can be

retrieved from the web based geothermal information system www.geotis.de. There are two major facies types in the Malm:

massive facies and bedded facies. It is generally agreed upon that a correlation exists between the total dolomite content of the well

and its transmissibility. The massive facies of the Malm carbonate is, in contrast to the bedded facies, more prone to dolimitization.

Dissolution processes in dolomitic carbonates along joints lead to comprehensive karstification in the massive facies. The thickness

of the massive and bedded facies through which the boreholes are drilled, accordingly, also has an impact on the productivity of the

wells (Böhm et al., 2013). The higher the percentage of massive facies, the higher the transmissibility. Steiner et al. (2011)

suggested based on analysis of image logs that significant karstification is mostly related to lithofacial boundaries and faults are less

affected by karstification. Small-scale changes in facies can occur locally, with swells of massive “reef” or biotherm complexes and

basins with stratified marly or micritic carbonate rocks. This picture known from outcrops of the Franconian or Swabian Alb was

largely confirmed for the deep Malm in the Greater Munich area. The seismic exploration of the geothermal reservoirs in the

greater Munich area is described in Lüschen et al. (2011), Thomas et al. (2010), Böhm et al. (2007).

Figure 4: Planned drill path of GT1 (black), final trajectory of GT1a (sidetrack) and GT3a. Contours and colors relate to

thickness of massive facies in the Malm formation (source: Schubert et al. 2012).

2 HYDRAULIC TESTS

2.1 General

Hydraulic test data of the Taufkirchen wells were obtained during several stages of the project:

well cleaning pre and post chemical stimulation, shortly after well completion

subsequent short-term air-lift production test after well cleaning (in GT3 only)

mid-term production tests of each well with simultaneous re-injection into the other well

The first two testing types are summarized as preliminary testing. The production/re-injection tests are referred to as circulation

tests, even though in hydrothermal system it is very unlikely that 100 % of the re-injected water is re-circulated through the

pumping well.

2.2 Methodology of Testing

The test layout and equipment used for hydraulic testing vary for the above mentioned project stages. Equipment set-ups specific

to certain tests are described in the respective sections.

The preliminary tests were conducted by means of air-lift production technique. During most of the preliminary testing the borehole

was equipped with downhole PT gauges near the casing shoe of the 3rd borehole section (Figure 3). The produced thermal water

was conveyed from the well head to the water-vapor-separator and then pumped from the intermediary tank to the cooler and the

storage basins. The three storage basins were interconnected and provided a total storage capacity of 4500 m3. The flow rate of the

liquid phase was measured using two magnetic flow meters (redundant measurement) installed between the intermediary tank (at

outlet of water-vapor separator) and the cooling unit. The mass flow of the vapor phase was not measured.

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The circulation tests were conducted after removal of the drill rig and were controlled by use of an electrical submersible pump

(ESP from Baker Centrilift). Pump installation depth was 600 m and 660 m., respectively, minimum 400/550 m below the

cold/warm water table of the borehole. The two well heads were connected by pipework. On the injection side, a temporary 7” pipe

of 250 m length was used to re-inject the pumped thermal water below the water table. The connecting pipe between the two wells

was instrumented with a magnetic flow meter and sensors for a continuous measurement of pressure, temperature and electric

conductivity.

The circulation tests were carried out in both directions: first by pumping from GT3a and re-injecting in GT1a, then pumping from

GT1a and re-injecting in GT3a. For all hydraulic tests both the drawdown/injection and the recovery/fall-off phases were analyzed.

During the circulation tests redundant PT gauges were used. During preliminary testing of GT1a the previously completed well

GT3a was equipped with pressured gauges which enabled to analyze these tests as interference (cross-hole) tests.

The data presented origin from three data acquisition systems: main surface DAQ provided by the drilling company Daldrup and

recording P, T, q and physico-chemical parameters; the DAQ of the ESP control system recording the pump parameters including

pump intake pressure and temperature; and the DAQ of the individual PT memory gauges. The recording interval varied between

15 seconds and 2 minutes depending on the specific DAQ and the test period.

2.3 Methodology of Test Interpretation

Data preparation prior to interpretation required synchronization and interpolation of data records form different DAQ systems,

either because of non-matching recording intervals or differing clock-speed. The size of the resulting data sets was reduced by

interpolation. As a result, the size of the data files was reduced by a factor of 50 to 100 (e.g. reducing from 380 000 to 4 700 data

rows for the 1st circulation test) but the short recording intervals for the periods with rapidly changing pressure/flow values were

preserved (e.g. at the beginning of drawdown or build-up). The flow history was simplified for the interpretation of the entire test

sequences in Saphir. Singular test events were analyzed using conventional straight-line analysis technique (SLA) and derivative

calculation using the (programmable) graphing software Igor Pro (Wavemetrics). Pressure build-up periods and entire test

sequences were simulated using AQTESOLV (Hydrosolve) and Ecrin/Saphir (Kappa Engineering).

Inverse parameter estimation was preceded by the definition of plausibility ranges for the varied well parameters (wellbore storage,

skin) and aquifer parameters (permeability and storativity). Plausible values for the storativity parameter were estimated based on

plausible ranges for porosity and rock compressibility. The storativity parameter cannot be varied as fitting parameter in Saphir but

is defined at model set-up. Therefore, a number of models were set-up to investigate ‘manually’ the influence of storativity on the

fit quality. Permeability results from the inverse parameter estimation were double-checked with the results from simple straight-

line analysis. A few diagnostic plots of singular test events suggested double-porosity flow and therefore, this reservoir model was

tested.

Multiple well concepts, aquifer models and boundary conditions can be combined in Saphir during test interpretation. Simple

model combinations were generally preferred over more detailed concepts involving large numbers of fitting parameters. Log-log

diagnostic plots (Bourdet et al. 1989) supported the use of a simple radial flow model in most cases. Multi-layer models, although

plausible from the findings of the temperature logs, were avoided in order to limit the number of fitting parameters.

In a number of simulation runs it was assumed that the perturbation of the pressure potential field during the simultaneous

production and injection from and to the aquifer at a relatively small lateral distance would form a linear constant head boundary

between the two wells (Figure 5).

The effect of pump activities of neighboring geothermal sites on the Taufkirchen hydraulic tests were analyzed, approximately

quantified and accounted for when rating the quality of the results, but not included in the computer aided well test interpretation.

Figure 5: Sketch showing conceptual linear flow boundary between pump and injection well.

2.4 Hole Cleaning, Stimulation and Preliminary Testing

After completion of each borehole and final logging runs, the open hole section was cleaned during short production periods using

air-lift technique and chemically stimulated by injection of 15% HCl. This procedure was repeated and followed by an additional

short-term production test of <2 days with subsequent recovery period. The total volume produced during the individual tests did

not exceed 6000 m3. In GT3a, an additional short-term production test of 1.5 days (Vtot = 7800 m3) was conducted that was

followed by a 7 day recovery period. In addition to the water quality measurements at surface, the hydraulic test parameters

pressure, flow rate and temperature were measured during hole cleaning and the preliminary production tests. The data quality

improved with ongoing testing is shown in Figure 6 for two air-lifts tests in GT3a. This improvement is a result of successful

stimulation and well cleaning measures which enable steadier production rates, but also due to a change in the flow measuring set-

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up. The unreliable downhole flow meter was replaced by magnetic flow meter installed at the outlet of the vapor-water separator.

The volumes of the vapor phase were not measured during preliminary testing. A rough thermodynamic calculation using enthalpy

values for the liquid and the vapor phases and assuming a maximum fluid temperature of 120 °C at the inlet of the separator

suggests that the vapor mass fraction was below 3.7 %. All pressures were measured using quartz PT memory gauges installed near

top of the Malm reservoir.

Figure 6: Pressure and flow rates recorded in GT3a prior to the first chemical stimulation (left) and during the short-term

pump test (right) which followed hole cleaning after the second chemical stimulation. The data quality improved

significantly between the two tests.

2.5 Circulation Test GT3a to GT1a

The first mid-term test started on 2nd July 2012 with relatively unsteady pumping rates varying between 65 and 100 l/s. After 2 days

a steady pump rate of 68 l/s was achieved (Figure 7). During the entire production period until 24th July 2012 (520 h), the pump rate

was increased stepwise to 74, 79 and 83 l/s. A total volume of 134600 m3 was produced and re-injected into the aquifer via GT1a.

Then the pump was stopped and the pressure recovery (build-up) was monitored during 7 days (169 h).

As a precaution, two redundant PT downhole probes were installed in each well. Unfortunately, one of the sensors in GT1a stopped

working in the middle of the fourth stage of the step rate test (Figure 7) and the all the data from the second (redundant) sensor

could not be read out. Surface pressure data were available during the circulation period from the pressure sensor in the pipework

connecting the two boreholes. During the fall-off period of GT1a, this sensor lost the contact with the water column of GT1a.

Figure 7: Overview plot of circulation test GT3a to GT1a

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2.6 Circulation test GT1a to GT3a

The 2nd circulation test started on the 7th August 2012. As expected, only modest pressure drawdown were observed. The 2nd

circulation test was split in two phases. In the course of the first phase which included the flow steps 1, 2 and 3, the pumping rate

increased from 61, 74 to 81 l/s. The EPS stopped on 20th August 2012 initiating a 15 day recovery period. Until mid of September

2012, the surface pipes connecting the two wells were replaced by conducts of increased pressure rating and a centrifugal pump

was installed to increase the pressure at the re-injection side (GT3a). The circulation test was restarted on 18th September 2012 with

a pump rate of 95 l/s. During the last step, a pump rate of 99 l/s was attained.

During the 3 steps of the first phase of the circulation test a total volume of 84300 m3 was pumped and re-injected within 317 h

(13 days). A similar volume of 82700 m3 was circulated during the two flow steps of the second phase within 239 h (10 days). The

recovery periods were recorded during 367 h and 257 h, respectively.

Figure 8: Overview plot of circulation test GT1a to GT3a.

3 RESULTS (I): PRODUCTIVITY AND INJECTIVITY (PI; II)

The performance of a well-aquifer-system can be expressed as production/injection rate per unit pressure change of drawdown or

build-up, respectively, e.g. in liters per seconds per bar [l/(s bar)]. Productivity and injectivity values are likely to be affected by

near borehole effects (skin, non-linear well entry pressure losses) and the friction loss along the production path (production

casing). Also, PI and II are usually evaluated assuming a steady-state flow regime. This criteria is often not met under field

conditions and PI and II values are accordingly tampered. Therefore, PI and II values should be used with care und preferably when

benchmarking geothermal systems of comparable well specifications and when applying a standardized evaluation procedure.

Differential pressure (P) versus flow rate (q) are shown for all preliminary tests in GT1a and GT3a in Figure 9. The shown

parameters were measured at the end of pumping stages when flow normalized drawdown indicated quasi steady-state flow

conditions. The maximum flow rate attained at the end of a pumping step was 65 l/s at a differential pressure of P = 25 bar in

GT1a and 85 l/s at P = 13 bar in GT1. A significant improvement of the flow vs. differential pressure was noticed after the first

HCL acidification (see green and blue arrow lines in Figure 9). The second chemical stimulation was much more effective in GT1a

than in GT3a. In GT1a, the productivity (q/P) improved from 3.0 l/(s bar) to 6.1 l/(s bar) whereas the q/P ratio in GT3a only

improved marginally from at 2.6 l/(s bar) to 2.7 l/(s bar), as indicated by the upward directed green arrow line in Figure 9. The

escaped vapor phase is not included in these figures.

Similar values were measured during the mid-term circulation tests. During the 1st circulation test with GT3a as pump well and

GT1a as injection well the step specific PI values varied from 2.5 to 2.1 l/(s bar) with the lowest value at qmax. The II values in

GT1a varied between 6.0 and 5.5 with the lowest value at qmax. The respective PI/II ranges for the 2nd circulation test are 6.9 to 5.4

l/(s bar) for the pump well GT1 and 2.7 to 2.3 l/(s bar) for the re-injection well GT3a. All these values refer to pressure changes

measured close to reservoir depth (~ 30 to 50 m above casing shoe of boreholes section 3, see Figure 3). The two circulation tests

confirmed largely the results from the preliminary testing and the extrapolated P-values (flow dependent) at that time.

The findings of the preliminary tests are shown in the regional context in Figure 10, comparing the rate-dependent drawdown

values of the Taufkirchen wells with the well characteristics of 13 deep geothermal wells in and around Munich (Böhm, 2012).

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Figure 9: Results of preliminary testing shown as quasi-steady state flow rates (q) and differential downhole pressures (P)

for GT3a and GT1a.

Figure 10: Productivity curves of 15 geothermal wells in the Munich area with facies tags (adapted from Böhm, 2012). The

extrapolated characteristics of the Taufkirchen wells, based on downhole P data, are projected as exponential fits

into the Böhm’s chart as thick dashed lines (GT1a: blue, GT3a: green, based mainly on measured downhole

differential pressures). Three facies regions are indicated: bedded facies (B), transition facies (Ü) and massive facies

(M). GT1a is in the region of the massive facies, GT3a within the transition facies. Please note that the exponential

fits (Pfit = C1 x + C2 x2) do not match perfectly the data in the low q domain where P vs. q data align mostly

linearly.

It is a rare case that for same doublet circulation test data for both directions are available which can be compared with respect to

injectivity and productivity (Table 1). The thermal water was re-injected without cooling. This means that temperature effects on

viscosity were not impairing the comparison. It can be seen that injectivity is approximately 10 % better than productivity in the

well with the smaller absolute PI value (GT3a) but the larger pressure differences, whereas injectivity II is equal to productivity PI

at the well with the larger absolute value and smaller pressure differences.

Table 1: Comparison of stationary PI and II values measured at 80 l/s

Steady-state well performance GT1a GT3a Units

Productivity index (PI) 5.7 2.1 l/(s bar)

Injectivity index (II ) 5.7 2.4 l/(s bar)

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4 TRANSIENT PRESSURE ANALYSIS VS. STEADY-STATE EVALUATIONS

Before presenting the results from reservoir-focused well test interpretation (section 5), the differences in analyses approach with

regard to the previous evaluation (section 3) is illustrated. The insertion of this short method statement aims to show where a

simple rule of thumb calculation provides reasonable results, and in which circumstances more advanced methods are appropriate.

In an ideal case for a radial flow concept, i.e.

borehole skin is absent

drawdown is not affected by friction loss along production casing

P and q at each end of step represent quasi steady-state flow conditions

both approaches, steady-state approximation (SSA) based on PI values and pressure transient analysis (PTA) yield similar

transmissivity values.

In the synthetic example shown in Figure 11, the three P - q pairs lead to equal PI values because their ratios (P/q) are equal, as

indicated by the straight line in the upper left chart of the figure. So any of the three P - q pairs can be entered in equation (1) for

steady-state flow to estimate transmissivity:

)ln ( 2

sr

R

P

gqT

w

i

[m2/s] (1)

T, q, g, P, Ri, rw and s are transmissivity, density, volumetric flow rate, gravity, differential pressure, radius of investigation,

well radius and skin factor, respectively. Skin is assumed 0. The radius of investigation is estimated by making a preliminary

assumption of Ri followed by a few iterations using the equation (2) and (1):

5.1S

tTRi

[m] (2)

where t is the elapsed time at the end of step 1 and S the aquifer storage.

In contrast to the SSA approach, the straight line analysis or PTA approach consists in determining the slope m of linearly aligned

data in a semilog plot (lower right chart of Figure 11) reflecting the transient pressure response of the formation. The transmissivity

is given by equation (3):

183.0m

gqT

[m2/s] (3)

In the special case with skin = 0, the two approaches (SSA and PTA) yield comparable results, as it is expected given that equations

(1) and (3) are derived from the same Theis equation (Cooper et al., 1946).

Figure 11: Steady-state approximation and pressure transient analysis approach for step rate test, ideal case.

Figure 12 illustrates that applying the above described SSA procedure would require to reliably estimate the skin factor. Skin is

difficult to estimate for single borehole tests because it strongly correlates with aquifer storage, another uncertain parameter.

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The result of the PTA straight-line analysis is not affected by skin as it is shown in the lower right graph of Figure 12. The mid/late

time data of the three steps in graph P versus log of superposition time align on the same straight line with slope m. Provided that

the radial flow model applies, the use of equation (3) gives the true formation transmissivity.

Figure 12: Effect of skin on PI values and PTA.

High flow rates are common in geothermal wells. Additional friction loss may occur near to the wellbore and increases non-linearly

with increasing flow (left graph in Figure 13, green dashed line). In a P versus q diagram, this effect results in a convex shape of

the data curve (upper right graph in Figure 13), similar to those curves presented in Figure 10. The definition of the rate depended

skin (s’) is shown in the lower right of the figure. The application of the SSA method would provide reasonable results for low flow

rates (provided that the constant part of skin s0 is negligible) but introduces important errors with increasing flow rate. Rate

dependent skin is further abbreviated to RDS.

Figure 13: Effect of rate dependent skin (RDS) on drawdown and PI values.

5 RESULTS (II): HYDRAULIC PROPERTIES OF THE THERMAL AQUIFER

5.1 Circulation Test 1, GT3a to GT1a

A selection of applied analysis models using Saphir is shown in Figure 14.

• Three of the shown models (B, C, D) include a linear constant head boundary to simulate the simultaneous re-injection in

GT1a; one of them includes RDS (D).

• The introduction of a CHB results in lower transmissibilty estimates.

• Discarding the result of the forward simulation (C) due to the bad visual fit quality, the transmissibilties still vary by a

factor of 3.

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• RDS effects are difficult to demonstrate on this data because of the small differences in flow rate between the steps.

• The forward simulation (C) using the results from preliminary testing but assuming a constant head boundary between the

wells (section 2.3, Figure 5) results in generally smaller drawdown values and, shortly after the beginning of each step, a

relatively rapid pressure stabilization as it is not seen in the data.

• Irregularly varying pressure impedes log-log derivative diagnostics on drawdown data.

• Derivative diagnostic on recovery data (not shown here) are impeded by cooling effects affecting the cable length and

depth position of the PT-sensor.

Figure 14: Circulation test 1, GT3a to GT1a: simulation results using different analysis models shown in a Cartesian plot.

5.2 Circulation test 2, GT1a to GT3a

A selection of applied analysis models in Saphir is shown in Figure 15, applied on the pressure responses in GT1.

• The shown simulations (#1, 5, 7 and 10) are fitted on phase 1 of the circulation test 2 (ST1- ST2- ST3; see Figure 8).

• The simulated flow regime is radial.

• Better fits are achieved when using RDS with strongly negative s0-part and an important flow dependent part (ds/dq) of

total skin.

• The highly negative s0-part of total skin could represent the effect of chemical stimulation.

• Clear indication of quasi steady state flow at the end of each flow step (in the data). Steady-steady flow conditions were

also seen in the late step data of the injection borehole GT3 (not shown here).

• The planar fault structure (identified in the 3D seismic cube) is not visible in the data.

• The implementation of a linear constant head boundary (to simulate the effect of simultaneous injection in GT3a)

required an increase of the aquifer storage parameters, and ct, to obtain a good visual fit quality.

• This is in contradiction with the results of interference tests which suggested a high aquifer diffusivity and therefore with

preferably low aquifer storage values.

• Recovery data were influenced by temperature effects (affecting length of PT sensor cable).

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Figure 15: Circulation test 2 (phase 1), GT1a to GT3a: simulation results using different analysis models shown in a

Cartesian plot.

6 DISCUSSION AND CONCLUSIONS

6.1 General

The focus of this work was to determine the hydraulic well properties and to assess the characteristics of the hydraulic formation

response during the mid-term circulation tests. For this purpose, multiple inflow zones identified in temperature logs were

simplified as a single aquifer in the analysis models. Fairly good matches of the data were achieved based on different parameters

sets. The introduction of a linear constant head boundary between the two wells to simulate the effect of simultaneous reinjection

lead to lower transmissibility estimates and higher aquifer storage estimates. Rather low storage values were expected from a few

(but short) cross-hole tests during the preliminary tests. This apparent contradiction was not solved during the interpretation. The

analysis models used are not representing the full complexity of the true aquifer conditions and therefore, certain discrepancies

have to be accepted. Also, only few reliable aquifer storage measurements from the deep Malm of the region are available which

makes it difficult to confine the plausibility range of this parameter. Conversely, better knowledge of the variability of the storage

parameter would enable to refine the analysis models.

6.2 Conclusions Concerning Test Design

The short-term preliminary tests proved to be useful for assessing the effect of chemical stimulation measures and to determine the

pump specifications for the subsequent circulation tests. Step rate tests are a very common method to assess both well performance

and aquifer properties and were successfully applied for the Taufkirchen wells. The circulation tests avoid the need for temporary

storage and treatment of the produced thermal waters. An additional constant rate cross-hole test of 2-3 day duration would be

beneficial to confine the aquifer storage parameter. This might be of minor interest for the future plant operation. However, a

confined plausibility range for the storage parameter could improve the interpretation quality in general and the accuracy of the

other estimated hydraulic parameters. Cross-hole testing requires sufficient storage capacity for the produced thermal waters which

has to be considered at an early planning stage.

6.3 Conclusions Related to Testing equipment

Significant effort was done to synchronize data sets that origin from multiple data acquisition systems (surface main

DAQ, ESP, PT gauges). GPS clock synchronization for the surface DAQ and time checks at selected events for downhole

gauges (e.g. when pull out of hole) are recommended.

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Both near surface and dowhole pressure sensors are recommended to assess both well performance and reservoir

properties. Redundancy of sensor installations proofed to be essential to obtain a good quality data set.

Use of downhole flow-meter is highly desirable especially in case of well production with air-lift technique, where the

escaped vapor phase is difficult to quantify. In the presented case, however, this kind of tool did not deliver reliable data.

Due to cooling of the water column in the well after hot water injection, shortening of the PT sensor cable occurred which

compromises the interpretation of the recorded data. In geothermal wells, a magnetic or mechanical fixation mechanism

attaching deep PT-sensors to the casing would be a welcome advancement.

6.4 Conclusions Related to the Reservoir

The large fault planes identified in the seismic images (with considerable displacement) do not seem to dominate the hydraulic test

responses. If the boreholes would be fully connected to the fault zones and act as distinctly planar, highly permeable features, then

their extension would still be limited in one direction by the overlying and underlying low permeable rocks (Molasse and Dogger).

In such a setting, the fault zones would likely to be detectable in hydraulic test data as partly confined reservoir. The tests analyses

indicate a basically radial flow regime with minor boundary constraints for GT3a only.

The above does not deny that the fault zones may or are likely to play a major role for increased permeability in the Malm

formation. The faults may interconnect large parts of the aquifer, possibly also along conjugate fault subsystems not seen in seismic

data, with a distribution of increased permeability not restricted to the main fault surfaces.

6.5 Conclusions Related to Future Plant Operation

GT1a: 120 l/s pumping rate is expected to be feasible with relatively small drawdown.

GT3a is only half as performant as GT1a.

The less performing well GT3a will be used for re-injection. In future operation, the thermal water will be cooled from

130 ºC to 55 ºC. Reinjection is eased by the higher density of the cooled water creating an increased pressure gradient

from well to formation (Figure 16).

The viscosity of the water will increase due to cooling and thus have an effect on the injectivity. The effect of viscosity is

expected to be masked by other processes, as it has been observed at other geothermal sites in the Munich region,

presumably due to CaCO3 dissolution in the formation and due to increased fracture apertures in the cooled part of the

formation.

The objective of 120 l/s flow capacity under the conditions of future plant operation seems achievable provided that an

additional injection pump is installed (at the surface) to support re-injection.

Figure 16: The shift of the well characteristics curve due to density effects when injecting cooled water suggests that

injection rates at GT3a can be increased up to 120 l/s whilst limiting injection pressure at surface to 40 bar. This

forecast assumes that increased flow impedance due to higher viscosity of the cooled water is compensated by other

effects such as CaCO3 dissolution in the formation and by increased fracture apertures in the cooled part of the

formation.

Fisch et al.

13

7 ABREVIATIONS AND NOMENCLATURE

CHB Constant head type boundary

DAQ Data acquisition system

ESP Electric submersible pump

LC Log cycle

MD Measured depth along the path of the borehole

PSR Pressure static recovery

PT Pressure and temperature (e.g. as combined measuring parameter in a PT gauge).

RDS Rate dependent skin

SLA Straight-line analysis

ST Step of step rate test

TVD True vertical depth

WBS Wellbore storage (C)

c = compressibility [Pa-1]

C = wellbore storage coefficient [m3/Pa]

C1, C2 = fit coefficients [bar/s]

ct = total compressibility [Pa-1]

cf = formation compressibility [Pa-1]

cw = water compressibility [Pa-1]

D = non-Darcy flow coefficient [-]

g = acceleration of gravity [m/s2]

H, h = hydraulic head [m]

i = gradient [-]

II = injectivity index [l/(s bar)]

L, h = thickness of aquifer, ‘pay or feed zone’ [m]

k = (intrinsic) permeability [m2]

K = hydraulic conductivity [m/s]

m = slope of semi-logarithmic straight line [Pa]

PI = productivity index [l/(s bar)]

P = pressure [Pa, MPa]

q = volumetric flow rate [m3/s, l/s]

r = radial distance [m]

rw = wellbore radius [m]

S = aquifer storage = rw g L (cw + cr) [-]

Ss = specific storage = S / L [m-1]

S’ = storativity = h ct [m/Pa]

S = storage coefficient = g h ct [-]

s = skin factor [-]

s0 = constant part of skin in RDS equation [-]

s’ = rate dependent skin (RDS) [-]

= porosity [-]

= dynamic viscosity [Pa s]

= density [kg/m3]

T = transmissivity [m2/s]

k h = transmissibility [m3]

t = time [s]

V = volume [m3]

Fisch et al.

14

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