m. Boedihardi, Suranto, s. Sudarman-evaluation of the Dieng Geothermal Field_ Review of Development Strategy-Indonesian Petroleum Association (1991)

© IPA, 2006 - 20th Annual Convention Proceedings, 1991 IPA 91-21.20

PROCEEDING INDONESIAN PETROLEUM ASSOCIATION Twentieth Annual Convention, October 1991

EVALUATION OF THE DIENG GEOTHERMAL HELD; REVIEW OF DEVELOPMENT STRATEGY M. Boedihardi* Suranto*

S. Sudarman*

ABSTRACT

INTRODUCTION

The liquid-dominated system of the Dieng Geothermal Field in Central Java can be divided at least into three areas: the northwestern Sileri area, the central Sikidang-Merdada area, and the southeastern Pakuwaja area. A total of sixteen exploration and development wells have been drilled, with one well at Sileri, thirteen wells at Sikidang-Merdada, and two wells at Pakuwaja.

The Dieng geothermal field is situated in Central Java, about 26 km north of Wonosobo, the nearest town. Presently, eleven exploration and five production wells have been completed at Dieng (Figure 1). Thirteen wells have been drilled in the central Sikidang-Telaga Merdada area, one well (DNG-10) at Sileri in the northwest, and two wells (DNG-5 and 11) at Pakuwaja to the southeast. Existing plans are to develop a 1x55 MW unit by 1995 located at the Sikidang-Merdada area. About 10 MW of electrical capacity are currently available at the wellhead for wells in this part of the field.

Interpretations of existing gravity, magneto-telluric and well data reveal that the Sikidang-Merdada area is separated from Sileri and Pakuwaja by zones of low permeability and low temperature. These low permeability barriers show up on the geophysics as regions of gravity highs and conductivity lows. A fumarolic area with an associated gravity anomaly known as the Siglagah prospect occurs in east of Sileri and represents a target for future exploration. The total geothermal potential is estimated as 355 MW, of which 175 MW are located in Sikidang-Merdada. A 55 MW project is planned for Sikidang-Merdada to come on line in 1995/96. Modular units are recommended, consisting of 2x20 MW and 1x15 MW units. About ten megawatts of capacity currently exist at the wellhead and future development wells are expected to produce 55 tons/hr steam per well. The reservoir at Sikidang-Merdada consists of a domeshaped steam zone overlying a brine reservoir with 2000-3000 ppm chloride and 0.5 to 1.6% noncondensible gas by weight. The reservoir temperature is as high as 320°C.

1

PERTAMINA-Geothermal Div., EP

Dieng has been the subject of many studies. The most comprehensive study, which was based on both exploration and production data, was done by Unocal in 1989. Unocal examined the results of thirteen Dieng wells. Since 1989, three additional wells have been drilled at Dieng, and these provide key data for the reinterpretation of the reservoir characteristics. The objective of this study is to utilize the new well data in order to determine whether the development plan for a 1x55 MW unit is still suitable for the Dieng reservoir. PRODUCTION DATA Table 1 summarizes the drilling results at Dieng. Of the thirteen wells in the Sikidang-Merdada area, the abandoned wells DNG-1, 4, 7 and 12 had the highest capacity of 5 to 10 MW, and they produced from about 1500-2000 mVD (metre Vertical Depth). Wells DNG2, 8 and 13 currently produce steam for a total of about 10 MW from productive zones between 1500 to 2000m. Three other wells, DNG-3, 6 and 9 are now unusable due to well damage. The recently completed wells, DNG-14, 15, and 16 are still undergoing testing. Wells DNG-5 and 11 at Pakuwaja have not been tested due to

348 well damage or low reservoir permeability, although the downhole temperatures are high. The well production characteristics, such as mass flow, enthalpy, gas content and dryness for the SikidangMerdada and Sileri areas are presented in Figures 2 and 3. At a WHP (Well Head Pressure) of 15 bars, the total mass flow in Sikidang-Merdada wells is about 75 tons/ hr, enthalpy is about 2200 kJ/kg and gas content and dryness are 5% by weight and 70%, respectively (Figure 2A). DNG-10 in the Sileri area has a slightly lower output of about 65 tons/hr with enthalpy of about 1500 kJ/kg and gas content of 4% by weight and 50% dryness (Figure 2B). A high gas content of up to 20% wt, mostly carbon dioxide, was recorded in DNG-2 at the Sikidang-Merdada area during the initial well tests. During ten years of production to a 2 MW monoblock, the gas content of DNG-2 decreased to 2% wt, and a computer simulation model indicates further decreases to about 1% as production continues (Figure 3A, from GENZL/SDB, 1991). This simulation analysis also indicates that the mass flow for a 1x55 MW unit will decrease from about 125 to 100 kg/s after 30 years of operation (Figure 3B). EXPLORATION AND SUBSURFACE WELLBORE DATA Surface exploration data, including geology, geophysics and geochemistry, are discussed in this section and linked to the wellbore data in order to clearly understand the subsurface conditions of the studied area. A simplified geological map of the Dieng field is shown in Figure 1. This field has 10 lithologic units which are composed of andesitic lava, tuffaceous breccia and quartz latite. Potassium-Argon age dating has been done for some units to give better control to the volcanic stratigraphy. The Sileri area which is associated with Gn. Pagerkandang at the north has an age of 460,000 years. Sikidang-Merdada is associated with Pangonan of 370,000 years of age. The Pakuwaja area in the southeast has an age of 90,000 years. These age dates show a magmatic migration from northwest to southeast. These three areas have gravity and magneto-telluric (MT) anomalies (Figures 4A and 4B, respectively). The gravity anomalies are characterized by low residual gravity at 1000 Mhos. The combined anomalies have sizes of 3 km2 at Sileri, 6.5 km2 at Sikidang-Merdada, and 2 km2 at Pakuwaja. The gravity data also shows an anomaly at Siglagah, an area east of Sileri which has fumaroles. Unfortunately, a few MT coverage is available for Siglagah. SikidangMerdada is clearly separated from Pakuwaja by an MT

conductance low and a gravity high. The geophysical data show a similar but less pronounced separation between Sikidang-Merdada and Sileri. Major lineaments, which have been interpreted from aerial photography as faults, are also shown in Figure t. The three main structural trends from the oldest to the youngest are E-W, NW-SE and NE-SW or N-S. These structural features are likely to control the boundaries of the three prospective areas. Circular features, which usually represent crater rims, are seen in all areas and typically have diameters ranging from 0.1 to 1.1 km. The hydrothermal surface manifestations on the Dieng plateau occur on high and low elevation areas. At high elevations such as volcanic peaks and plateau, the manifestations consist of fumaroles, acid sulphate boiling springs, mud pools and intensively altered ground. At lower elevations, the thermal areas are mainly hot and warm springs with temperatures ranging from 35 to 55°C and neutral pH. These springs are numbered on the map in Figure 1, and the corresponding spring and borehole chemistry appear in the ternary diagram on Figure 1 and in Table 2. The warm and hot springs have bicarbonate-sulphate chemistry, while the wells produce sodium chloride water. This chemistry suggests that there is no direct connection between the springs and the deep reservoir fluids, thus making chemical geothermometry of little use in estimating reservoir temperature. However, the high chloride content in the feed zones indicates that the reservoir is water dominated. The dissolved gas in the brine reservoir of the SikidangMerdada area has been computed by Suwana (1986) and Fauzi (1987) from well discharge data. They conclude that the brine contains 0.5 to 1.6% by weight non-condensable gas. The chloride concentration in the reservoir brine is between 2000-3000 ppm. The hydrothermal alteration in the 16 Dieng wells has been studied. Ganda and Suroto (1985) found three main alteration assemblages with a vertical zonation that could be correlated with temperature. Argillic alteration is the shallowest, propylitic occurs at intermediate depths, and phyllic at the greatest depthThese assemblages correspond to temperatures of 150-250°C, 250-300°C and >300°C, respectively. The production casing shoe is set towards the bottom of the argillic section. Exceptions to the alteration zonation are found at DNG-10 at Sileri, where the phyllic alteration is absent, and in DNG-15, where the high temperature minerals are undeveloped. Maps of temperature and pressure of the main feed zones for the entire Dieng field are shown in Figures

349 300°C, but at Pakuwaja measured temperatures only reach 290°C. This relationship suggests that Pakuwaja may be in cooling phase.

350 Well DNG-9 which was originally low in productivity and has now become unproductive penetrates the same permeability structures encountered by wells DNG-1 and 4 which were originally good producers. The low productivity of DNG-9 might be due to formation damage which occurred as a result of lost circulation or might be attributable to calcite and silica scaling. This well would be a good candidate for an acid job followed by hydraulic fracturing in order to improve permeability. Wells DNG-1, 4 and 7 (which was also originally good producers), had to be abandoned because the casings are suffered from severe corrosion damage. Poor cement jobs seem to contribute to the corrosion problems by allowing shallow acidic fluids to contact the casing. Cementing practices and the cement formula should be re-examined to overcome this problem. The potential geothermal reserves of each of the three areas have been calculated by multiplying the areal extent of the anomaly by two factors. The first factor is the temperature differential, taken as the difference of the measured reservoir temperature and a base temperature, in- this case 180°C. The second factor, referred to as the "C" factor, is an energy conversion factor with the units of MWe per area-temperature. C has been given one of two values: 0.1 MW/°C km2 for energy stored in fluid and 0.19 MW/°C km2 for energy in fluid and rock. That latter value is based upon experience at the Kamojang geothermal field. Table 4 shows the results of the potential reserve calculations. The largest reserves of up to 175 MW occcur in the Sikidang-Merdada area. The total resource is estimated to be as high as 355 MW. Because the Sikidang-Merdada area has the highest potential reserves, good well deliver ability (55 T/hr of steam from total flow of 75 T/hr per well), and 10 MW of existing capacity, this area is the most suitable for a 55 MW development. In order to accelerate the resource develoment, the development plan should be changed from the 1x55 MW unit to the following combination of units: 1x15 MW plus 2x20 MW. The first 1x15 and 1x20 MW units should be fed by boreholes drilled in the eastern part of SikidangMerdada and the other 1x20 MW from the western part, as shown in the summary map in Figure 8. Future exploration should be directed towards the Siglagah area. This area has both fumarolic manifestations and a low gravity anomaly. At Dieng, the gravity anomalies are interpreted as a result of thick zones of argillic alteration. Based upon the size of the anomaly, Siglagah has an area of 2 km2 and up to 45 MW of potential resource.

The high elevation area surrounding the Dieng Plateau is mostly composed of older volcanics, such as Gn. Prahu, Gn. Bisma, and Gn. Nagasari, which date between 2.5 - 3.6 million years. These older volcanics probably represent the recharge area for the Dieng reservoir. The meteoric water could penerate the basement rocks which are probably composed of marl or limestone. CONCLUSIONS AND RECOMMENDATIONS

Provided below are the main findings and recommendations from this study. (1) Drilling has defined three distinct, separate prospect areas at the Dieng field: Sileri, SikidangMerdada and Pakuwaja. A gravity anomaly and fumarolic manifestations suggest the existence of a fourth prospect area at Siglagah. The total potential resource at Dieng is 355 MW, of which the Sikidang-Merdada area provides 175 MW. Barriers of low permeability and low temperature separate Sikidang-Merdada from Sileri and Pakuwaja. (2) The Sikidang-Merdada area has a dome-shaped steam zone overlying the 320°C brine reservoir containing 2000-3000 ppm Cl and 0.5-1.6% wt dissolved gas. The reservoir at Sileri is two phase, while Pakuwaja is single-phase liquid. All systems have reservoir temperatures ranging from 280°C to >320°C. Reservoir modelling suggests that lateral permeabilities at Sileri could be twice as high as at Sikidang-Merdada. The permeability at Pakuwaja is still unknown, but appears to be low, and any future drilling at Pakuwaja should be considered as exploration holes. (3) To accelerate commercial operation, a modular scheme consisting of 1x15 MW and 2x20 MW units is favourable for installation at Sikidang-Merdada rather than a single 1x55 MW unit. The 1x15 MW unit and a 1x20 MW unit should be fed by boreholes in the eastern portion of the area, and the other 1x20 MW unit should be fed from western wells. 10 MW of capacity currently exist, and each future well is expected to supply 55 T/hr steam. (4) DNG-9 is a good candidate well for an acid job and hydraulic fracturing in order to improve the well s productivity. (5) In general the Dieng field has low reservoir permeability. In order to reduce formation damage by drilling mud, consideration should be given to drilling with aerated mud. Furthermore, many

351 wells at Dieng have experienced casing damage. To reduce well failures, investigations are recommended for changing the composition of the cement and to setting the intermediate 13 3/8 inches casing deeper to perhaps 750m. ACKNOWLEDGMENTS The authors express their appreciation to the Management of PERTAMINA for allowing us to present this paper. We also acknowledge D. Rohrs of Unocal, who assisted with the English version of this paper, and Djauhar Fuad, who helped preparing the paper.

Ganda and Suroto, 1985, Subsurface Hydrothermal Alteration in the Dieng Prospect, Central Java, Internal Report, Lodged in PERTAMINA Geothermal Division. GENZL/PT. Sumber Daya Bumi, 1991, Computer Simulation of the Dieng Geothermal Field, Internal Report, Lodged in PERTAMINA Geothermal Division. Fauzi,A., 1987, Mineralogy and Fluid Composition at the Dieng Geothermal Field, Thesis, Victoria University.

REFERENCES

Suwana,A., 1986, Chemistry of the Dieng Geothermal System, Diploma Report, Geothermal Institute, University of Auckland.

BEICIP, 1975, Vulcanology of the Dieng Area, Indonesia, Internal Report, Lodged in PERTAMINA Geothermal Division.

Unocal, 1989, The Dieng Geothermal Prospect, A Technical and Economic Assessment, Unocal Geothermal Division.

352 TABLE 1 BOREHOLE DATA AND STATUS OF THE DIENG WELLS

SIKIDANG - MERDADA

SILERI

PAKUWAJA

WELLS Depth (mVD) A. PRODUCTIVE DNQ - 2 DNG - 8 DNQ - 10 DNQ - 13

BHT

Dryness Output ( x ) (MWe)

1662 1806

275 295

63 55

3 -2.5

1732

255

70

>4

Depth (mVD)

2300

BHT (°C)

325

Sub Total >9.5

Dryness Output ( X ) (MWe)

65

Depth (mVD)

BHT (°C)

Dryness Output ( X ) (MWe)

-3

Sub Total 3

Sub Total

0

B. NON PRODUCTIVE 1. Low T*K DNG-15

1905

225

2. High T.Low K DNG-5 DNG-11

2500 2431

3. Blocking DNG-3 DNG-6 DNG-9

1950 2501 2450

275 300 320

4. Abandoned DNG-1 DNG-4 DNG-7 DNG-12

1901 1855 2401 1996

325 325 365 325

C. NOT YET TESTED DNG-14 DNG-16

1768 2283

250 320

67

-2 -4.5 - 2

80 90

-5.5 -5.5 -5.5 -to

285 295

TABLE 2 CHEMICAL ANALYSIS OF RESERVOIR FLUIDS AND HOT SPRING WATER AT THE DIENG GEOTHERMAL FIELD. FLUID SAMPLE

WELLS DNG DNG DNG DNG DNG DNG DNG

: -

6 7 8 9 10 11 13

AREA/LOCATION

SIKIDANG-HERDADA SIKIDANG-HERDADA SIKIDANG-MERDADA SIKIDANG-MERDADA SILFRI PAKUWAJA SIKIDAHG-HERDADA

pH

5.7 6.4 7.2 4.6 5.9 5.8 4.1

Na'

K'

Ca"

525

166 478 17 68 919 161 93

110 89 40 140 405 148 83

1 16 18 57 60 53

11 22 72 167 226 159 90 59

1649

229 562 3351

821 1062

Hg"

2

4 1 7 67 2 1

HCOS'

348 71 1054

23 366 381 1

cr

S04=

802

370 243 119

3110

373 979

724

sio2

190 84 83 43 70

6741 1539 1508

288 189 450

3 9 15 386 429 439 135 388

68 87 152 156 45 162

34 175 175

148

27

3

1

* Aft

110 280

HOT SPRINGS :

1 2 3

4 5 ® 1 OUTSIOE ' f MAP 8 >

T. WAKNA BII1NGAN SILEIU NGANDAM PULASARI KALIANGET GARUNG WANARATA

3.9 7 6.4 5.7 6.9 6.4 6.4 5.9

7 31 53 123 183 249 101 259

24 27

0 104 340 313 208

3 10 32 47 48 153

1060

55 117

1053

409

67 164 * ttA . . 1

124 4

353 TABLE 3 CORRELATIONS : PERMEABILITY, AGE AND PRODUCTION DEEPER PERMEABILITY ( mD )

A R E A

AVERAGE WELL PRODUCTION AT 15 BAR (T/h)

AGE ( year )

STEAM

TOTAL FLOW

460,000

35

65

5

370,000

55

75

<

—

...

—i

—i —•-

2" ,

I—( 1 11—1 1 11 1

-r;

1—t

...

-*-

15

10

20

- 0 - -o- —0-o- -o-

—0 —r

25

year

200

FIGURE 3 - Prediction of gas concentration and steam flow changes up to 30 years production (1 x 55 MW) calculated from computer simulation modelling for the Sikidang - Merdada area (after GENZL/SDB, 1991).

30

357 OOtfGOZ'6

000'002'6

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6 u

99> E

2 "53 IS

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s 000'S03'6

000'00Z'6

T3

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358 ooo'eoz'6 T

000*002 '6 1

O O O

c o

6

s

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000*0026 3 t/j in U

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8

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359 OOO'OOZ* 6

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JO

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OOO'CO2' 6

OOO'003^6

op •u

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3

360 TENTATIVE MODEL -—S|NW—

-—SE

DIRECTION OF MAGMATIC MIGRATION ACCORDING TO THE AGE 370,000 Ye

•H

\—

190,000 Yr

SIKIDANG - MERDADA AREA

{>"6.5km*)

> 90,000 Yr

KENDIL—f-PAKUWAJA —f-PAKUW AREA-* BLOCK .

T.MERMOA PP3lti20MWJ

Solfotara / fumarolt MoUtt Main (e«d lont Cotlng shoe 9^8 Inch

| I

| Argllllc zone I Propjrlitlc zone Phylllc zone ^Sedimentary basement 3

4 9 Dlttanc* (km)

PERMEABILITY STRUCTURE 10 1,600

RESIDUAL GRAVITY ANOMALY

M.T. TOTAL CONDUCTANCE

FIGURE 7 - The Dieng geothermal field cross sections.

361

380,000

376,000

r

SIGLAGAH PROSPECT

SIKIDANG MERDADA AREA

Low K 6 T barrier 1 X 15 + 1X20 MW 1 X 20 MW

PAKUWAUA AREA

Power plant site Well DNG6/16

FIGURE 8 - Prospective areas and sugested geothermal development at the Dieng field. Explanation for other symbols see figure 1.