ASSESSMENT OF STEAM SUPPLY FOR THE EXPANSION OF GENERATION CAPACITY FROM 140 TO 200 MW, KAMOJANG GEOTHERMAL FIELD, WEST JAVA, INDONESIA

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1 ASSESSMENT OF STEAM SUPPLY FOR THE EXPANSION OF GENERATION CAPACITY FROM 14 TO 2 MW, KAMOJANG GEOTHERMAL FIELD, WEST JAVA, INDONESIA Subr K. Sanyal 1, Ann Robertson-Tat 1, Chrstopher W. Klen 1, Steven J. Butler 1, James W. Lovekn 1, Phlp J. Brown 1, Sayog Sudarman 2 and Syafe Sulaman 2 1 GeothermEx, Inc., 5221 Central Avenue, Sute 21, Rchmond, Calforna 9485 USA 2 Pertamna Geothermal Dvson, Jl. Merdeka Tmur 6, Jakarta 111, Indonesa Key Words: Indonesa, Kamojang, geothermal reservor engneerng ABSTRACT The Kamojang geothermal feld has an nstalled generaton capacty of 14 MW and another 6 MW s planned to be added. Ths paper presents an assessment of the feasblty of ths expanson from the pont of vew of resource supply. Volumetrc assessment of reserves ndcates an equvalent of at least 21 to 28 MW generaton for 3 years, suffcent for the exstng capacty plus the proposed expanson. The noncondensable gas content n the steam s low (<1%) wth a modest amount of H 2 S (<3 parts per mllon by weght). Slca scalng n flow lnes and the turbnes s beng effectvely handled and only one well produces corrosve steam. Therefore, the flud chemstry presents no barrers to capacty expanson. To date, 68 wells have been drlled wth a hgh success rate (about 8%). Thrty-one wells (ncludng 3 stand-by) are presently used to supply the exstng plant, and more than suffcent wellhead steam capacty s avalable from the remanng wells for the expanson. Ample drllng stes are avalable for future make-up wells. The productvty of ndvdual wells has declned at a very low rate (1 to 5% per year). Over the 15-year producton hstory of the feld, reservor pressure has shown a modest drop (5 bars), mplyng that the reservor storage and flow capactes are adequate for at least the exstng generaton level. The produced steam has not shown any sgnfcant superheatng to date; ths fact and the relatvely small pressure drop mply that the reservor stll contans water saturaton. The present average well productvty declne rate s about 4.2% and we have estmated that the capacty expanson wll change ths to approxmately 6.4%, whch s stll a low declne rate. Ths rate of declne mples the need for 2 to 3 make-up wells per year followng capacty expanson. At ths rate of make-up well drllng, the well-spacng wll be reduced over 3 years to 35 m whch should not cause undue nterference between wells. Therefore, we have concluded that the planned expanson of generaton at Kamojang s entrely feasble from the pont of vew of response supply. 1. INTRODUCTION 1.1 Background Ths paper presents the results of assessng resource supply at the Kamojang geothermal feld n Central Java, Indonesa. Ths feld supports an nstalled capacty of 14 MW, operated by PT PLN, the natonal electrc utlty. The feld s operated by PN Pertamna, the natonal ol company. The goal of assessment was to determne whether the Kamojang resource would be able to support another 6 MW development (Unts IV and V), to dentfy potental problems assocated wth the expanson, and to determne the lkelhood that the steam supply can be mantaned over the project lfe. Most of the wells to be dedcated to the expanson have already been drlled and tested. 1.2 Project Hstory The Kamojang feld was dscovered early n the 2th century, and fve exploraton holes were drlled n the late 192s. Exploraton resumed n the 196s, and n late 1982 producton of 3 MW (Unt 1) was started, suppled by 6 wells n the central part of the feld (fgure 1). Development drllng contnued and two 55 MW unts (Unts II and III) were added n At present, 31 wells, ncludng stand-by wells, supply steam to the three unts. The total number of producton wells, ncludng both actve and deal wells, s now 68. The upper plot on fgure 2 shows versus tme the total producton from the feld and the number of producton wells on lne. Snce Unts II and III started up, the total producton from the feld has remaned relatvely constant at about 1,1 tonnes per hour of steam. Producton has been mantaned by drllng make-up wells. As n other steam-domnated felds, productvty declne (and hence the need for make-up wells) s most severe n the frst year or two, and stablzes thereafter. Wellhead temperature and pressure data ndcate that most wells are showng a dryng trend over tme; ths s also ndcated by the fact that condensate njecton decreased from an average of about 2 tonnes/hr durng the perod from 1988 to 1992 to 15 tonnes/hr n md Slca scale has been found on the Unt I turbne blades and n the gatherng system. Pertamna uses a combnaton of chemcal and mechancal means to remove the ppelne scale perodcally, and the turbne scale s mechancally removed durng annual plant mantenance overhauls. 1.3 Summary of Resource Characterstcs The Kamojang reservor s steam domnated and ncludes an area of 14 km 2 whch s commercally proven by drllng productve wells; these presently supply on average about 5 MW per well wth 7-nch producton lner. The addtonal drllng needed to complete the steam requrement for Unts IV and V wll take place wthn ths zone of proven producton. Based on MT resstvty data and a lmted amount of drllng data, t s lkely that an addtonal area of 7 km 2 lyng largely to the west of the area of present development s also commercally productve. Based on these

2 areas and a typcal reserve s range of 15 to 2 MW per km 2 for such volcanc systems, the proven reserves for the Kamojang feld are estmated to range from 21 to 28 MW for 3 years. Producton s obtaned from fractured andestes, wth the reservor top at an elevaton of about +9m (msl) n the northeast part of the proven area, slopng from there gently to the west and south, and droppng sharply n the extreme south to about +2 msl (fgure 3). Subsurface temperature data support ths geometry (fgure 4), as do electrcal resstvty data from standard traverses and soundngs, MT, and CSAMT (fgure 5). Resstvty also shows a strong ncrease along the far east sde of the project area (fgure 5), whch may ndcate a feld boundary to the east. Chemcal data from the project wells are typcal for steam wells, wth total non-condensable gas content generally below 1% by weght and H 2 S gas content less than 3 parts per mllon by weght. 2. ANALYSIS OF WELLTEST AND PRODUCTION DATA 2.1 Well Productvty Hstory Fgure 2 shows the total feld steam producton rate (upper graph), the number of wells beng produced (upper graph), and the average producton rate per well (lower graph) as a functon of tme. Average well productvty declned sharply n 1987 when generaton capacty was ncreased from 3 MW to 14 MW. Ths declne s a result of drllng wells n areas of lower productvty than encountered for the frst 3 MW project, and of nterference between the wells. Fgure 6 shows the producton hstory of a typcal well (KMJ-17), wth steam flow rate, flowng wellhead pressure, flowng wellhead temperature and the superheat n steam. The unt steam requrement for the exstng plants s 8 tonnes per hour per MW. The wells supplyng these plants have a combned capacty of about 142 MW, and there s spare capacty of about 9 MW from 2 more wells that are used nfrequently. Many of the wells are beng produced at wellhead pressures hgher than the mnmum acceptable (about 1 kscg), gven the turbne nlet pressure and the pressure drops experenced n the ppelnes. These wells can produce more f they are opened more, but some are kept throttled to prevent wellbore scalng, whch s known to be an operatng problem. There s lttle evdence of superheatng n the produced steam, whch mples that sgnfcant lqud water saturaton stll exsts n the reservor. If the lqud n the reservor bols off completely, there can occur a strong decrease of well productvty and potentally an ncrease n steam corrosvty. 2.2 Well Delverablty Test Data Pertamna has tested 37 producton wells for mult-pont delverablty characterstcs (flow rates at varous wellhead pressures) before puttng them nto routne producton. In addton to these ntal delverablty tests, a sngle-pont delverablty test (measurement of flow rate at a sngle wellhead pressure) has been conducted once a year as a means of montorng changes. We frst defned the ntal delverablty curve of each well by usng numercal wellbore smulaton to ft an approprate curve to the ntal delverablty data. Then, usng the sngle pont delverablty data for each well n January 1996, and agan usng wellbore smulaton, we also defned the recent delverablty characterstcs of each well. Fgure 7 (well KMJ-36) s an example whch shows the delverablty curves determned from the ntal mult-pont test n 1985 and annual sngle-pont tests to (In ths case, some data ponts are explaned only by hgher reservor pressure than was ntally apparent; ths s not lkely and some data may be n error.) Table 1 presents for each well the estmated ntal delverablty at 1 klograms per square centmeter gauge (kscg) wellhead pressure (and on the date shown), as well as the delverablty as of January 1996 (also at 1 kscg). From table 1, t s clear that all wells have declned n productvty over tme, as to be expected n any geothermal feld. The sgnfcance of the ntal annual harmonc declne rate shown n table 1 s dscussed below. 2.3 Productvty Declne Trend of Wells Table 1 and plots such as fgure 6 are not suffcent to assess the true productvty declne of a well, because wellhead pressure has not been constant. Instead, an analyss of productvty declne trend requres several steps, as proposed n Sanyal, et al (1989). The frst step s to calculate the statc (that s, shut-n) wellhead pressure for each well usng the equaton: W = C ( 2 p - p 2 n f ) where W s the steam flow rate, p s the statc wellhead pressure, p f s the flowng wellhead pressure, C s an emprcal parameter and n s a second emprcal parameter ("turbulence factor") havng a value between.5 and 1.. As producton from a well declnes, p declnes steadly, n remans nearly constant and C declnes relatvely slowly. If C s assumed to be constant, t can be replaced n (1) by C, the ntal value of C, as follows: W = (2) C 2 n ( p - p f 2 ) where the subscrpt '' denotes ntal condtons. A representatve value of C for each well s determned statstcally based on data from the frst few weeks of producton. Then the statc wellhead pressure of a well can be calculated as a functon of tme usng ths C and any chosen n value; ths method has proven vald at The Geysers steamfeld n Calforna. Fgure 8 shows the calculated statc wellhead pressure plot for a typcal Kamojang well (KMJ-17), usng an n value of.75, along wth measured shut-n wellhead pressures. The match between the measured and calculated statc wellhead pressures s excellent for most wells. In cases where the measured pressure s low compared to the calculated, the measured data smply reflect nsuffcent buld up tme. Ths analyss showed that the statc wellhead pressure has been declnng for all wells, whch s a drect reflecton of declnng reservor pressure. Snce the turbne nlet pressure has remaned nearly constant wth tme, declnng reservor pressure has resulted n declnng steam producton rate. (1)

3 The next step n our analyss was to "normalze" the producton rate hstores of the wells to a constant wellhead pressure of 1 kscg (Sanyal et al, 1989). Once the normalzed producton rate hstory of a well s calculated, ts productvty declne trend can be assessed. The long-term declne trend n well productvty n steam reservors has been observed to be harmonc. Harmonc declne mples that the declne rate (D) s not constant, but tself declnes wth tme. Harmonc declne can be expressed as: W W = (3) 1+ D t where W s the ntal producton rate, W s the producton rate at tme t and D s the ntal declne rate. Assumng harmonc declne, one can calculate the ntal harmonc declne rate for each well usng (3) and the ntal delverablty and January 1996 delverablty values shown n table 1 for that well. The declne rates thus calculated are shown n the last column of table 1. The table shows that a majorty of the wells have relatvely low declne rates (18% or less), typcal of steam wells. However, eght wells are showng unusually hgh declne rates. These wells are suspected of declnng rapdly n productvty due to contnung wellbore scalng or some other form of gradual wellbore damage. Records ndcate that at least wells KMJ- 35, KMJ-42 and KMJ-44 were known to be damaged. We suspect that the other three wells wth unusually hgh declne are also beng affected by well damage and the declne does not reflect reservor pressure drawdown alone. It should be noted that (3) mples that a plot of log W versus cumulatve producton s lnear, from the slope of whch one can calculate the ntal annual declne rate (Sanyal et al, 1989). Fgure 9 presents a plot of normalzed producton rates (on a logarthmc scale) versus the cumulatve producton (on a lnear scale) for a typcal Kamojang well. Several wells wth an apparent tendency towards well damage were not consdered for such analyss. On each plot there are two lnear trends, one correspondng to the 3 MW generaton level (pre-1988) and one correspondng to the 14 MW generaton level (post-1988). The slope of the pre-1988 data trend s much less than that of the post-1988 trend, reflectng the hgher level of producton after (In the case of well KMJ-17 on fgure 9, the slope of the pre-1988 data trend s effectvely zero, and 1988 starts at 2 klo Tonnes.) A lnear trend lne has been ftted to the post-1988 perod on each plot lke fgure 9 and, from the trend, the ntal declne harmonc trend (D ) for each well has been calculated (table 2). The ntal harmonc declne trend of 1.6% to 9.5% and the average ntal harmonc declne trend of 4.2% are typcal of wells n a steam reservor, provdng that the wells are not undergong scalng or other forms of contnuous well damage and that the reservor has not been developed beyond ts sustanable capacty. Table 2 also compares the annual harmonc declne trend estmated from the declne curve wth that estmated from the delverablty curve (table 1; adequate delverablty data are not avalable for wells KMJ-45 and KMJ-46). Except for wells KMJ- and KMJ-34, whch appear to be sufferng from well damage, the declne rates calculated by both methods are smlar. In fact, for the 14 comparable wells n table 2, the average declne rate from declne curves (3.8%) s close to that from delverablty curves (4.%). Therefore, wth mechancal control of well damage and for 14 MW of generaton, the wells at Kamojang can be expected to declne at an ntal annual harmonc rate of 4.2% startng Analyss of Pressure Buld-up Data Pressure buld-up data were avalable from many wells and from these we have calculated the ndvdual well flow capacty. The results show a three orders of magntude varaton, from 328 to 35, n mlldarcy meters, whch s not uncommon n a geothermal reservor. Ths wde varaton n reservor flow capacty s reflected n the wde varaton n well productvty, from.4 to 14.8 MW (two orders of magntude). There s a weak postve correlaton between flow capacty and well productvty, but no areal dstrbuton pattern of reservor flow capacty or of well productvty could be decphered. Therefore, certan randomness n drllng success s unavodable. Indvdual well pressure buld-up data could also be used to determne the statc pressure dstrbuton n the reservor. From ths, t was concluded that the ntal reservor pressure was about 35 kscg, and after 15 years of producton the reservor pressure has declned by about 5 kscg. Ths change s relatvely modest. 3. STEAM SUPPLY FOR CAPACITY EXPANSION 3.1 Introducton The 31 wells connected to the exstng plants (14 MW) have a maxmum total capacty of about 155 MW, mplyng a 1.7% excess capacty. Nne exstng wells are not connected to the exstng plants and are, therefore, avalable for steam supply for the 6 MW expanson. These 9 wells have a maxmum total capacty of 68.8 MW. If all nne wells are avalable for producton, the needed steam supply for the startup of the new 6 MW plant s theoretcally avalable already. However, n practce t would be more prudent to drll a few more wells to allow operaton at a hgher wellhead pressure than 1 kscg (presumably to reduce slca scalng), to allow some stand-by capacty, and to provde for the needed njecton capacty. Development drllng at Kamojang has enjoyed a hgh success rate (over 8% to date) and wthn the dedcated area ample drllng stes are avalable for addtonal development wells. Therefore, the needed producton and njecton capactes for the expanson can be readly secured, and the resource rsk s mnmal up to the tme of start-up for the new plant. However, as at any geothermal project usng a steam reservor, there are some resource rsks once addtonal generaton starts. These rsks are dscussed below. 3.2 Man Resource Rsks After Plant Capacty Expanson The productvty of the wells at Kamojang has declned at a low rate (about 4% per year ntal harmonc rate). Over the 15-year producton hstory of the feld, reservor pressure has declned by about 5 kscg, whch s a relatvely modest pressure drop, mplyng that the reservor storage and flow capactes are adequate for at least the exstng generaton level. The produced steam has not shown any sgnfcant

4 superheatng to date; ths fact and the relatvely small pressure drop mply that the reservor stll contans water saturaton. However, there s some uncertanty about the extent of the lqud saturaton at ths tme. Therefore, W D W W W ln ( ) = ln ( ) (8) W D W a a The above uncertanty notwthstandng, we beleve that the expanded capacty of 2 MW can be supported for the project lfe for the followng reasons: And from (8), we get W ( lnw - lnw a ) D = ( ) (9) D W ( lnw - lnw a ) the recoverable geothermal energy reserves appear to be adequate for the expandng capacty; the unusually small pressure drawdown n the reservor and declne n well productvty ndcate a relatvely hgh storage capacty n the reservor; there s lttle ndcaton of superheat n the produced steam to date, ndcatng the presence of lqud saturaton; there are no major problems assocated wth steam chemstry; and there has been a hgh success rate n development drllng. However, the expanson may present certan operatonal challenges; therefore, the operatons and mantenance costs per MW may be greater than htherto experenced. One operatonal challenge may be an undue ncrease n the well productvty declne rate, whch would also ncrease the makeup well requrement. Ths ssue s dscussed below. Another operatonal challenge may be superheatng of steam, wth consequent aggravaton of slca scalng and ncreasng the chances of corroson. 3.3 Forecastng Productvty Declne Followng Plant Capacty Expanson We have estmated approxmately the expected ncrease n the productvty declne rate of wells followng the capacty expanson. Whle ths approxmate calculaton must be verfed and refned by numercal smulaton, t does gve an estmate of at least the mnmum lmt of the expected declne rate. The approxmate methodology that we have used s as follows. The reserves of steam avalable from any well can be estmated as: t a Reserves (R)= Wdt, (4) where t a = abandonment tme. Therefore usng equaton (3), ta dt R (5) =W 1+ D t W = D W ln ( ) (6) W a Smlarly, one can show that f a hgher ntal producton rate W s mposed, and D s the correspondng ntal harmonc declne trend, then W W R = ln ( ) (7) D W a One can then calculate from equaton (9) the ncrease n the harmonc declne rate from D to D n consequence of an ncrease n the producton level from W to W. To verfy the above equatons, let us note that the ntal harmonc declne rate at Kamojang has been 4.2% per year at a 14 MW generaton level. Usng equaton (9) we can calculate the ntal harmonc declne rate for a 3 MW generaton level. Let us assume an abandonment level of 1 MW. Then D I s calculated at.6%. Indeed, lnear ft through the pre-1988 data ponts n the plots of normalzed flow rate versus cumulatve producton for the wells verfy that the ntal harmonc declne of approxmately.6% prevaled between 1982 and Fgure 9 clearly shows, for well KMJ-17, the change from.6% pror to 1988 to about 4% after the capacty ncreased to 14 MW. Gong forward, from equaton (9) we can make an approxmate estmate of the ntal harmonc declne rate followng capacty expanson from 14 MW to 2 MW: ths s 6.4%. Therefore, the declne rate wll reman relatvely low even after a capacty expanson by 6 MW. Assumng that the 2 MW capacty wll be suppled by 4 producton wells (31 supplyng the exstng plants and 9 avalable for the new plant), ths rate of harmonc declne wll requre about 2.5 make-up wells per year (that s, 2 to 3 make-up wells wll need to be drlled each year). In 3 years, the total number of make-up wells would reach about 75. The 4 wells at start up plus 75 make-up wells would mean 115 wells n 3 years. Gven that the proven productve reservor at Kamojang s 14 km 2, 115 wells would have an average dranage area of 122, m 2 per well. Ths mples an ultmate average well spacng of about 35 m. Ths level of ultmate well spacng should not gve rse to undue nterference between wells. In fact, we often use the rule of thumb of 3 m mnmum well spacng to assess the adequacy of dedcated productve area for a geothermal project. In concluson, we beleve, subject to verfcaton by numercal smulaton, the above analyss ndcates that the resource rsk assocated wth productvty declne s low. ACKNOWLEDGEMENTS The authors would lke to thank Pertamna for ther support of ths project and permsson to publsh these results, and Dr. Keshav Goyal of Calpne Corporaton for revewng ths paper. REFERENCES Sanyal, S.K., Menzes, A.J., Brown, P.J., Enedy, K.L, and Enedy, S. (1989). A systematc approach to declne curve analyss for the Geysers steam feld, Calforna. Transactons, Geothermal Resources Councl. Vol.13, pp

5 Intal Delverablty Estmated January 1996 Delverablty Intal Annual Well Tonnes/hour Date MW Tonnes/hour MW Harmonc Declne (%) KMJ / KMJ / KMJ / KMJ / KMJ / KMJ / KMJ- 27 8/ KMJ / KMJ / KMJ / KMJ / KMJ / KMJ / KMJ / KMJ / KMJ / KMJ / KMJ / KMJ-39 1/ KMJ / KMJ / KMJ / KMJ / Table 1: Analyss of Well Delverablty at 1 kscg Wellhead Pressure Well Intal Harmonc Declne Rate (%) Name From Declne Curves From Delverablty Curves KMJ KMJ KMJ KMJ KMJ KMJ KMJ ? KMJ KMJ KMJ KMJ KMJ ? KMJ KMJ KMJ KMJ KMJ KMJ Average 4.2 Table 2: Estmated Rates of Well Productvty Declne 15 Total Producton (tonnes/hr) , 14, 17, 18 24,, 39, , 26, 27, 3, 31, 36 38, 41, 42, 45, , off Total Producton Rate Producton Well Count off 33 4 Numbers along well count lne ndcate whch wells are gong on-lne or beng shut n Producton well count Ave. w ell productvty (tonnes/hr) Year Fgure 2: Total producton, number of wells on lne and average well productvty Fgure 1: Well locaton map Fgure 3: Elevaton of hghest lost crculaton zone Fgure 4: Temperature dstrbuton at +6 m (msl)

6 Flow rate (tonnes/hr) Pressure (kscg) Temperature ( C) Temperature Superheat Year Superheat ( C) Fgure 6: Producton hstory, Kamojang well KMJ-17 Fgure 5: MT apparent resstvty at 1 Hz Total flow rate (tonnes/hour) Reservor Pressure (kscg) Statc Pressure (ksca) Aprl - July 1985 test Annual test Wellhead pressure (kscg) Fgure 7: Delverablty data, Kamojang well KMJ Calculated, n=.75 measured Year Fgure 8: Changes to statc pressure vs. tme, Well KMJ-17 1 Normalzed flow rate (Tonnes/hr) 1 Calculated, n= Cumulatve producton (klo Tonnes) Fgure 9: Log normalzed flow rate vs. cumulatve producton, Well KMJ-17