Efficient Well Testing at Ulumbu Field, Flores, Indonesia - Hole, Grant

8/10/2019 Efficient Well Testing at Ulumbu Field, Flores, Indonesia - Hole, Grant

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PROCEEDINGS, Twenty-Second Workshop on Geothennal Reservoir EngineeringStanford University, Stanford, California, January 27 -29, 1997SGP - TR - 155

EFFICIENT WELL TESTING AT ULUMBU FIELD, FLORES, INDONESIA

Malcolm A Grant', Hagen Hole', Markos Melaku' PT PLN (Persero)I Geothermal Energy NZ Ltd., Newmarket, Auckland, New Zealand.

2Jakarta, Indonesia

ABSTRACT

lumbu field is a candidate for a mini - geothermal3wer project, to replace the existing diesel generation.uch a small project cannot support the drilling andsting programs involved with large developments.

fter the successful drilling of a well, producing largeDWS of dry steam, a testing program covering a totalFless than 9 days' discharge over a period of twoonths provided definition of the reservoir fluid stateid permeability distribution.

he resource can support a larger discharge of dryearn from the currently drilled area than is needed fore present project, and it is indicated that drillingsewhere will find greater resources of two-phaseuid.

TRODUCTION

u field is one of several geothermal prospects on

g prospect on the island, and is the site of athermal power project carried out by P.T.

asdi 1996, Mahon et al 1992). The threedrilled deviated from a single pad. Figure 1n view of the well tracks and figure 2 a

tion. The wells cover a spread of about half a

and of Flores in Indonesia. It is the most

metre at bottomhole. They thus give informationithin a single region of the field.distance and difference between the

11s to give some indication of gradients within thisa as well as conditions at one point.

permeabil ity has been found and there ise than sufficient production for current plans.

very strong indications that the resourceof supporting a considerably greater

ent, if there is demand for the electricity.

Due to limited water supplies it was not possible tocarry out completion tests by injecting cold water. Allthree wells fill with steam in the upper part.

A mini project cannot support much overhead ofdrilling or testing. The program of well testing wasdesigned and executed to determine reservoir

parameters with minimum time and cost.

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-eo- U L E -

W i E m)

Figure 1 Plan view ofwells ULB-01 to ULB-03drilledfrom a single pad.

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U p p e r Vo l c an i c s

M i d d l e Vo l c a n i c s

L o w e r Vo l c a n i c s

Te r t i a r y S e d rn e n t sa n d Vo l c a n i c s

Horizonta l Dis tance from We l l h e a d ( rn)

pre 2. Cross section through the wells.

IWNHOLE MEASUREMENTS

mperatures and pressures

asurements in all three wells show superficiallyd a r results. Figures 3 4 show temperature andssure profiles measured after the discharge testinghe wells.

Temp era tu re s a f te r d i sch a rg e

I 9 0 21 0 230 250170

0

200

400

600

800

I

I 2 0 0

1400

1600

1 8 0 0

2

U L B 0 1

UL B -02U L B 0 3

~~

i

rre 3. Downhole temperatures in wells ULB-01toF03 11 -13 July 1996.

three wells show a stable wellhead pressure ofind 30

bar. There is a column of steam in the wells

down to around sea level or somewhat lower.this there is liquid in the well.

Downhole pressures after disc harge

20 40 60 80 100 120

0

600

.r3 1200

1400

1600

1800

2000

L B 0 3

ULB - 02-L B 0 1~

~ ~

Beneath

Figure 4. Downhole pressures in wells ULB-01 to -03,11 -13 J u ~1996.

In the steam column temperatures reach around 240C.In the liquid zone measured temperatures are similar,or a little cooler. Earlier runs in ULB-01 showed afairly constant profile with depth, suggesting adownflow. Stable temperatures in ULB-02 are veryconstant within the liquid column, also suggesting adownflow of water at 240C.

The downhole pressure profile can be representative ofreservoir conditions, ie corresponding a reservoir ofliquid water with a vapour -dominated zone above.Alternatively the liquid columns could beaccumulations of condensate and the reservoir entirelyvapour -dominated. The presence of downflowswithinthe liquid columns, and the discharge of liquid fromULB-0 1, demonstrates that the reservoir is liquid -dominated in the lower sections of the three wells.

DISCHARGEEach well was discharged separately, and the dischargemonitored. Wellhead pressures were monitored on allthree wells throughout the test period.

The test was designed to achieve the maximum ofresults with minimum effort and environmental effort.

It was very economical o f :

reservoir fluidwaste production

testing effort.84


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I

ell was discharged for 8 days, flowing dry steamnce producing no waste water. The other wells

flowed for periods of and hour to a few hoursWith this small test, excellent definition of

ficant reservoir parameters was obtained.

the discharge performance of each well

n, depending on the nature of the well and thethe following may be measured (and were in

well permeability from transient changes in pressure andlor flowreservoir permeability from interference

e test sequence proved to be a very efficient meanscollecting an unusually wide range of measurementsreservoir parameters.

ormally instrumentation limitations or reservoirc aracteristics mean that considerably less interpretedi formation is available at such an early stage of field

STHIS TO RY

TimeDate25 May 11:20

14:35

development, with such a short test period. With thedownhole conditions indicating a steam zone in theupper part of each well, and a liquid region beneath, adischarge may consist of dry steam from the upperzone, low -enthalpy fluid from the liquid zone or amixture of the two. Which may be produced dependsupon the permeability of the well in the two zones, andthe wellhead pressure.

Well ActionULB- 02 Start output testULB- 02 Co m d e teoutput test

The water level in each well lies around sea level orsomewhat below. Water cannot enter the dischargeuntil the water level rises above the steam feed, whichrequires a downhole pressure drop of 10 -20 bar. Ifthere is high permeability in the steam zone its pressurewill not drop by this much, and it will then beimpossible to obtain any fluid from the liquid zone inthe discharge, irrespective of the permeability there.

6 June

Conversely in a well with low permeability in thesteam zone, liquid would be expected in the dischargeexcept at high wellhead pressure. It should be possibleto obtain a dry discharge in all wells at sufficientlysmall flow rates (high WHP) that there is littledrawdown of the steam zone.

15:28 ULB-02 Open15:4 ULB-02 Shut

ble 1 and figure 5 show the history of the discharge tests conducted over May -July 1996.i . 9A b l e I Ulumbu test history sequence

I~~._ I

15:50 I ULB-02 ODen

ULB- 02 Shut I

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Flow of the Ulumbu wells

250

Well

c

E

50

0

4-Jun

liquid steamzone zone

ULB - 02

ULB - 03Interference 02 - 0 1

ULB -01

v. small 0.7 - 1.6230

9-Jun 14-Jun 19 -Jun 24-Jun 29-Jun 4-Jul 9-Jut 4 J u l 1 9 J u l 2 4 J u l

Date

TEST RESULTS

ULB - 02

ULB-03ULB - 01

Figure 5. Test histog

Wellhead pressure was logged on all wells throughthe test. ULB -02 discharge a large flow of dry steam.The decline ULB -02 was monitored during itsdischarge, and the pressure buildup when the wellwas shut. In addition interference with ULB-01 wasobserved.

ULB -03 also discharged dry, while ULB-01 produceda low-enthalpy (1 100 kJ/kg) discharge. For both ofthese wells a decline analysis of the discharge wasused, and for ULB -03 also a buildup.

The following table summarises the results of the welltesting:

Table 2. kh measurementsfor Uliimbu wells andreservoir

ULB-01 1.5 v. smallULB - 02 not meas. 150

RESERVOIR CHARACTERISTICS

Steam zone

The steam zone has high permeability, 100 -250 dm.This permeability was intersected by ULB -02. ULB-03

intersected a smaller fracture.ULB-0 I

although

unproductive in the steam zone, lies sufficiently closeto good permeability to respond to ULB-02

Drilling results indicate that a greater thickness of permeability was intersected in ULB -02 than ULB-01, consistent with a trend back toward the presumedcentre of the field.

Gas correlations were used to estimate saturation ofthe steam zone, using the method of Grant et al.

(1996), as extended by Grant Hanano (1997).ULB-02 is extremely permeable and consequentlythere was little drawdown and hence little contrast ingas contents. No significant estimate was obtained.

Liquid zone

Wells in the liquid zone have found moderate tonegligible permeability. Liquid pressures differ

between ULB-01 and ULB -02, and between shallowand deeper zones of ULB-0 1,which indicates that

permeability in this zone is not generally high.

If there is a larger development in the field it willneed to draw on the liquid zone whioh is the bulk ofthe resource, rather than just produce from the steamzone. Well performance for liquid - fed wells is likelyto be better elsewhere in the field where there will behigher temperatures. With production of 230 -240Cwater from an underpressured reservoir producingwellhead pressures will be low and will fall asreservoir pressure ultimately falls. Higher reservoirtemperatures will give higher discharge enthalpiesand better pressures at wellhead. The problem of anunderpressured liquid resource is exacerbated by the

poor liquid permeability, as was clearly shown by thefailure of ULB-01 to sustain continued discharge at18 lip pipe


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The liquid reservoir found in the area of the existingwells does not constitute an attractive resource,3ecause of the poor performance expected of liquid-Fed wells.

Reservoir concept ual model

The steam zone and liquid zone intersected by the:hreewells are both outflow zones from elsewhere in:he field. In the upflow region there must be risingRo-phase fluid from which steam and water flowlaterally away. Temperatures in this upflow mustrherefore reach significantly higher temperatures than240C. This upflow zone is presumably locatedupslope from the existing wells.

ACKNOWLEDGMENT

We thank PLN and MFAT for the use of the fielddata and information.

REFERENCES

Grant, M. A., Hanano, M., 1997 MonitoringMatsukawa reservoir changes through gas content of produced steam in prep.

Grant, M.A., Irwin, W.T. Ibrahim, R.F., 1996Vapor - dominated fields: fluid reserves andgeothermometry Proceedings 2 1st Workshop onGeothermal Reservoir Engineering, StanfordUniversity.

Mahon, T., Modjo, S., Radja, V.T., 1992 Theresults of a joint scientific study of the Flores Ulumbugeothermal area Transactions Geothermal ResourcesCouncil, v16 pp97-104

Sulasdi, D., I996 Exploration of Ulumbu geothermal

field, Flores -East Nusa Tenggara, Indonesia.Proceedings, 2 st Stanford workshop on geothermalreservoir engineering, pp5 1-54

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Efficient Well Testing at Ulumbu Field, Flores, Indonesia - Hole, Grant

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