SPESPE7480

WELLTEST

byAlain C.

NG INTWO-PHASEGEOTHERMALWELLS

(%ingarten,Member SpE-AIME, B.R.G.M.

~Copyright 1978, American Institute of Mining. Metallurgical. and Petroleum Engineers. Inc

This paper was presented al the 53rd Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers ot AIME. held in Houston. Texas. Oct. 1-3,1978 The malerial ISsubject to correctionby the author Permias!on 10 COPY la restricted to an abstract 01 not more than 390 worda. Write 6200 N. Central EXPY. Dallae. Texaa 75206

ABSTRACT of bottom hole data gathering that often resultsin mechanical damage of the measuring devices be-

This paper deals with the possibilityof using cause of the high temperaturesinvolvede and of theusual oil well transient preaaure testing methods high production ratea of boiling geothermal fluids.in hot water wells when flashing occurs in the well-bore. Multiple rate analysis techniqueshave been

A two rate flow teste conducted on one well

successfullyapplied to available data on a well dril- in the Cerro Prieto field was recently presented

led by B.R,G.M. in the former French Territory ofby Riveratand RsmeylO. The data were satisfactori-

Afars and Issas. Practical recommendationsconcer- ly interpretedb meana of models sug ested byRusselll, Seliml1 !/ning such teats are also included. , and Odeh and Jones 3e14.

To the authors knowledge, no other example of

INTRC2UCTIONtransient test in a cwe-phasewell haa been pub-lished in the literature. Unpublished data, how-

Well established pressure trans~ent analysisever, were available from a well drilled in 1975

techniques are routinely used in oil and gas wellsby B.R.G.M. in the French Territory of Afars and

for the determinationof reservoir parameterslz. :ssas (now Republic of Djibouti).In recent years, improvementsin these techniques Analysis of these data, which include pressurehave allowed a better understandingof complex reser- build-up as well as two rate flow tests is pre-voir and well behaviore as in the presence of akine sented hereafter.wellbore storage3, or fracturesab.

Only recently have these methods been applied ASAL WELLto geothermal systems. For a number of years,geothermalwell testing was based on empiricalmeth-

TheAsal rifte located 80 km west of Djiboutieisone of the active rifts-in rifttstructuresof

ods5~ that, although useful for eatimsting geother-mal field production pote~.tialegave limited knowl-

the Afar depreasionea transitionbetween the Gulf

edge on the reservoir itself.of Aden and the Red Sea ridges.

Pressure transient techniqueshave been appliedAttention was drawn to this zone because of

successfully in vapor dominated geothermal reaer- the presence of a g:aben structure and of geochem-

voira6,7. It was found that gas well analysis meth-ical particularitiesof various hot springs.

ods apply to steam wells and that wellhead measure- Two wells were drilled on the S-W margin of the

ments were generally adequate for analysia, thus rift, at locations chosen mainly from geological

eliminating the need for downhole instrumentation. considerations. The first hole (Asal 1) reached ahot water geothermal reservoir, the second, oneIn water dominated systems, on the contrary, kilometer away, was dry.

transientwell testing can be far more complex,depending upon whether two-phase flow develops, Both wells found, from top to bottom, a recent

with flashing in the wellbore or in the formation. basaltic seriese then a thick rhyolitic volcanic

Usual oil well methods have already been shown to aerieae and finally an old tectonice tiltede

apply when no flashing develops8. On the otherbasaltic series, where the reservoir is located,

hand, they apparently cannot be used when flashing The first well was drilled to a depth of 1130 me

occurs in the formation. Very little informationwhere heavy mud losses occurede while the second

was available until recently on the case of hot wa-well reached 1550 m. An important normal fault

ter wells in which flashing occurs at some depth in appears to separate the two wells.

the wellhcre. This is mainly due to the difficulty A schematic of Asal I wellhead is presentedin figure 1. The well could be produced either

References and illustrationsat~end of paper. vertically through a 6 tube, or horizontally.-

2 WELL TESTING IN TWO-PHASE GEOTHERMALWELLS SPE 7480

through a calibratedpipe. Four sizes were avail-11and ln) which were used to

able (namely,6, 4, 2control the well flow rate. In?Idition, a 2stainless steel tubing was lowez-d into the well-bore to a depth of 650 m, for wireline temperatureand pressure measurementswith bourdon-tube typegauges. As the well could not initially start byitself, the same tubing was used for inducing it:.ntoproduction by air lift. The initial water lev-.1 in the well was at an approximatedepth of 200 mJlow the well head.

The flow rate history of ASAL 1 well is pre-sented on figure 2. After completion, the wellwas produced through a 6 outlet for 6 days in pre-paration for a build-up test for obtaining reser-voir parameters (flow period+1). Mechanical prob-lems, however. were encounteredwith the stuffingbox on the lubricator attached to the 2 stainlesesteel tubing, and these required the closing ofthe well for about 9 hours in order to retrievethe measuring davices (flow period.#2). Duringthis unplanned pressure build-up, temperature andpressure could only be measured at the wellheadand are shown on figure 3,

It can be noticed on figure 3 that the well-head temperaturedrops suddenly after 400 minutes.This apparently corresponds to steam condensationin the wellbore and defines the time limit afterwhich the well has to be induced into production.

The well was again started by air-lift andflown through the 6 outlet for approximately 4days (flow period#3) before being shut-in for20 hrs (flow period# 4). In order to obtain thewell characteristiccurve relating wellhead pres-sure and mass-flow rate, the well was again star-ted by air-lift, flown for 43 hours with a 4outlet (flow period~5), shut-in for 3 hours(flow period#6), and open again with the 6outlet. Because of the ehort 4 pressure build-up duration, the well was able to start by itself.After about 3 hours of 6 production (flow period# 7), the 6 outlet was changed into a 2 outlet;the well was then produced for 3 days (flow period# 8), shut-in for 7 hours (flow period#9), openagain with the 6 outlet (flow period#lO) whichwas changed into a 1 outlet after 3 hours (flowperiod#ll). During this final teet, there wasevidence that the flow could not be sustained, andno build-up was attempted.

Although the primary objective of these lasttests was to obtain the well characteristiccurve,bottomhole measurementswere taken throughout theduration of most individual flow period, in orderto check the results of the eecond 6 build-uptest. Temperatureand pressure measurements werealso performed at various depths in the wellbore,and are shown on figures 4 and 5, respectively.They indicate a reeervoir temperatureof 253C,and show the presence of a flashing front atapproximately870 m with the 6 outlet, 850 mwith the 4 outlet and 700 m with the 2 outlet.

TYPE - CURVE ANALYSIS

Aaa 1 bottomhole pressure data that weru avail-able for flow periods 4, 6, 7, 8, 9, IO and 11are ehown on the log-log plot of figure 6. Foreach individual flow period, the differencebetween the bottomhole pressure during the testand that at the beginning of the test is plottedversus the time since the beginning of that test.

Although no unique quantitativeestimate ofthe reservoir parameters can be deduced from fig-ure 6, important quantitative informationcan beobtained.

By comparing the log-log plot of figure 6with the skin and storage type curves published inref. 3, it becomes apparent that none of the 6flowing tests hatibeen run long enough for theradial flow logarithmicapproximationto apply.On the other hand, that approximationdoes applyto the 2 flowing test, which had a much longerduration, and to the variom build-up tests.

The comparisonalso suggests that the earlytime behavior of the well is of the changing well-bore storage type, going from liquid level con-trolled storage to compressibilitycontrolled stor-age when flashing or condensationoccurs.

Figure 6 further indicates that the radialflow logarithmicapproximationapplies sooner forbuild-up than for drawdown tests. This is some-what opposite to what is generally observed insingle phase wells, and may be due to the two-phase nature of the flow.

Anothez comment regarding figure 6 is that,although a dry well had been drilled one kilometeraway from ASAL 1, no impermeableboundary is ap-parent on the log-log plot.

Figure 6 was also used to estimate the massflow rates under resenroir conditions during thevarious flow periods.

As no separatorwas available, these werefirst deduced from Jameslip pressure method15 :James observed that, as a fairly large flow ofsteam or steam-watermixture was expanded along apipe to the atmosphere, the pressure at the ex-treme end (lip) of the pipe was greater than theatmospheric pressure, and proportional to themass flowing, and the enthalpy, as in the follow-ing formula :

~ E1*102

0.96 = 11 400.(1)

Lwhere G is the mass velocity (in lb/sq ft.see),E is the stagnationenthalpy (in Btu/lb, at res-ervoir conditions),and PL the critical lip pres-sure (in psi absolute).

Jamess formula was transformed for the pre-sent study into a more convenient system of unitsto yield :

w=692.33PL096 E-]02d2 (2)

where w is expressed in tons/hr*, PL in bars*, andd in inches.

Data for mass flow rate calculationsare sum-marized in Table 1. The enthalpy value used inEq. 1 was that for pure water, although the geo-thermal fluid salinitywas of the order of 200,000ppm. Enthalpy valuee for highly saline brines arenot readily available in the literature.

As indicated in Table 1, the lip pressure wasuniquely defined only with the 6 outlet, corre-sponding to a rate of approximately83 tlhr.With the 4 and 2 outlets, on the contrary, the

l Metric units are used throughout this paper :1 ton = 103 kg; 1 bar = 105 Pa

.-* TI.on &.(. 12RThllURT!?hl axn I*OU . . . -. -------------- .J

lip pressure was oscillatingbetween two extreme A straight line of slope in= 3.6 10-3 is clear-values, because of the slug nature of the flow, and ly evidenton figure 7. Substituting= 3.6 10-3only a range of rate values could be obtained byapplying James formula (71 -83 tlhr and 40-54

into Eq. (4) and taking v= 1.25 10-3 m./kg andu = 0,2 cp yields :

tlhr, respectively). According to James16,suchpulsations could have been eliminated by using a kh = 15,9 Dm.

glycerine-dampedpressure gauge with a needle valve. (P = 0,2 cpwas obtained by extrapolatingpublishedBy throttling the valve, the lip pressure stabilizes 1 to a temperatureof 253C and aviscosity curvesat the correct value, between the two extreme ones. salinity of 200,000 ppm).

However, by comparing the relative position of The calculated initial pressure is 76,6 bars,the different curves on figure 6, it appears that which compares reasonablywell with the value ofthe r~tes with the 4 and 2 outlets were more like- 77,4 bars measured before the test.ly 71 ad 23 tlhr, respectively. In the same way,the rate ~ith the 1 outlet seems to be equal to It can be observed in figure 7 that data points11 t/hr. for build-up times greater than 6 hours deviatefrom the straight line. This is a consequenceofMULTIPLE RATE ANALYSIS the condensation%ffect already noticed on figure 3

during the first 6build-up (flow period#-2).Quantitativeanalysis of Asal 1 bot~~-zihole

pressure data was performed with a variable draw- The early time deviation (for build-up timesdown model, similar to that proposed lJyOdeh and less than 20 minute?) is caused by wellbore storageJones 13. The well was col.sideredas ..lavingbeen effects.

produced at different flow rates, q), qz,... q . In the same way, analysis of the 4 build-upAseuming the radial flow Iogarithm;,capproxima/ion test (flow period#6) yields a kh value of 15,0 Dmvalid for each one of the flow pe:iods,the bottom- and a pi value of 76,4 bars (figure 8).hole well.pressure during the nt~ test is equal to : kh and p. from the 2 build-up (flow period#9)

are equal to 16,8 Dm and 77,2 bars, respectively

qnu

1

n q.-q. (figure 9).PWf Pi- 411kh

~~ In (tn-t. )n j=ln

J-l Flowing test analgsis------ ---------- ---

A plot of p f versus the multiple rate functionfor the two 6 f~owing tests (flow periods&7 and

\

10) is shown on figure 10. The slope for the first+ ln~=+ 0.80907 + 2S (3) one (+#7) is equal to 7,2 10a , which yields

w kh = 8 ~, while that for the second one (#IO) iaequal to 10,2 10-3 (kh = 5,6 Din). Aa expected from

in Darcy units. figure 6, these kh values are much lower than thoseobtained before, and are not correct. On the other

With geothermalmetric units, Eq. (3) becomes : hand, the analyais shown on figure 11 of the 2flowing test (flow period#O) yields a slope equal

P=Wf to 3,5 10-3 , correspondingto kh = 16,3 ~, whichn ia consistentwith the results of the ?mild-up test

nanalyses.

-0.228 ~kh j~l (Wj-wj-l) in 60 (tn-tj-l) The sudden increase in bottomhole pressureindicated on figure 11 is likely to be caused by a

wnv!Jdecrease in the flow rate. The same phenomena

+ Pi - 0.228 F(ln* - 8.434 + 2S)(4) appears on figure 12, in the analysis cf the 1flowing test (flow period~ll), during which thewell was not able to sustain the flow : the lastrecorded pressure is roughly equal to the initial

In Eq.(4), k is expressed in darcy, t in min- pressure. All the other pressure points fall on autes, and rw in meters. straight line, whose slope is equal to 3,2 10-3.The correspondingkh is equal to 17,8 Dm, which

Therefore,by graphing ~wf versus E (wj~j-l) agrees well with the build-up testa results.In 60 (t - t. ,) in carteszan coordinatesone shoul~ obt& a straight line, the slope of SKIN EVALUATIONwhich is equal to : It is not possible to obtain the sk~ factor

m=directly from the multiple rate analysis plot,

- 0228 E (5) because of insufficientreservoir information. TheThis slope is independentof the flow rate. skin factor,however, can be evaluated from a type

The intercept of the straight line can be used to curve match, with a dimensionlesspressure given by ~

calculate the skin, if pi and @ are known. Inthe case of a build-up test (wn = O), the intercept kh AP

D = 0.228 Wn::VIJ=~(6)

is equal to the initial pressure pi.for a build-up test, or

Build-u~ test analysis------- --------.- --- kb AP APA plot of pwf during the second 6 build-up D = 0.228 (wn - Wn-l)vp= 2(wn Wn-l)m (7:

test (flow period~4), versus the multiple ratefunction, (expressedin tonafhr), is shown on for a flowing test.figure 7.

k WRT,T. TFXTTNG TN TW(I-PMASR f3?fWFTTi!RMAT. WRT.T.R cm7 Vt,Qn

The type curve match is shown on figure 13, andindicates a skin approximatelyequal to 25 . Thiehigh skin value is probably due to rocks plugging thebottom of the hole : although the well was drilled to1130 m, the pressure gauge could only be lowered to adepth of approximately 1050 m. This would also ex-plain why, although the formation is known to befractured,no fracture controlled flow period is ap-parent on the log-log plot of figure 6.

CONCLUSIONS

The following conclusions can be reached fromthe present study :

1) Two phase geothermalwell testing can becarried out with standard bourdon-tubetype instrumentsfor extended periods oftime at temperaturesae high as 253*C andsalt content of the order of 200,000 ppm.

2) Usual multiple rate techniques can be usedin two phase geothermalwells for obtain-ing reservoir characteristicsfrom bottomhole transient pressure data. Type curvematching also provides useful qualitativeand quantitativeresemoir information.

3) Early time bottom hole pressure appear tobe wellbore storage controlled. Thereare indicationsthat wellbcre storagechanges from liquid level type to compres-sibility type, as a consequence of waterflashing or steam condensing in the well-bore. These wellbore storage effectsappear to last longer in drawdown anddouble-rate tests than in build-up tests(200mn compared to 20mn for the wellAeal I studied in this paper). This hasto be taken into account when planningsuch tests.

4) When the well is not artesian, there isno practical avantage of running longbuild-up tests, because of the effects ofsteam condensation in the wellbore (6hours was the practical limit in the caseof well Asal 1 ).

NOMENCLATURE

C2= total fluid compressibility,bar-1E= enthalpy,kcallkgh= formationnet thickneaa,mk= permeability , darcym= slope of straight linen= constant rate intervalsP= pressure,barq = volumetric flow rate, m3/hrr= wellbore radius, mSw = skin factort= producing time, minw= mass flow rate, tlhru= viscosity, cpv= epecific volume, mslkgQ= poroeity, fraction

Subscri@s------- --

i= initial conditionj= constant rate intervalw= wellbore or waterWf = flowing conditions at well bottom

---- ---------- ------ *I.. ,-?

.ACKJ70WLEDGEMENT

The work reported in this study was supportedin part by the Commission of the European Communi-ties (Contract081-76 EGF, GeothermalEnergy Pro-gram). The author,is grateful to the Conunissionofthe European Communities and to the management ofthe Bureau de Recherches G6010giques et MiniSres forpermission to publish this paper.

REFERENCES

1. Matthews, C.S., and Russell, D.G. :!Ipressurebuild-up and flow tests in welleMonograph series, Society of Petroleum Engi-neers of AIME, Dallas,(1967),Volume 1.

2. Earlougher, R.C. : Advances in well test anal-ysis Monograph series, Society of PetroleumEngineers of AIME, Dallas, (1977),Volume 5.

3. Agarwal,R.G.,Al-Hussainy,R.ad Ramey, H.J., Jr:t~~ investigationof wellbor~ storage and skinin transient liquid flow - 1 analytic treat-ment, Sot. of Pet. Eng. J, Sept. 1970, 279.

4. Gringarten,A.C., Ramey; H.J., Jr and Rag avan:Applied pressure analysis for fracturedwellsPet. Tech., July 1975, 887.

5. James, R. : Factors controllingborehole per-formanceU.N.Symposium cn the Development andUtilization of Geothermal Resdurces, PissProceeding (Geothermics,Spec.Iss.2)V.2 pt.2p 1502.

6. Ramey, H.J., Jr and Gringarten A.C. : Effectof high volume vertical fractures on geothermalsteam well behavior, Proceedings, 2nd U.N.Symposium on the Development and use of geo-thermal resources, San Francieco , California(USA),MsY 20-29, 1975, V.3, p. 1759.

7. Barelli, A., Msnetti, G., Celati, R. andNERI, G. : llBuild-up and baCk-pX.e66ure ests

on italian geothermalwells, Proceedings,2nd U.N. Symposium on the Development and useof geothermal resources, San Francisco,t%lifornia (USA), May 20-29, 1977, V.3, p.1537,

8. Witherspoon, P.A., Narasimhan, T.N. and McEdwards, D.G. : Results of interferencetestsfrom two geothermal reservoirs, SPE paper 605251st Annual Fall Meeting, New Orleans, Ott.3-6,1976.

9. Gulati, M.S. : Pressure and temperaturebuild-up in geothermal wells Proceedings, Stanfordgeothermalworkshop, Stanford University,Dec. 15-17, 1975.

10. Rivers, J.R. and Ramey, H.J., Jr : Applicationof two-rate flow tests to the determinationofgeothermal reservoir parameters, SPE paper6887, 52nd Annual Fall Meeting of SPE, Denver,=9-12, 1977.

11. Russell, D.G. : )JDete~inationOf fo-tioncharacteristicsby two-rate flow tests,J. Pet. Tech., Dec. 1962, 1349.

12. Odeh, A.S. and Jones, L.G. : Two-rate flow 14. Odeh,A.S. and Jones, L.G. : Pressure drawdowntest, variable rate case, J. Pet. Tech,, Janv. analysis, variable rate casa, J. pet. Tech.,1974, 93. Aug. 1965, 960.

13. Selim, M,A, : Modification of the two-rate 15. James, R. : Alternativemethods of determiningflow nethod for determinationof reservoir enthalpy and mass flow Proceedings,U.N.parameters, J. Institute of Petroletaq, Conference on new sources of energy, RO=,1961.V. 53, N527, NOV, 1967, 343.

I 16. J-es, R. : Sprivateco~unication or16an8(France),Nov. 1977.

TABLE 1

MASS FLOWRATE MEASUREMENTS---------------. -----

SPE 7fb80 A.C. GRINGARTEN

[[

%

OUTLETDIAMETER(inch) [pgy[~sj(fr=~l,l]~?-

-.----..-- .--------- ------....-.---

6 0.5 1.5 83 I 834 2-2.5 3-3.5 71 - 83 712 6-8.5 7.9.5 40- 54 23,11 not 11available I

I

100I

5,

I=4-!=

4111

- :!1:-7 5/8133/8Fig. 1 - Schematic of ASAL 1 well.

TIME ,OAYS

Fig. 2 - Mass flow rate changes during ASAL 1 tests.

\w H TEMPERATuRE

wELL IWAO PRESSURE

, t..

IWRO 20090 3WO0 40000 D*O.00

WILO -UP TIME , MINUTES

Fig. 3- ASAL 1 pressure buiId-up test (6 outlet, flow period #2).

oor~Zwlw

00.00

I

v 6X1 70 (tOUTLET)l 2X1 n (t OuTbEl )

1d6 31.x70($ 0UTL27)

++0 28 X.75($ WTIST)

o 21.X.?6 (d OUTLET}

+ D 1?. x.76[s7ATIO)a AO.IX.70R7A71C)0 WRWA G31LLIW0+ WEALLOWwSLL9 1972

Io

1aw.w 4oom W090 8CW0 Iwono 1800.00

0EP7H, METERS

Fig. 4- Temperature profiles in well ASAL 1.

800.00 . v 2.W.7S (IWTL2T)

O 3.X1.75 (*OUTLET)7om

6 51.X.7614OU1LE1)

o W.x .75(0 OLWMT )80.00

0 t4.x.70 (qullm )

D ID.X.TS (mATIC )

II

00.00 a 20. IX.70 (mmIi

o 2W.00 mm 40000 8W.W IOw.w Uoo.wmmn , M2TCX2

Fig. 5- Pressure profiles in well ASAL 1.

VALIDITY OF SEMI-WQ METHOD

-t-1

I : BUILD-UP oRAW;OWN

I ~ -

II ~ 6 2UL0 .UP (n04)OROO

4W1L0 -UP [II*6) l OWAWWWN(0111

,,+-+.+ ..+ -+-+-H -...+.--. +...++ +.+.+++ -+-++

k ORMIOOWN(,*8),-. .-

/*4

\*@ +

, ~:

;

B

2 IuILOAW (*91

+

;.

/

i

..i

/+..

/

I 10&t, MlhUTES

,.2 Id

Fig. 6- ASAL 1 pressure tests.

W.w

o0

%000

00

00

00

00

00

0

.

26000 aoo.oo moo 40000 42000 Soo.oo 2W.00 ew.w wow 7W.00 7&Xlo

MUL71PL2 RATE FUNCTION

Fig. 7- ASAL 1 pressure build-up test (6 outlet, flow period #4).

Ww

* PIS76.6 -2m. 3,0 to73W u \.

a

a 4 %66

naA

row .

:

nn

An

..

I ~

o0

A00

24W%.

3b

o

~ 2om -0

Ao

n

Woo

29-20W0 5WO0 22000 40000 4WQ0 Sww Swm 2WW Wo.oo 70000 7wno

WJL?WLC RATE FUNCTION

Fig. 8- ASAL 1 pressure build-up test (4 outlet, flow period #6).

sem * , , ,

70.00

00

eo

moo

!.

II W.w

!4

I 00.CO

sow

m.m , ,

0 Oaoo 10090 160.W 200.00 mm 800Q0 mom 400.CQ 480.00 Sw.ooMuLnPLE mm FUHCTION

ao.oo

70.00

mao

i.

16090

3

~ 0.00

0600

Oosm

Fig. 9- ASAL 1 pressure build-up test (2 outlet, flow period #9).

Wooo 4mw Wo.m 700Q0 7m.oo wow mono Wow Wom 1000.00 IMOBOWLTIPU RATE

Fig. 10 - ASAL 1 pressureflowingtests

FUNCTION

(6 outlet, flow perids #7 and 10).

9090

7D.W

$.

Q

o

00 ee

, J

m m 4mm 4mm 9V2)J0 mom Omao Wow 7m.eo lmm -MLwl?lx KATEFLwc71an

Ftg. 11 - ASAL 1 pressure flowing test (2 outlet, flow period #8).

e

v

v

VVG v v v

mm Zwm am> mwv Me.m 4aam 4wile mono worn mom Uvne

wLTIRC RAIE fW310N\

Fig. 12- ASAL 1 pressure flowing test (1 outlet, flow period #11).

10

[ C=s *U---------- ----- . . . . . . . . . . . . . . . . . . . . . ----- D. . . . . . . . . . . ----- . . . . . I

-.

a--------- ----- .. ... . .. ... ... .. ... .. .. . ..

. ... .. .. ... ----- .. . ..

---------

.. .. .. ... .. ---------.21

t--------- ---- -----1s

........... ----- ----=s

--p~isjj~--;;::..::...-.-

--.-----

--. -0-.

1.--- I

L10 I105 106 109 10

Fig. 13 -Type curve matching with skin and storage type curves (ref.3).