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PRESSURE BUILDUP TESTS OF WELL CPC-CS-4T,

CHINSHUI GEOTHERMAL FIELD

K. Y. Shen and Carl R. Y. Chang

Taiwan Petroleum Ex plor ati on Divis ion

Chinese Petroleum Corporation

Miaoli, Taiwan 360

Well CPC-CS-4T i s located i n t he Chinshui Geothermal F ie l d i n t henor th-eas t o f Taiwan.

t o t a l d e pt h is 1,505 m.

completed with 7 i n . l i ne r hanger and s lo t ted l iner on June 17, 1976.

The production hole i s from-539 to 1,505 m .

The f i r s t deep w e l l d r i l l e d i n t h i s area, i t sThe w e l l was spudded on April 16, 1976, and

E i g h t c en t r a l i z e r s were used during s e t t i ng of th e 9-5/8 i n , pro-

d u c t io n ca s i ng i n a 12-1/4 i n . ho le a t 529 m.

j ob was performed with s i l i c a fl ou r cement, wi th a s l ur ry weight o f 1.60

sp ec i f ic g rav i ty . About 10 k i l o l i t e r s of c ement s l u r r y was flowed out

a t th e surf ac e through th e annulus during cementing. This implied th a t

a good cement job had been done.

The f u l l cas ing cement

I$r i l l i n g the 8-1/2 in . p roduct ion ho le , much l o s t c i r cu l a t i on

The main

was encountered.

between the d r i l l in g i n t er va l f rom 724 m t o t h e t o t a l d e pt h.

loss zone w a s from 750 t o 800 m.

a s p e c i f i c g r a v i t y of 1.06 and 1.15.

A t o t a l o f 740 k i l o l i t e r s of d r i l l i n g mud w a s l o s t

The weight of the l o s t mud was between

The reservoi r i s wi thi n t he Lushan formation, of Miocene age. The

I t

l i t ho l og i es encountered are a l l dark gray s la te , s l i g h t l y i n t e r c a l a t e d

wi th l i g h t g ray f ine-grained t o ver y fine-gra ined compact sandstone.

i s a water-dominated geothermal 'reservoir.

emperature and pressure-depth surveys were performed during flow

tests .poi n ts between th e s urfa ce and 230 m were observed c lear ly .

phase f low i n the r e se rv o i r i s evident , making the pr essure an aly s is

These d a t a were p l o t t ed on the phase diagram of water. Flash

Thus single-

BUILDUP TEST

A f t e r t h e w e l l w a s completed, many kinds of flow tests were per-

formed.

a cumulat ive r ese rvo i r f l u i d of 133,000 tons had been produced.

A pressure bu i ldup t e s t was performed on October 2 0 , 1976, when



-106-

Table 1 presents the pressure buildup data.

lowered to 1,500 m, and the wellhead valve was completely closed. Fig-

ure 1 graphs pressure increase versus shut-in time on aelog-log scale.initial 45" line shows the after-flow effect.

flat straight line indicates a probable homogeneous reservoir.latter part should be used'to estimate the flow conductivity, skin effect,

and flow efficiency, etc., on a Horner plot.

The pressure gauge was

The

The other part of the

The'

Figure 2 is a Horner pressure-time semilog plot. From the straight

line on the graph:

162.6 qUB = (162.6) (20,000) (0.14) (1.13)

m 49h =

= 10,500 md-ft

+ 3 . 2 3' 2

1h + 3.232h- h gm

Wctrw

1,916-k,357 - Eog 10,500

-5 8 59

(0.2) (0.14)(10 )(24)2,600)

S.= 1.151

= 1.151

= + 6.6

"skin = 0.87 ms = (0.87) (49) (6.6) = 281 psi

p*'pwf'hps 1,967-1,357-281 = o.54-1,967 1,357

E =P*-Pwf

The porosity and thickness of the geothermal reservoir are not known.We assume porosity to be 20% of the reservoir bulk volume, and the thick-

ness to be 2,600 ft. If porosity were only 2%, then the argument of thelog term in the skin effect calculation would increase by ten times, and

the skin effect would decrease by 1.15.

the thickness in the calculation of skin effect.

SECOND PRESSURE BUILDUP TEST

The same observation applies for

A small oil boiler steam turbine generator was installed at the well

site for demonstration.plant by the end of October, 1977. Another pressure buildup test was run

on September 2, 1979, after about 1.1 million tons of reservoir fluid had

been produced.

just at the top of the main lost circulation zone. After testing several

different flowrates, the well was shut in.

bleeding through a 2 in. pipeline with a ehoke.to prevent hydrogen embrittle-

ment of the wellhead system.Figures 3 and 4 are again plotted by the same procedures as in Figs. 1 and

2.

to indicate that pressure transmission is the same at any water phase

depth, because the water is almost incompressible.

Well4T

began supplying steam for the power

This time, the pressure gauge was lowered te only 750 m,

Then we kept the wellhead

Table 2 shows the pressure buildup data.

The two log-log Ap-time plots are exactly coincident. This appears



-107-

By ana ly si s of Fig. 4.we obtai n:

kh = 17,600 md-ft

S= + 12.6

"skin = 350 p s i

FE = 0.46

Before the mult iple f lowrate and second pressure buildup tests werebegun, many different sizes of Go-Devil were r un f o r d e t e c ti n g t h e i n t e r -

na l d iameters of the cas ing p ipe .

bottomhole sample w a s taken out by sand bailer through a wire l i n e o pe ra -

t io n .

sloughing had oc curred i n t he 8-1/2 in. production hole.

th a t t h e h e igh t of th e f in e s l a t e p rec ip i t a t e i n th e ann ulus sh ou ld b e

g rea te r th an th e h e ig h t o f th e p r ec ip i t a t e o f 4 5 m in t h e in n e r 7 in .s l o t t e d p i p e.

crease of f low ef f i c ie ncy compared wi th t he f i r s t p ressure bu i ldup tes t .

However, i t is no t evi den t why th e kh incr eas ed between t he two tests .

No Go-Devil would pass 1,460 m. A

I t was completely f ine dark gray s la te cu t t in g s , implying that

I t is bel ieved

This probably caused the increase of sk in eff ec t and de-

DISCUSSI O N

During mult iple f lowrate tes ts p r i o r t o s h ut -i n, temperature-depth

surveys were run fo r each ra te i n w e l l 4T.

the temperature of the produced f lu id s drops with increa sing production

ra te .ducing rates. Th is ap pea rs t o in d i ca te th a t as the f lowra te increases and

w e l l p ressu re d ro ps , co ld e r f lu id en te r s th e w e l l in some manner and

blends wi th t he ho t water.w e l l s completed l a t e r i n t h i s area.

A s t r a n g e r e s u l t was observed:

Thus there was alsg a drop in wellstream enthalpy a t high pro-

The same phenomenon was observed in o th e r

The most obvious reas on f o r cold water en tr y with reduced wellbore

pressure would be a poor cement bond t o th e casing. This would al low

co ld e r water from formations nea r th e s ur fac e t o dump-flood th e w e l l .

Well 4T was not cement bond logged, but other w e l l s were.of cement bond lo gs did in di ca te some poor cement bonds.

An i n sp ec t io n

Another reason for cold water en tr y would be th at th ere i s no cap

rock on th e top of t he geothermal res er voi r. Most w e l l s d r i l l e d arealong th e bank of th e Chingshui River. Thus i t i s a l s o i n f e r r e d t h a t t h e

f a l l i n temperature could be caused by a v e r t i c a l f r a c t u r e i n fl ow of

cold water beneath the r iver bed.

The communication between cold water and hot water forma tion s would

obviously a f fec t the p ressure bu i ldup ana lyses .

venting cold water en t ry i s underway a t t h i s area.

Remedial work f o r pre -



-108-

REFERENCES

Matthews, C.S., and Russell, D.G.: "Pressure Buildup and Flow Tests in

wmey, H.J., Jr.: "Pressure Transient Analysis for Geothermal Wells,"

Wells," SPE Monograph Series (1967), I.

-*Proc 9 Second United Nations Symposium on the Development and Useof Geothermal Resources, San Francisco, California, May 20-29,1975.

..



-109-

*

Table 1 Pressure Buildup Data of First Test

Flowing Period: 0630 - 1630, October 20, 1976Shut-in Period: 1630 - 0630, October 20-21, 1976

= 126 Tons/hr = 20,000 BBL/D

/.\ = 0.14 CP

B = 1.13 res.bbl/surface bbl

C& = 1 x 10-5 p

t = 600 minutes .

Pf = ,1957 sig at reference depth of 1500 meters

At ,Min

0

3

6

9

12

1530

4s

60

90

105

120

150

180

75

AP,psi+A tA t pws, psig

- 1357 0

201 1417 60

101 1549 192

67.7 1647 290

51 1731 374

41 1796 43921 1877 520

14.3 1892

11 1901

9 1906 549

7.7 1913 556

6.7 1918 561

6 1920 563

4.3 1932 575

5 1932 575

t+At.

st ,Min 4 t pws p i g P ,psi

210 3.9 1937 580

240 3.5 1940 583

270 3.2 . 1942 585

300 3 1944 587

330 .2.8 1944 587'

360 2.7 1944 587390 2.5 1947 590

420 2.4 1947 590

450 2.3 1949 592

48 2.25 1949 592

510 , 2.2 1949 592

540 2.1 1949 592

570 2.05 1952 5 5

600 - 2 1952 595

630 1.95 1954 597



-110-

Table 2 Pressure Buildup Data of Second Test

Flowing Period: 0 9 0 0 August 1 8 , 1 9 79 - 1000 September 2 , 1 9 7 9

(Continued Multiple flow rate tests)I

.

Shut-in Period: 1000-0800 September 2 - 3 , 1 9 7 9

q = 1 3 8 Tons/hr = 2 1 , 9 0 0 BBL/D

t = 20,076 Minutes (Equivalent flowing time of 1 3 8 tons/hr)

Pi = 9 8 8 psig at reference depth of 7 5 0 meters

At Min

0

5

10

1 5

2 0

25

3 0

35

40

4 5

5 0

6 0

70

8 0

9 0

100

110

1 2 0140

1 6 0

1 8 0

t+At

At

401

2009

1 3 4 0

1005

8 0 4

6 7 0

575

5 03

447

403

336

288

252

2 2 4

202

1 8 4

168144

126

113

- 3 50444

5 8 0

7 0 3

7 7 8

8 2

83 3

848'

850

855

862

8 7 4

887

896

9 0 1

906

9 1 1

9139 2 0

923

9 2 8

0

9 4

2 3 0

353

42

4 5 2

4 8 3

4 9 8

5 0 0

5 0 5

5 1 2

5 2 4

537

546

5 5 1

556

5 6 1

5635 7 0

573

5 7 8

@,psi+ At

At,Min Pws,PSig

2 1 0240

3 0 0

3 6 0

420

4 8 0

5 4 0

6 0 0

6 6 0

7 2 0

7 8 0

8 4 0

9 0 0

9 6 0

1 0 2 0

1 0 8 0

1140

1 2 0 0

1 2 6 0

1 3 2 0

97'

85

6 8

57

49

43

3 8

3 4

31

29

27

25

23

22

2 1

19.6

18.6

17.7

16.9

16.2

9 3 2 5 8 2 .93 585

9 4 0 , 5 9 0

9 4 0 5 9 0

9 4 0 - 5 9 09 4 4 5 9 4

947 597

949 599

949 599

9 4 9 5 9 9

9 4 9 5 9 9

949 ,599

952 6 02

952 6 02

9 5 4 6 0 4

9 5 4 6 0 4

9 5 4 6 0 4

9 5 4 6 0 4

957 607

957 607



-111-

100

AP

(Psi)

10

Fl6.IPRESSURE BUILDUPW E L L N O . C P C - C S - 4 1OCTOBER 20,197SREF. DEPTH l 5 O O M

IO 100 ICA t , Mln

0

0

0

FlO. 2PRESSURE BUILDUP CURVE&ELL NO. C P C - C S - 4 fOCTOBER 20,1976R E F . DEPTH 1600M

- Pw f E 1357

1 4 o ( y ' " ' I

01 1 1 1 I I I , 1 I l l 1 I 1 1 ,

I

IO IOOO 100

*+At

At-



I

-112-

A t , Min

~ o o o o 0 0 goo a

FIG . 3PRESSURE BUILOUPWELL NO. C P C - C S - 141SEPTEMBER 2.1979REF. OEPTM 7 5 0M

m

60

600

0

0

0

0

Fi6.4

PRESSURE BUlLDUP CURVE

WELL NO. C P C - CS- 4 TSEPTEMBER 2, 19t9REF. D E P T H 7 6 0 M

P w f=550I I l l 1 I I I I I I l l I I I * I I

1000 t00 10

t + A t

A t-

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