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Land Subsidence (Proceedings of the Fourth International Symposium on Land Subsidence, May 1991). IAHS Publ. no. 200, 1991.

Subsidence Due to Oil Production in Western Venezuela: Engineering Problems and Solutions

JUAN MURRIA Maraven S .A. , Apar tado 173, Codigo P o s t a l 4016A, L a g u n i l l a s , Venezuela

ABSTRACT Ground subsidence associated with oil pro­duction in the east coast of Lake Maracaibo (Western Vene­zuela) was detected as early as 1929. Maximum cummulative subsidence as of April 1990 is 5.03 m. Typical subsidence rates range from 5 cm/year along the coast to 20 cm/yr in inland areas subjected to intensive oil production. The geomorphology of the area (low, swampy lands) prompted the need to protect inhabitants and oil industry installations from lake waters. The construction of a coastal protection system was started in 1932 and is s t i l l in progress. The system consists of coastal and inner dikes as well as drainage networks, al l conforming the "polders" of Tia Juana, Lagunillas and Bachaquero, which correspond to the oil­fields of the same names, collectively know as the Costa Oriental oilfields. This paper summarizes the origin, development, and moni­toring of the subsidence describes the aforementioned coastal protection system, highlighting design and construction problems and the engineering solutions implemented and, finally, describes four subsidence pre­diction models (two operational and two being developed).

BRIEF HISTORICAL SUMMARY

Geomorphologically the eastern coasts of Lake Maracaibo are typical of lacustrine environments-, flat and swampy (Lagunillas in Spanish means small lagoons or marshes), barely above lake level and composed mostly of sandy-silty soils . These swamps were separated from the lake by a comparatively narrow strip of land slightly higher than mean lake water level, so that these strips were flooded during high tides, storms, and strong on-shore winds.

Venezuelan Oil Concessions (VOC), Ltd., a subsidiary of the Royal Dutch Shell, was the concessionaire of the on-land area and started oil operations in the area in 1926. This company was renamed Com-paîiia Shell de Venezuela in 1953, and, in 1976, became Maraven, S.A., as a result of the nationalization of oil industry (Abi-Saab S Murria, 1985).

VOC established their base of operations in Lagunillas-, because of the region's topography, a small earthen dike, a few meters wide, less than one meter high and several hundred meters long was built by hand to protect the installations and dwellings from lake waves.

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Juan Murria 130

In 1929 the Lagunillas dike was breached and the r e s u l t i n g flood­ing led to suspicion of the ocurrence of ground subsidence, s ince the foreshore became permanently submerged.

Almost from the s t a r t of the subsidence i t became necessary to construct inner dikes and a drainage system to dispose of the run-off by pumping i t i n to the Lake.

As new o i l was discovered, the o i l companies extended the i r ope­ra t ions and es tabl i shed new o i l f i e l d s , Tfa Juana to the north and Bachaquero to the south (Figure 1 ) , which had to be a l so protected by means of the construct ion of polders s imilar t o the one in Laguni l las .

FIG. 1 Costa Or ienta l o i l f i e l d s .

I n i t i a l l y , these dikes were b u i l t along the coast as "simple" elevated roads behind shee tp i les (Col l ins , 1935). The continued sub­sidence made i t necessary to cont inual ly ijp.se and widen the d ikes . With t ime, the i n i t i a l simple elevated roads became ful ly developed ear th dams. Figure 2 shows the progressive r i s i n g of the coas ta l d ikes .

On the bas i s of present subsidence pred ic t ions i t i s expected

131 Subsidence due to oil production in Venezuela

DATE CUMMULATIVE SUBSIDENCE

1932 0 S 8 ' " 2MB, niniz

1939 *•»*" SOB

I960 W 4S2-

F I G . 2 P r o g r e s s i v e r a i s i n g o f t h e d i k e .

t h a t t h e T f a J .uana a n d B a c h a q u e r o d i k e s w i l l h a v e t o b e r a i s e d o n l y a n a d d i t i o n a l 1 . 0 0 - 1 . 5 0 m e t e r s , w h i l e t h e L a g u n i l l a s d i k e may h a v e t o b e r a i s e d a s much a s 4 . 0 0 m e t e r s a s a d d i t i o n a l s u b s i d e n c e i s e x p e c t e d due t o t h e e x p l o i t a t i o n o f t w o s u p e r p o s e d r e s e r v o i r s , L a g u n a a n d Lower L a g u n i l l a s .

As c o n s t r u c t i o n p r o c e e d e d , t h e c o a s t a l p r o t e c t i o n s y s t e m g r a d ­u a l l y t o o k s h a p e a n d c o n f o r m e d p r o p e r " p o l d e r s " i n T i a J u a n a , L a g u ­n i l l a s a n d B a c h a q u e r o ( M u r r i a & A b i - S a a b , 1 9 8 8 ) c o n s i s t i n g of-. ( a ) The c o a s t a l d i k e t o p r o t e c t t h e s u b s i d e d a r e a f rom L a k e f l o o d i n g . (b ) The i n n e r d i v e r s i o n d i k e s t o p r e v e n t r u n - o f f f rom t h e a r e a o u t ­

s i d e g e t t i n g i n t o t h e s u b s i d e d p o l d e r a r e a .

( c ) Drainage c h a n n e l s t o convey t h e wate r t o t h e pumping s t a t i o n s c o n s t r u c t e d a l o n g t h e c o a s t .

( d ) P u m p i n g s t a t i o n s t o d i s p o s e o f t h e w a t e r o v e r t h e d i k e a n d i n t o t h e L a k e .

F i g u r e 3 shows t h e m a i n f e a t u r e s o f t h e L a g u n i l l a s p o l d e r . The B a c h a q u e r o a n d T f a J u a n a p o l d e r s show s i m i l a r f e a t u r e s .

SUBSIDENCE MONITORING

A l e v e l l i n g n e t w o r k was e s t a b l i s h e d i n L a g u n i l l a s i n 1929 a n d l a t e r

e x t e n d e d t o c o v e r t h e C a b i m a s , T i a J u a n a , B a c h a q u e r o , a n d Mené G r a n d e

133 Subsidence due to oil production in Venezuela

as well as nearshore lake o i l f i e l d s . lb date the network covers an area of about 800 km2 and comprises over 1.600 bench na rks .

The subsidence monitoring surveys are s t i l l conducted a t two year i n t e r v a l s . The h i s t o r i c records kept since the very beginning cons is t b a s i c a l l y of the bench mark e leva t ion , r e l a t i v e subsidence between the two most recent surveys, and the cumulative subsidence s ince the beginning of explota t ion of each f i e l d . This information has been represented graphica l ly in the form of i s o l i n e maps showing the cumulative and the biannual subsidence Figure 4 shows cummulative subsidence as of March, 1990.

In 1988 GPS (Global Pos i t ion ing System) techniques were incorpo­ra ted in the l e v e l l i n g campaign (Figure 5) with encouraging r e s u l t s (Chrzanowski e t a l , 1988), but problems developed because of poor s a t e l l i t a r geometry. In 1990 GPS was again used together with t r a d i ­t i o n a l l e v e l l i n g and the r e s u l t s improved considerably.

The computational scheme includes a model t ha t allows the combi­nat ion of GPS with l e v e l l i n g in a geodynamic adjustment. For more d e t a i l s on the computational methodology the reader i s r e fe r red to Leal (1989).

The maximum subsidence r a t e i s about 20 cm per year inland in the area of El Polvorin, Laguni l las . The maximum subsidence r a t e along the coas ta l dikes i s 7 cm/yr. The maximum cumulative subsidence up to March 1990 has reached a value of 5 m in Laguni l las .

There a re 60 years of subsidence h i s t o r y , s i g n i f i c a n t t r e a su re from a s c i e n t i f i c point of view and probably one of the few cases in the world.

DESCRIPTION OF THE COASTftL PROTECTION SYSTEM

Design and construct ion of coas ta l dikes

The gradual nature of subsidence, together with the p o s s i b i l i t y of p red ic t ing future subsidence, has allowed for staged construct ion of the coas ta l dikes as shown in Figure 2 .

The c h a r a c t e r i s t i c s of the coastal dikes have been influenced, not only by subsidence but a l so by geotechnical , hydrographie, and s e i s -mici ty and seismic geology aspects (Abi-Saab e t a l , 1982).

Geotechnical aspects

The Delft Soil Mechanics Laboratory has s tudied the s t a b i l i t y of the dikes based on s o i l inves t iga t ions and the r e s u l t s showed t h a t , for a dike with a he ight of 8-10 m on a subsoil of s i l t y sand, an outer (Lake) slope i s required not s teeper than 1:2.5, and an inner (land) slope not steeper than 1-.3.

Hydrographie aspects

The height of the dike i s determined by the water level and wave run-up, a l l in accordance with the expected subsidence-, as a c r i t e ­r ion for the he ight of the dike i t has been assumed t h a t no more than 2% of the waves during a storm should reach the c r e s t .

Based on t h e o r e t i c a l ca lcu la t ions confirmed by model t e s t s a mini-mum dike he ight of 1.8 m above MLWL was defined.

Juan Murria

134

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G

•d

•H

tn

ai >

•H

J->

rH

0 ni +J o

En

U

H

135 Subsidence due to oil production in Venezuela

SCALE

FIG. 5 Original and GPS aided subsidence monitoring main l eve l l i ng network..

In 1982, i t was decided to r a i s e t h i s minimum freeboard to 2.30 m to account for t i d a l v a r i a t i o n s (+ 30 cm) and the p o s s i b i l i t y of lake body o s c i l l a t i o n s (+ 20 cm) in an eas t /wes t d i r e c t i o n .

Seismicity and seismic geology aspects

The coas ta l dikes a re located in a seismic area of low to moderate i n t e n s i t y , corresponding to Zone 2 of the 5 zones in which Venezuela i s divided for seismic design purposes.

Seismic geology and se i smic i ty s tud ies were car r ied out from 1985 to 1988. These s tudies showed a moderate seismic r i s k , mainly due to the p o s s i b i l i t y of l iquefac t ion of a f a i r l y loose, s a tu r a t ed , s i l t y sand layer in the dike foundation s o i l s .

Mit igat ive measures, cons is t ing b a s i c a l l y of downstream berms, with or without so i l improvement, and, in some s e c t i o n s , an upstream a r t i f i c i a l beach, are being implemented in about 25 of the 47 km of coasta l dikes (see Figure 6 ) .

136

DOWNSTREAM BERM

DOWNSTREAM BERM WITH SOIL IMPROVEMENT

ARTIFICIAL BEâÊM

FIG. 6 Mitigative measures.

Work in the th ree polders was s t a r t e d in 1988 and i t i s est imated tha t i t w i l l be completed by 1995.

The implementation of the mi t iga t ive measures wi l l increase the re turn period of the design earthquake from about 130 to 3000 years , the re tu rn period t y p i c a l l y used worldwide for the design of earth dams in seismic a reas .

Drainage masterplan

The nonuniformity of subsidence i s obviously r e f l ec t ed in topography and, the re fo re , in surface drainage p a t t e r n s . In recogni t ion of t h i s problem, Maraven undertook in 1984 the development of a Drainage Master Plan ( I razabal e t a l 1986) with a 50 year planning scenario with an intermediate 25 year scenar io , both based on subsidence fo r e ­cas ts using the "ANALYTICAL" model (see below).

This masterplan was f ina l ized in 1986 when de ta i l ed drainage p r o j ­ec t s were undertaken. Actual construct ion of the f i r s t p ro jec t s t a r t e d in 1988.

SUBSIDENCE PREDICTION MODELS

In the case of the Costa Oriental o i l f i e l d s , subsidence predic t ions allow appropriate planning and scheduling of the engineering a c t i v ­i t i e s needed to guarantee the pro tec t ion function of the coas ta l and diversion dikes as well as the s a t i s f ac to ry performance of the drainage system.

Maraven and i t s predecessors have been studying subsidence, as i t r e l a t e s to rese rvo i r compaction, p r a c t i c a l l y s ince the beginning of o i l production in the l a t e 1920 's . For a thorough and extensive

137 Subsidence due to oil production in Venezuela

d e s c r i p t i o n of t h e s t u d i e s pe r fo rmed , t h e r e a d e r i s r e f e r r e d t o Van der Khaap and Van der V l i s (1967) and Nunez and Esco j ido (1976) and Mendoza and Murria (1989) .

Since 1980 , Maraven has been working in t h e development of two s u b s i d e n c e p r e d i c t i o n mode l s , HUNDCALC and t h e "ANALYTICAL" mode l . I n 1983, and b a s e d on an approach s u g g e s t e d by De l f t G e o t e c h n i c s , The N e t h e r l a n d s , work was s t a r t e d i n t h e development of t h e SINK model . F i n a l l y , INTEVEP, t h e r e s e a r c h and development s i s t e r company, h a s been d e v e l o p i n g t h e SUB 3D model s i n c e 1984.

A g e n e r a l d e s c r i p t i o n of t h e s e models i s given be low.

The HUNDCALC Model

The HUNDLAC model was t h e f i r s t e m p i r i c a l method deve loped by Maraven t o e s t i m a t e f u t u r e ground s u b s i d e n c e i n t h e Cos ta O r i e n t a l o i l f i e l d s . The t e c h n i q u e for p r e d i c t i o n of s u r f a c e s u b s i d e n c e i n v o l v e s c a l c u l a t i n g r e s e r v o i r compaction from a c t u a l or e s t i m a t e d p r o d u c ­t i o n . Subsidence i s t hen c a l c u l a t e d from t h e s e compaction r e s u l t s . The model w i l l a l s o c a l c u l a t e t h e maximum compara t ive v a l u e of t h e sand and s h a l e and t h e r e f o r e t h e maximum s u r f a c e subs idence e x p e c t e d .

The a r e a was cove red wi th a g r i d system of 8 2 8 , 1.4 x 1.2 km r e c t a n g l e s ( " b l o c k s " ) . The s u b s i d e n c e i s c a l c u l a t e d for each b l o c k . The i n d i v i d u a l g r i d b lock r e s u l t s a r e fused t o g e t h e r and c o n t o u r e d t o produce an a r e a l l a n d s c a p e .

The "ANALYTICAL" model

The ana ly t i c a l model i s the most widely used for the Costa Or ien ta l . I t i s the r e s u l t s of combined h i s t o r i c a l da ta , laboratory measure­ments and the usual ana ly t i c a l and simple equations of surface defor­mation due t o pore pressure reduct ion in the subsurface layers with va r i ab le compress ib i l i ty .

This methodology has been applied successful ly i s easy to handle, with low cost and does not use excessive da ta .

The SINK model

The SINK model i s an approach suggested in 1983 by Delft Geotechnics. The object ive i s the simulation of surface subsidence due t o f lu id

withdrawal from the subsurface. A simple solut ion i s not ava i lab le due to i t s complex na tu re , mechanical e f fec t s in a heterogeneous rese rvo i r with environment or surface consequences. The behavior i s t y p i c a l l y non- l inear and r e l a t e d t o the pore f lu id behavior .

The SINK model i s an ana ly t i ca l formulation of the consolida­t ion/deformation process e x p l i c i t y in terms of rese rvo i r p ressure , which in tu rn are determined numerically in order to account for heterogenei ty in the geological system, whereas the t rue coupling between both i s es tab l i shed i t e r a t i v e l y . The in tegra t ion in time i s not r equ i red , reducing the cost of long term p red i c t i ons .

The SUB-3P model

The Sub-3D i s a fully-coupled model t h a t uses f i n i t e element t e c h n i ­ques to ca lcu la te the compaction of the r e se rvo i r and associa ted

Juan Murria 138

s u r f a c e s u b s i d e n c e . The model fo l l ows t h e p r i n c i p l e t h a t t h e s o i l above t h e r e s e r v o i r w i l l n o t be on e q u i l i b r i u m a f t e r f l u i d e x t r a c t i o n and t h e c o r r e s p o n d i n g d e c r e a s e of t h e p o r e p r e s s u r e . This w i l l c ause de format ion u n t i l t h e whole sys tem a c h i e v e s s t e a d y s t a t e c o n d i t i o n s . C o n s o l i d a t i o n i s t h e p h y s i c a l law which governs t h e compaction phenomenon. An e l a s t o p l a s t i c c o n s t i t u t i v e law b a s e d on c r i t i c a l s o i l mechanics i s u s e d .

The c a l c u l a t i o n of t h e p l a s t i c b e h a v i o r of t h e s o i l i s done by u s i n g a flow r u l e , a f i e l d s u r f a c e and an e v o l u t i o n law. A s o l u t i o n a l g o r i t h m t a k i n g i n t o a c c o u n t t h e e q u i l i b r i u m e q u a t i o n s , C a r e y ' s Laws and an assumed r h e o l o g i c a l model for t h e s o i l h a s been chosen .

The model i s i n e x p e r i m e n t a l s t a g e b u t h a s been t e s t e d on t h e Costa O r i e n t a l o i l f i e l d s i n smal l s e c t i o n s of t h e c o a s t a l a r e a . A p r e l i m i n a r y e v a l u a t i o n i n d i c a t e s t h a t t h e model may be u s e f u l for sma l l a r e a s where s p e c i f i c de fo rmat ion problems a r i s e . An obse rved d i s a d v a n t a g e i s t h e e x c e s i v e computing t ime needed s p e c i a l l y i n 3D p rob lems . I t h a s t h e a d v a n t a g e , on t h e o t h e r h an d , of r e p r e s e n t i n g many f e a t u r e s of s o i l b e h a v i o r u s i n g a smal l number of p a r a m e t e r s which can be o b t a i n e d from s t a n d a r d s o i l t e s t i n g p r o c e d u r e s .

FINAL REMARKS

This paper has a t t e m p t e d t o summarize t h e development of t h e Cos ta O r i e n t a l C o a s t a l P r o t e c t i o n System, t h e s t u d i e s pe r fo rmed , t h e prob lems e n c o u n t e r e d , and t h e e n g i n e e r i n g s o l u t i o n s implemented .

Pe rhaps t h e most s a l i e n t c h a r a c t e r i s t i c of t h i s sys tem i s i t s m u l t i d i s c i p l i n a r y d i v e r s i t y . D i s c i p l i n e s a p p a r e n t l y a s d i v e r s e a s geodesy and e a r t h q u a k e e n g i n e e r i n g , h y d r a u l i c s and s e i s m i c geo logy , o c e a n o g r a p h l y and subs idence mode l ing , have been s u c c e s s f u l l y i n t e g r a t e d and , a s a r e s u l t , t h e Venezuelan o i l i n d u s t r y can p r o u d l y s t a t e t h a t t h e main o b j e c t i v e of t h e sys t em, p r o t e c t i o n of l i f e and i n s t a l l a t i o n s , h a s been s u c c e s s f u l l y a c h i e v e d .

REFERENCES

Abi-Saab Soto J . , R o e s t , P . W., V e l s i n k , H. " P o l d e r s and dykes of t h e B o l i v a r C o a s t , V e n e z u e l a " . I n t . Syrnp. on P o l d e r s of t h e WarId, October 1982, N e t h e r l a n d s , V o l . I , 1 3 4 - 1 4 5 .

Abi-Saab S. J . and J . Murria (1985) "Origen y d e s a r r o l l o de l s i s t e -ma de p r o t e c c i o n c o s t a n e r a , Costa O r i e n t a l de l Lago de Mara-c a i b o " . I J o r n a d a s de Tecnologfa de P r o d u c c i o n , INTEVEP, Los Teques , V e n e z u e l a .

Chrzanowski A . , Y. p . Chen, R. Leeman, and J . Leal (1988) " I n t e g r a ­t i o n of t h e g l o b a l p o s i t i o n i n g sys tem wi th g e o d e t i c l e v e l l i n g s u r ­veys in ground subs idence s t u d i e s " . P r o c . of t h e 5 t h I n t . (FIG) Symp. on Deformation Measurements and 5 t h Canadian Symp. on Mining Survey and Rock Deformation Measurements , F r e d e r i c t o n N. B . , Canada , 1 4 2 - 1 5 1 .

C o l l i n s , J . J . "New t y p e s e a - w a l l b u i l t fo r s u b s i d i n g l a k e s h o r e i n V e n e z u e l a " . Eng inee r ing News Record, V. 114, No. 3 , 1935 , 4 0 5 - 4 0 8 .

I r a z a b a l A . , Abi-Saab J . , Murria J . , G r o o t , J . "Drainage problems i n a r e a s s u b j e c t t o s u b s i d e n c e due t o o i l p r o d u c t i o n " . P r o c . of t h e 2nd I n t . Conf. on H y d r a u l i c Design ifeter Resources Eng inee r -

139 Subsidence due to oil production in Venezuela

ings Land Drainage, Southampton Univers i ty , U.K., April 1986, Springer Ver l ag , Berlin Heidelberg-New York-Tokyo, 545-554.

Leal J . " In tegra t ion of GPS and l e v e l l i n g in subsidence monitoring s tudies a t the Costa Bolivar o i l f i e l d s " . M.S.C. Thesis in prep. Department of Surveying Engineering, Univ. of New Brunswick, Freder ic ton, N. B . , Canada, 1989.

Mendoza, H. and Murria J . "Ground subsidence modelling in western Venezuela". Submitted for acceptance to the Organizing Committee of the I n t . Symp. on Land Subsidence, Dhanbad, Bihar, Ind ia , 1989.

Murria, J . and Abi Saab J . "Engineering and construct ion in areas subjected to subsidence due to o i l product ion". 5th I n t . (FIG) Symp. on Deformation Measurements and 5th Canadian Symp. on Mining Surveying and Itock Deformation Measurements, Freder ic ton, N. B . , Canada, 1988, pp. 367-373.

Nunez, 0. and Escojido, D. "Subsidence in the Bolivar Coast", Pub­l i c a t i o n No. 121 of the IAHS, Proc . of the Annaheim Symp., December 1976, 257-266.

Van der Khapp, W. and Van der Vl is , A. C. "On the cause of s u b s i ­dence in o i l producing a reas" . Proc. of the 7th World Petroleum Congress, Mexico Ci ty , 1967, 85-95.