SPE 97385

Nitrogen Injection in the Cantarell Complex: Results After Four Years of Operation J.L. Snchez, SPE, A. Astudillo, F. Rodrguez, SPE, J. Morales, and A. Rodrguez, PEMEX E&P

Copyright 2005, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Latin American and Caribbean Petroleum Engineering Conference held in Rio de Janeiro, Brazil, 20 23 June 2005. This paper was selected for presentation by an SPE Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract The nitrogen injection project in the Cantarell Complex, offshore Mexico, implemented in May 2000, has been the most ambitious pressure maintenance project around the world regarding incremental oil, production rate, nitrogen injection rate, and investment.

In this paper, the main drivers and objectives of the project, along with key issues that have been taken into account to guaranty success, are reviewed. A description of the data acquisition program implemented to monitor reservoir performance during nitrogen injection is discussed and results analyzed.

The pressure-production behavior of the reservoir during nitrogen injection is presented and results obtained after four years of operations are assessed, along with additional field development and benefits from updating and increasing facilities. Introduction Cantarell, a complex of offshore naturally fractured carbonate fields located in the Bay of Campeche, about 50 Miles of the Yucatn Pennsula, is the most important complex in Mxico, see Figure 1. It is comprissed of five fields, being Akal the largest, with 32 billion standard barrels of oil, Billion barrels, BB, 86 percent of the OOIP of the complex. Detailed information of the characteristics of the reservoirs and fluids in the complex have been presented elsewhere1 and here we will highlite only those more relevant.

Main pay zones in Cantarell are highly fractured-vuggy carbonate formations from Jurassic, Cretaceous and Lower Paleocene geological ages. Within Akal, formations are hidraullicaly continuous and have an average thickness of about 4,000 feet and structural relief of over 7,000 feet.

The reservoirs contain a 22 API oil, initially undersaturated; pressure at the reference depth of 7,544 feet below sea level, BSL, was 3,970 psi. Typical total porosity in

the reservoirs is 8% and it has been determined that up to 35% of it is secondary porosity, fractures and vugs. Typical absolute permeability in the matrix and fractures is 1 and 3,000 md, respectively.

Akal field has produced under gravity dominated flow conditions throughout its life: Under natural depletion, a secondary gas-cap formed and was subject to water encroachment from an associated regional aquifer shared with other nearby fields.

Production started in Akal in 1979 and by 1981 reached a peak of 1.156 million barrels per day, MMBPD. Since then and up to 1995 the Cantarell complex produced at an almost constant rate of 1 MMBPD of oil; while pressure declined, production was maintained by drilling additional wells. In 1987 the implementation of gas lifting was required. Since 1997 production started to climb up, when a long term optimization program2 was launched; this consisted of infill drilling, the modernization and expansion of production facilities and the implementation of a pressure maintenance program by nitrogen injection in year 2000.

In what follows, main results obtained by the implementation of the pressure maintenance program by nitrogen injection in the Cantarell complex are presented.

The Cantarell Oil Recovery Optimization Project.

Main drivers for PEMEX in 1995 to consider the implementation of an optimized scheme to recover remaining reserves of oil in the complex were the trends being imposed by natural depletion regarding the increasing number of wells required to maintain the oil production rate of Cantarell which would eventually come to a limit and uneconomical, as compared to other alternative recovery schemes, see Figure 2.

Initially, when average pressure was 3840 psi, Cantarell produced from Akal field at an average oil rate of about 29,000 BPD per well; the 1.156 MMBPD produced in 1981 required only 40 non-assisted wells1, while the 1.02 MMBPD produced in 1995, when the Cantarell Project was conceived, required 150 gas-lift assisted wells, producing about 7,000 BPD per well.

Reservoir simulation studies3,4 indicated that implementing a pressure maintenance program in the fields would yield optimized oil recovery in the Cantarell complex. Forecasts showed that under natural depletion, Akal would have reached an average pressure of 1180 psi and oil production rates of 3,200 BPD per well by year 2004. Under these circumstances, long times, in the order of 80 years, would have been required to produce Cantarell oil reserves with replacement of production facilities; water encroachment would have also

2 SPE 97385

continued in the reservoirs with poorer oil recovery efficiency as compared to gas invasion.

The above considerations, along with resultso from other technical and economical studies,5 led to conclude that pressure maintenance by nitrogen injection was required to maximize the economic value of the Cantarell fields. The production optimization project included the drilling of 205 production and 9 injection wells, as well as modernization and expansion of production facilities.

The Nitrogen Injection Project. Among gas injection technologies, nitrogen was selected under the basis of availability, cost, handling infrastructure, environmental and safety issues. Preliminary estimate of the cost of nitrogen, delivered at the wellsite, was 0.54 USD/MSCF which compared favorably against 2.66 for natural gas which was the second option found. The final cost of nitrogen1, obtained through the bidding of services for nitrogen supply was 0.36 USD/MSCF.

Technical and economical analysis of various simulated production-injection scenarios3,4 indicated that best results would be obtained when producing 2 MMBPD of oil for 4 years, which in turn would require the injection of 1.2 BSCFD of nitrogen. Oil production rate would then start declining as well as nitrogen requirements; excess nitrogen could then be injected in other offshore fields. Nitrogen would be injected at the top of Akal reservoir in the gas cap and pressure would be maintained within the oil column.

Estimated additional oil and gas recoveries are 2.324 Billion barrels and 870 TCF, respectively, considering the economic limit of the project.

The pressure maintenance program started with the injection of 300 MMPCD of nitrogen in May 2000 and by December of the same year injection reached the maximum programmed injection rate of 1.2 BSCFD. Nitrogen is injected into the gas cap through seven wells, drilled and completed at the top of Akal. The response of the fields has been followed up through a program designed and implemented to monitor pressure, nitrogen concentration and the gas-oil contact movement.

Monitoring Program. A carefully designed program was designed and implemented to monitor the pressure response of the fields to nitrogen injection, the distribution of nitrogen in the gas cap of Akal, gas-oil contact movement, and composition of the produced gas during nitrogen injection. Recently, a paper was documented6 on practices followed in Cantarell on this regard.

Pressure Monitoring. Pressure has been measured in 10 wells strategically located in the gas cap, oil column and the aquifer, see Figs. 3 and 4. The measured pressure response of the wells has been found consistent with that predicted by reservoir simulation, as shown in Figure 5. Bottomhole pressure data measured in typical wells located in the gas cap and oil column of Akal are shown in Figs. 6 and 7 respectively. Detailed analysis of the pressure response to changes in the nitrogen injection rate shows very short transient flow periods, followed by pseudo-steady state flow

regimes, which confirms the extremely high transmissibility in the Akal reservoir.

Figure 6 shows that pressure in the gas cap has been steadily increasing with time as a result of the intention to maintain pressure at the water-oil contact. Notice that this condition is met if pressure in the oil column is also maintained. For this to happen, pressure in the gas cap must be increased as oil column is removed, since higher pressures are found by the gas-oil contact as it moves down the structure. Notice in this figure the shift in the rate of pressure change in Well C42 as it becomes invaded by the gas cap.

Figure 7 shows pressures measured in the oil column since nitrogen injection started. Notice that until about the end of 2001, pressure in the oil column increased with time, since the target nitrogen injection rate of 1.2 BSCFD was achieved before the 2 MMBPD target oil production could be established. During this period, an enhancement in the production rates of the existing wells was evidenced. This overpressuring was then compensated by producing over the target oil rate, as shown by the pressure decrease, starting 2002.

Nitrogen Concentration Monitoring. Gas chromatography has been applied to monitor the concentration of nitrogen, CN2, in wells strategically located accross the Akal field.

Being nitrogen one of the components in the original Cantarells fluids, sampling and analyses of the gas-cap gas and the produced associated gas were conducted prior to nitrogen injection: Concentrations of 1.1 to 1.3 mole percent were measured in the gas-cap gas, and 0.4 to 0.6 mole percent in the produced gas.

The buildup of nitrogen concentration in the gas-cap gas has been measured in gassed out wells, as reached by the gas-oil contact and afterwards. Measurements have also been conducted in old gassed-out and recompleted wells located up in the reservoir. Figure 8 shows concentrations measured at various locations within the gas cap. Notice that large nitrogen concentration values are found as we approach the gas-oil contact, and that as we move up in Akals gas-cap, small concentration values, tending to the concentrations of the original gas-cap gas, are found. This indicates that the injected nitrogen is segregated in the gas cap and that distributes in a cone-shaped fashion, as has been verified by reservoir simulation studies7.

Figure 9 shows the evolution of CN2 in the gas produced from Cantarell. Notice that during the first year after injection started, the concentration of nitrogen in the produced gas remained basically the same as before injection. Notice also that after this period, intermittent increasing-decreasing values of CN2 are found due to the use of nitrogen in the gas-lifting operations of Cantarell. Then, at about middle 2002, a trend of increasing CN2 values are found due to continuous use of nitrogen for gas-lifting. Notice, interestingly, that beginning 2004, when nitrogen usage for this purpose ceased, N2 concentrations started declining and tended to approach the original values.

Gas-Oil Contact Monitoring. The gas-oil contact in Akal has been monitored through the years and with special emphasis since nitrogen injection started. Figure 10 shows information

SPE 97385 3

on the position of the gas-oil contact through the years along with oil production rates. Notice that after nitrogen injection started, in May 2000, and oil production rate increased, a noticeable increase in the rate of descent of the gas-oil contact is seen, averaging about 230 feet per year. This was important information used in the calibration of Akals numerical model, and more important, to anticipate the invasion of wells by the gas-cap.

Special Tests. One key issue in forecasting the evolution of nitrogen concentration in the production stream is the proper characterization of nitrogen diffusion in the vecinity of the gas-oil contact and dispersion within the oil-column through the fractures. In order to characterize this phenomenon, a field test was designed and implemented, which consisted of monitoring changes in the composition of the oil in a well producing in the neighborhood of the gas oil contact, as the contact approached perforations. Downhole oil samples were taken in different wells at different times and compositional analysis on the sampled oils was conducted. Figure 11 shows an example of the composition of oil samples taken four months apart in a given well completed close to the gas-oil contact. As seen in this figure, no noticeable changes in composition were found within this time lapse, thus indicating very limited, or practically unexistent, transfer of nitrogen from the gas-cap into the oil column. Results of the Pressure Maintenance Project.

The cummulative volume of nitrogen injected in Akal four years after pressure maintenance operations started was 1,400 BSCF. During the same period of time, a slightly bigger equivalent amount of reservoir oil and its associated gas have been produced. As expected, given the gravity dominated flow conditions of Akal through the years, the gas-oil contact has been moving steadily during nitrogen injection: no channeling of the injected gas has been detected, and the invasion of wells by the gas-oil contact has been very predictive, based on reservoir simulation and field results. The gas oil contact has moved at a pace of about 230 feet per year in the last four years. Due to the high vertical permeability of the fractures, gas coning has not been a problem in Akal. Current position of the gas oil contact is 7,446 feet BSL and thickness of the gas cap is 3,838 feet. Current position of the water-oil contact is 8,700 feet BSL as compared to its 10,500 feet BSL original position.

Figure 12 provides information about the effect on oil

production rate of the two main components of the Cantarell optimization project: pressure maintenance and additional drilling-expansion of production facilities. Discrimination of effects was obtained by using the numerical model of Cantarell7. The base case of continuing operations with no implementation of the production optimization project is also shown. Notice that as of beginning 2004, an increase of about 1,402 MBPD in the rate of oil production was estimated, as compared to the base case. From this rate, 628 MBPD corresponds to pressure maintenance and the rest, 774 MBPD, to the drilling of new wells and expansion of production facilities.

As per the above description of experiences and results, it follows that the pressure maintenance project by nitrogen injection in Cantarell has been very successful, both technically and economically.

Conclusions 1. In the first four years of the pressure maintenance project

in Cantarelll, a cummulative volume of 1,400 BSCF of nitrogen has been injected into Akal. The volume of gas in the secondary gas cap before injection was 1,800 BSCF.

2. Pressure in the oil column has been kept at 1455 psi. Had not the pressure maintenance project been implemented, reservoir pressure would have dropped to 1220 psi by 2004, as updated estimations indicate.

3. The gas-oil contact has continued moving into a flat surface at a rate of 230 feet per year during the nitrogen injection process.

4. Overall nitrogen concentration in the produced gas of Cantarell increases only when the gas-oil contact reaches the producing wells, or when nitrogen is used in gas-lifting operations.

5. Data obtained from the monitoring program indicates that nitrogen is segregated in the gas cap and that its concentration in the oil column has not changed.

6. Contribution of pressure maintenance to oil production rate, as of beginning 2004, was estimated in 628 MBPD.

7. The pressure maintenance project by nitrogen injection in Cantarell has proven to be very successful, both technically and economically.

Acknowledgement We thank the management of PEMEX Exploration and Production for permission to publish this paper. Nomenclature

BSL = Below sea level BB = Billion Barrels BSCF = Billion of standard cubic feet CN2 = Nitrogen concentration, Mole percent MBPD = Thousand barrels per day MMBPD = Million barrels per day OOIP = Original oil in place

4 SPE 97385

References 1. Rodrguez, F., Ortega, G., Snchez, J.L. and Jimnez, O.:

Reservoir Managemet Issues in the Cantarell Nitrogen Injection Project, paper OTC No. 13178, presented at the 2001 Offshore Technology Conference held in Houston, Texas, 30 April-3 May, 2001.

2. Limn-Hernndez, T., Garza-Ponce, G. and Lechuga-Aguaga, C.: Status of the Cantarell Field Development Program: An Overview, paper OTC No. 13175, presented at the 2001 Offshore Technology Conference held in Houston, Texas, 30 April-3 May, 2001.

3. Advisory study of the Cantarell Complex fields (Akal, Chac, Kutz and Nohoch), located offshore Campeche Mexico, prepared for PEMEX E&P by NSAI, June 1996.

4. Cantarell Complex Special Simulation Runs, Schlumberger-Geoquest, Denver, CO., August 1998.

5. Feasibility Study of Gas Injection in Offshore Mexican Oil Reservoirs, a Unigas, Co. study prepared for PEMEX E&P, Norman, OK., December 1996.

6. Astudillo-Abundes, A., Miguel-Hernndez, N., Urriza-Vergara, A. and Jimnez-Bueno, O.: Methodology to Detect Nitrogen Concentration at Surface Facilities and in the Reservoir Gas Casp in the Akal Field During Nitrogen Injection, Paper SPE 92112 presented at the 2004 SPE International Petroleum Conference in Mexico, hel in Puebla, Mxico, 8-9 Nov. 2004.

7. Rodrguez, F., Snchez, J.L. and Galindo-Nava, A.:Mechanisms and Main Parameters Affecting Nitrogen Distribution in the Gas-Cap of the Supergiant Akal Reservoir in the Cantarell Complex, paper SPE 90288 presented at the 2004 Annual Technical Conference and Exhibition of SPE, held in Houston, Texas, USA, 26-29 Sept., 2004.

Figure 1. Offshore location of the Cantarell complex in Mxico

Fig. 2 Production history of Cantarell previous to the planning of the Production Optimization Project

Fig. 3 Areal location of wells for pressure monitoring.

Fig. 4 Vertical location of wells for pressure monitoring

AKALCHAC

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SPE 97385 5

Figure 5. Field and reservoir simulation static pressures for Akal field.

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Figure 7. Evolution of pressures measured in wells at various locations in the oil column.

CONTACTO GAS ACEITE 2180 mv CONTACTO GAS ACEITE 2180 mv bmrbmr

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Figure 10. Monitoring of the gas-oil contact movement in Akal.

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PG

S

C

RN - A N - C

N - B

A

D

IO

J

B

N D BBN

C-57A

C-269D

EC-52CICI-67

C-42

C-75C-39I

AKAL

CHAC

NOHOCH

KUTZ

AKAL

CHAC

NOHOCH

KUTZTM M

MBTM M

MBLTJ LLTJ

F

PG

S

C

R

F

PG

S

C

RN - AN - A N - CN - C

N - BN - B

AA

DD

IIO

J

O

J

B

N D BBN B

NN D BBN

C-57A

C-269D

EEC-52CICI-67

C-42

C-75C-39I

C-57A (D)C-269D (P)

C-52 (E)

C-67 (CI)

C-42 (G)

C-75 (D)

C-39 (I)

73.1

77.6

82.1

86.6

2400

1800

1200

600

01150

1050

950May/00 May/01 May/02 May/03

Time

NN22

inj e

ctio

nin

jec t

ion

rate

rat e

( MM

S CFD

)(M

MSC

F D)

Oil

Oil

p rod

u ctio

npr

oduc

tion

rate

rate

(MB

P D)

(MB

PD)

Dow

nhol

e pr

essu

re (p

si)

Dow

nhol

e pr

essu

re (p

si)

1400

1410

1420

1430

1440

1450

1460

1470

1480

1490

1500

1510

1520

1530

1540

1550

1560

1570

1580

May/00 Jul/00 Oct/00 Ene/01 Abr/01 Jul/01 Oct/01 Ene/02 Abr/02 Jul/02 Oct/02 Ene/03 Abr/03 Jul/03 -3000

-2700

-2400

-2100

-1800

-1500

-1200

-900

-600

-300

0

300

600

900

1200

1500

1800

2100

2400

C-77D (DB)

71 psi/year

C-58 (O) C-18 (O)

C-42 (G)

AKAL

CHAC

NOHOCH

KUTZ

N - A

N - B

A

TM MMB

LTJ

F

PG

S

C

RN - C

D

I

O

J

B

N D B

BN

GP

C-77D

C-18C-58

28 psi/year

28 28

N2 Injection starts: May/00

HURRICANE ISIDORE

2104 MBPD 2243 MBPD

1658 MBPD 1667 MBPD

2 psi/year

1400

1420

1440

1460

1480

1500

2400

1800

1200

600

0

May/00 May/01 May/02 May/03Time

NN22

inj e

ctio

nin

jec t

ion

rate

rat e

( MM

S CFD

)(M

MSC

F D)

Oil

Oil

p rod

u ctio

npr

oduc

tion

rate

rate

(MB

P D)

(MB

PD)

Dow

nhol

e pr

essu

re (p

si)

Dow

nhol

e pr

essu

re (p

si)

1400

1410

1420

1430

1440

1450

1460

1470

1480

1490

1500

1510

1520

1530

1540

1550

1560

1570

1580

May/00 Jul/00 Oct/00 Ene/01 Abr/01 Jul/01 Oct/01 Ene/02 Abr/02 Jul/02 Oct/02 Ene/03 Abr/03 Jul/03 -3000

-2700

-2400

-2100

-1800

-1500

-1200

-900

-600

-300

0

300

600

900

1200

1500

1800

2100

2400

C-77D (DB)

71 psi/year

C-58 (O) C-18 (O)

C-42 (G)

AKAL

CHAC

NOHOCH

KUTZ

N - A

N - B

A

TM MMB

LTJ

F

PG

S

C

RN - C

D

I

O

J

B

N D B

BN

GP

C-77D

C-18C-58 C-42 (G)

AKAL

CHAC

NOHOCH

KUTZ

N - A

N - B

A

TM MMB

LTJ

F

PG

S

C

RN - C

D

I

O

J

B

N D B

BN

GP

AKAL

CHAC

NOHOCH

KUTZ

AKAL

CHAC

NOHOCH

KUTZ

N - AN - A

N - BN - B

AA

TM MMB

LTJ

F

PG

S

C

RN - C

D

I

O

J

B

N D B

BN

GP

TM MMB

TM MMB

LTJ LLTJ

F

PG

S

C

RN - CN - C

DD

II

O

J

O

J

B

N D B

BNB

NN D B

BN

GP

C-77D

C-18C-58

28 psi/year

28 28

N2 Injection starts: May/00

HURRICANE ISIDORE

2104 MBPD 2243 MBPD

1658 MBPD 1667 MBPD

2 psi/year

1400

1420

1440

1460

1480

1500

2400

1800

1200

600

0

/00 May/01 May/02 May/03May

6 SPE 97385

0

5

10

15

20

25

30

35

40

H2S CO2 N2 C1 C2 C3 I-C4 N-C4 I-C5 N-C5 C6 C7 C8 C9 C10 C11Component

% m

ol

Jul 5th-2001 9-Nov 9th-2001

0

5

10

15

20

25

30

35

40

H2S CO2 N2 C1 C2 C3 I-C4 N-C4 I-C5 N-C5 C6 C7 C8 C9 C10 C11Component

% m

ol

Jul 5th-2001 9-Nov 9th-2001

Figure 11. Special testing: Composition of downhole sampled oil at different times, conducted in a well as the gas-oil contact approached perforations

1,554 MBD1,554 MBD

628

774

780 MBD780 MBD

Dic / 03 2,182 MBDDic / 03 2,182 MBDProduccin BaseProduccin de Obras y PozosProduccin N2

Ene / 97 1,146 MBDEne / 97 1,146 MBD

500

750

Figure 12. Impact of the pressure maintenance and new wells and facilities expansion on oil production.

1000

250

500

750

000

250

94 95 96 97 98 99 00 01 02 03 04

Year

1,554 MBD1

1

1

2

2

628

774

780 MBD

Dec-03: 2,182 MSTBD

Qo

(MB

PD)

Jan-97: 1,146 MSTBD

Base productionNew wells and facilitiesNitrogen injection

1,554 MBD1,554 MBD

628

774

780 MBD780 MBD

Dic / 03 2,182 MBDDic / 03 2,182 MBDProduccin BaseProduccin de Obras y PozosProduccin N2

Ene / 97 1,146 MBDEne / 97 1,146 MBD

500

750

000

250

500

750

000

250

94 95 96 97 98 99 00 01 02 03 04

Year

1,554 MBD

1

1

1

1

2

2

628

774

780 MBD

Dec-03: 2,182 MSTBD

Qo

(MB

PD)

Jan-97: 1,146 MSTBD

Base productionNew wells and facilitiesNitrogen injection

Base productionNew wells and facilitiesNitrogen injection