gas+lift+with+nitrogen+injection+generated+in+situ

Copyright 2000, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 2000 SPE International Petroleum Conferenceand Exhibition in Mexico held in Villahermosa, Mexico, 1–3 February 2000.

This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to an abstract of not more than 300words; illustrations may not be copied. The abstract must contain conspicuousacknowledgment 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

Gas lift is a feasible option as an artificial lift system in a

depleted field. In the Bellota field there is no substructure

means to install any type of artificial lift system, therefore

the use of nitrogen as a gas lift source is necessary to

keep the Bellota wells producing. After evaluating

different options we implemented a nitrogen generated

in situ project using the membrane technology.

This paper analyzed the gas lift process design by using

nodal analysis and optimum allocation of nitrogen in

each well. Special emphasis and consideration was

given to this project from an economical, operational,

technical and environmental points of view. It is olso

compares this option with differents alternatives

including the traditional gas lift method using natural gas

as a source as well as the use of stored nitrogen from

storage trucks (Tanks). Our evaluation of results

obtained from the different options investigated in this

study clearly indicates that this method is a good option

in this particular situation.

Nitrogen injection generated in situ with membrane

technology can be a feasible and profitable alternate

source of gas lift as shown in this study.

INTRODUCTION

Bellota – Chinchorro is one of the seven producing areas

in the Southern Region of Mexico. The current

production of this field is 105 000 STB/D of oil and 150

MMSCF/D of gas produced from dolomite formations,

belonging to Jurassic and Cretaceous age.

The objective of this study are the wells drilled in the

Bellota field. Initial production of the bellota field started

in 1982 reaching a peak production of 44 000 STB/D in

1995. Current production is at 20 000 STB/D, mainly

attributed to natural depletion. Year- to- date production

from this field is estimated at 140 MMSTB, and it is

expected to produce another 50 MMSTB from calculated

total reserves.

This field is divided in two different sections. The wells

that will be discussed in this gas lift application belong to

the north section. These wells are currently depleted

below saturation pressure. It is presumed that a gas cap

has been already formed in the top of the reservoir,

since GOR has been decreasing gradually. In addition,

the reservoir pressure has decline drastically making it

necessary to provide some form of artificial lift

assistance to keep the wells producing.

Those wells that have been converted to gas lift, are

deep wells, which have so many disadvantages for any

SPE 59028

GAS LIFT WITH NITROGEN INJECTION GENERATED IN SITUMiguel A. Lozada Aguilar, M.del Remedios Arredondo Monarrez, SPE, Pemex, PEP.

2 MIGUEL A. LOZADA AGUILAR, M.DEL REMEDIOS ARREDONDO MONARREZ SPE 59028

artificial lift system. Some electric submergible pumps

have been tested before with poor results, and then, a

gas lift system by using nitrogen as source was installed,

since no facilities were available to handle natural gas as

a source for gas lift.

In the very beginning, storage trucks were used to

deliver nitrogen, but since this method was so

expensive, an in situ nitrogen generation project was

implemented, using the membrane technology. In this

way, it was possible to reduce 50% the total costs in

three gas lift wells.

In March 21 of 1998, gas lift started in wells Bellota 136,

138 and 158-D using nitrogen generated in situ with

membrane technology as a source.

PRINCIPIA OF MEMBRANE EQUIPMENT FOR

NITROGEN GENERATION

Nitrogen generation through membrane equipment is

carried out by pumping an air current into membrane,

which due to its especial material design let the air to be

separated into nitrogen and oxygen mainly. This is

achieved basically because the oxygen flows faster than

nitrogen through it, being expulsed to the atmosphere,

as long as the nitrogen is absorbed into the membrane

to be delivered to the next compression stage. Before

the separation process, air composition is 78% of

nitrogen, 21% of oxygen and 1% of rear gases;

Neverdeless just after the separation process, gas

mixture will be expulsed into the atmosphere with 40% of

oxygen content, and the one that has been absorbed

into the membrane has from 95% to 98% of nitrogen

content.

Membranes are built up from a polymeric thin cap, which

has special physical properties that make the separation

efficiency to have a variation base on: pressure,

temperature, permeability, membrane aria and

selectivity. Since 1987, when they have been reported

for its use in petroleum industry, they have evolved until

now, having 60% of better permeability and 30% of

better selectivity, besides, a significant reduction on

consumption energy has been achieved (Figure No.1).

Equipment to be installed on location together with

membrane unit is the following: two air compressors

pack, an air – nitrogen compressor pack, flow meter unit

and additional equipment. A brief functional description

for each component is given next to it.

a).- Two air compressor pack.

This pack has the function of comprise the air, which

comes from the atmosphere to be deliver into the next

compression stage. Air pressure is increased from

atmosphere pressure to 200 PSIG. Those two

compressors handle 5.6 MMSCF/D with 1100 HP of

potency.

b).- Air – nitrogen compressor pack.

This pack has the function to raise the air pressure that

comes from the previous stage to be deliver into the

membrane unit at 400 PSIG, and it has also the function

to increase the pressure of nitrogen which comes from

membrane unit in two stages, one of them from 400

PSIG to 900 PSIG and the other one from 900 PSIG to

2000 PSIG, to be deliver into the gas line for gas lift

purpose.

c).- Membrane unit.

This pack has the function of separate 5.6 MMSCF/D of

air to obtain 2 MMSCF/D of nitrogen with 95% to 98% of

quality. This unit is built up of 36 cylinders, the ones has

the membrane element inside of them.

d).- Flow meter unit.

This unit has the function of measure the amount of

nitrogen that is deliver into the gas line for gas lift.

e).- Additional equipment.

This equipment helps in order to let the main equipment

accomplish its function. Some of the most important

devices are: filter system, coolers, start on compressor,

energy plant and fuel storage.

SPE 59028 GAS LIFT WITH NITROGEN INJECTION GENERATED IN SITU 3

GAS LIFT NETWORK AND WELLS CONVERSION

Those three wells, which will be converted from natural

flow to gas lift, were drilled in the same location, thereby

gas lift line construction was cheap and quickly, as

distance from each well was no longer tan 500 FT. It

was also necessary to install flow meter and regulation

valves for each well. The nitrogen generation equipment

was installed in the same location (figure No. 2).

In the other hand, in order to make the wells conversion

cheaper, it was concluded that no workover rigs should

be necessary, as operation conditions for each well

permitted start the wells on production just by injecting

gas in one point, that means not to use any upper

injection valve. In this way a puncher charge was shot,

taking in account the equivalent diameter for an specific

drop drown from casing pressure to tubing pressure.

This was achieved basically, because a high pressure

was available in the gas line (2000 PSIG).

EQUIPMENT DESIGN

In order to design the equipment dimensions, it was

necessary to use three different software: nodal

analysis, gas lift design and equal slope method to

allocate the amount of gas for each well.

Based on the results getting from the equal slope

method and gas lift design software, it was concluded

that 2 MMSCF/D of N2 will be necessary to be injected

in those three wells, and 1600 PSIG will be required on

surface pressure; thereby, according to manufacture

specifications, one equipment for 2 MMSCF/D and 2000

PSIG was selected for this purpose (figure 3 and 4).

• Gas lift design criteria was to find out the deepest

injection point, thereby, with static conditions was

possible to start the wells on production with 2000

PSIG of surface pressure. Injection points were

located just above of packers, due basically, that

reservoir pressure was low enough, end in the other

hand, wells productivity index were high enough to

reach dynamic conditions with only one gas injection

point. Thanks to those conditions mentioned before,

it was possible to shoot the tubing by using puncher

charges, rather than use workover rigs to pullout

tubing string and put it back with gas lift valves.

• Due basically, that there wasn’t any available

surface control valve for high pressure, it was

necessary to install chocks in order to allocate the

optima amount of gas for each well; thereby it was

necessary to design the right diameter for each one

of them, using Bernulli equation.

ECONOMICAL ANALYSIS

a) Assumptions for different scenarios

In order to asses the feasibility of this project, three

scenarios were made in a period of time of five years:

1) build up a gas lift network in order to use natural gas

as a source, installing compressors in the location to

increase gas pressure to that one which is

necessary for each well.

2) Inject nitrogen as a gas lift source by using storage

trucks.

3) Inject nitrogen as a gas lift source, by generating it

with membrane technology, with leasing option.

4) Inject nitrogen as a gas lift source, by generating it

with membrane technology, with purchase option.

Those assumptions used for this analysis are refereed to

July, 1998 (table No.1).

The economical premises are defined as following:

• Oil price: It is refereed to July 1998.

• Production race: For comparison purpose, according

to allocation of gas for each well, getting from equal

slope method, it was possible to increase 3725


STB/D by injecting 2 MMSCF/D of nitrogen, and

4125 STB/D by injecting 2 MMSCF/D of natural gas.

• Discount rate: It was used that one from Pemex

projects, which is 10%.

• Initial investment: Initial investment was considered

in this way:

⇒ For gas lift network option, it was considered to

built up 13 miles of gas lift line of 6 in. and 3 in..

⇒ For membranes leasing option, it was

considered to invest on nitrogen line from

membrane equipment to each well.

⇒ For membrane purchase option, it was

considered to invest on nitrogen line from

membrane equipment to each well.

⇒ For storage trucks option for nitrogen injection,

there wasn’t any investment.

• Operational costs: In order to obtain those costs, it

was considered the production increase for each

option, as well as the total operational costs for each

option, getting, in this way the cost for each

produced barrel.

⇒ Operational costs for that to build up gas lift

network, is an addition of: differential cost

between to buy 2 MMSCF/D of natural gas and

to sell the same amount of sour gas; leasing of

compressors to increase gas pressure from gas

line pressure to that which is required to inject in

to the well; and comprising costs to inject sour

gas toward sweeter station; which yield

657.8+48+184.2 = 1249 USD/D.

⇒ Operational cost for that to leasing storage

trucks for injecting 2 MMSCF/D of nitrogen is

equal to 18,650 USD/D.

⇒ Operational cost for leasing membranes to

generate 2 MMSCF/D of nitrogen is equal to 11,

275 USD/D

⇒ Operational cost for membrane purchase is alike

to that of comprising costs in gas lift network

option, which is equal to 184.2 USD/STB.

• Field depletion: it was taken that one which

represents field performance.

b) Interpretation of economical indicators.

Some of the indicators shown here are quite far good for

any petroleum project, basically because the high

production rate to be expected (table 2). Some

comments are summarized as following:

- Giving the risk approach form internal rate of return

all of the four option are excellent, as it is unlikely to

reach the same value for discount rate in any bank.

- Regarding to investment efficiency all the values are

quite high, so it means that all the four options are

profitable.

- Pay back period is very short for every option, so

cash flow will be available since the early stages.

- Net present value could be the indicator to be

considered for making a good decision, since option

1 and 4 represent the highest values, and they also

represent an important difference between the

others options.

- As net present value for option 1 and 4 are quite

similar, best option should be that which has some

else benefits; whether environmental, technical or

operational aspects concerns.

TECHNICAL, OPERATIONAL AND ENVIRONMENTAL

ISSUES

• Technical comparisons:

⇒ According to equal slope method, figure 6 shows

how injecting the same amount of gas, whether

nitrogen or gas at the same depth, it is possible to

obtain 400 STB/D more injecting natural gas than

nitrogen. The explanation of this is because nitrogen


is heavier than natural gas, thereby getting a higher

gradient all along the tubing; therefore, the higher

flowing bottom hole pressure, the lower liquid rate

based on productivity index ecuation. For this

specific case a 10% production rate increase could

be achieved (figure 5).

⇒ In the other hand, shows how the higher nitrogen

weigth in the annulus, the surface requirements

pressure is lower from that with natural gas injection,

therefore less potency is required. For this particular

case it is expecting to reduce 10% of total costs due

to potency reduction (figure 6).

• Operational and environmental comparisons:

⇒ Nitrogen is an unfinished source available in the

atmosphere, thereby its use doesn’t have to deal

with hydrocarbon exploitation.

⇒ Nitrogen plants can be installed in the most

convenient place, as they don’t need natural gas

supply.

⇒ As nitrogen is an inert gas, safety problems are

reduced enormously.

⇒ Petrochemical plants can only handle 3% of impurity

as a total amount of gas, thereby nitrogen uses as a

gas lift source is constrained by the total processed

gas. For this particular case the impurity percentage

was no higher than 0.5%.

⇒ As it is known nitrogen generated with membrane

technology has from 5 to 2% of impurities, mainly

oxygen. Somewhere during membrane operation,

there were some corrosion problems in the process

facilities, but unfortunately by that time it was

necessary to stop membrane operation for budget

reasons, without giving the chance to evaluate

oxygen impact on this problem, specially because

sour gas is produced in those wells. So this is a big

concern as this technology is a good option for this

particular case, anyway, if it was true, it is possible

to use chemical products to avoid this phenomena.

CONCLUSIONS

• Nitrogen injection as a gas lift source is feasible, as

it is an unfinished and available source in the

atmosphere.

• Nitrogen injection as a gas lift source has a similar

profitability as that with natural gas injection.

• At this moment with the current conditions on the

leasing contract, it is more profitable for PEMEX to

buy and install its own plant.

• It is possible to save 10% of potency injecting

nitrogen rather than natural gas.

• There is a significant reduction on risks, as nitrogen

is an inert gas, besides of that a significant reduction

of gas line mileage is achieved.

• Reduction of 10% of production rate is expected as

a result of inject nitrogen rather than natural gas.

• Further investigation will be needed to evaluate the

nitrogen impurities on corrosion problems.

• The amount of nitrogen used for gas lift is

constrained by the total gas handled in

petrochemical plant, which shouldn’t be no higher

than 3%.

REFERENCES

- Nodal analysis software, “PIPESIM”, Baker Jardine

and Associates Limited.

- The technology of artificial lift methods – volume 2ª -

kermit Brown.

- Gas lift optimitation and design software – GLOP-

Cealc.

- Temas selectos sobre bombeo neumático continuo.

Colegio de Ingenieros Petroleros de México.


- Optimización de la distribución de gas en la red de

bombeo neumático del campo Cunduacán –

Oxiacaque, AIPM, Miguel Angel Lozada Aguilar y

Maria del Remedios Arredondo M.

- Technical and operational manual of membrane

plants.


Figure 1

Figure 2

MEMBRANE DIAGRAM

FIBER

NITROGEN

AIR FEED

WASTE OXYGEN VENTWASTE OXYGEN VENT

FIELD MEMBRANE DIAGRAM

BELLOTA 158-D WELL

BELLOTA 138 WELL

BELLOTA 136 WELL

MEMBRANE EQUIPMENTGAS METER

ME

MB

RA

NE

COMPRESSOR

COMPRESSOR

COMPRESSOR

FLOW LINE

NITROGEN LINE

PROCESSFACILITIES


Figure 3

Figure 4

WELL PERFORMANCE WITH NITROGEN INJECTION

0

200

400

600

800

1000

1200

1400

1600

1800

0 0.2 0.4 0.6 0.8 1

NITROGEN INJECTION RATE (MMSCF/D )

LIQ

UID

RA

TE

(S

TB

/D)

BELLOTA 136

BELLOTA 138

BELLOTA 158-D

Iny.P.=1437 PSIG

Iny. P.=1380 PSIG

Iny.P.=1671PSIG

EQUAL SLOPE METHOD

CHARACATERISTIC CURVE FOR THREE WELLS WITH NITROGEN INJECTION

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5

Nitrogen injection rate (MMSCF/D)

Liq

uid

rat

e (ST

B/D

)

41004100


Figure 5

Figure 6

E Q U A L S L O P E M E T H O D

N A T U R A L G A S V S . N I T R O G E N I N J E C T I O N C O M P A R I S O N

0

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

2 5 0 0

3 0 0 0

3 5 0 0

4 0 0 0

4 5 0 05 0 0 0

0 0.25 0.5 0 .75 1 1 .25 1.5 1 .75 2 2 .25 2.5

G A S I N J E C T I O N R A T E ( M M S C F / D )

LIQ

UID

RA

TE

(S

TB

/D)

4 1 0 04 5 0 0

N A T U R A L G A S

N I T R O G E N

S U R F A C E R E Q U I R E M E N T S P R E S S U R E

N A T U R A L G A S V S . N I T R O G E N C O M P A R I S O N

1 5 0 0

2 0 0 0

2 5 0 0

3 0 0 0

3 5 0 0

4 0 0 0

4 5 0 0

5 0 0 0

5 5 0 0

7 0 0 9 5 0 1 2 0 0 1 4 5 0 1 7 0 0 1 9 5 0

P r e s s u r e (Ps i )

De

pth

(m)

N I T R O G E N N A T U R A L G A S


Table 1

Table 2

BASIS FOR ECONOMICAL ANALYSIS

OPTIONOPTION CONCEPTCONCEPT PRICE PERPRICE PERBARRELBARREL

(USD)(USD)

PRODUCTIONPRODUCTIONINCREASEINCREASE

(STB/D)(STB/D)

DISCOUNTDISCOUNTRATE (%)RATE (%)

CAPITALCAPITALINVESTMENTINVESTMENT

(USD) (USD)

OPERATIONALOPERATIONALCOSTS (USD/STB)COSTS (USD/STB)

FIELDFIELDDEPLETIDEPLETION (%)ON (%)

GAS LIFTGAS LIFTNETWORKNETWORK

NITROGENNITROGENINJ. WITHINJ. WITHSTORAGESTORAGETRUCKSTRUCKS

MEMBRANEMEMBRANELEASINGLEASING

MEMBRANEMEMBRANEPURCHASEPURCHASE

22

33

44

11 1010

1010

1010

1010

41254125

37253725

37253725

37253725

1010

1010

1010

1010

1010

1010

1010

1010

0.30280.3028

5.00065.0006

3.0273.027

0.0490.049

1’253,0001’253,000

150,000150,000

1’710,9601’710,960

00

PROFITABILITY INDICATORS

OPTIONOPTION CONCEPTCONCEPT NET PRESENTNET PRESENTVALUE (DLS)VALUE (DLS)

INVESTMENTINVESTMENTEFFICIENCYEFFICIENCY

EFFICIENCYEFFICIENCYRATE (%)RATE (%)

INTERNALINTERNALRATE OFRATE OF

RETURN (%)RETURN (%)

PROFITABILI-PROFITABILI-TY RATETY RATE

(%)(%)

PAY OUTPAY OUTTIMETIME

(YEARS)(YEARS)

GAS LIFTGAS LIFTNETWORKNETWORK

NITROGENNITROGENINJECTIONINJECTION

WITHWITHSTORAGESSTORAGES

TRUCKSTRUCKS

MEMBRANEMEMBRANELEASINGLEASING

MEMBRANEMEMBRANEPURCHASEPURCHASE

22

33

44

11 44’380,05844’380,058

29,481,60729,481,607

40’575,59240’575,592

3636

10,00010,000

198198

2525

8282

347347

116116

7474

0.08650.0865

0. 01580. 0158

0.12810.1281

900900

539’353,083539’353,083

49954995

0.030.03

11551155

63106310

780780

678’996,700678’996,70021’221,89021’221,890 1.47 x 101.47 x 10-7-7

gas+lift+with+nitrogen+injection+generated+in+situ

Documents