otc19451multilateral wells to improve production performance in heavy oil reservoirs-the challenges...

8/12/2019 Otc19451Multilateral Wells to Improve Production Performance in Heavy Oil Reservoirs-The Challenges of the ZAM

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OTC 19451

Multilateral Wells to Improve Production Performance in Heavy-OilReservoirs: The Challenges of the ZAM-408ML WellD. Baldini, L. Tealdi, F. Okassa, L. Riccobon, D. Isella, A. Baioni, G. Obondoko, H. Malonga, F. Itoua Konga, M.Rampoldi, Eni Congo

Copyright 2008, Offshore Technology Conference

This paper was prepared for presentation at the 2008 Offshore Technology Conference held in Houston, Texas, U.S.A., 58 May2008.This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract

The Zatchi field, located in the Lower Congo Basin offshore, is a multi-layer reservoir of Cenomanian/Albian ageoperated by Eni Congo in partnership with Total Congo.The Zatchi B reservoir is 30 m thick sand characterized by a large accumulation of heavy and highly viscous oil(15 API, 1000 cP) trapped in the marine-transgressive sands of the Gres de Tchala formation. Three aspectsmake the successful development of such reservoir an extreme challenge: the presence of both bottom water anda gas cap, the very low reservoir pressure, the very high viscosity of the oil. As a matter of fact, over the 27-yearlife of the field, only three wells were put in production from the B layer, with not satisfying results due to theheaviness and viscosity of the oil and the severe problem of gas coning and cresting.The multilateral technology generally allows increasing the reservoir exposure with fewer wellbores, reducing andspreading the drawdown along the drains reducing the potential for coning. This technology has been proveneffective in several heavy oil deposits recovery worldwide. For this reason, in order to improve the reservoirdrainage reducing times and costs, the layer B was selected as the optimal candidate for the first multilateral wellin the Congo basin: well ZAM 408 ML.This paper will review the Zatchi B reservoir history and development challenges with a focus on the multilateralwell reservoir modeling, the TAML6 completion and artificial lift design. Furthermore, the challenges encounteredduring the operations in terms of reservoir properties sampling, operations geology, drilling, completions andproduction will be described in detail.

Introduction

Heavy oil has become an important theme in hydrocarbon industry with an increasing number of operators gettinginvolved or expanding their plans in this market around the world.

A huge number of non conventional oil reservoirs have been discovered worldwide, but only a small percentageof them is producing or is under active development.

Heavy oil represents a massive world resource, but the great challenge is to find the best way to produce,transport and process it.Eni Congo is taking part to this challenge, concentrating the efforts on the heavy oil of the Zatchi B reservoir.The Zatchi field is located in the Congo offshore basin, with water depth ranging from 55 to 57 meters and anareal extension of about 34 km

2(fig.1)

The field is characterized by multiple stacked reservoirs (from A down to E) belonging to the Gres de Tchala andCarbonates de Sendji formations (Cenomanian/Albian age).Like many other commercial reservoirs in offshore Congo, the Zatchi fied is characterized by sandy and dolomiticrocks deposited during the early drift phase in the mid-Cretaceous, with structural-stratigraphic traps created bymovement of the transition-phase Aptian salts (fig. 2 and 3).The Zatchi field was discovered in 1980 with the first exploratory well ZAM-1, which found oil accumulations in theCenomanian/Albian section.


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Since then, around 90 wells were drilled from four different platforms, mainly to produce from C, D and Ereservoirs.The A layer (gas bearing) was never taken into consideration because it was uneconomical, while the Breservoir, in spite of the interesting hydrocarbon accumulations, was targeted only by three wells mainly due tothe heaviness of the oil.

Reservoir B Description and production challenges

Sedimentological Model and Diagenesis

The Zatchi B is a shallow reservoir (average top @ -400 mTVDSS) of Cenomanian age characterized byhomogeneous tchickness of about 90 meters.The whole Albian-Cenomanian series of the Zatchi field is interpreted as transgressive complex of littoral bars thatdelimit areas with lagoon characteristics landward; inside the lagoon areas, there are some minor delta areas andzones characterized by deposition in typical low energy sand conditions, The complex of littoral bars is locally cutby tidal channels.

A larger deltaic system that cannot be recognized in the reservoir area is probably responsible for the letting in ofthe most of the clastic sediments in the area. These sediments, redistributed along the coast by littoral currents,give origin to the longshore bar complex.

From a lithological point of view, it is possible to observe a cyclic alternation of mainly sand and dolomite levels;this alternation respectively corresponds to periods of high and poor terrigenous supply in the main depositionsystem. In general, the main reservoir levels of Zatchi field correspond to significant regressive episodes in amain transgressive framework.

A schematic representation of the sedimentilogical interpretation of the deposits of Zatchi Reservoir B is shown infigures 2, 3, 4 and 5.

Production Challenges

The Zatchi B, although it is characterized by good porosity (> 30% in sandy facies) and permeability, in somecases is higher than 1 Darcy, is a very difficult reservoir to deal. In fact the extreme challenge is the high density(15 API) and viscosity (1000 cP) of the oil, the presence of both bottom water and gas cap and the very lowreservoir pressure.

Since 1980, Reservoir B was put in production only on three Zatchi well: ZAM 116, ZAM 406 and ZAM 111 ST.The production from these wells was soon stopped due to the early increase in GOR (Gas Coning effect).

The huge oil accumulation within the B reservoir induced the partnership to search for alternative-innovativesolutions to exploit the potential of this layer. On the base of this, the Pilot project ZAM-408ML was launched inorder to evaluate the potential for multilateral wells in such reservoir.

The ZAM-408ML multi lateral well design

Reservoir modeling and Geological aspect

The multilateral technology has been one of the most rapidly evolving over the last decade.It has been proven effective worldwide in terms of costs reduction, improving sweep efficiency (by delaying gas

and water breakthrough), facilitating drainage of heterogeneous reservoirs, increasing reservoir exposure andreducing coning effects.In few words, multilateral are wells in which a single hole (parent wellbore) is drilled to a pre-determined depth,then multiple branches are drilled out from the original wellbore. These laterals may extend in opposite directionsfrom each other in the same zone, or they may be drilled into different zones or formations.

The lesson learn from former wells put in production from this level has pushed to considering, at the projectimplementation fase, in opposition to what was previously done, to optimize the positioning of the well in order totake into consideration the artificial lift system which will be installed.Due to the high viscosity of the oil implementing the drilling of multilateral well will allow the increase the surfacecontact between the well and the formation which will therefore homogenize and reduce the drawdown apply tothe formation. In a nutshell, the major purpose is to enhance reservoir contact and thereby enhancing the wellproductivity.


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In practice, the well has been projected to be in the North Est part of the structure of the Zatchi where the gas capis localized far enough from the projected well. Each leg of the multilateral will be on opposite each other in orderto minimize interferences between them.The maximum length of each leg has been defined as been 1,380 ft.The presence of zones of low dolomite with low porosity, due to phenomenon of secondary dolomitizationoccurred close to the oil water contact (dolomite bacteria), have been taken also into consideration to optimize thewell trajectory (see FIG.5 bis).

Driling & Completion design

At first glance the multilateral option offers immediate advantages over single bore wells since it providesincreased reservoir exposure at a reduced cost. The critical component of a multilateral well is the Junction, whichis the interface between the main bore and the branches

Multilateral wells are classified into different forms or levels namely on the basis of the junction structure.The Level 6 Formation Junction run on ZAM-408ML consists of two adjacent tubular joints at the bottom end of aparent casing. The dual-leg junction is run in the hole at the bottom of the parent casing, with leg #1 in thecollapsed position. The junction leg was expanded downhole to their original outer diameter, using expandablemetal forming techniques. The system provides a wide range of completion options for the two branches.The purpose of the junction is to achive a structural link between a parent casing and the two lateral legs, whileproviding hydraulic isolation from the wellbore. During the forming process, the geometry of the junction iscollapsed to fit entirely inside a 13 3/8 68# L-80 BTC casing and only the ductile legs sustain plastic deformationwhile the stiffener remains underformedThree tubulars are used to build the indexing casing coupling. The assembly is essentially a short casing pup jointwith an orientating keyway and a locating profile in its inner upper diameter. The tools that need to be oriented

with respect to the junction outlets have an orienting key that rides the casing couplings orienting keywayThe drilling diverter is set in place and the 6 drilling BHA is run in the hole to drill each one of the two laterals.These drilling phases use standard techniques and equipmentBoth branches were cemented at the same time, leg #2 has cement shoe already installed when running in hole.

The ZAM-408ML mul tilateral well execution

The ZAM 408 ML was drilled from the ZAF-4 Platform by the SSD 18which is a Modular Offshore & fast MovingRig.The well spudded on April 8

th, 2007 and the rig was released 65 days later. The whole operations sequence is

described in the following paragraphs.


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Reservoir Evaluation While Drilling

The formation evaluation on ZAM-408ML well was entirely performed by means of Logging While Drilling (LWD).The main challenge related to this decision was to obtain a valid CPI in both 6 sections without the support ofwire-line logs, sparing on rig time and costs.

A Gamma Ray-Resistivity tool (GR-ARC) was run during the 12 section, mainly for correlation purpose. Thetop of the 9 5/8 formation junction assembly was set 3 meters below the top of B reservoir, in order to keep the

two drains as far as possible from the overlaying gas cap.A Density-Neutron tool (ADN) was added in both 6 legs, with the aim of computing a detailed formationevaluation.The final output was more than satisfying. The quality of the logs was very good for both 6 sections, allowing thepetrophysics team to compute a reliable CPI.From a reservoir point of view, all formation tops (including the top of Reservoir B) were encountered in accordwith the initial well prognosis.Reservoir B showed very good petrophysical properties (Poro Av = 26-30%, So Av = 84-86%) and a total Net Payof about 385 mMD for each leg, with corresponding Net/Gross ratio of 98%.Legs A and B were stopped respectively 12 and 14 verical meters above the OWC, in order to keep watersaturations as low as possible.

Drilling

Well schematics

Main Wellbore

The 26 phase was drilled to 184m and a 20 CP was landed and cementedA 12 Pilot Hole was drilled to 460m (63 angle) and the section was enlarged to 17 with a dedicated under-reaming BHA. The 13 3/8 casing was landed & cemented at 451m

The 12 slant well was drilled to measured depth of 760m MD (439m TVD 89 angle) using standardtechnology and drilling practices. A 70m (ROP: 5-8m/hr) Interval of the drilled section was under-reamed to 17 using standard under-reaming technology and procedures. After POOH under-reaming assy a clean out BHA with7 Bull Nose was run to where Formation Junction would be set

Landing & Orienting the 9 5/8 Formation Junction Assy

The Double Leg Junction plus the 9 5/8 casing was Run in Hole to Total Depth of 12 Section (751m BottomLeg # 1 and 685m Bottom Leg # 2). The orientation of the Casing String was done by Formation Junction Deviceand checked by MWD taking as a reference the casing collar locator

Swagging & Cementing the Formation Junction


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The Formation Junction Expansion Tool was activated and the lowest part of Leg # 1 was expanded with aWorking Pressure between 2,000psi and 3,500psi. The entire operation from beginning to the end takes 35min. Acement stinger assy was run to 724m and after sting-in a cement job was performed using double slurry: 1.9sgLead Gas Tight plus 1.97sg Tail Latex cement slurry. After Bump Plug the 1,800psi Pressure was held for 5min.This cement off both legs of the formation junction and drilling of the legs could now be performed once thecementing string was removed from the well.

Building and Completion of the Two Parents / Branches

The First 6 Section was drilled through Leg # 1 to 1,173m MD (449m TVD 86 angle) and after landed a 3 Poromax Screens below a 7 Packer, a cleaning acid job was performed. The Leg # 1 was displaced to filtered1.1sg BrineThe Second 6 Section was drilled through Leg # 2 to 1,104m MD (446m TVD 90 angle) and after landed a 3 Poromax Screens below a 7 Liner Hanger packer, a cleaning acid job was performed. Also Leg # 2 wasdisplaced to filtered 1.1sg BrineThe 2 7/8 Completion string was run in parent well with an ESPCP Gearbox mod. 9-1 Submersible Pump andboth strings were put on production without isolating the 2 zones ( fig.6).

Well challenges

The most challenging part of the project was starting from the 12 TD section:1) Enlarge the last 100ft to 17 and be sure to clean very well the hole just opened (If the hole collapsed

the entire project could be jeopardized)2) Run and land at the correct / required depth the Multilateral Junction without bend or break it3) Perform a correct cement job with a specific Plastic Slurry

All that above-mentioned was performed in the best way possible thanks to the Professional work done bypersonnel present in Eni Congo offices and who phisically perform / supervise the job on-board We have tokeep in mind that all the operations have been performed for the first time in Eni Congo successfully.

0

200

400

600

800

1,000

1,200

1,400

0 10 20 30 40 50 60Time (days)

Depth(m)

12 1/4" PH

26" Section

ESP Completio n

(Contingency 15%)

ZAM 408 ML - Time Vs Depth

17 1/2" HO

12 1/4" Section

6" Section

6" Section

17 1/2" HO


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6 OTC 19451

DATE:

SINGLE COMPLETION VERTICAL DEVIATED HORIZONTAL SELECTIVE

ICGP OHGP GRAVEL SIZE: 20/40 40/60

String weight up [ t ] Casing Size: [in] Type of packer fluid:

String weight down [t] Top: [m] Bottom: [m] Density: [kg/l]

Make up report yes [y/n]

SHEAR RELEASE VALUE: 60.000 -lbs.

SHEAR RELEASE VALUE: lbs.

Steel Gr. %

MAKE UP TORQUE : GREASE :

REF Tools PTH

ADJUST. SUB 3-1/2" HYD CS pin x pin

Thd S. Grade Top (m)

CSG/LIN OD

TOP AT mt PUMP 538 PM FER 6 MNL 118P31

SHOE AT mt SEAL SECTION Mod GSB3DB FER HL H6 AB PFSA

CEM UP mt GEAR BOX SERIE 525/562

LINER HANGER : CENTRILIFT MOTOR KMHTC 63 HP 721 V 53A

LINER HANGER :

LINER HANGER : LEG B

OD NOM

679.8

PUP JOINT 3-1/2" NUTop (mMD) Bottom (mMD O - RING SEAL SUB 3-1/2" NU X 2-1/2" ID

BLANK PIPE 3-1/2'' NU

TYPE OF CHARGE :

SHOOT DENSITY :

Top (m) Bottom (m)

BLANK PIPE 3-1/2" HYD-CS 1 jt

X-OVER 3-1/2" HYD-CS X NEW VAM

HALLIBURTON POROMAX SCREENS 3-1/2" N.VAM N

TD

41 BLANK PIPE 3-1/2'' NUO RING SEAL SUB 3-1/2" NU X 2-1/2" ID

1 098.18

3.500 1 096.862.500 3.500 1 098.03

NO -GO SUB BELOW

LEG # B NEW SC-1R 70B-40 PACKER

683.77

CASING SUB 5-1/2" SLHT box X 5" BTC pinCOUPLING 5" BTC Box x 5" VAM Box

5.500

CASING PUP JT 5-1/2" SHLT pin x pin

682.305.000

OPEN HOLE ZONE ZONE

35

2.992

2.990

Level

683.50

5.437

36

34

32

3031

Rig Supervisors

3.500

684.45

685.01

694.51

33

"B" / LEG B1104 X-OVER 5" VAM box x 3-1/2" HYD-CS pin

FRANCOIS POUNGA

Superintendant

2.992

2.9903-1/2" NU PUP JOINT 2.990X-OVER 3-1/2" NEW VAM box x 3-1/2" NU pin

2.990

2.990 3.5003-1/2" NU CIRCULATING SHOE

383940

29

LEG A

569.15

711.00

20"

684.46

F.J = Formation Junction top @ 673,55 m

for more details see Baker drawing

CASING

9-5/8"53,5

#L-80AMS

9- 5/ 8" A MS 53 ,5 / 4 7 L -80 W HD/ 668 66 8 / 6 73

7"/LE GB

H.MANTINOU/-PRADA/ELSAYED

N.VAM

OPENIG Press = psi Press = 600 psi

NOTES:

COMPLETION JOB PURPOSE

LEG # A SLPR HANGER

29 L-80 F.J 684

ID mm

182 K-55 AMS 106.5

3.900 695.0737

1 104.00

3.900 1 096.33

753 1173 "B" / LEG A 3.500

1 100.60

PERFORATED ZONE

28 New SC-1R 70B-40 W/5-1/2" 17# SLHT Box Down 4.00 6.010 679.76

NO -GO SUB ABOVE

TYPE OF GUNS :

ISOLATION METHOD

682.82

CMT RETAINER DEPTH SIZE

1 173.0027 3.50 1 171.30

3.50

3.50

CIRCULATING SHOE 3-1/2" NU26

3.50 1 168.0125Level24

Old Perforated intervals 231 166.84

X-OVER 3-1/2" N.VAM box x 3-1/2" NU PIN 2.99 3.9022 HALLIB. POROMAX SCREENS 3-1/2" N.vam N 67 jts

3 .50 7 58 .2 62.99 3.90 758.83

21 X-OVER 3-1/2" HYD-CS box x 3-1/2" NEW VAM pin20 BLANK PIPE 3-1/2" HYD-CS N 4jts 2.99 3.50

29 157.17"/LEG A 7537"/LE GB 684

L -8 0 N .V AML -8 0 N .V AM

9-5/8" 673 L-80 N.VAM

AMS 53.5 216.8

29 157.1

47 220.5

9-5/8" 668

42

2.99

2.99

2.992.502.99

L-80

13-3/8" 451 L-80 MS482.6

68 315.3

TO m STEEL THRD lbf/ft

568.73

16 L , CENT 2' ' L P,W/DOUB LE INJ CHECK VAL VE _ 8.20 569.15

15 _ 5.90

_ 5.13 565.83

14 5.13 566.23_

553.29

12 _ 5.13 563.73

20" 13- 3/8" 9- 5/ 8"

552.81

11 X-OVER 3-1/2" EU BOX X 2-3/8" PIN 2.44 2.88

3 .75 5 52 .3 1

182 451 673

10 UPPER CENT. 3-1/2EU P X BOX 2. 44 8. 26

9 X-OVER 3-1/2" EU pin X 2-7/8" HYD-CS box 2.44

7 2.44 2.88 542.502-7/8 # 6,5 HYD CS PUP JOINT

2.44 2.88 552.03

2.44 4.64 539.58DPG-TA' DOWN HOLE CARRIER 2-7/8 " HYD

TD

8 2-7/8 # 6,5 HYD CS TUBING L-80 N 1

13

CASINGS CHARACTERISTICS

17

WHD WHD WHD

2.44 2.88 536.60

7"/LEG A N.VAM 29 L-80 F.J 753

6Nom OD lb/ft Bottom (m)

PRODUCTION CASING

1 BREDA TBG-HGR 3-1/2" EU UP X3-1/2 hyd DOWN

4 2-7/8 # 6,5 HYD CS TUBING L-80 N 55

3.50 13.35TUBING HANGER 11'' X 2-9/16 EU U X VAM D.WELL HEAD 3 K CONVENTIONAL BREDA

2 2.99

3 2.44X-OVER 3-1/2" HYD-CS BOX x 2-7/8" HYD-CS PIN

11.8011 12.092.99BACK PRES, VALVE Size 3" Type TSB2

BONNET 11" x3-1/8" 3,000-psi RT SSD 18 TIE DOWN ELEVATION

WELL HEAD DESCRIPTION 2200 ftlb

X-MAS TREE 3-1/8" x 3,000-psi ID OD

API 5A-2

GEOG COORD LONG m6.5 L-80 7 552.00GEOG COORD LAT m 2,875" HYD

TOTAL DRILL 1500 MM/RT N om . O .D. T hr ea d l b/ ft Down toANNULUS FLUID 1.03-Kg/lt filtered S-Water TubingBRIDGE PLUG mMDORKB COMPLETION STRING

BRIDGE PLUG M/RT SIZE1500 mMDORKB

RT/TIE DOWN 11,8 mMDORKB 7" BKR SLP 4.42 711.0

TOTAL DEPTHWELL HEAD RATING 3,000-psi working pressure

Model type I.D. DepthNew SC-1R 4.00 679.76RIG USED SSD 18 7" BKR

DRILLING PERIOD juin-06 Nom. O.D. Manufact.

1.13

Well deviation [max.] @ m MD/RT Well deviation

GENERAL INFORMATIONS PACKER

11

11J5B9002

ESP

14 7" NACL

ARPO 20 / C Accoun t

ZATCHI

ENI CONGO WELL NAME ZAM - 408 ML

FIELD NAME

E & P7 juin 2007

SAND CONTROL ASSEMBLY

3.75 13.70

2 .88 5 34 .7 02.44

5 2-7/8 # 6,5 HYD CS PUP JOINT

5.96

5.565.441819

5"x7" 26-29# LINER HANGER PACKER BTC pin

CASING EXTENSION 5" BTCX-OVER 5" B TC box x 3-1/2" HYD-CS pin

4.42

4.40

711.00

715.40720.01720.33

1 166.32

1 168.17

COMPLETION SKETCH

Fig 6.: Well Schematic

Completion & Artificial Lifting Design

The multilateral well ZAM-408ML represents the fourth well which is attempting the production of such reservoir.

The well architecture and trajectory was designed to optimize reservoir recovery, productivity index while reducing

the drawdown at the wellbore to avoid gas coning phenomena. In parallel, it has been decided to develop an

innovative system to improve the ESPCP performances by increasing and homogenising the temperature

distribution around the artificial system planned to be installed. The original scope was not to improve the well

productivity, but only to have a better handling of the flow at the pump interface and maximizing the pump

efficiency.

ESPCP are pumps made especially for ash environment or handling high viscous fluids but handling a viscousfluid of 1300 cp or 500 cp is not the same thing. An ESPCP will react better handling a fluid with 500 cp than 1300cp if we consider the same equipment (see table 1 and graph.1).


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OTC 19451 7

Frequency Freq Hz 44.2 45.1 47.5 52.5

Dead Oil Viscosity@BHT DOilVisc Cp 502 1004 1507 2009

Total DynamicHead TDH FT 2093 2275 2449 2620

Viscosity Pumpintake VISin Cp 333 649 959 1266

Viscosity Pumpdischarge VISout Cp 228 434 636 837

Motor Load MtrLoad % 57.41 62.69 67.02 70.68

Pump Efficiency PmpEff % 64.83 61.59 57.56 51.58

Table 1: relationship between the increase in fluid viscosity compared with the pump efficiency &Motor load at a given speed. The flow rate is kept constant at 610bfpd.

T

Data from this level show a viscosity of 1343 cp @ 34.5C (reservoir temperature)

Graph.1: Relationship between the decreases in viscosity compared with the temperature.

Therefore, to improve the pump efficiency, one solution can be reducing the oil viscosity surrounding the ESPCPin order a have a lighter fluid at the pump suction section. The fluid temperature distribution around the

electrical motor could give important clues on this perspective.An electrical motor immerged in a viscous oil put in motion by a pump in this upper part of the system induces athermal propagation due to conduction and forced convection.The tool has been made of different pieces of sleeve with external fins. The geometry of the fin has been made ina way to give them an edge shape. A total of 11 external fins per sleeve.The number of fins has been chosenaccording to the minimum fluid velocity in order to cool the motor (1ft/sec is the recommended value from themanufacturer). A total of 5 sleeves have been threaded along the motor (this association of sleeves with fins iscalled Motor Jacket). Each fins sleeve positioning has been set in such a way to allow the distortion of the fluidin motion. To avoid any motion of the sleeve, eight (8) screws were used to tighten up along the sleeve withoutimpacting on the motor internal structure.Therefore the Motor Jacket has no impact on the motor structure.It is

just a heat exchanger. The Motor Jacket is able to protect the motor lead extension (MLE) during the running ofcompletion (Fig7).

Temperature VS Viscosity

0

200

400

600

800

1000

1200

1400

1600

3 0 40 5 0 60 7 0 80

Temperature (C)

Viscos

ity(Cp)


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8 OTC 19451

Fig.7: Motor Jacket

The thermal source is coming from the motor external temperature. Principally we are using the motor inefficiency

or in other words, the Joules effect losses. I could be noticed that the more the motor is inefficient, the higher will

be the motor external temperature. Hence, to have a sufficient thermal source from the motor.The solutions could

be playing with the motor inefficiency or reducing the fluid velocity in order to reduce the cooling of the motor.

Pratically, it is nessesary to install an oversized motor (ranging from 20 to 30% HP extra) although at the same

time making sure not to shorten the motor run life and also avoiding electrical insulation losses of the downhole

motor.In June 2007, the well ZAM-408ML has been completed with an artificial lift system composed of:

- Electrical submersible progressive cavity pump (ESPCP)- Gas separator,- Seal section- Motor shrouded by Motor Jacket

The well is now producing an average fluid rate of 760 bfpd, with 570 bopd of oil production (see graph.2).

ZAM /408ML B HOR

300

350

400

450

500

550

600

650

700

750

800

1-juin-07 16-juin-07 1-juil.-07 16-juil.-07 31-juil.-07 15-aot-07 30-aot-07 14-sept.-07 29-sept.-07 14-oct.-07 29-oct.-07 13-nov.-07 28-nov.-07

tot&

oil

0

10

20

30

40

50

60

70

80

90

100

W

C

Qt ot [ b fp d ] Qh ui l e [ bo p d] WC[ %]

0

100

200

300

400

500

600

janv-

07

janv-

08

janv-

09

janv-

10

janv-

11

janv-

12

janv-

13

janv-

14

janv-

15

janv-

16

janv-

17

janv-

18

janv-

19

janv-

20

Dbitjournalier[BOPD]

GOR lim = 250 Sm3/Sm3

NO GOR limit

GOR lim = 1000 Sm3/Sm3

Graph.2: well production history Graph.3: .Production forecast


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OTC 19451 9

Such results are very encouraging since they are above the production forecast (see Graph.3).In thispilot project of the first Motor Jacket these main results are observed:

The pump hydraulic efficiency obtained is 70% instead of the 35% on the well ZAM-406 and 30% onZAM-116.

The temperature increment of the fluid given by the motor shrouded with the Motor Jacket is of an

average of 5C which implies that the thermal propagation expected is positive for the ESPCP MotorJacket application.

The ZAM-408ML multilateral well production performance

A 56hour hour build up of the well has been done in order to evaluate precisely the well potentially. The analysishighlights showed the following results:

The pressure derivative trend is similar in its final part of a normal single drain well having 5000 ft longlength (see graph.4).

The skin factor is equal to zero

The productivity index is 6.7m3/d/bar.

Graph.4: Log-log plot of the pressure and the derivative

With respect to the former producing wells from this level, the PI is twice bigger which shows the positive resultsin terms productivity index of the ZAM-408ML TAM6.

The Zatchi B potential

At the present time an ongoing study is being done in order to increase reservoir recovery factor. In fact, the levelB has an estimate OOIP of 435 MMstb and a GOIP of 1262 MMsm

3for only a total volume produced of 370,000

stb @ 2005. It has been evaluated that the maximum recovery factor with a traditional method will not go beyond2%. Hence, the study was oriented on the EOR methods which will allow to have, as objective, a minimumrecovery factor of 10%.

10

100

1000

0.1 1 10 100

PressureChangeandDerivative(psi)

Elapsed time (hrs)

Log-Log Match - Flow Period 2

Effective length

Distortion due to interference b/w drains


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10 OTC 19451

The actual EOR methods which are under instigation are the following:

Thermal methods: CSS CSS + gas injection in-situ combustion Steam assisted gravity drainage

Chemicals:

Polymer injection VAPEX

Complex well geometry (CHOPS)

All these different options will be addressed by taking into account the cost issues learned from previous studies.Since the best hope for recovery of this viscous oil appears to be a thermal application, a special care will betaken on the core analysis with special analysis on rock compressibility, wet ability, thermal capacity andtransmissibility. In addition, the fluid sampling and pressure testing of the B zone will be performed in order tohave a clear view on viscosity, barrier location and depletion. This study will increase the field perspective.

Conclusions

With the purpose of optimizing heavy oil production from Zatchi B Reservoir, ZAM 408 ML was the first successful

multilateral well ever drilled in West Africa.The most critical drilling challenges were faced with extremely satisfying results and the whole operationssequence from spud until rig release were performed in only 65 days.The formation evaluation process, performed by means of Logging While Drilling technology, allowed thecomputation of a reliable CPI which pointed out very good reservoir properties in both drilled legs (high porosityand oil saturations).

An innovative system called Motor Jacket was also installed in order to optimize the artificial lift system.The first production data are encouraging, the well performances are satisfying and the TAML6 technology allowsmitigating premature water and gas breakthrough, or coning.The ZAM 408 ML well opens new development perspectives for the Zatchi B Reservoir and the ongoing studiesshould support the optimization of the heavy oil recovery, with new dedicated wells to be drilled in the near future.

AcknowledgementsThe authors wish to thank the Eni Congo management, its different technical departments for permission andencouraging the implementation of the ZAM-408ML project.We would also like to thank everyone who has contributed to the said project with their constructive comments.

References

1. Fluid mecanics, L.Landau, E. Lifchitz Mir edition Moscow.2. webb,Eckert, ERG and golstein, JR. heat transfert and friction in tubes with repated-rib roughness, int

J. heat and Mass transfer, vol 14, pp601-617, (1971)3. Gee, DL, , and Webb, RL, Forced convection heat transfert in helically rib-roughened tubes, Int,

J.Heat and Mass Transfer, vol.23 pp1127-1136, (1980)

4. Garimella, S., Chandrachood, V., Christensen, R.N and Richards, D.E, Investigation of heat transfer andpressure drop augmentation for turbulent flow in spirally enhances tubes, ASHRAE transactions, Vol. 94,Part 2, pp. 1119-1131 (1988).

5. Farina, A. and Palgiarini, G., un apparato per lanalisi sperimentale dello scambio termico convettivo incondotti con parete corrugata, atti del XII congresso Nazionale sula trasmissione del Calore, pp.151-162,LAquila, 23-24 giugno 1994.

6. Piero De biase, Andrea Fedullo, Nazmul Hug, Zatchi Marine field Layer B (November 2005).7. Sada D.Joshi, Horizontal well technology.8. A. Banioni, D. Baldini, R. Riccobon; Drilling and completion program of ZAM-408ML (Eni Congo).


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Fig. 1: Zatchi Location Map: the field is located in the Fig. 2: Schematic stratigraphic chart showing theCongo offshore, with water depth ranging between 55 gross litho logical compost ion of the Zatchi field.and 57 meters .

Fig. 3: Schematic cross section highlighting the main tectono-depositional domains of the West African margin. The lowersedimentary sequences are strongl y deformed by salt tectonics w hile the upper ones are relatively undeform ed or characterized bya low tectonic contro l (from Brownfield and Charpentier, 2006)

Zatchi Field

ZatchiZatchi


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Fig. 4: Facies Sequence of the typical Zatchi w ell, with GR and Dendity/Neutron response.The B Reservoir is characterized by the prevalence of sands alternated with thinner dolomit icLayers.

Fig. 5: Deposition Model for Zatchi field. The serie isinterpreted as a transgressive complex of littoral bars that delimitareas with lagoon characteristics landward


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Fig. 5 bis: NS Cross section passing along the two legs of ZAM 408ML well. The top of the 9 5/8 Multiju nction assembly was set 2.5meters below the top o f B reservoir in order to keep the 2 drains as far as possible from the gas cap. Leg A and B endedrespectively 12.5 and 14.5 above the OWC.

OWC -434

BLayer

N S

20 CSG @ -155

13 3/8 CSG @ -380

9 5/8 multijunction Ass. @ -409

LEG A / TD -421.5 LEG B/ TD -419.5

Top B @ -406.5


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Composite showing the logs recorded on the LEG A of ZAM 408 ML (6 phase). Record of the Resistivi ty and Density/Neutron whiledrilling.The overall qualit y of the logs is good and allows the computation of a reliable CPI (last three tracks of the composi te).The B Reservoir characteristics are excellent, with average porosity around 26% and oil saturation hi gher than 80%..In this reservoir section o nly Sand seems to be present with negligible amount of Carbonates


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Composite show ing the logs recorded on the LEG B of ZAM 408 ML (6 phase). Record of the Resisti vity and Density/Neutron whiledrilling.The overall qualit y of the logs is good as in LEGA allowing the compu tation of a reliable CPI (last three tracks of the composit e).The B Reservoir characteristics are excellent, with average porosity around 26% and oil saturation hi gher than 80%..In this reservoir section o nly Sand seems to be present with negligible amount of Carbonates

otc19451multilateral wells to improve production performance in heavy oil reservoirs-the challenges...

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