PETROLEUM SOCIETY OF CIM PAPER 98-73

DOWNHOLE HEATING DEVICE TO REMEDIA TENEAR- WELLBORE FORMA nON DAMAGE

RELATED TO CLAY SWELLING AND FLUID BWCKING

A.K.M. Jamaluddin'Noranda Technology Centre

! Presently wid1 Hycal Energy Research Laboratories Ltd

S.A. Mehta and RO. MooreUniversity of Calgary

This paper is to be prcscntcd at die 4~ Annual T cchnical Meeting of The Petroleum Society of aM in Calgary. Alberta,Canada, June 8-10,1998. Discussion of this paper is invited and may be presented at the meeting if tiled in writing with thetechnical program chainnan prior to die conclusion of die meeting. This paper and any discussion filed wi" be consideredfor publicatim in CIM journals. Publication rights arc reserved. This is a pre-Print and is subject to ~m.

ABSTRACT power input and the carrier gas flow various desirabletemperatures can be achieved.

It is a well-known fact in the petroleum industrythat thermal treatment (wellbore heating) could resultin significant benefits to the producing wells byremedyingfluid related problems. Recently, it has beenshown in the literature that thermal stimulation alsoimproves fluid flow characteristics of the near-wellbore

porous region.

INTRODUCTION

Conventional hydrocarbon producing wellsencounter various problems during their production life.These problems are either related to fluid characteristicsor related to fonnation rock characteristics. Fluidrelated problems are primarily related to deposition ofwax and asphaltene materials and creation of water-m-oil emulsion and these problems can occur in thewellbore tubulars and accessories (i.e. surfaceequipment, wellbore pUDlps, pumping rods etc.). Inaddition, these problems could also occur in the nearwellbore regions of the rock. The reservoir rock relatedproblems are primarily caused by fluid invasionresulting in clay swelling and fmes migration.

To conduct wellbore and near-wellbore thermaltreatments, a simple retrievable electrical downholeheating device has been designed, constructed; andtested The downhole heating system contains aseparate heating chamber, wiring chamber, and coolingchamber. The cooling chamber is inserted between thewiring chamber and the heating chamber to preventheat propagation from heating chamber to the wiringchamber. 77Ie heating is carried out at the surface totest the heater's capability. An inert gas at the ambienttemperature condition is injected through the tubing andheated to the desirable temperature as it passes throughthe heater. Results indicate that by controlling the

Acid treabnents are very popular in remedying theproblems related to formation rock characteristics,especially, for carbonate reservoirs. However, acidtreatments have had mixed success in remedying

PAPER 98-73

pump has also been proposed earlier.I'1n diis design. ifdie heater is powered dieo pressurized water is directedthrough die heater and to die fonnation where it will cutaway at the rock fonnation and thermally stimulate diewell. If the heater is oot activated then the pressurizedwater is to turn a turbine and assist in die downholepumping of production fluids. The use of pressurizedwater also prevents the heater from overheating andburning out the elements. The method should preventheat losses along dte pipe from pumping steam from thesurface.

fonnation rock-related problems in sand stonereservoirs.

Thennal treatment (weUbore heating) could resultin significant benefits to the producing wells byremedying fluid related problems. Recently, it has beenshown in the literature'.2 that thermal stimulation alsoimproves fluid flow characteristics of the near-weUboreporous region. The foremost application for heating aformation has been to reduce the viscosity of the oil,specifically heavy oil!-'J A second importantapplication for heat treatment has been the prevention orremoval of waxes or asphaltenes build-upJ.4.6.14.ls.'6. inthe weUbore and near-weUbore region. Other benefitsresulting from thermal treatments include claydehydration. J thermal fraCbJring at high temperature,J

prevention of thermal fraCbJring in water zones at lowtemperaturesll and sand consolidationl7 inunconsolidated formation. In the case of downholeelectrical heating some of the current may be diverted toprevent the corrosion of tubing, casing, pwnp rods andother downhole components and to prevent build-up ofcorrosion products.

A heating technique based on supplying electricalpower at the thermal hannonic frequency of theformation is proposed by Gill and GiWo. The designconsists of converting the three-phase AC power to DCand then chopped to single phase AC at the hannonicfrequency. The ham1onic frequency heating occurs inaddition to the normal ohmic heating.

A fuel-fired downhole radiant heater has beenproposed by Hemsath9. In this design, the well is firstflooded with water from an injection well to a desiredpressure then the fuel-fired heater is ignited to heat thefonnation and water_The heater consists of threeconcentric cylindrical tubes. A burner within theinnennost tube ignites, and bums a source of fuel andair_Apertures are sized and positioned to developlaminar flow of the combustion products from d1eburner such that the heat transfer is effective along itsentire length- The combustion products are removedfrom the annular space between the two outer tubes.The design of the heater minimizes local hot spots andshould heat the reservoir evenly.

Most downhole heating devices have been designedto apply to combustion or downhole steam generationpurposes. A downhole electric heater is designed to useto ignite the in-situ fuel3 The heater is removed and airis supplied to maintain a combustion front. An ohmic4heating system has also been applied for an oil, whichhad a viscosity of 160 mPa.s. A low-frequency electriccurrent is projected into the formation returning to thecasing. The electric power caused ohmic heating whichdissipated as the inverse square of the distance from theelectrode.

A downhole steam generator involving combustinga fuel downhole is described by Meeks and Rhodesl9.The heat generated during combustion heats an array ofwater tubes converting the water to steam. The steam isinjected downwardly into the borehole. The combustionproducts are retrieved from the annulus between theheater and the casing. A packer force the steam into thefonnation preventing it from mixing with thecombustion products.

Various downhole heating systems have beenproposed in the literature. Among these heating systems,a downhole heater design which uses the casing ortubing as electrodes has been proposed in the literature.6In this design, one electrode is aligned with the payzone. The opposite electrode exists outside the pay zoneand preferably at least three times the diameter of thehole away from first elecb"ode. For cunent to pass fromone elecb"ode to the other it must pass through the payzone. The current will be carried either by a conductiveformation or by the water in the formation. The highresistance to CUn'ent flow will result in local ized heating.

A downhole steam generator specifically designedfor horizontal wells has been described by Meshekow.12A chamber having water in the bottom is heated with

electrodes to form steam above the water. The steam isdischarged into the formation.A combination of downhole heater with a water

PAPER 98-73

An apparatus for use in a horizontal weU but forproducing tar sands has been discussed by MimS.20 Thefirst step involves a preheating period to get the bitumento such a temperature d1at it will flow. Then steam isinjected through perforations at the toe of the horizontalweU only. The steam wiU gravitate upwards andtowards me perforations in the production (heel) end ofthe horizontal well. The injection and productionsections of the horizontal well are separated from eachother by a packer. As the toe end becomes depleted thepacker can be moved toward the well head to stimulateadditional sections of the reservoir.

with one end of each of heater elements being commonand neutral. They also supplied DC current to theheater.

Van EgmondD advises using a copper-nickel alloycore cable for downhole heating. This 6% by weightnickel alloy is less prone to failure because it has a lowtemperature coefficient of resistance. It maintains thelow resistance of copper. The cable is capable ofwithstanding temperatures to lOOO°C and utilizingvoltages to 1000 volts. The cable is especially usefulfor heating long intervals. Van Egmond2J describes aheater using this material which is cemented into anuncased borehole. The savings on casing stable to high-temperature corrosion would reduce the cost ofinstallation considerably. The heater can provide heat toabout 250 watts per foot of length.

A downhole packed-bed electric heater has beendescribed by Nenningerl4. The heater comprises twoelecb'ode which are displaced from each other. The gapis filled with conductive balls. Resistive heating occurswhen current is passed through the heater. The multiplepaths of current flow through the heater prevent failureof the heater due to element burnout. The heaterprovides a large surface area for heating whilemaintaining a low pressure drop between the inlet andoutlet of the heater. The length and diameter can beeasily adjusted to satisfy well design and heatingrequirements. F onnation heating is achieved by passinga solvent through the heater which is heated up, passesinto the formation and transfers dte heat to dtefonnation.

The present design is concerned with an electricaldownhole heating system for formation heat treatment.The heater contains separate wiring chamber, heatingchamber, and cooling chamber, the latter being insertedbetween the wiring chamber and the heating chamber.The heat treatment is carried out by inserting the heaterin a borehole to be treated. A gas, preferably nitrogenor air, is brought to the heater with a hose or tube. Thegas goes through the wiring chamber and coolingchamber, and is heated by following a tortuous path inthe heating chamber before it is expelled from theheater.Rice et aI.4 describe the process required to heat up

a reservoir slowly to avoid hot spots and materialproblems. They ran a slotted fibregiass liner (rated to150 °c) with a 5m long steel elecb'ode such that theelectrode terminated opposite the producing zone.Electric power at a frequency of 27 Hz and maximumcurrent of 400 A was conducted through an armourinsulated 1" -diameter copper cable. Power losses in thecable were estimated to be in the range of 10-20%. Adownhole electric heater has been used by Stahl et aptto covert hot water to steam. Instead of producing stearnon the surface and pumping it downhole, they suggestheating water on the surface, pump dris downhole wherean elecbic heater converts the hot water to steam. Theelectric heater is a series of V-tubes disposedcircumferentially around the water injection tube. EachV-tube can be individually controlled. The injectiontube is closed at the bottom with orifices displacedradially. Water flows out the injection tube and past theheater tubes where it is vaporized. Electric power issupplied via a three-phase grounded neutral "V" system

FORMATION HEAT TREATMENT CONCEPT

The formation heat treatment (FHT) processinvolves the application of heat for the treatment ofnear-wellbore damage. The FHT process. 1,2 consists of

exposing the fonnation to an elevated temperature tocause:

>- vaporization of blocked water,>- dehydration of the clay structure,>- partial destruction of the clay minerals, and>- possibly, micro-fracture of the fonnation in

the near-wellbore area due to thenna1 inducedstresses.

The dehydration and vaporization of bound andblocked water occur at temperatures higher than thesaturation temperature corresponding to the reservoirpressure. The extent of clay destruction also depends onthe heating temperature.

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PAPER 98-73

The electric downhole heating system designed forthe FHT process is particularly suitable for stimulatingthe production of oil and gas formations containing claymaterials and those formations that are susceptible tofluid blockage. Other uses include in situ steamgeneration, initiating in situ combustion, near-wellboreheating for heavy oil viscosity reduction, stimulation ofwater injection well, near-wellbore emulsion breakingsetc.

which are resistive, conver1s electricity to heat in theheating chamber.

Each heating element is inserted in a tube, which isconnected to a heater extension with bolts. The heaterextension is also made of dielectric material. so that verylittle heat, if any, is transferred from heating chamber orheating element to cooling chamber and wiring chamber.The heater extensions are combined by groups of threein wiring chamber to form three wires which areconnected to an appropriate power source at the surface.OFDETAILED DESCRIPTION

DOWNHOLE HEATING DEVICETHE

The heating elements are connected to the outersurface of the intermediate tubing. The heating chamberis equipped with a thermocouple to monitor thetempeniture at each end of each chamber.

A schematic diagram of die downhole heatingdevice is presented in Figure I. As seen in Figure I, thedownhole heater having a wiring chamber, a coolingchamber and a heating chamber, contained in an outershell. The chambers are threaded for joining themtogether. The heater is closed at one end with a cap andis provided widi a connector at the opposite end. forconnection with any conventional tubing means,including coiled tubing, used in the oil and gas industry.The connector has a centered channel extendingdtroughout its length and emerging into production tube,which is inserted in heater and extends through thewiring chamber, the section of the pipe in die coolingchamber being cut and removed.

A closer look at the heating chamber shows that theintermediate tube has one end closed while the other endis also closed by the metal slab that connects the heatingchamber to the cooling chamber. However, just belowthe upper connection, slots are being cut to allow thepassage of gas from the first annular space to the secondannular space. To insure that the gas is unifonnlydispersed. the slots should be distributed at regularintervals at the same end around the intermediate tubing.The outer shell also has slots to allow even disbibutionof hot gas radially from the heater to the sandface.Spacers are placed in between the inennost tubing andthe intermediate tubing and also between theintermediate tubing and the outer shell. Spacersmaintain the pipes in place.

The heating chamber is comprised of threeconcentric pipes. The central pipe is open at both endsand the intennediate pipe is closed at the bottom end andconnected to the metal block separating the heatingchamber from the cooling chamber. Slots are cut in theupper part of the intermediate tube for gas to flowthrough. The outer tube is the continuous shell thatextends from the wiring chamber to the heatingchamber. The bottom part of the outer shell is closedand slots are cut at the bottom of the outer shell for thehot gas to exit the heater.

In operation, as illustrated in Figure 2, the heater islowered in wellbore provided with a conventionalinternal metal casing, in the area of the zone of interest,heating elements are heated and the hot gas, preferablynitrogen. is injected from the surface, generally anitrogen truck if the gas is nitrogen, in the productiontube. Since the section of pipe has been removed fromcooling chamber, the gas is allowed to flow freelytherein and act as a coolant As the gas enters theheating chamber, its temperature starts to increasebecause of the presence of heating elements on thesurface of the intennediate pipe. The gas follows thetortuous path indicated by the arrows before beingexpelled from die heater dlrough the openings at dieouter shell at the desired temperature. Such tortuouspath provides adequate residence time for die gas to heatup at the desired temperature. The ability to manipulate

Conventional heating elements are briefly desm"bedas follows: Each comprises a first section made of twowires of nickel extending from the wiring chamberthrough the cooling chamber. The second section is inthe heating chamber and comprises two wires ofINCONEL electrically connected to the wires of nickel.Both sections are contained in a casing filled with adielectric materia1like magnesium oxide. The resuh isthat little heat is generated in the cooling chamberbecause of the nickel wires, while the INCONEL wires,

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PAPER 98-73

the gas flow me at die surface also allows flex.ibility ofdie gas residence time widiin die heating chamber. Itshould also be noted that nitrogen is also injectedthrough dte tubing-casing annulus to maintain a positivepressure downward, so that the heated gas isconcentrated in die zone of interest, dius reducing theheat losses to die top of die zone (Figure 2).

If one heating element fails, the heater maystill be operated at lower power; there istherefore no need to retrieve it from thewellbore;It may be used in harsh wellbores, whichcontain brine, oil and gas.

All the pieces of the present heater are made ofstainless steel, except for the heating elements and theheating extensions, which are sealed in INCONEL 600sheets.

Each heating element has a power of 7.2 kW. Inthe heater, 9 heating elements are used, thereforeallowing a total power of the equipment of65 kW. Theheating elements are connected by groups of d1ree inparallel connections, so that if one group fails, the heaterwill still be able to operate with six elements.

SURFACE TEsnNG OF THE DOWNHOLEELECTRICAL REA TING DEVICE

The electrical downhole heater design presented inthis paper was constructed and tested on the surfaceseveral times. To emulate the field condition during thesurface testing, the heater was hoisted vertically andplaced inside a 12.7 cm (5") casing. Severalthermocouples were attached to the heater to monitor theheat transfer characteristics. The thermocouplelocations are indicated by numbers in Figure I. Severalthermocouples were also attached to the enclosingcasing to monitor the heat distribution around the outersurface of the heater. Nitrogen gas was use to flowthrough the heater and also through the heater-casingannulus.

Gases suitable for injection in the above heaterinclude air, oxygen, methane, steam, inert gases and thelike. Inert gases are preferred, nitrogen being the mostprefened. The flow rate of gas may vary from 5000m3/dayto 57,000, or higher, m3/day (standard conditionsof 15°C and 1 atm). Accordingly, a 65 kW power anda nitrogen flow rate of about 10,000 m3/day wouldcorrespond to a temperature increase of up to 800°C. Atemperature above 600°C is generally sufficient for theapplications of the present electric heating system. It isthus possible to control the tempel'atW'e both by varyingthe flow rate of gas, or by regulating the power output.

Before reaching the heating chamber, the injectedgas is at ambient temperature, and cools the wiringchamber and the cooling chamber, thus avoidingundesirable overheating in these chambers. The wiringchamber is also preferably fluid sealed to peTnlit theapplication of the heater in any environment in thewellbore, such as water, oil, gas and mixtures. Formaterial safety issue, the heater should include anautomatic shutoff system to cut the power off andprevent overheating of the cooling and wiring chambers.

During the surface testing. the heater was poweredup slowly inorder not to provide a sudden electricalshock to the heating elements. The power up sequenceis presented in Figure 3. As seen in the figure. initially,the power was controlled by variac. After reachingabout 90 kW power the power control was switched toautomatic Halmar controller and sequentially power wasincreased to about 45 kW. This power corresponds toa current of 42 Amps and a voltage of 630 Volts.

The temperature of the gas exiting the heater ispresented in Figure 4. As seen in the figure, the exit gastemperature increased slowly as the power wasincreased slowly. The heater temperature reached thehighest temperature of720 DC corresponding to a powerof 45 kW and a total nitrogen flow rate of3.0 m3/min ataround 180 minutes after the experiments started.Subsequently, dIe power was cut off and the nitrogenflow rate was increased to cool the heater quickly. Theaverage exit gas temperature was calculated using theaverage gas properties and the calculation indicates that

The total length of an electric heater according tothe present design and illustrated in Figure I is about462 cm (182"), 3/4 of which being the length of theheating chamber, and the wiring and cooling chambereach representing 1/8 of the length of the heater. As thediameter of deep wellbores generally does not exceed 12cm (5"), the diameter of the heater should be around 8-9cm (3.5") to facilitate its introduction and positioning.

The design of dte present elCCbic heater has several

advantages:

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PAPER 98-73

Hancock, J. Neurorf, and B. Miles of the University ofCalgary. We would also like to convey our specialthanks to R.G. McGuffm of Stimulus for his valuablecontribution in designing the tool.

the exist gas temperature should have been 748°C. Asexpected, the measured exit gas temperature was lowerthan the calculated value and this is because of the heatlosses. Thermocouple #14 (shown in Figure 4) waslocated at the inner side of the casing opposite the exitgas apparture of the heater. REFERENCES

Jamaluddin, A.K.M., Vandamme, L.M., Nazarko,T.W., Bennion, D.B., "Heat Treabnent for Clay-Related Near WeUbore Fonnation Damage",Journal of Canadian Petroleum Technology (in

press).

Temperatures of junction box (thermocouple # 1),cold box (thermocouple #2), outside of the junction box(thermocouple #8), and thermocouple #12 which waslocated across the cold box are presented in Figure 5.As seen in the figure, the temperatures in thesethermocouples did not increase over 50°C.

Jamaluddin, A.K.M., Hamelin, M., Harke, K.,McCaskill, H., Mehta, S.A., "Field Testing of theFormation Heat Treatment", paper presented at the47th Annual Technical Meeting of the PetroleumSociety of ClM, Alberta, Canada, June 10-12,1996.

2.

Temperatures in the heating chanbers at variouslocations are presented in Figure 6. As seen in thefigure, gas temperature at the top of the heating elementis low in the second annular space. This is because ofthe fact that thermocouple #3 was closed to the cold gasentry to the heater. It is also interesting to note that asthe gas goes down the second annular space, it reacheshigher temperatures (thermocouple #5) and slightlycools down at the close to the exit (thennocouple # 6).This is also because the bottom part of the heatingelement directly contacts the cold gas entry from thefirst tubing to the first annular space.

White, P.D. and Mass, J.T., "High TemperatureThermal Techniques for Stimulating Oil Recovery",Journalo/Petroleum Technology, September 1965,pp 1007-1015.

3;

Rice, S.A., Kok, A.L. and Neate, C.J., "A Test ofthe Elecbic Heating Process as a Means ofStimulating the Productivity of an Oil Well in theSchoomebeck Field", paper presented at the 43rdAnnual Technical Meeting of the Petroleum SocietyofCIM, Calgary, Canada, June 7-10,1992.

4.

SUMMARY

To conduct wellbore and near-wellbore thermaltreatments, a simple retrievable electrical downholeheating device has been designed, constructed, andtested. The downhole heating syStem contains aseparate heating chamber, wiring chamber, and coolingchamber. The cooling chamber is inserted between thewiring chamber and the heating chamber to prevent heatpropagation from heating chamber to the wiringchamber. The heating is carried out at the surface tosest the heater's capability. An inert gas at the ambienttemperature condition is injected d1rough the tubing andheated to the desirable temperature as it passes d1roughthe heater. Results indicate that by controlling the powerinput and the carrier gas flow various desirabletemperatures can be achieved.

Bridges, J.E., Dubiel, G.T. and Bajzek T.J.,"Corrosion Inhibition Apparatus for DownholeElectrical Heating", U.S. Patent 4,919,201, April24, 1990.

5

Bridges, JoE., Bajzek, T.J., Hofer, KoE., Spences,H.L., Smith, L.G. and Young, V.R., "RobustElectrical Heating Systems for Mineral Wells",U.S. Patent 5,070,533, December 3,1991.

6.

Bridges,J.E. and Dubiel, G.T., "Power Sources forDownhole Electric Heating", U.S. Patent5,099,918, March 3, 1992.

,.

ACKNOWLEDGEMENTReed, M.G., "Method for Inhibiting SilicaDissolution Using Phase Separation During OilWell Steam Injection", U.S. Patent4,913,236,April3,1990.

8.

The authors wish to thank Noranda TechnologyCentre for permission to publish this work. We alsoacknowledge the contribution of M. Ursenbach, M.

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PAPER 98-73

9, Hemsath, K.H., "Enhanced Oil Recovery Systemwith a Radiant Tube Heater", U.S. Patent5,020,596, June 4, 1991.

4,508,172, April 2, 1985

21. Stahl, C.R., Gibson. M.A. and Knudsen, C. W.,"Thennaliy-Enhanced Oil Recovery Medtod andApparatus", U.S. Patent 4,694,907, September 22,1987.

10. Gill. W.C. and Gill, H., "Technique for ElectricallyHeating Fonnations", U.S. Patent 4,951,748,August 28,1990.

22. Van Egmond. C.F.H., "Heater Utilizing Copper-Nickel Alloy Core", U.S. Patent 5,060287, October22,1991.

11. Sanchez, M.J., "Wellbore Hearing Process forInitiation of Below Fractue Pressure SteamStimulation from a Horizontal Well Located in anInitially Immobile Tar Sand", Canadian Patent2,026,483, April 12, 1991.

23. Van EgmODd, C.F.H.. "Subterranean Heater" U.S.Patent 5,065,818. November 19, 1991.

12. Meshekow R.L.. "Horizontal Steam Generator forOil Wells". u.S. Patent 5.142.608. August 25.1992.

13. Glandt, C.A., Vinegar, H.J. and Prats, M., "Methodof Producing Tar Sand Deposits ContainingConductive Layers", U.S. Patent 4,926,941, May22, 1990.

14. Nenninger, J.E., "Method and Apparatus for OilWell Stimulation Utilizing Electrically HeatedSolvents", U.S. Patent 5,120,935, June 9,1992.

15. Winckler, E. and McManus, J.W., "Method andApparatus for Removal of Oil Well Paraffin", U.S.Patent 4,911,239, March 27, 1990.

16. Galloway, D.R., "Method and Apparatus forRemoval of Oilwell Paraffm", U.S. Patent5,168,929, August 12, 1992.

17. Friedman, RH., Surles, B.W. and Kleke, D.E.,"High Temperature Sand Consolidation", SPEProduction Engineering, May 1988, p. 167.

18. Erickson, J. W., "Electric and Hydrau1ic PoweredThermal Stimulation and Recovery System andMedtod for Subterranean Wells", U.S. Patent4,285,401, August 25, 1981.

19. Meeks, T. and Rhodes, G.A., "Downhole StearnApparatus", U.S. Patent 4,243,098, January 6,1981.

20. Mims, D.S. and Allen, R.S., "Tar Sand ProductionUsing Thermal Stimulation", U.S. Patent

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