Note: Descriptions are shown in the official language in which they were submitted.
10152O2530CA 02264354 1999-03-04TH 1175ELECTRICAL HEATERFIELD OF THE INVENTIONThis invention relates to a electrical heating methodand apparatus useful in a borehole.BACKGROUND TO THE INVENTIONU.S. Patent Nos. 4,640,352 and 4,886,118 discloseconductive heating of subterranean formations of lowpermeability that contain oil to recover oil therefrom.Low permeability formations include diatomites, lipidcoals, and oil shales. Formations of low permeability arenot amiable to secondary oil recovery methods such assteam, carbon dioxide, or fire flooding. Floodingmaterials tend to penetrate formations that have lowpermeabilities preferentially through fractures. Theinjected materials bypass most of the formationhydrocarbons. In contrast, conductive heating does notrequire fluid transport into the formation. Oil withinthe formation is therefore not bypassed as in a floodingprocess. Heat injection wells are utilized to provide the"heat for such processes.Heat injection wells can also be useful indecontamination of soils. U.S. patents 5,318,116 and5,244,310, for example, disclose methods fordecontamination of soils wherein heat is injected belowthe surface of the soil in order to vaporize thecontaminates. The heaters of patent '31O utilizeelectrical resistance of spikes, with electricity passingthrough the spikes to the earth. Patent '116 disclosesheater elements passing through the wellbore to thebottom of the formation to be heated. The wellboresurrounding the heater includes a catalyst bed, which isheated by the heater elements. Heat conductively passesthrough the catalyst bed to a casing surrounding thecatalyst bed, and then radiantly from the casing to thel01520253035CA 02264354 1999-03-04_ 2 _soil surrounding the wellbore. Typical alumina basedcatalysts have very low thermal conductivities, and asignificant temperature gradient will exist through thecatalyst bed. This significant temperature gradient willresult in decreased heat transfer to the earth beingheated at a limited heater element temperature.U.S. patent no. 5,065,818 discloses a heater wellwith sheathed and mineral insulated ("MI") heater cablescemented directly into the wellbore. The MI cablesincludes a heating element surrounded by, for example,magnesium oxide insulation and a relatively thinsheathing around the insulation. The outside diameter ofthe heater cable is typically less than one half of aninch (1.25 cm). The heater well optionally includes achannel for lowering a thermocouple through the cementedwellbore for logging a temperature profile of the heaterwell. Being cemented directly into the wellbore, a needfor a casing (other than the sheathing of the cable) iseliminated, but the outside diameter of the cable isrelatively small. The small diameter of the heater cablelimits the amount of heat that can be transferred to theformation from the heater cable because the area throughwhich heat must pass at the surface of the cable islimited. A cement will have a relatively low thermalconductivity, and therefore, a greater heat flux at the.surface of the cable would result in an unacceptably highheater cable temperature. Multiple heater cables may becemented into the wellbore to increase the heat transferto the formation above that which would be possible withonly one cable, but it would be desirable to furtherincrease the heat that can be transferred into earthsurrounding the heaters.U.S. patent 2,732,195 discloses an electrical heaterwell wherein an "electrically resistant pulverulent"substance, preferably quartz sand or crushed quartzgravel, is placed both inside and outside of a casing ofa wellbore heater, and around an electrical heating101520253035CA 02264354 1999-03-04element inside of the casing. The quartz is placed thereto reinforce the casing against external pressures, and acasing that is sealed against the formation is required.The casing adds considerable expense to the installation.It is therefore an object of the present invention toprovide a wellbore heater wherein the heater has agreater surface area at the temperature of the electricalresistance element than those of the prior art, and inwhich a substantial casing is not required. This heateris useful as a well heater for such purposes as thermalrecovery of hydrocarbons and soil remediation.SUMMARY OF THE INVENTIONThese and other objects are accomplished by anelectrical heater comprising an electrical insulatingmaterial surrounding an annular heating elementconfiguration, wherein there is no casing surrounding theheating element configuration.The casingless design of the present heatersignificantly reduces the cost of a heat injection well,which is significant in an application such as heatinjectors for recovery of hydrocarbons from, for example,oil shales, tar sands, or diatomites. Heat injection canalso be used to remove many contaminates.It is preferred that the annular heating elementconfiguration is selected from the group consisting of anannular porous metal sheet, one or more expanded metalplates, a wire mesh, and strips wires, rods or filamentsconnected by spacers.In accordance with another aspect of the inventionthere is provided a method to heat a portion of theearth, the method comprising the steps of:providing a borehole within the portion of the earthto be heated;placing an annular heating element configurationwithin the borehole; andsupporting the heating element configuration withinthe borehole with electrically insulating material,1O1520253035âCA 02264354 1999-03-04wherein a metal casing is not provided between theheating element configuration and the earth to be heated.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 shows first embodiment of a heater accordingto the present invention within a wellbore.FIGS. 2A, 2B, and 2C show details of an electricalcable attached to the top of a heater according to thepresent invention.FIG. 3 shows an alternative embodiment of a heateraccording to the present invention within a wellbore.FIG. 4 shows a cross sectional View of the heater ofFIG. 3 in a borehole.FIG. 5 shows an apparatus for installing the heaterof the present invention.DETAILED DESCRIPTION OF THE INVENTIONOptionally the annular heater of the presentinvention has a mesh heating element which can be formedto conform to a wall of a wellbore to maximize thesurface of the heating element which is provided and tomaximize the heat flux leaving the wellbore. Anelectrically insulating filler is placed around andinside of the heating element to essentially eliminateâelectrical shorting of the element to the formation. Thiselectrically insulating material could be a material thatis initially wet, and therefore electrically conductinguntil it is dried. The drying step could be accomplishedby passing electricity through the heating element andinto the wet material, and heat generated by theelectrical energy would gradually heat the soil andeventually vaporize liquid water initially present. Theremaining dry sand is an acceptable electrical insulator.Optionally, a hydraulic cement could be used in place ofthe sand. Hydration of the cement reduces free liquidwater, and the cured cement can be an acceptableelectrical insulator. Other materials could be used asthe insulating material. Preferred materials are easilyplaced and inexpensive. An ideal material would also101520253035CA 02264354 1999-03-04either be or readily become an electrically nonconductingmaterial. A material such as sand could be placedpneumatically or as a slurry.A plurality of electrical heating elements arepreferably placed in the wellbore to form the heater,with the elements connected at the lower portion of thewellbore, and different phases of alternating electricalpower applied to each of the elements. Two or threeelements are preferred.The heating elements can be expanded metal, oranother porous metal element such as a wire screen orwire mesh. A porosity of between about forty percent andabout eighty percent is preferred, where porosity isdefined as the percent of open area looking at thesurface of the sheet of material. Providing this openarea considerably increases the total area contacted bythe element, without reducing the thickness of theelement. A thicker element provided greater allowancesfor corrosion. Thickness of the element is chosen toresult in a voltage requirement at the targeted heat fluxwhich is not excessively low or high. For example, avoltage differential of about 120 to about 960 volts ofalternating current between the upper ends of twoelements within a wellbore which have connected lowerends would be preferred._Generally, for longer lengths ofmeter (100 to 700 meters) from 480 to 960 volts ispreferred and for shorter meters (2 to 200 meters) from120 to 480 volts is preferred. To accommodate greaterthicknesses of elements, multiple heaters could beprovided in series, but the extent to which this can bedone is limited by the expense of the cables leading tothe heater elements. Power is preferably applied betweentwo symmetrical heater elements wherein the net voltageis zero. Thus the voltage applied at one time to oneelectrode is the negative with respect to ground of thevoltage applied to the other heater element.101520253035CA 02264354 1999-03-04The elements are preferably formed into a curvedshape either at the surface or within the borehole toconform to the walls of the wellbore. The curved shapecould be provided at the surface by a die through whichthe metal is passed as it is passed into the wellbore.The curved shape could be provided within the wellbore bya passing a mandril past the element. The mandril could,for example, be provided as a part of an apparatus whichspreads the elements and places the electrical insulatingmaterial around and between the elements. When theelements are formed into a curved shape at the surface,centralizers and spacers can be added to the elements tokeep the elements separated within the wellbore. Use ofthe mandrel as described above is preferred becausecentralizers and spacers can be eliminated, reducing thecost of materials. Flat mesh elements could be provided.The advantage of providing curved elements is that heatcould be transferred from almost the entire circumferenceof the borehole, with two flat elements, heat could betransferred from a surface area of only about twice thediameter of the wellbore, but installation of the flatelements could be simplified compared to the semicircularshaped elements.Generally, heater elements of stainless steel of, forexample, grades 304 or 316 are preferred. INCLOY 600could also be useful (INCLOY is a trademark). 316stainless steel is preferred when the elements will beexposed to brines because of the greater resistance of316 stainless steel to chloride stress corrosion.Stainless steels are not excessively expensive, and wouldwithstand exposure to elements that may be present duringstartâup phases for long enough to get the elements up toelevated temperatures, and sufficiently low corrosionrates when exposed to most borehole environments forextend periods of time at elevated temperatures.Typically, stainless steels are not utilized as heaterelements because of limited high temperature corrosion101520253035CA 02264354 1999-03-04resistance, but because of the relatively large surfacearea from which heat is transferred in the heater of thepresent invention, the elements surface temperature canbe suitable for stainless steels. Carbon steels couldalso be used as heater elements for applications wherehigh levels of heat do not have to be provided forextended periods of time.Although in a preferred embodiment of the presentinvention includes the use of stainless steel as theheater element material, higher alloys could be useful insome applications of the present invention. For example,when the heater is applied in a relatively deep wellbore,the costs of providing the well could be much greaterthan the costs of the heater element material, andtherefore a higher alloy could reduce total costs bypermitting operation at higher temperatures and thusreducing the number of wells required for the same totalheat duty.Alternatively, the heating elements could be coatedwith a more corrosion restive metal surface, or arefractor surface to provide additional electricalinsulation and protection.Thermocouples for control of the heaters could beprovided within the wellbore, either inside of curvedheater elements, outside of the elements, or attached tothe heater elements (through an electrically insulatingconnection). The thermocouple could be used to monitorthe operation, or to control electrical power applied tothe heater element. When thermocouples are used tocontrol the electrical power, multiple thermocouplescould be provided and the a control temperature selectedfrom the thermocouples. The selection could be based on amaximum temperature, an average temperature, or acombination such as an average of the highest two orthree temperatures.The heat elements of the present invention can bemade to a wide variety of lengths because of the101520253035CA 02264354 1999-03-04flexibility to select different combinations of voltagesand porosities of the heater elements. Heaters as shortas two to six meters can be used, and as long as twohundred to seven hundred meters could be provided.A borehole within which the heater of the presentinvention is placed may be cased and cemented for atleast a portion of the borehole above the heater, toensure isolation of the formation to be heated. In ashallow well, the borehole may be filled with sand to thesurface.Referring now to FIG. 1, a schematic drawing of theannular heater of the present invention is shown. A meshheater element 1 is shown as two semicircular expandedmetal plates within a wellbore 2. An electricallyinsulating filler 3 such as sand is shown surrounding andbetween the heating elements. The borehole is within aportion of the earth to be heated 4, such as a formationof oil containing diatomite, tar sands or oil shale.Alternatively, the earth to be heated 4 could becontaminated soil in a thermal desorption remediationprocess. Electrical leads 5 extend to each of the heaterelements and the heater elements are electricallyconnected at the lower portion of the elements byconnector 6. Alternatively, the elements could all begrounded at the base of the borehole. Electrical leadsextend through the portion of the overburden which is notâto be heated 7 through sheathed cables 8, the sheathedcables separated by spacers 9. A transition portion ofthe wellbore will be heated by the heater elements, butnot to the temperatures that result in the portion of theborehole which contains the heater elements. Thistransition portion of the borehole is shown as cased by acasing 10, which may be of a metal such as stainlesssteel, which will have an acceptably long useful lifewhen exposed to elevated temperatures. The corrosionenvironment within this transition volume may be moresever than the corrosion environment near the heatersl01520253035CA 02264354 1999-03-04because of the dew point temperature being within thisregion. Above the transition zone, the casing could be acarbon steel casing 11. The casing within the transitionzone and the overburden 7 could be filled with a filler12 such as sand or cement, or left void.Referring now to FIG. 2A, 2B, and 2C, three viewswith partial cutaways are shown of fittings forelectrical cables and connections to the heater elementof the present invention. The top of the heater element21 is connected to a high temperature lead cable 22 by aweld connection 33. A waterproof interface between thecable and heater A is within a transition zone. Above thetransition zone, an inexpensive cable such as apolyethylene coated copper wire could be used. Anelectrically insulated high temperature section B extendsfrom the waterproof interface to the heater element. Astiffener 24 provides support for the electricalconnection to the heater element. The stiffener isattached to the cable by a collar 25. The collar is anelectrically insulating collar. The water proof interfaceincludes a coupling 26 around a soldered connection 27,the soldered connection providing continuity between thehigh temperature lead cable 22 and a low temperature leadcable 28. The coupling is threaded to swedge fittings 30,which may be brass fittings, and which provide a frictionfitting to each of the high temperature lead sheath 31and the low temperature lead sheath 23. Cable 23 goesfrom the surface to just above the top of the heater andcam be a copper core-copper sheathed mineral insulatedcable. This type of cable is preferred because of itsability to carry very large amounts of electrical power,and because it is waterproof. Although the cable canwithstand high temperatures, it is used at temperaturesbelow the boiling point of water due to corrosion rates.A waterproof splice (A) terminates the mineral insulatedcable 23 and forms a transition to a nickel or nichromeclad-nickel electrode 22 that is welded 33 to the upper101520253035CA 02264354 1999-03-04_ 10 _part of the heater 21. The nickel hot electrode 22 can beinsulated with a TEFLON sleeve 31 to prevent corrosion ofthe nickel electrode and provide a waterproof seal at thelower end of the cable transition 30 (TEFLON is atrademark). Stiffening arm 24 provides support to theTEFLON sleeved nickel electrode 22 during installation ofthe heater into a wellbore. The waterproof splice A canbe about two to twenty feet above the top of the heaterelement. The water proof splice is far enough away fromthe heater so that the water proof splice remains at atemperature below the boiling point of water. The TEFLONcoated high temperature lead is, at one point, exposed tothe boiling point of water, and is easily capable ofhandling this environment. The lower (hotter) portion ofthe high temperature lead sheath 31 will eventually meltaway, leaving exposed high temperature lead. Providingthe TEFLON coating to this point ensures that the TEFLONextends past the point where the temperature is at theboiling point of water.The high temperature lead sheathing could be anycoating which would protect the high temperature leadfrom corrosion at temperatures of the boiling point ofwater or less, and would either withstand highertemperatures or melt away and not cause any corrosion athigher temperatures. Heat resistant resins are preferredbecause they provide a greater length of protected hightemperature lead which could be helpful if the point atwhich the temperature is the boiling point of watermoves. Acceptable high temperature resins includepolyimide, polyamide-imide, and polyetheretherketone.The high temperature lead sheath is separated fromthe high temperature lead by mineral insulation such asmagnesium oxide. Copper leads are acceptable andeffective for the low temperature leads, but nickel ornickel-chromium clad nickel are preferred for the hightemperature leads.l01520253035CA 02264354 1999-03-04-11-Alternatively a plurality of elongate electricalheating elements are placed in the wellbore to form theheater, with the elements connected at the lower portionof the wellbore, and different phases of alternatingelectrical power applied the elements. At least sixelements are preferred in order to provide heat aroundthe entire circumference of the wellbore.The heating elements can be, for example, stainlesssteel wire, nickel-chrome alloy wire or carbon fiberelements. The wires are preferably between about 0.2 andabout 0.8 mm in diameter and more preferably about 0.3 mmin diameter. Thicker elements provided greater allowancesfor corrosion, but at the expense of greater currentrequirements and greater material costs. Thickness of theelement is chosen to result in a voltage requirement atthe targeted heat flux which is not excessively low orhigh. For example, a voltage differential of about 60 toabout 960 volts AC between the upper ends of two elementswithin a wellbore which have connected lower ends wouldbe preferred. For shorter heaters (2 to 200 meters),voltages of 60 to 480 volts AC are preferred, and forlonger heaters (100 to 700 meters) a voltage of 480 to960 volts AC is preferred. To accommodate greaterthicknesses of elements, multiple heaters could beprovided in series, but the extent to which this can bedone is limited by the expense of the cables leading tothe heater elements. VAGenerally, heater elements of stainless steel of, forexample, grades 304, 316, or 310 are preferred. Stainlesssteels are not excessively expensive, and would withstandexposure to elements that may be present during startâupphases for long enough to get the elements up to elevatedtemperatures, and sufficiently low corrosion rates whenexposed to most borehole environments for extend periodsof time at elevated temperatures. Carbon steels could beused as heater elements for applications where heat doesnot have to be provided for extended periods of time. For101520253035CA 02264354 1999-03-04_12-shallow applications such as soil remediation,nichrome 80 is preferred.Thermocouples for control of the heaters could beprovided within the wellbore, either inside of the ringof heater elements, outside of the elements, or attachedto the heater elements. The thermocouples could be, forexample, secured to one of the electrically insulatingspacers. The thermocouple could be used to monitor theoperation, or to control electrical power applied to theheater element. When thermocouples are used to controlthe electrical power, multiple thermocouples could beprovided and the control temperature selected from thethermocouples. The selection could be based on a maximumtemperature, an average temperature, or a combinationsuch as an average of the highest two or threetemperatures.The heater elements of the present invention can bemade to a wide variety of lengths because of theflexibility to select different combinations of voltagesand diameters of the heater elements. Heaters as short astwo meters can be used, and as long as 700 meters couldbe provided.A borehole within which the heater of the presentinvention is placed may be cased and cemented for atleast a portion of the borehole above the heater, toensure isolation of the formation to be heated. In ashallow well, the borehole may be filled with sand or abentonite slurry to the surface. The bentonite slurryprevents water ingress from above.Referring now to FIG. 3, a schematic of the heater ofthe present invention is shown. Heater elements 101 (twoshown) are provided with electrical leads to theelements 102 which are larger in diameter than the heaterelements, but can be of the same material. The number ofelements is preferably between two and six. Theelectrical leads are shown extending to individual heaterelements, but a spacer could be provided wherein only one101520253035CA 02264354 1999-03-04_l3._electrical lead is provided for each phase of electricalenergy, and the power is applied in parallel or series todifferent heater elements. The borehole within which theheater is placed is preferably between about 5 and about20 centimeters in diameter, and the heater element arepreferably placed between about one half and about onecentimeter from the wall of the borehole. The elementsare preferably separated by between about four and abouteighteen centimeters. Fewer elements generally reducesthe cost of the heater, but a larger number of elementspermits greater heat flux into a formation from theheater at limited heater element temperature. The heaterelements are not individually electrically insulated, butrely on the electrical insulating properties ofelectrically insulating filler material surrounding theelements. A casing 103 is provided at the surface forisolation, but preferably does not extend to the soil tobe heated 104, but only through an overburden 106. Sandor a hydraulic or ceramic cement 105 is shown surroundingthe heater elements. When the soil is to be heated to thesurface, a short tube could be provided to provide astable flange for securing the tops of the heaterelements.A flange 107 is shown with insulating sleeves 108around the electrical leads to the heater elements. Powersupply wires 109 provide electrical power to theelectrical leads, and are secured by nuts 110.An electrical insulating spacer 111 providesseparation of the electrical elements within theborehole. One electrical insulating spacer is shown, butmore than one can be provided, and preferably, one isprovided each three to ten meters within the wellbore.Further, the electrical insulating spacer is shown withinthe heater section, but one or more can also be providedin the electrical lead-in section about the heaters. Theelectrical insulating spacers can be made from aninexpensive plastic, and do not necessarily have to101520253035CA 02264354 1999-03-04_]_4..withstand the elevated operating temperatures. Thespacers only need to hold the heater elements in placewhile the filler material is placed around the elements.Alternatively the spacers could be made from ceramicssuch as alumina, or machineable ceramics such as MACOR(MACOR is a trademark).The lower ends of the heater elements can beconnected with an electrically conducting connector 112.The electrically conducting connector can connect all ofthe elements, or a combination of elements such that eachof the elements has electrical continuity necessary forcurrent to pass through the elements. The electricallyconducting connector optionally has a cup 113 forsecuring the connector to a tube for lowering theelements, connector and spacer down the borehole. Atubing from, for example, a coiled tubing unit, could beplaced within the cup 113, and the cup held to the coiledtubing either by, for example, a friction fit which couldbe broken by pressure from with the coiled tubing, or thetubing could be held to the cup by tension from theheater elements as the connector is lowered into theborehole.The electrically conducting connector is shown at thebottom of the wellbore, with each heater elementextending uniformly down the heated portion of thewellbore. But the number and/or heat duties of the heaterelements can vary along the length of the heater. Thediameters of the heating elements can vary along thelength of the heater to tailor the heat deposition to adesired profile. 4Referring now to FIG. 4, a view looking down at theelectrically insulating spacer is shown. Heaterelements 101 (six shown) are separated by insulatingspacer 111, with the electrically insulating filler suchas sand or cement 105 surrounding the spacer and heaterelements. The soil to be heated 104 surrounds the heater.The electrically insulating spacer 111 is shown as being101520253035,CA 02264354 1999-03-04_15_in two parts, with mating tongues and groves to allow thespacers to be slipped inside the heater elements andaround a tube when the tube is being used to lower theheater elements into the borehole. A tie wrap 201 can beused to secure the heater elements in notches within thespacer. The spacer may be secured vertically to theheater elements by friction, or may be held vertically byclamps (not shown) placed above, or above and below thespacer on one or more of the heater elements.Referring now to FIG. 5, an apparatus which can beused to place the heater of the present system into awellbore is shown. Heater elements 101 (two shown) arestrung over pullies 301, the pullies mounted on brackets302 which are set on a flange 303. The flange 303 ismounted on the casing 103, which is equipped with amating flange. The heater elements 101 are rolling offspools (not shown) and can be maintained in slighttension to prevent entanglement of the heater elementswithin the borehole. A coiled tubing 304 is shownextending into the borehole. The coiled tubing can beused to place the heater elements and electrical leadswithin the borehole, and then used to fill the borehole"with the electrically insulating filler as it is removed.The heating elements can be of a wide variety oflengths and a wide variety of distances down a borehole.For example, for heating an oil shale formation, theheater may be 400 meters long. For remediation ofcontaminated soil, the heater may be only two or threemeters long, although longer heater elements are moreadvantageously provided by the present invention. Theheaters may be provided an extended distance down theborehole. For example, an oil shale formation may beheated which lies under 400 meters of overburden. As thelength of the heater and electrical leads become verylong, the heater elements and/or electrical leads may berequired to be of larger diameter or may need to be madeof a material which has greater strength because theseCA 02264354 1999-03-04_]_6_elements must be self supporting until the electricallyinsulating filler is placed around the elements. Theheater elements therefore do not have to be selfsupporting at operating temperatures because frictionwith the electrically insulating filler will providevertical support for the elements.
