Abstract:
A method for manufacturing an electrical cable provides an electrical cable suitable for use in heating wells. An elastomeric jacket is extruded over insulated conductors. A stainless steel plate is rolled around the jacket to form a cylindrical coiled tubing having a seam. The seam is welded, then the tubing is swaged down to a lesser diameter to cause the tubing to frictionally grip the jacket. A recess may be formed in the jacket adjacent the seam to avoid heat damage from the welding process.

Description:
This application is a continuation-in-part of application Ser. No. 09/939,902, filed Aug. 27, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to applying heat to wells and in particular to a heater cable that is deployable while the well is live. 
     BACKGROUND OF THE INVENTION 
     Occasions arise wherein it is desirable to add heat to a hydrocarbon producing well. For example, U.S. Pat. No. 5,782,301 discloses a heater cable particularly for use in permafrost regions. The heater cable in that instance is used to retard the cooling of the hydrocarbon production fluid as it moves up the production tubing, which otherwise might cause hydrates to crystalize out of solution and attach themselves to the inside of the tubing. Also, if water is present in the production stream and production is stopped for any reason, such as a power failure, it can freeze in place and block off the production tubing. 
     Another application involves gas wells, which often produce liquids along with the gas. The liquid may be a hydrocarbon or water that condenses as the gas flows up the well. The liquid may be in the form of a vapor in the earth formation and in lower portions of the well due to sufficiently high pressure and temperature. The pressure and the temperature normally drop as the gas flows up the well. When the vapor reaches its dew point, condensation occurs, resulting in liquid droplets. Liquid droplets in the gas stream cause a pressure drop due to frictional effects. The pressure drop results in a lower flow rate at the wellhead. The decrease in flow rate due to the condensation can cause a significant drop in production if the quantity and size of the droplets are large enough. A lower production rate causes a decrease in income from the well. In severe cases, a low production rate may cause the operator to abandon the well. 
     Applying heater cable to a well in the prior art requires pulling the production tubing out of the well, strapping a heater cable to the tubing and lowering the tubing back into the well. One difficulty with this technique in a gas well is that the well would have to be killed in order to pull the tubing. This is performed by circulating a liquid through the tubing and tubing annulus that has a weight sufficient to create a hydrostatic pressure greater than the formation pressure. However, in low pressure gas wells, killing the well is risky in that the well may not readily start producing after the killing liquid is removed. The killing liquid may flow in the formation, blocking return of gas flow. 
     The heater cable of the type in U.S. Pat. No. 5,782,301 does not have the ability to support its own weight. It must be supported by another structure, such as the production tubing. Proposals have been made for installing a coiled tubing with a heater cable located therein. Coiled tubing is a metal continuous tubing that is deployed from a reel to the well. The diameter is typically from about 2 to 2⅞ inch. Coiled tubing is normally made of a mild steel in a seam welding process. After welding, it is annealed to provide resistance to cracking as it is wound on and off a reel. produced by rolling a flat plate. If heater cable is to be located within a string of coiled tubing, it will be pulled through the cable after the annealing process because the temperatures employed during annealing would damage the insulation of the heater cable. A variety of techniques, including standoffs, dimples and the like have been proposed to cause the power cable to grip the coiled tubing to transfer its weight to the coiled tubing. Because of the standoffs, the outer diameter of the coiled tubing is larger than desirable. When deployed within production tubing, coiled tubing reduces the flow area of the production tubing, increasing pressure drop and frictional losses. 
     SUMMARY OF THE INVENTION 
     The heater cable for this invention has at least one insulated conductor. An elastomeric jacket is extruded over the insulated conductor, the jacket having a cylindrical exterior that has a longitudinally extending recess formed thereon. A metal tubing having a cylindrical inner wall and a longitudinally extending weld seam is formed around the jacket. The seam of the metal tubing is welded in a continuous process and is located adjacent the recess so as to avoid excessive heat to the jacket while the seam is being welded. The coiled tubing initially has a greater inner diameter than the outer diameter of the jacket. After welding the seam, the coiled tubing is swaged to a lesser diameter, causing its inner wall to frictionally grip the jacket. 
     The coiled tubing is preferably formed of a stainless steel that provides sufficient strength and toughness to be used as coiled tubing without an annealing process. Preferably, the outer diameter of the coiled tubing after swaging is no greater than one inch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an electrical cable installed within a coiled tubing, shown during a manufacturing process in accordance with this invention. 
     FIG. 2 is a sectional view of the cable of FIG. 1 after the coiled tubing has been swaged. 
     FIG. 3 is a schematic view of the manufacturing process for the electrical cable of FIGS. 1 and 2. 
     FIG. 4 is a schematic sectional view illustrating a well in the process of having the cable of FIGS. 1 and 2 installed therein. 
     FIG. 5 is a sectional view of the lower end of the cable of FIGS. 1 and 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, heater cable  11  has a plurality of conductors  13 . Conductors  13  are preferably fairly large copper wires, such as 6 AWG. Each conductor  13  has at least one layer of high temperature electrical insulation and in the preferred embodiment, two layers  15 ,  17 . Insulation layers  15 ,  17  may be of a variety of materials, but must be capable of providing electrical insulation at temperatures of about 60 to 150 degrees F. above the bottom hole temperature of the well. In one embodiment, inner layer  15  is formed from a polyimide such as Kapton, marketed by Du Pont. Outer layer  17  protects inner layer  15  and is formed of a fluoropolymer, preferably MFA, which is a copolymer of tetrafluoroethylene and perfluoromethylvinylether. Layers  15  and  17  are formed on conductors  13  by extrusion. 
     The three insulator conductors  13  are twisted together and an elastomeric jacket  19  is extruded over them. Jacket  19  provides structural protection and also is an electrical insulator. Jacket  19  also must be able to withstand temperatures of about 60 to 150 degrees F. above the bottom hole temperature of the well and can be of a variety of materials, the preferred being an EPDM (ethylenepropylenediene monomer) material. Generally, bottom hole temperatures in wells in which heater cable  11  would be deployed would not exceed about 250° F. 
     Jacket  19  has a cylindrical exterior  21  that has a plurality of grooves  23  thereon. Grooves  23  extend longitudinally along the axis of jacket  19  and in this embodiment are rectangular in cross-section. Grooves  23  are separated from each other by lands, which are portions of the cylindrical exterior  21 . The width of each groove  23  is approximately the same as the distance between each groove  23 . 
     Also, preferably jacket  19  has a flat or recess  25  formed on a portion of its cylindrical exterior  21 . Recess  25  in this embodiment has a flat base  25   a  with two inclined sidewalls  25   b  and  25   c  on each side of recess  25 . Recess  25  extends longitudinally, parallel with the axis of jacket  19 . The width of recess  25  is proportional to an angle a, which is the angular distance from side edges  25   b  to  25   c.  In this embodiment, angle a is between 50 and 90°, and preferably about 70°. In this range, base  25   a  is a distance b from an outer diameter line that is the same as the outer diameter of cylindrical exterior  21 . Distance b divided by a radius of cylindrical exterior  21  is in the range from about 0.15 to 0.35 and preferably 0.25. 
     A metal tube or tubing  27 , also referred to as coiled tubing, extends around jacket  19 . Tubing  27  is preferably formed from stainless steel, such as 316L stainless steel. Tubing  27  is formed from a flat plate that is rounded to form a cylinder with its side edges abutting each other to form a seam  29  that is welded. Initially, tubing  27  will be formed to a great inner diameter than the outer diameter of jacket  19 . FIG. 1 exaggerates the difference, and in the preferred embodiment, the difference in diameter is in the range from 0.030 to 0.050 inch and preferably about 0.040 inch. This difference creates an initial clearance between jacket cylindrical exterior  21  and the inner diameter of tubing  27 . 
     FIG. 3 schematically illustrates the manufacturing process, with forming rollers  31  deforming a flat plate into a cylindrical configuration around jacket  19  in a continuous process. Then, a torch  33  welds seam  29  (FIG.  1 ). Recess  25  (FIG. 1) is oriented under seam  29  so as to protect jacket  19  from excessive heat during the welding procedure. After welding, tubing  27  undergoes a swaging process with swage rollers  35  to reduce the diameter. This process causes the inner diameter of tubing  27  to come into tight frictional contact with jacket cylindrical exterior  21 . The outer diameter of jacket exterior  21  will reduce some, with the deformed material of jacket  19  being accommodated by grooves  23  and recess  25 . Preferably the outer diameter of tubing  27  after swaging is less than one inch, and preferably about 0.75 inch. In an embodiment with an outer diameter of 0.75 inch after swaging, jacket  19  had an outer diameter and tubing  27  had an inner diameter of about 0.620 inch, which places base  25  a distance b of about 0.077 inch from the inner diameter of tubing  27 . 
     Tubing  27  is not annealed after the welding process, thus heater cable  11  is ready for use after the swaging process. The 316L stainless steel material of tubing  27  has been found to be capable of handling a large number of flexing cycles without undergoing an annealing process. In one test, tubing  27  was able to undergo 5,000 flexures without fatigue causing cracking in tubing  27 . The tight grip of the inner wall of tubing  27  with jacket  19  after swaging causes the weight of conductors  13  and jacket  19  to be transferred to tubing  27 . Spaced apart supports between jacket  19  and tubing  27  are not necessary. 
     FIG. 4 illustrates one method for installing heater cable  11  within a well. A Christmas tree or wellhead  37  is located at the surface or upper end of a well for controlling flow from the well. Wellhead  37  is located at the upper end of a string of conductor pipe  39 , which is the largest diameter casing in the well. A string of production casing  41  is supported by wellhead  37  and extends to a greater depth than conductor pipe  39 . There may be more than one string of casing within conductor pipe  39 . In this example, production casing  41  is perforated near the lower end with perforations  43  that communicate a gas bearing formation with the interior of production casing  41 . A casing hanger  45  and packoff support and seal of production casing  41  to wellhead  37 . Conductor pipe  39  and production casing  41  are cemented in place. 
     In this embodiment, a string of production tubing  47  extends into casing  41  to a point above perforations  43 . Typically production tubing  47  is made up of sections of pipe screwed together. Production tubing  47  has an open lower end for receiving flow from perforations  43 . A tubing hanger  49  lands in wellhead  37  and supports production tubing  47 . A packoff  51  seals tubing hanger  49  to the bore of wellhead  37 . Production tubing  47  may be conventional, or it may have a liner of a reflective coating facing inward for retaining heat within tubing  47 . 
     In the embodiment shown in FIG. 4, heater cable  11  is lowered into production tubing  47  to a selected depth while the well is live. That is, the well has not been killed by circulating a heavy kill fluid, thus has pressure in wellhead  37 . The depth of heater cable  11  need not be all the way to the lower end of production tubing  47 . Preferably, heater cable  11  has a closed lower end and its interior is free of any communication with production fluids. A shorting bar  55 , shown in FIG. 5, electrically joins the three conductors  13  to each other. Shorting bar  55  is located at the lower end of heater cable  11 . 
     Wellhead  37  has a valve  57 , such as a gate valve, that may be closed to block well pressure in wellhead  37  above tubing  47 . During the preferred installation procedure for heater cable  11 , valve  57  will be initially closed, and a set of coiled tubing rams  58  will be mounted to the upper end of wellhead  37 . Rams  58  are sized to close around the smooth exterior of heater cable  11  to form a seal. A coil tubing injector  59  is mounted above rams  58 . Tubing injector  59  is of a conventional type that will grip the exterior of coiled tubing  27  and push it downward into the well. Coiled tubing injector  59  also has a conventional blowout preventer or pressure controller (not shown) that seals around coiled tubing  27  while pushing it downward. 
     During the installation procedure, heater cable  11  will be inserted through tubing injector  59  and rams  58  while valve  57  is closed. After coiled tubing injector  59  forms seal on heater cable  11 , valve  57  is opened, and heater cable  11  is pushed into production tubing  47 . Injector assembly  59  prevents leakage of gas pressure as heater cable  11  is inserted into production tubing  47 . 
     When at the desired depth, the operator will close rams  58  around coiled tubing  11  to form a static seal. The upper end of heater cable  11  is cut and injector assembly  59  is removed. A coiled tubing hanger (not shown) will be mounted above rams  58  to provide a permanent seal around heater cable  11 , which enables rams  58  to be opened. Valve  57  remains open and will not be closed while heater cable  11  is in the well except in the event of an emergency. In an event of emergency, valve maybe closed, resulting in heater cable  11  being sheared. 
     To avoid excess energy requirement, it is beneficial to insulate production tubing  47  against heat losses. In the embodiment of FIG. 4, this is handled by a vacuum. Production tubing  47  has a production flow line or outlet  61  with a valve  63  at wellhead  37 . A tubing annulus  65  surrounds production tubing  47  between tubing  47  and production casing  41 , with the lower end of tubing annulus  65  being at a packer  67 . Packer  67  is located at or near the lower end of tubing  47  and seals production tubing  47  to casing  41 . Tubing Annulus  65  communicates with a port  69  in wellhead  37 . A valve  71  at port  69  is connected to a line leading to a vacuum pump  73 . Vacuum pump  73  causes pressure in tubing annulus  65  to reduce below atmospheric pressure. This provides insulation to retard heat loss from tubing  57 . The vacuum level may be monitored with vacuum pump  73  periodically operating to maintain a desired level of vacuum. 
     Conductors  13  (FIG. 1) are connected to a voltage controller (not shown) that supplies electrical power to heater cable  11  to create a desired amount of heat. The electrical power supplied should provide an amount of heat sufficient to raise the temperature of the gas to reduce any condensation levels that are high enough to restrict gas flow. The temperature of the gas need not be above its dew point, because gas will still flow freely up the well so long as large droplets do not form, which fall due to gravity and restrict gas flow. The large droplets create friction which lowers the production rate. Some condensation can still occur without adversely affecting gas flow, particularly condensation in a cloudy state with small droplets. The amount of heat needs to be only enough to prevent the development of a large pressure gradient in the gas flow stream due to condensation droplets. Eliminating condensate that causes frictional losses allows the pressure to remain higher, increasing the rate of production. Increasing the temperature far above the necessary level to avoid losses would not be economical because it requires additional energy to create without reducing the detrimental pressure gradient. An adequate amount of heat has been found to be enough to create a temperature in tubing annulus  65  that is about 60 to 150 degrees F. above the temperature in the well. The water and hydrocarbon vapors that remain in the gas will be separated from the gas at the surface by conventional separation equipment. 
     The invention has significant advantages. The insulated conductors are installed in a continuous process while the coiled tubing is being formed. This avoids the need for pulling electrical cable through preformed tubing. By utilizing stainless steel, the conventional annealing step required for coiled tubing is omitted, which otherwise would result in temperatures that would be too high for the electrical cable to withstand. The coiled tubing has a smooth outer diameter for sealing with conventional coiled tubing injector equipment. Since the cable does not need internal supports for transferring weight of the insulated conductors to the coiled tubing, the outer diameter may be quite small. This provides a greater flow area in the production tubing for the production fluids as well as making sealing on the outer diameter of the cable easier. Evacuating the tubing annulus reduces loss from the production tubing. Installing the heater cable in a live well avoids risking killing procedures. 
     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention. For example, if the initial inner diameter of the coiled tubing is sufficiently greater than the heater cable jacket, it is possible to eliminate the recess adjacent the weld seam.