Abstract:
Tubular apparatus for delivering steam or other hot fluids to an oil well comprises an inner tubular having an outer surface and defining an inner space for conveying fluids at a temperature greater than 400 degrees F., and an outer tubular disposed around the inner tubular and defining an annular space with the inner and the outer tubulars being connected together. The annular space is closed to atmospheric pressure. A vacuum is established within the annular space. A getter material for absorbing at least one active gas is disposed within the annular space. Active gases which are absorbed by the getter material include hydrogen formed by corrosion of the outer tubular which hydrogen migrates through the outer tubular into the annular space and gases such as nitrogen, carbon monoxide, and hydrogen that are released from the inner tubular at elevated temperatures.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates in general to insulated hot fluid injection tubing and more particularly to a new and useful arrangement for maintaining a vacuum in an annular space between inner and outer tubulars forming an insulated tubing. 
     Heavy oil and tar sands represent huge uptapped resources of liquid hydrocarbons which will be produced in increasing quantities to help supplement declining production of conventional crude oil. The deposits must, however, be heated to reduce the oil viscosity before the oil will flow to the producing wells in economical quantities. A dominant method of heating is by injection of surface generated steam in either a continuous (steam flood) or intermittent (steam stimulation or &#34;huff and puff&#34;) mode. 
     When steam is injected down long injection pipes or &#34;strings&#34;, a significant amount of thermal energy is lost to the rock overburden (500 to 7,000 feet) which covers the oil deposits if the strings are not properly insulated. In initial steam injection projects, the price of oil did not justify the prevention of this heat loss, but with the price of oil at $30 or more a barrel, insulation systems for the well injection pipe become economically justified. 
     It is known to use insulated steam injection tubing for the injection of steam into oil wells and the prevention of excessive heat loss. 
     Tubing of the insulated steam injection type is formed of coaxial inner and outer tubulars that are connected together whereby an annular space is formed there-between. The annular spaces are typically insulated by products such as fiber and layered insulation with air or inert gas typically in the annular spaces. 
     The provision of a vacuum within an annular space between inner and outer tubulars is disclosed in U.S. Pat. No. 3,680,631 to Allen et al. and U.S. Pat. No. 3,763,935 to Perkins. Both of these references deal with the conveyance of warm fluid, such as oil over cool environments such as permafrost zones wherein the fluid, specifically liquid petroleum, is to be conveyed typically at a temperature of 160 degrees F. 
     Both of these patents suggest the use of special coatings, such as nickel or chromium alloy coatings, on the tubular surfaces to reduce gas diffusion into the space so that the vacuum which is originally established in the annular space can be maintained. Both patents generally suggest the use of a getter material for absorbing gases which may invade the annular space. 
     While the problem of gases diffusing or leaking into an evacuated annular space of a double walled tube is treated generally in the Allen et al and Perkins patents, neither of these patents address additional problems which are faced in the rugged environment of an oil well undergoing steam injection. The outer surfaces of the outer tubular in such an environment are exposed to corrosive water under pressure, which pressure increases with well depth. The tubing is generally made of carbon steel for economic reasons, and the corrosive environment drastically increases the generation of nascent hydrogen that permeates the outer tubular wall in particular at the greatly increased pressures encountered in typical water depths of 4000 to 6000 feet or more. 
     In addition, under the high temperature conditions of the inner tubular, the outgassing of objectionable gases such as oxygen, carbon monoxide, hydrogen, and nitrogen into the annular space increases in the order of an estimated ten times or more over the outgassing rate when the fluid in the inner tubular is at a temperature of merely 160 degrees F. Again, for economic reasons, the inner tubular should normally be made of relatively inexpensive metals such as carbon steel. While baking is known for the purposes of outgassing the surface of such steel, it is estimated that sufficient degassing of the inner tubular would require a temperature of 1,800 degrees F. for a period of about a day. Such processing is generally impractical, however. 
     During the life of an insulated steam injection tubing, which is estimated to be at least five years, an increase in hydrogen partial pressure within the annular space of up about four torr can be expected due to hydrogen diffusion. An increase in partial pressure from other active gases of 100 torr can be expected from outgassing of the inner tubular. Such increases in pressure defeat the insulating function of the annular space. Although partial pressures of other gases of up to 0.1 torr can normally be tolerated, a partial pressure above 0.01 torr cannot normally be tolerated for hydrogen due to its greater mobility. 
     In an article entitled New Double-Walled Tubulars Can Aid Thermal-Recovery Operations, B. V. Traynor, Oil and Gas Journal, Feb. 18, 1980, the problems of insulating the annular spaces of double-walled tubing in a rugged oil well environment are discussed on p. 106. It is noted that the use of a vacuum in the annular space for insulation purposes was found to be economically impractical so that the vacuum approach has been abandoned. 
     SUMMARY OF INVENTION 
     The present invention provides a solution to the problems of maintaining a vacuum in the annular spaces of tubing having inner and outer tubulars and used to inject steam into an oil well. 
     According to the invention, getter material is placed in the annular space during assembly of the tubulars. The getter material is advantageously positioned adjacent a surface which will achieve a high temperature during service. This increases the capacity and pumping speed of the getter material. The inner and outer tubulars are assembled using connecter means such as plates which are welded to the tubulars whereby an annular space is provided therebetween. The space is sealed and evacuated. The getter material is preferably simply and automatically activated by heating when the tubing is utilized for steam injection. 
     Accordingly, an object of the present invention is to provide a tubular apparatus for the delivery of steam or other hot fluids to a well comprising an inner tubular having an outer surface and defining an inner space for conveying fluids at a temperature of greater than 400 degrees F., an outer tubular disposed around the inner tubular and defining an annular space therewith, means for connecting the inner and outer tubulars, the annular space closed to atmospheric pressure and a vacuum established in the closed annular space, and a getter material for absorbing at least one active gas in the closed annular space, the getter material being disposed preferably adjacent the surface of the inner tubular or another high temperature component in the apparatus. 
     A further object of the present invention is to provide such a tubular apparatus whereby an acceptable vacuum may be maintained in the annular space when the inner tubular is made of material which releases at least one active gas by outgassing, which outgassing is increased with elevated temperatures. 
     A further object of the present invention is to provide such a tubular apparatus whereby an acceptable vacuum may be maintained in the annular space when the outer tubular is made of material which corrodes in a corrosive environment of a well to generate nascent hydrogen which penetrates the outer tubular and enters the annular space. 
     The invention advantageously can help avoid the use of costly corrosion resistent materials as coatings for the inner and outer tubulars. 
     The getter material may be made of titanium, zirconium, or other gas absorbing materials. However, titanium and zirconium are preferred for absorbing hydrogen gas. 
     A further object of the invention is to provide a tubular apparatus for the delivery of steam to a well in which a vacuum environment used for insulation is maintained within desired limits throughout the life of the tubular apparatus, and which is simply designed, rugged in construction, and economical to manufacture. 
     For an understanding of the principles of the invention, reference is made to the following description of a typical embodiment thereof as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the Drawings: 
     FIG. 1 is a longitudinal sectional view of insulated steam injection tubing according to the invention; and 
     FIG. 2 is a cross-sectional view of another embodiment of the insulated steam injection tubing according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, in particular, the invention embodied therein comprises an insulated hot fluid injection tubing apparatus generally designated 10, which apparatus can be assembled with other similar tubing to form a string which is lowered into an oil well for the purposes of injecting steam or other hot fluids therein. The tubing 10 is typically about 40 feet in length and comprises an outer tubular 12 disposed around an inner tubular 14 and defining therewith an annular space 16. Annular space 16, as shown in FIG. 1, can be provided with multi-layered insulation 18 wrapped around an outer surface 20 of inner tubular 14, or, as shown in FIG. 2, fibrous insulation 22. However, other suitable insulation may be used. Connecting means such as flanged connecting members 24 and 26 are connected, preferably welded, between the inner and outer tubulars at axially spaced locations. In addition to connecting inner and outer tubulars, members 24 and 26 may serve to advantageously seal the annular space 16. A fluid supply device 34 may be connected to tubing 10 by means of the line schematically illustrated at 36 to supply steam or other hot fluid to the tubing. Device 34 may be of any type known in the art for that purpose, and such devices are commonly known to those of ordinary skill in the art to which this invention pertains. 
     Annular space 16 is vacated in known fashion to a level preferably at or below 10 -3  torr. 
     With an estimated service life of about five years or longer, it is desired that this level of vacuum be maintained at or below 0.1 torr including a partial pressure below 0.01 torr for hydrogen. Above these pressures, the thermal insulating function of the annular space is degraded. This is due, in the case of hydrogen, to the light weight and fast movement of hydrogen molecules that undesirably transfer heat from the inner tubular 14 to the outer tubular 12. The vacuum, as noted above, also deteriorates due to outgassing of active gases such as oxygen, nitrogen, hydrogen, and carbon monoxide from the material of the tubulars, in particular the inner tubular 14 which is exposed to steam conducted in its inner space 28, which inner tubular is typically at a temperature of about 650 degrees F. and generally within a temperature range of 400 to 700 degrees F. 
     As will be discussed in greater detail hereafter, during service life of a tubular apparatus 10, the atmosphere within annular space 16 can be expected to increase by up to about four torr due to diffusing hydrogen generated by corrosion, and by about 100 torr due to outgassing of gases from the hot inner tubular 14. Thus, even without otherwise mechanical leakage of gases into the annular space which may occur, the upper limits at a total pressure of 0.1 torr and a hydrogen partial pressure of 0.01 torr for the annular space 16 is far exceeded. 
     To eliminate such undesirable active gases from the annular space 16, and according to the invention, a getter material 30 is provided within the annular space and preferably adjacent the outer surface 20 of inner tubular 14 or another surface such as the surface of a connecting member 24 or 26 which may also be at a temperature of more than 400 degrees F. In this location, the getter material which is preferably activatable by heat of steam in space 28, absorbs both hydrogen permeating through the outer tubular wall and gases outgassed from the inner tubular 14. To increase the surface area of the getter material, the getter material can, for example, advantageously be of a sponge form which is a form providing a large surface area and is commonly known to those of ordinary skill in the art to which this invention pertains, or it may be provided on a corrugated metal strip, shown at 32 in FIG. 2; that surrounds and is adjacent the outer surface of inner tubular 14. The corrugated strip 32 may, for example, be iron or another metallic alloy, with the getter material formed in a coating on the strip. 
     The getter material is advantageously titanium, an alloy of titanium, zirconium or an alloy of zirconium for absorption of hydrogen as well as other active gases. Getter material such as aluminum may be added for absorbing other active gases. These getter materials may be activated, or increased in activity at elevated temperatures, typically the temperatures within space 28, to absorb the objectionable active gases that would otherwise migrate into space 16. 
     In this way, relatively inexpensive mild carbon steel can be used for constructing inner and outer tubulars without corrosion resistant diffusion coatings such as chromium or nickel plating or stainless steel. This results in significant &#34;savings&#34; in manufacturing of the tubular apparatus 10 while at the same time maintaining a sufficient vacuum insulation during its service life. 
     While the tubing is primarily useful in the rugged environments of an oil well for oil recovery, the tubing is also useful in other similarly rugged environments such as those of oil-coal slurry transportation and steam and high pressure hot water circulation in geothermal wells. 
     In calculating the amount of hydrogen which may permeate into the annular space 16, an outer surface of outer tubular 12 is assumed to have an area of 232.7 cm. 2  per inch of tube length. The water volume in the annular space between the well casing (not shown in FIG. 1) and the insulated tube is assumed to be 369.86 cm. 3  per inch of the tube length. The volume of the evacuated space 16 in the insulated tube is assumed to be 133.27 cm. 3  per inch of the tube length. These figures are based on typical sizes for the inner and outer tubulars. 
     The density of carbon steel, of a type advantageously used according to the invention, is 7.86 g/cm. 3 . 
     A corrosion mechanism in acid or alkali solution is as follows: 
     
         Fe→Fe.sup.2+ +2e: anodic reaction 
    
     
         2H.sup.+ +2e→H.sub.2 (gas): cathodic reaction 
    
     Corrosion of 1 g-ion of Fe (55.85 g) evolves 1 g-mole of H 2 . 
     In addition to the above, the following assumptions are made: 
     Corrosion rate of carbon steel: 1 mpy (mil per year) 
     Outer surface area of the insulated tube: 91.16 cm 2  per inch of tube. 
     Inner surface area (exposed to steam) of the inner tubular: 48.12 cm 2  per inch of tubular. 
     Wall thickness of the outer tubular: 0.635 cm. 
     Wall thickness of the inner tubular: 0.483 cm. 
     Permeation coefficient U can be calculated according to the equation 
     
         U=U.sub.o e(-K/RT) 
    
     where 
     U o  =2.83×10 -3  cm 3  /cm-sec-atm 1/2   
     K=8,400 cal/g-mole., 
     R=the universal gas constant, and 
     T=temperature in degrees Kelvin. 
     At temperatures of 150,400, and 650 degrees F., the permeation coefficient U is found to be as follows: 
       U  150 degrees F.=1.1×10 -8  cm 3  /cm-sec-atm. 1/2   
       U  400 degrees F.=4.1×10 -7  cm 3  /cm-sec-atm. 1/2   
       U  650 degrees F.=2.95×10 -6  cm 3  /cm-sec-atm 1/2   
     The hydrogen partial pressure in the steam side of the inner insulated tubular is assumed to be zero. With Q 1  being the hydrogen flux from water to the vacuum space of the insulated tube, and Q 2  being the hydrogen flux from the vacuum space of the insulated tube to the steam, the steady state condition is Q 1  =Q 2 . 
     Q is calculated according to the following equation: ##EQU1## where A=surface area in cm 2   
     P=the difference in pressure in atmospheres between inside and outside of a tubular, and 
     t=thickness in cm. of the tubular wall. 
     Based on the above assumptions and setting Q 1  =Q 2 , the partial pressure of hydrogen in the vacuum space of the insulated tube under steady-state conditions is estimated for an inner tubular temperature in the range of 400 to 650 degrees F. to be as listed in Table I or greater. 
     
                       TABLE I______________________________________HYDROGEN PARTIAL PRESSURE AS A FUNCTION OFDISTANCE BELOW WATER SURFACEDepth (ft)  H.sub.2 Partial Pressure (Torr.)______________________________________  0         2.1 × 10.sup.-2 500        3.2 × 10.sup.-11000        6.4 × 10.sup.-12000        1.274000        2.556000        3.82______________________________________ 
    
     From this table, it may be observed that the annulus pressure due to permeation of hydrogen into the annulus under steady-state conditions is unacceptably high (above 0.01 torr) even at the water surface if an effective means is not provided for absorbing the hydrogen gas. 
     While various corrosion inhibitors can be utilized to inhibit corrosion or prevent the passage of hydrogen into the vacuum space as noted above, this increases the cost of using insulated tubing. 
     By using titanium or zirconium, each of which have a strong affinity for hydrogen gas, as a hydrogen getter in a vacuum space, hydrogen as well as other gases can be absorbed from the vacuum space to prolong the useful life of the insulated tubing according to the invention. 
     The reaction of titanium as a hydrogen getter is as follows: 
     
         Ti(s)+H.sub.2 (g)→TiH.sub.2 (s) 
    
     (s) signifies a solid and (g) signifies gas 
     The free energy of formation and the dissociation pressure of titanium hydride are shown as a function of temperature in Table II. The large negative value of free energy and the low dissociation pressure indicate that titanium can serve as an efficient hydrogen getter in the vacuum space of the insulated tube. 
     
                       TABLE II______________________________________FREE ENERGY OF FORMATION ANDDISSOCIATION PRESSURE OF TITANIUMHYDRIDE AS A FUNCTION OF TEMPERATURE    Free EnergyTemperature    of Formation                Dissociation Pressure of TiH.sub.2(degrees F.)    (cal)       (Atm.)______________________________________ 80      -25,067     10.sup.-18.3260      -28,845     10.sup.-11.9400      -20,813     10.sup.-9.5440      -18,518     10.sup.-8.1620      -15,144     10.sup.-6.0650      -10,579     10.sup.-5.2______________________________________ 
    
     Assuming the service life of tubing 10 to be about five years, and to preserve a final vacuum within annular space 16 of no more than 0.1 torr including a hydrogen partial pressure no more than 0.01 torr, this space should first be evacuated to a level of 10 -3  torr or less as noted above. Assuming hydrogen will contribute a partial pressure of about four torr and all of the gases will contribute about 100 torr pressure, the use of a getter material within the annular space should maintain the vacuum level at the end of the service life of the tube well below 0.1 torr including a hydrogen partial pressure no more than 0.01 torr. 
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that this invention may be embodied otherwise without departing from such principles.