BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to lubrication of components for use in a wellbore. More particularly, the invention relates to the lubrication of wellbore components with a fullerene. More particularly still, the invention relates to reducing friction encountered during operation of a downhole tool in a wellbore.
2. Description of the Related Art
Galling of wellbore components due to friction has always been a problem in wellbore operations. Galling is surface damage to mating, moving, metal parts due to friction between the parts. In a wellbore, galling can take place between moving parts of a single component, like slips and cones of a packer or between a component and some other surface in the wellbore that is necessarily contacted as a component operates. Galling is also a problem for threaded connections that may be made up on the surface of the well and then utilized in the wellbore. Soft metals are more susceptible to galling than hard metals, and similar metal surfaces are more prone to galling than dissimilar metal surfaces.
Wellbore threads are often specialized and perform functions other than simply holding parts together. For example, in production tubing, the threaded connections between sequential lengths of tubing are frequently required to form gas tight seals. There are many of these “proprietary” threads that are capable of providing a gas tight seal for tubular connections. Examples of proprietary threads include Hydril connections, Atlas Bradford connections, and VAM connections. Generally, these threads have special geometric designs including shoulders that form metal to metal seals to prevent the migration of gases though the threaded connection. Because of the gas-sealing connections, tolerances are especially close and the surfaces of the threads come into contact with each other frequently as they are threaded together. Furthermore, the proprietary threads are commonly formed of a metal that is relatively soft, such as corrosive resistant alloys. With unlubricated threaded connections, galling often results as the connection is made up or taken apart.
Repair of the galled threads means reworking or replacing the threads or the component upon which they are formed. Because the threaded connection is typically made or unmade during assembly or disassembly of a component and after most of the value has been added to a component, galling can result in a complete loss of a tool or assembly.
Galling is also a problem when expanding tubulars in a wellbore. Expansion technology enables a tubular to be expanded and its diameter to be increased in a wellbore. Using this method, a liner, for example, can be hung off of an existing string of casing without the use of a conventional slip assembly. Tubulars can be expanded with a swedge or tapered cone that is physically pushed through the inside of the tubular with enough force that the inside diameter of the tubular is increased to at least the outside diameter of the cone. More recently, expander tools are fluid powered and are run into a wellbore on a working string. The hydraulic expander tools include radially extendable rollers which are urged outward radially from the body of the expander tool and into contact with a tubular therearound. As sufficient fluid pressure is generated upon a piston surface behind these rollers, the tubular is expanded past its point of plastic deformation. By rotating the expander tool in the wellbore and moving it axially, a tubular can be expanded along a predetermined length in a wellbore.
FIG. 1 is an exploded view of an exemplary expander tool 100 for expanding a tubular (shown as 200 in FIG. 2). A tubular is expanded by an expander tool 100 acting outwardly against the inside surface of the tubular. The expander tool 100 has a body 102 which is hollow and generally tubular with connectors 104 and 106 for connection to other components (not shown) of a downhole assembly. The connectors 104 and 106 are of a reduced diameter compared to the outside diameter of the longitudinally central body part of the tool 100. The central body part 102 of the expander tool 100 shown in FIG. 2 has three recesses 114, each holding a respective roller 116. Each of the recesses 114 has parallel sides and extends radially from a radially perforated tubular core (not shown) of the tool 100. Each of the mutually identical rollers 116 is somewhat cylindrical and barreled. Each of the rollers 116 is mounted by means of an axle 118 at each end of the respective roller 116 and the axles are mounted in slidable pistons 120. The rollers 116 are arranged for rotation about a respective rotational axis that is parallel to the longitudinal axis of the tool 100 and radially offset therefrom at 120-degree mutual circumferential separations around the central body 102. The axles 118 are formed as integral end members of the rollers 116, with the pistons 120 being radially slidable, one piston 120 being slidably sealed within each radially extended recess 114. The inner end of each piston 120 is exposed to the pressure of fluid within the hollow core of the tool 100 by way of the radial perforations in the tubular core. In this manner, pressurized fluid provided from the surface of the well, via a working string 310, can actuate the pistons 120 and cause them to extend outward whereby the rollers 116 contact the inner surface of a tubular to be expanded.
In one example of utilizing an expansion tool, a new section of liner is run into the wellbore using a run-in string. As the assembly reaches that depth in the wellbore where the liner is to be hung, the new liner is cemented in place. Before the cement sets, an expander tool is actuated and the liner is expanded into contact with the existing casing therearound. By rotating the expander tool in place, the new lower string of casing can be fixed onto the previous upper string of casing, and the annular area between the two tubulars is sealed.
FIG. 2 is a partial section view of a tubular 200 in a wellbore 300. The tubular 200 is disposed coaxially within the casing 400. An expander tool 100 is attached to a working string 310 and visible within the tubular 200. Preferably, the tubular 200 is run into the wellbore 300 with the expander tool 100 disposed therein. The working string 310 extends below the expander tool 100 to facilitate cementing of the tubular 200 in the wellbore 300 prior to expansion of the tubular 200 into the casing 400. A remote connection (not shown) between the working, or run-in, string 310 and the tubular 200 temporarily connects the tubular 200 to the run-in string 310 and supports the weight of the tubular 200. For example, the temporary connection may be a collett (not shown), and the tubular 200 may be a string of casing.
FIG. 2 depicts the expander tool 100 with the rollers 116 retracted, so that the expander tool 100 may be easily moved within the tubular 200 and placed in the desired location for expansion of the tubular 200. Hydraulic fluid (not shown) is pumped from the surface to the expander tool 100 through the working string 310. When the expander tool 100 has been located at the desired depth, hydraulic pressure is used to actuate the pistons (not shown) and to extend the rollers 116 so that they may contact the inner surface of the tubular 200, thereby expanding the tubular 200.
FIG. 3 is a partial section view of the tubular 200 partially expanded by the expander tool 100. At a given pressure, the pistons (not shown) in the expander tool 100 are actuated and the rollers 116 are extended until they contact the inside surface of the tubular 200. The rollers 116 of the expander tool 100 are further extended until the rollers 116 plastically deform the tubular 200 into a state of permanent expansion. The working string 310 and the expander tool 100 are rotated during the expansion process, and the tubular 200 is expanded until the tubular's outer surface contacts the inner surface of the casing 400. The working string 310 and expander tool 100 are then translated within the tubular 200 until the desired length of the tubular 200 has been expanded.
Galling takes place during expansion due to friction between an outside surface of an outwardly extended roller and an inside surface of a tubular being expanded. Friction between the surfaces increases the amount of torque needed at the surface of the well to rotate the expansion tool in the wellbore and complete the expansion process. Increased friction causes galling of the contacting surfaces leading to even greater friction and less efficiency of the expansion tool.
In order to reduce friction and prevent galling in a wellbore, lubricants have been used on threads and on surfaces between moving parts, like the rollers of expander tools and tubulars to be expanded. Lubricants have included grease and oil. Sometimes, soft metals such as copper, lead, zinc, or tin are added to the material making up contacting surfaces. The reasons for adding the soft metals are two fold. First, the soft metals provide a barrier that prevents galling and second, they deform under pressure and act as a lubricant.
Methods of reducing friction and preventing galling are disclosed in two related patents, U.S. Pat. Nos. 4,527,815 and 4,758,025, which are herein incorporated by reference. The two patents disclose the use of electroless metal coatings on tubular goods to eliminate galling of the threads, provide a tortuous path as a sealing surface, and provide porous lubricant reservoirs. While these solutions reduce friction and the likelihood of galling, they are not completely effective.
There is a need, therefore, for a method and apparatus to reduce the friction encountered during the operation of a downhole tool that operates by contacting other surfaces. There is a further need for a method and apparatus for preventing galling created by friction between a downhole tool and other surfaces. There is yet a further need for a method and apparatus for preventing galling in threaded connections between tubulars and/or downhole components.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for reducing friction and preventing galling between surfaces in a wellbore. In one aspect of the invention, mating threads are coated with fullerene to reduce galling of the threads during make up and break down. Preferably, the fullerene is a spherically shaped carbon 60 molecule otherwise known as buckyball or C60. The fullerene coating provides an intermediate surface between two metal surfaces, thereby preventing galling between the two surfaces. In another aspect of the invention, the fullerene is placed between the roller of an expander tool and the surface of the tubular to be expanded in order to reduce friction and prevent galling.
In one aspect, the present invention provides a method for expanding a tubular in a wellbore. Initially, a tubular is disposed in the wellbore. The tubular is then expanded using an expander tool. The expander tool and the expanded area of the tubular include a coating of fullerene to prevent galling of the components. Furthermore, the fullerene coating reduces the friction forces between the tool and the tubular, thereby increasing efficiency.
In another aspect, threaded connections are coated with fullerene to prevent galling. Specifically, threaded connections for gas tight seals are coated with fullerene.
Many methods exist for producing fullerenes. One such method is described in U.S. Pat. No. 5,227,038, which is herein incorporated by reference. Fullerenes are commonly derived by contact-arc vaporization of a graphite rod, which results in the formation of raw soot. The raw soot produced by this process primarily comprises a mixture of two fullerenes, C60 and C70 in a ratio of about 10 to 1 respectively, accounting for about 5 to 10% of the total soot. Other methods of deriving fullerene-containing soot, primarily from sooting flames, also exist. Fullerenes such as C60, C70, C76, C78, C84, etc., can be deposited on a substrate alone following purification, or as a mixture with one or more fullerenes.