Patent Publication Number: US-2009235730-A1

Title: Method for cleaning an oil field capillary tube

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional patent application 61/037,733, filed on Mar. 19, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods for cleaning and maintaining an oil field capillary tube. 
     2. Description of the Related Art 
     Capillary tubes are used in oil field applications to inject various treatment chemicals into a production well. A capillary tube typically has a diameter from about one eighth (0.125) inch to about 2 inches and may extend several thousand feet down into a wellbore. Treatment chemicals are pumped into one end of the capillary tube at the surface and exit the other end of the capillary tube situated deep in the wellbore, or at a subsea tree in the case of an offshore subsea design. Accordingly, the treatment chemicals are allowed to contact the production fluids, wellbore casing, and other equipment associated with the production stream. 
     The treatment chemicals selected for pumping into the wellbore may differ from well to well depending upon the properties of the fluid(s) being produced. For example, if the production fluid is corrosive to the well tubing and also tends to form scale deposits, then the treatment fluid may include both a corrosion inhibitor and a scale inhibitor. Other properties of a production fluid may lead to the use of yet other treatment chemicals, as is generally known in the art. The one or more treatment chemicals pumped into the wellbore are selected to maintain or optimize the profitability of the well. Generally, the profitability of the producing well is a function of the rate of production, the cost of treatment chemicals and other well operating costs. 
     Therefore, the ongoing addition of treatment chemicals through the capillary tube is important to the continued operation of the well. However, capillary tubes are prone to plugging. Stoppage and/or reduction of capillary flow can be in the form of a localized plug of solid or semi-solid material, a layer of material (thus reducing the effective internal diameter of the capillary tube), or both. Formation of the plugging/flow-reducing build-up can stem from many causes. When the desired treatment chemicals are no longer added to the wellbore, production rates can drop significantly or even come to a halt and cause a substantial loss of revenue. Also the applications of certain production chemicals, such as corrosion inhibitors, are needed to reduce the risk to the asset which can be costly over the producing life of the well. 
     Conventional techniques for unplugging a capillary tube include removing the capillary tube from the well and pumping water through the capillary tube at high pressure until the plug is overcome and water begins to flow from the other end. The unplugged capillary tube may then be placed back into service within the well. 
     However, the present inventors have observed that a capillary tube that has been recently unplugged may develop a new plug rather quickly. Investigating the problem further, the inventors identified that the conventional technique for unplugging a capillary tube may serve to remove a localized plug, but does little or no cleaning of the inner wall of the capillary tube. As a result, simply unplugging a capillary tube using a high pressure fluid was found to leave solid deposits over large amounts of surface area within the capillary tube, and these deposits could quickly cause the capillary tube to again become plugged. 
     Therefore, the present inventors identified a need for a method of cleaning a capillary tube. It would be desirable if the method removed solid deposits from throughout the capillary tubing so that the capillary string performed similarly to a new capillary tube. In particular, it would be desirable that the method removed sufficient amounts of solid deposits so that the capillary string would not become plugged more rapidly than a new capillary tube. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention provides a method and apparatus for removing deposits formed within a capillary tube that has been used to deliver treatment chemicals into a well. The method includes pumping a cleaning solution through the capillary tube coil (after the capillary tube has been removed from the well and is directed to form a coil for cleaning purposes). The cleaning solution comprises at least 20 weight percent of a surfactant or dispersant, at least 5 weight percent of a coupling agent, and at least 10 weight percent solvent. A preferred surfactant or dispersant is selected from the group consisting of an alkyl-aryl sulphonate and a phosphate ester. Dodecylbenzenesulphonic acid is particularly preferred. Depending on the pH of the cleaning formulation, surfactants having ionizable moieties may be in hydrogen form, or salt form. If in salt form, the counter cation will correspond to the conjugate base, or mixture of conjugate base species that were present before blending the components. pH can be controlled in all such formulations described in this invention, either by adding a weak acid, weak base, strong acid, or strong base in amount sufficient to maintain formulation stability, that is, to keep all components in solution, and to maintain proper pH for optimum capillary string cleaning and minimal metal loss, if any, from the capillary string. A specific embodiment of the cleaning solution includes dodecylbenzeneylsulphonic (DDBSA) acid as the surfactant or dispersant, ethylene glycol monobutyl ether as the coupling agent and toluene as the solvent. 
     Generally embodied in this invention is the use of any type of surface-active agent (surfactant) or admixture of surfactants combined with a suitable solvent system. Said solvent system could be aqueous and/or organic based. These surfactants can be anionic, cationic, nonionic, zwitterionic, or amphoteric. Any other compound having surface active properties, such as hydrotropes, would also be within the scope of this invention. Hydrophobic moieties of said surfactants can be linear, branched, contain unsaturated regio-chemistry, and have regions of polar atom substitution to the extent that the chain can still be considered hydrophobic. Furthermore, the action of the cleaning mixture, while being described as a surfactant, dispersant, or wetting agent, can be described in more general terms as an emulsifier, solubilizer, detergent, or anti-redeposition agent. 
     Another embodiment of the invention provides a method for removing deposits formed within a capillary tube that has been used to deliver treatment chemicals into a well, comprising flowing a cleaning solution through the capillary tube and into the well, wherein the cleaning solution comprises at least 20 weight percent of a surfactant or dispersant, at least 5 weight percent of a coupling agent, and at least 10 weight percent of a solvent. Optionally, hydrocarbons may be produced from the well while the cleaning solution flows through the capillary tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a perspective view of a capillary tube coil secured on a spool. 
         FIG. 2  is a schematic diagram of a system for cleaning a capillary tube coil. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventors have discovered that the solids that restrict and plug a capillary tube are formed from combinations of well treatment fluids that were previously believed to be stable under a full range of operating conditions. Now it has been discovered that well treatment fluids may experience conditions in which the combinations of well treatment chemicals interact to form solids that adhere to the inside surface of a capillary tube. The conditions that give rise to the formation of solids in a capillary tube may not be uniformly experienced in all fields, in all wells within a field, or at all positions within a single capillary tube. For example, the hardness of the source of water used to blend with the treatment chemicals may affect solids formation. Furthermore, the temperature of the well treatment fluids rises considerably with the increasing depth of the capillary tube and the depth of the wells can vary significantly. Still further, variations between producing formations may lead to the use of different combinations of the treatment chemicals from one well to another. Accordingly, the properties of the treatment chemicals, the exact ratios of the chemicals, and the range of conditions that may be experienced in a capillary tube can cause the deposition of solids. The exact nature of the solid may be crystalline, semi-crystalline, amorphous, or liquid crystalline (lamellar, cubic, etc.). The solid may also be in the form of an aggregate, having dispersed crystalline particles in an amorphous or liquid crystalline matrix. 
     One embodiment of the invention provides a method of cleaning a capillary tube that has developed solid deposits that restrict or plug the flow of treatment chemicals into a well. Optionally, the method may be performed by pumping a cleaning solution through the capillary tube in situ and into the well so that the capillary tube does not have to be removed. If the well can continue to produce without treatment chemicals during the period of the in situ cleaning, then hydrocarbons may continue to be produced from the well during the performance of the cleaning method. However, the method is preferably performed by pumping a cleaning solution through the capillary tube after the capillary tube has been removed from the well. The capillary tube is typically coiled onto a spool or other device for compact and organized handling of the capillary tube. Having removed the capillary tube from the well, the scope and duration of the cleaning method are not constrained by the well conditions. If desired, a different capillary string may be run into the well to continue the supply of treatment chemicals. 
       FIG. 1  is a perspective view of a capillary tube coil  10  secured on a spool  12 . The spool  12  has two circular ends  14  that can be used to handle the spool and protect the capillary tube coil. A generally cylindrical body  18  (shown by dashed lines) extends between the circular ends  14  about a central axis  16 . The diameter of the cylindrical body  18  is about one foot so that the initial layers of capillary tube coiled onto the spool are not bent or kinked. A hole  20  is made through a circular end  14  radially adjacent the outer surface of the cylindrical body  18  to receive a first end of the capillary tube prior to coiling. The first end  22  of the tube  10  is secured through the hole, such as using large staples. The capillary tube is then coiled about the body  18  such that successive layers of capillary tube are coiled about the initial layers up to a maximum diameter of about four feet. A second end  24  of the capillary tube  10  is secured to the spool to prevent the tube from accidentally coming uncoiled during handling. 
     Once a capillary tube has been removed from the well for cleaning, the capillary tube may need to be conditioned for receiving a cleaning solution. For example, where the capillary tube has been in service delivering aqueous treatment fluids to the well, as is typically the case, and the cleaning solution would cause the formation of a stable emulsion, then a substantial amount of the water remaining in the capillary tube should be removed. The water may be removed from the capillary tube by passing an alcohol through the tube. Although various alcohols may be used, isopropyl alcohol is particularly preferred. It is generally not necessary to completely fill the entire volume of the capillary tube or pass multiple volumes through the capillary tube. Rather, the water may be sufficiently removed by pumping a small plug, such as 10 percent of the capillary tube volume, of an alcohol into the tube ahead of the cleaning solution. 
     In accordance with the invention, a cleaning solution is pumped through the capillary tube. Preferably, the method includes at least one period of pumping the cleaning solution through the capillary tube at a velocity of between 0.1 and 2.5 meters per second, which may be accomplished by controlling the volumetric flow rate. The high velocity of the cleaning solution and the interactions at the interface between the cleaning solution and deposits within the tube are an important part of the cleaning process. 
     One embodiment of the invention provides a cleaning solution that comprises at least 20 weight percent of a surfactant or dispersant. Preferably, the cleaning solution comprises between 20 and 70 weight percent of the surfactant or dispersant. A particularly suitable surfactant or dispersant is selected from the group consisting of alkyl aryl sulphonates and phosphate esters. These surfactants can be anionic, cationic, nonionic, zwitterionic, or amphoteric. A preferred alkyl-aryl sulphonate is dodecylbenzenesulphonic acid (DDBSA). 
     Optionally, the cleaning solution may further comprise at least 5 weight percent of a coupling agent. Preferably, the cleaning solution comprises between 5 and 30 weight percent of the coupling agent. A preferred coupling agent is ethylene glycol monobutyl ether. Examples of other types of glycol ether coupling agent include, but are not limited to, short-chain alkyl ethers of ethylene glycol, propylene glycol, or butylene glycol, phenolic ethers of ethylene glycol, propylene glycol, or butylene glycol, and the like. Acetates, ethers, and other derivatives of these coupling agents could also be employed, provided that they impart coupling action, such as a low molecular weight alcohol cosolvent. 
     Still further, the cleaning solution may comprise at least 10 weight percent solvent, such as toluene. Preferably, the cleaning solution comprises between 10 and 20 weight percent of the solvent. Other aromatic solvents like xylenes and others are useful. Branched, cyclic, linear, unsaturated hydrocarbons, and the mixtures thereof, can also be used in this application. Aliphatic solvents alone or in combination with aromatic solvents may also be used. These solvents may also include olefinic functional groups. 
     Cleaning formulations within the scope of this disclosure can be composed of any type of surface-active agent (surfactant) or admixture of surfactants combined with a suitable solvent system. Said solvent system could be aqueous and/or organic based. These surfactants can be anionic, cationic, nonionic, zwitterionic, or amphoteric. Any other compound having surface active properties, such as hydrotropes, would also be within the scope of this patent. Furthermore, the action of the cleaning mixture, while being described as a surfactant, dispersant, or wetting agent, can be described in more detail as an emulsifier, solubilizer, detergent, or have anti-redeposition properties. Still further, the initial mode of action for the cleaning operation on a micro-scale, is the formation of a surfactant interface on the plugging material. This interface is formed by surfactant molecules that spontaneously align and form a single mono-layer, or multi-layered structure on the plugging material. Solvent molecules can also be involved in helping to break up the plugging material in the presence of surfactants. The dispersed plugging material is then solubilized and/or dispersed by various mechanisms, such as roll-up, micellar solubilization, and the like. Surfactant aggregates, such as micelles, can act to maintain solubility of the removed plugging material by internal sequestration inside the structure. It is anticipated that, depending on the polarity of the liquid medium used, that reversed micelles or normal micelles will be in preponderance, thus acting to solubilize polar and non-polar plugging components as needed. Surfactant aggregates may consist of spherical micelles, worm-like micelles, or liquid-crystalline aggregates that are sufficiently mobile to remove the plugging material from the capillary tubing. Furthermore, the internal structure of these surfactant aggregates can act as a solubilization locus for the solvent component and coupling agent of the cleaning system to a given extent. It is further known that a certain fraction of the solvent, surfactant, and coupling agent alike will have a definite baseline free solubility in the aqueous medium. Moreover, there will be an equilibrium between free solvent and coupling agent in solution and that found solubilized in surfactant aggregates. 
     The duration of pumping the cleaning solution through the capillary tube coil may be variable or fixed. For example, the cleaning solution may be pumping through the tube for a fixed period of at least two hours. However, another embodiment includes the steps of measuring the pressure drop and flow rate of the cleaning solution pumped through the capillary tube coil, and continuing to pump the cleaning solution through the capillary tube coil until the capillary tube coil is determined to be clean. For example, the capillary tube coil may be determined to be clean if the measured pressure drop and flow rate are within 15 percent of a pressure drop and flow rate measured on a new capillary tube coil having the same nominal length and diameter under the same nominal pressure and other conditions. Other embodiments may determine the tube to be clean on the basis of a total volume of solids removed/filtered out of the cleaning solution or a reduction in the rate of solids removal/filtration. 
     A further embodiment comprises pumping the cleaning solution into the capillary tube string at a constant supply pressure, measuring a flow rate of the cleaning solution being pumped through the capillary tube coil, and continuing to pump the cleaning solution through the capillary tube coil until the measured flow rate is within 15 percent of a flow rate that would be expected through a new capillary tube coil having the same nominal length and diameter under the same nominal pressure and other conditions. 
     Similarly, a separate embodiment comprises pumping the cleaning solution through the capillary tube coil at a constant flow rate, measuring the pressure drop of the cleaning solution between the ends of the capillary tube coil, and continuing to pump the cleaning solution through the capillary tube coil until the measured pressure drop is within 15 percent of a pressure drop that would be expected through a new capillary tube coil having the same nominal length and diameter under the same pressure and other conditions. 
     In yet other embodiments, the step of pumping the cleaning solution through the capillary tube may be simultaneously or sequentially combined with an additional step or process selected from (1) reversing the flow direction of cleaning solution being pumped through the capillary tube coil, (2) changing the orientation of the capillary tube coil, (3) heating the capillary tube coil or the cleaning solution so that the capillary tube coil or the cleaning solution have a temperature greater than ambient temperature, and combinations thereof. Still further, the cleaning solution may remain in the capillary tube coil (i.e., soaking) for a period greater than one hour without pumping. It should also be recognized that one or more of the foregoing steps may be repeated any number of times to achieve the desired result of cleaning the capillary tube. During these operations of flowing, soaking, reverse flow, pulsed flow, or heating, the cleaning mixture will be effectively wetting, emulsifying, dispersing, and solubilizing components of the plugging material. Two main processes come into play in these scenarios. One being the kinetics of removal, and the other being energy input to physically aid in removal of sequestered/solubilized plugging material. 
     The capillary tube may be heated by thermal conduction, convection, radiation, or a combination thereof. Furthermore, the method may include the direct or indirect heating of the tube or the cleaning solution. For example, an entire capillary tube coil may be placed inside a heated chamber, which facilitates the heating of the cleaning solution while it is flowing through the coil. 
     The deposits within the capillary tube may be soaked with the cleaning solution by pumping the cleaning solution into the tube, then shutting off the pumps. Accordingly, the time duration of contact between the cleaning solution and the deposits may be increased without the expense of continued pumping. 
     Optionally, the direction in which the cleaning solution is flowing through the tube may be reversed in order to introduce a new dynamic to the cleaning mechanisms operating within the tube. The fluid experiences alternate locations of turbulence in reverse flow which increases the frictional interactions along the surface area between the solution and the solids. 
     In a further option, ultrasonic waves may be induced on the capillary tube so that the mechanical energy of creating and collapsing microscopic cavitation bubbles within the fluid inside the string will help break up particle adhesions. Other methods for causing vibration of the deposits may also be utilized. 
     In a still further option, the capillary tube coil may be repositioned and/or rolled about one or more axis of rotation to vary the fluid trajectory within the capillary tube and assist with particle removal. Repositioning and/or rolling may also increase the removal of particles by overcoming the force of gravitational, frictional, and velocity limiting effects. 
     Cleaning the capillary tube while the tubing is coiled around a spool complicates the cleaning process by dramatically increasing the pressure drop through the tube from about 100 psi for a linear capillary tube to perhaps 6,000 psi for the same capillary tube disposed in a coil of about 1-4 feet in diameter. The absence of a reliable flow model for coiled tube having such a small diameter makes it difficult to determine if the capillary tube is clean. Accordingly, certain embodiments of the invention utilize a collection or database of fluid pressure and fluid flow measurements as a function of diameter and length for new (clean) coiled capillary tubes. Used capillary tubes are preferably cleaned until the pressure and flow measurements meet established criteria, such as a setpoint percentage or standard deviation, relative to the benchmark values for a new capillary tube of substantially the same length and diameter under substantially similar conditions. 
     One embodiment of the invention uses measurements of the cleaning solution pressure and/or flow rate through the capillary tube as a quantitative indicator of the degree of cleanliness for an in-service or out-of-service capillary tube. A database of pressure and/or flow rate measurements are collected, wherein each database record includes, for example, the pressure, flow rate, temperature, fluid composition, capillary tube diameter, capillary tube length, and pump specifications. An application program utilizes pressure and/or flow rate measurements from a given capillary tube to calculate/estimate the cleanliness of the given tube based upon a comparison against data previously collected from capillary tube cleaning runs involving identical parameters, such as pressure, flow rate, temperature, fluid composition, capillary tube diameter, capillary tube length, and pump specifications. The database may be populated with data from large numbers of previous cleaning runs, but preferably includes at least the pressure and flow rate measurements of a new (entirely clean) capillary tube of the same diameter, substantially the same length (for example, within 10 feet), and the other parameters being substantially the same. A numerical scale can be established with the flow rate through the new capillary tube being considered, for example, 100% flow (the highest cleanliness rating) and a hypothetical plugged tube being considered, for example, 0% flow (the lowest cleanliness rating). The numerical scale may or may not be linear and the end points of the scale may differ from this example. 
     Optionally, the database will include a second record that includes the pressure and flow rate measurements of a heavily restricted or substantially plugged (but not completely plugged) capillary tube with all other conditions or variables being substantially identical to the conditions or variables under which the measurements of the new capillary tube were made. If the two records for the new and heavily restricted capillary tubes were made at the same pressure (such as using the same constant pressure pump), then the respective flow rates might establish end points of a broad range of flow rates (or scale of cleanliness) that are possible. Other types of scales are similarly possible. For example, if the records for each capillary tube cleaning run include the initial flow rate and the total volume of solids removed, then it is possible to calculate an effective volume for a capillary tube and a function of flow rate. Accordingly, an effective volume scale could be adopted in which a new capillary tube represents 100% of the effective volume and a hypothetical plugged capillary tube represents 0% effective volume. 
     The amount of data necessary to establish a useful database can be reduced by standardizing the parameters that are controlled by the cleaning system. For example, a heating system (or even moderate ambient conditions) may be used to assure substantially the same temperatures are used from one cleaning run to another, such that temperature is eliminated as a variable. Using the same or identical pump for each run may eliminate another variable, and yet another variable is eliminated if the pump is a constant pressure or constant flow rate pump. Using the same cleaning solution or a cleaning solution having substantially similar flow characteristics eliminates a further variable. Accordingly, the database should include at least one new capillary tube record and at least one used capillary tube record for each combination of capillary tube diameter and capillary tube length that are likely to be encountered. Only a few nominal diameters of tubing are expected to be used as capillary tubes (i.e., ⅛ and ¼ inch) and capillary tube length can be varied in increments, such as every 10 feet or every 50 feet. 
     In another optional embodiment, particle size analysis (PSA) is performed on the solids removed from the capillary tube. It is believed that particle size will show a correlation with cleanliness of the capillary tube. For example, if there is a decline in the size of solid particles being removed from the capillary tube as the flow rate increases, then particle size measurements may serve as a beneficial primary or secondary indicator of cleanliness. 
     The capillary tube cleaning methods may be performed in closed or open loop systems. Methods that involve cleaning the capillary tube in situ (without removal from the well) are one example of an open loop system. In the latter methods, the cleaning solution is not reused and the pressure and flow rate of the cleaning solution must be measured at the inlet (up hole) end of the tube. It should be noted that the up hole pressure measurement of the cleaning solution is a back pressure and not a differential pressure. However, if a constant pressure pump is used in this application, then the primary measurement is a flow rate measurement which should be the same at both ends of the tube. 
     In an optional embodiment, the cleanliness of an in situ capillary tube may be preliminarily assessed using pressure and flow rate measurements of the treatment fluids, rather than a cleaning solution. Accordingly, a cleaning solution does not need to be introduced into the capillary tube unless the preliminary assessment determines that the tube contains restrictions that require removal. 
       FIG. 2  is a schematic diagram of a system  30  for cleaning a capillary tube coil. A spool  12  securing a capillary tube coil is positioned for coupling the opposing ends  22 ,  24  of the tube in fluid communication with the system  30 . Valves  32  are disposed on the upstream and downstream ends of the tube to facilitate isolation of the system while capillary tube coils are being exchanged. A set of tanks  34 ,  36 ,  38  and associated pumps  35 ,  37 ,  39  are provided to selectively supply a fluid to the capillary tube being cleaned. These tanks will generally include a cleaning solution tank, an isopropyl alcohol tank, and a water tank. 
     Water is not used in the cleaning process but is used in combination with an IA plug to displace the cleaning solvent out the tubing. The cleaning solution is acidic and its removal is necessary for the safety of those handling the string after cleaning. Also the water could be used for flow rate and pressure drop measurements. These fluids may be pumped into a drain  40  or passed through a filter  42  to a high pressure pump  44  that is capable of flowing fluid through a heavily restricted capillary tube. The pipe section on the outlet side of the pump  44  includes a set of pressure relief valves  46 , a pressure gauge or sensor  48  and a temperature gauge or sensor  50 . 
     In the embodiment shown in  FIG. 2 , the spool  12  securing the capillary tube is secured in a heated chamber  52  that also includes a mechanism for automatically rotating the spool about one or more axis  54 . Heating and/or rotation may be used to help loosen and remove solids from the walls of the capillary tube. 
     The pressure and temperature of fluid exiting the capillary tube (to the right of the chamber  52 ) is measured by a pressure gauge or sensor  56  and a temperature gauge or sensor  58 . The fluid may then be optionally passed through one or more filtration unit  60 , a particle size analyzer  62 , and/or a flow meter  64  before being directed either to a drain  66  or back into a tank containing the same fluid. Accordingly, the system  30  can recycle any of the fluid used in the process, but mixtures of different fluids may be directed to the drain or waste container. 
     The measurements made by the components of the system  30  may be made and recorded manually, but it is preferable to make the measurements with electronic sensors that communicate with a process controller (not shown). The process controller may be a specialized or multipurpose computer having memory for storing the measurements in a database. Furthermore, the process controller may run an application program that includes computer executable instructions for controlling the system  30  in a manner consistent with the cleaning methods of the present invention. Accordingly, the process controller may control the operation of valves, pumps, heated chambers and rotation mechanisms shown in  FIG. 2  and receive measurements and other input from the sensors and other components shown in  FIG. 2  in order to execute any or all of the cleaning methods of the present invention. Furthermore, the application program should accept user input, such as through the use of a computer keyboard, mouse, or touch screen, regarding the capillary tube length, capillary tube diameter, and any user setpoints regarding the degree of cleanliness required before the tube is determined to be clean. Optionally, the user may instruct the application program to determine cleanliness on the basis of flow rate, total solid volume removed, or the like by selecting from one or more scales. 
     The terms “comprising,” “including,” and “having,” as used in the claims and specification herein, shall be considered as indicating an open group that may include other elements not specified. The terms “a,” “an,” and the singular forms of words shall be taken to include the plural form of the same words, such that the terms mean that one or more of something is provided. The term “one” or “single” may be used to indicate that one and only one of something is intended. Similarly, other specific integer values, such as “two,” may be used when a specific number of things is intended. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.