You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
   The present invention relates to downhole tools. More specifically, the invention relates to tools run into a wellbore and apparatus and methods to facilitate their insertion. More particularly still, the invention relates to a centering device having friction reducing members to reduce contact of a tool with the walls of a non-vertical wellbore. The invention also facilitates “pumping” a tool into a wellbore with fluid when gravity is not available. 
   BACKGROUND OF THE INVENTION 
   Various operations require tools to be inserted into a well and fixed there temporarily. In some instances, packers are run into a wellbore and then set using slips and cones that fix the packer at a predetermined location to isolate an annular area of the bore. In other instances, bridge plugs or “frac” plugs are similarly installed to temporarily block the wellbore and provide a barrier against which pressure can be developed to treat a hydrocarbon-bearing formation adjacent the wellbore. In all of these instances, the tool is typically disconnected from a run-in string of tubulars and left in place during the operation. Thereafter, some of the tools can be retrieved to the surface while others must be destroyed with a milling device. 
   Increasingly, hydrocarbons are collected from wellbores that are not vertical but extend outward, sometimes horizontally from a central wellbore. These non-vertical wellbores are cased and completed just like their vertical counterparts and are also subject to the same treatments and tools. Tools can always be run into a non-vertical wellbore on rigid tubing but that requires a rig and complimentary equipment to connect the tubing as it is inserted and removed from the wellbore. Coil tubing is thin-walled, removable, continuous tubing without joints. Coil tubing is available for running tools into a well but must be transferred to the well site on large reels and then requires some type of injector to be installed in the wellbore. 
   Because of the above disadvantages of tubing, the preferred way to install many downhole tools is with wireline. Wireline is a cable comprising one or more conductors which provides real-time communication with a downhole tool and can also bear the weight of the tool. Wireline is designed to be reeled into a wellbore with the tool on one end. In operations requiring many tools to be placed in the wellbore, like fracturing operations including multiple zones, wireline installation saves time and money. 
   Problems with wireline installations arise with non-vertical wellbores simply because gravity is not available to help urge the tool down the wellbore. Rather than move along the center of the wellbore, the tools tend to rest on the low side of the bore, coming into contact with any debris that has settled there. 
   Various means have been used to overcome the problem of wireline delivered tools and non-vertical wellbores. In some instances the tools are “pumped down” with fluid pumped past the tool. This is partially effective but due to the position of the tool on the low side of the wellbore, a large annular gap extends between the top of the tool and the upper wall of the wellbore, making the pumping process partially ineffective. In other instances, tractors are used to help move a tool along a non-vertical portion of a wellbore. Tractors typically have at least one moving member that either rotates or oscillates against a wellbore wall. However, tractors are expensive, cannot be left in a well and add another layer of complication to a tool installation job. 
   There is a need therefore for a method and apparatus that can facilitate the installation of a tool into a wellbore, particularly a non-vertical portion of a wellbore. There is a further need for a tool that has a friction-reducing component to reduce the friction that necessarily arises as the tool moves along a non-vertical wellbore. There is a further need for a tool that has centering capabilities to reduce its tendency to sit on a low side of a non-vertical wellbore. There is yet a further need for a tool that can better utilize an annular area created between the tool and the wellbore to facilitate pumping down the tool with circulating fluids. 
   SUMMARY OF THE INVENTION 
   The invention relates to a system for facilitating the insertion of a tool into a wellbore, especially a non-vertical wellbore. In one embodiment a tool is fixable in a wellbore and includes centralizing, friction-reducing members that serve to keep the body of the tool off the walls of the wellbore wall. In another embodiment the tool includes a wiper ring that partially fills an annular area formed between the centered tool and the wellbore walls. The surface of the ring facing the upper end of the wellbore provides fluid resisting piston surface and permits the centered tool to be pumped down the wellbore more effectively. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a view, partially in section of a wellbore, showing a tool being run in on wireline. 
       FIG. 2  is a section view of a tool including the centralizing, friction reducing members of the present invention. 
       FIG. 3  is a section view of the tool of  FIG. 2  after it has been set in the wellbore. 
       FIG. 4  is a section view of the tool along a line  4 - 4  of  FIG. 3 . 
       FIG. 5  is section view of another tool showing additional embodiments of the invention. 
       FIG. 6  is an end view of  FIG. 5 . 
       FIG. 7  is an enlarged section view illustrating the flow of the fluid through and around the tool of  FIG. 5  as it is being pumped down a wellbore. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a typical completed well with a wellbore  100 , a wellhead  105 , a vertical wellbore section  107  and a non-vertical wellbore section  110 . The wellbore is lined with casing  112 . Installed over the well is a rig  115  placed there to facilitate the insertion of a tool or tools into the wellbore. A truck  120  is shown with a reel  122  of wireline that can be directly placed in the wellbore via a block and tackle assembly  125  of the rig. 
   At a lower end of the wireline  130 , in the non-vertical section  110  of the wellbore is a tool  135 . Like those described herein, the tool is designed to be located via the wireline at a predetermined location in the wellbore and then fixed to the wall of the wellbore by remotely actuating a slip and cone assembly (not shown) built onto the tool. In one instance, the downhole tool is a plug with a central bore that can be temporarily blocked in a single direction during an operation. In a wireline installation, the plug is typically actuated or set using a setting tool  137  schematically shown at an upper end of the tool. The setting tool includes a charge or some chemical compound that creates a force used to cause one part of the tool to move in relation to another part, thereby setting the slip. The action is initiated from the surface of the well by a signal that travels down a conductor in the wireline  130 . Setting tools are readily available and one setting tool is a Baker E-4 wireline setting assembly sold by the Baker-Hughes Company of Houston, Tex. 
     FIG. 2  is a section view of a tool  200  shown in a wellbore  100  prior to being set. For illustrative purposes, the setting tool and wireline string is not shown. The tool includes a first portion and a second portion that are designed to move axially relative to each other in order to compress portions of the tool and set the tool in the wellbore ( FIG. 3 ). The main components of the tool are well known. For instance, there is a deformable sealing member  202  and a set of slips  205  that move across conical surfaces  207  to increase an outer diameter of the tool  200  and place the slips  205 , with their toothed outer surfaces, into contact with the walls of the cased wellbore  100 . 
     FIG. 3  shows the tool set in the wellbore. Relative movement between the first portion of the tool and the second portion has caused the sealing member  202  and slips  205  to contact the wellbore  100  and fix the tool  200  in the wellbore. Visible in both  FIGS. 2 and 3  is a bore  210  of the tool and a ball  215  that is seated in the bore to block the flow of fluid through the bore in at least one direction. Typically, the bore  210  is temporarily blocked to permit pressure to be developed above the tool in order to carry out an operation, like fracing the well. After the operation is complete, some tools are designed to be removed from the wellbore and reused. Others however, are designed to be milled and destroyed and are thus irretrievable. In one instance, the tools are constructed largely of a non-metallic material that can withstand certain extremes of temperatures and pH conditions and can be more easily drilled when the tool&#39;s use is completed. An example of such a non-metallic tool is disclosed and claimed in U.S. Pat. No. 6,712,153, assigned to Weatherford/Lamb, Inc. of Houston, Tex., and that patent is incorporated herein by reference in its entirety. 
     FIGS. 2-7  all illustrate various aspects of the invention designed to facilitate the insertion of a tool  200  like the one shown, into a wellbore, especially a non-vertical wellbore. In the embodiment shown in  FIGS. 2-4 , the tool is provided with a friction reducing system including friction reducing members in the form of rollers  300  that are outwardly extended and radially disposed around a front end of the tool  200 . The relationship of the rollers  300  to the body of the tool  200  and to the wellbore  100  therearound is illustrated in  FIG. 4 . Visible is the body  301  of the tool, bore  210  of the tool and the rollers  300  that are mounted on axles  304  and operate to center the tool in the wellbore, provide a uniform annular space around the tool and prevent substantial contact between the body of the tool and the wellbore  100 . In  FIG. 4 , the rollers  300  contact the wellbore casing  101 , leaving an annular space  302  between the body of the tool  200  and the casing wall. 
   The advantage of this arrangement when a tool is run into a non-vertical wellbore on wireline is obvious. Rather than lay on the lowest side of the wellbore  100 , the tool  200  is held off the sides of the wellbore and only the rollers  300  with their friction reducing qualities are exposed to the wall. Additionally, because of the stand-off, the tool is less likely to be slowed by sediment and other debris that settles on the low side of the wellbore  100 . Finally, the uniform annular space  302  around the tool  200  improves its “pump down” characteristics. The position of the rollers  300  towards the leading end or front of the tool  200  increases their effectiveness. Rather than being installed on some other component, like the setting tool, the rollers are as close as possible to the leading edge of the tool that will be fixed in the wellbore. The rollers are also installed in a manner that ensures the outer diameter of the tool  200  will “draft” through the wellbore  100 . Alternatively, the rollers could be spring-mounted to permit some compliance but in all cases they are designed to maintain the tool coaxially in the wellbore. 
     FIGS. 5 and 6  illustrate another embodiment of the invention that includes an additional feature also designed to facilitate the insertion of the tool into a wellbore.  FIG. 5  shows another version of the tool  200  previously described with a wiper ring  400  installed around an outer perimeter of the tool  200  in a manner whereby the ring  400  extends into the annular space  302  between the tool  200  and the wellbore  100 . The purpose of the wiper ring  400  is to increase back pressure on and around the tool as fluid is pumped past it and used to urge the tool along the wellbore  100 . 
   Also shown in  FIG. 5  are flow ports  500  radially extending around the tool just behind the wiper ring  400  to direct a portion of the fluid from the annular space  302  to an area in front of the tool  200 . The redirection of some of the fluid helps wash debris from the front of the tool while permitting adequate fluid flow to act on the wiper ring  400  as discussed above. 
   The wiper ring  400  increases that back pressure and its use with the centralizing rollers  300  is especially effective since the tool  200  is centered in a way that permits the wiper ring  400  to circumferentially extend into the annular space  302  around the tool rather than assuming an eccentric position due to the effect of gravity in a non-vertical wellbore. 
     FIG. 7  uses arrows  600  to illustrate the flow of fluid through and around the tool  200  as it is urged along the wellbore  100 . The arrows show for example, that a certain portion of the fluid flow is directed to the wiper ring  400  and another portion flows into the ports  500  and out the front tool which includes a “mule shoe” shape  208  at its front end to avoid obstructions in the wellbore. The combination of the various optional features of the invention act together to increase the effectiveness of fluid pushed past the tool in order to urge it along a wellbore, especially a non-vertical wellbore. 
   The system of the present invention is especially useful with tools made substantially of non-metallic material since these are typically lighter than metallic tools and have even less inclination to move in a non-vertical wellbore on their own. The parts of the system including the rollers, axles and the wiper ring are easily and typically made of non-metallic, drillable material and hence do not impede the milling and destruction of a non-metallic or composite bridge plug, like the one described in the ′153 patent incorporated previously herein. Additionally, the components can be made of material effective in uses in extreme pH conditions. 
   As described and as shown in the FIGS., the present invention overcomes many problems associated with running tools into a non-vertical wellbore, especially on wireline or other non-rigid run-in strings. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Summary:
A system for facilitating the insertion of a tool into a wellbore, especially a non-vertical wellbore. In one embodiment a tool is fixable in a wellbore and includes centralizing, friction-reducing members that serve to keep the body of the tool off the walls of the wellbore. In another embodiment the tool includes a wiper ring that partially fills an annular area formed between the centered tool and the wellbore walls. The surface of the ring facing the upper end of the wellbore provides fluid resisting piston surface and permits the centered tool to be pumped down the wellbore more effectively.