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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional application Ser. No. 61/101,235, filed Sep. 30, 2008, entitled “Plug Catcher,” the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to completion and stimulation of oil and gas and more particularly, but without limitation, to filtering return well fluids in a plug drill out operation. 
     BACKGROUND OF THE INVENTION 
     There are many situations while completing or performing remedial work on a well where it becomes necessary to isolate particular zones of a well. One reason for isolating a zone is for performing multiple stage downhole stimulations. Industry available products that will isolate the well bore to prevent passage of fluid to other zones are called “plugs.” 
     Essentially a plug isolates some part of the well from another part of the well. There are several types of plugs, including bridge plugs and frac (fracture) plugs. A bridge plug or frac plug is placed within the wellbore to isolate upper and lower sections of a zone. Bridge plugs hold pressure from both directions, while a frac plug holds pressure from above but allows upward flow. Plugs may be temporary or permanent. 
     A plug is removed by drilling or milling through it with a bit or blade in combination with circulating a drilling fluid through well to bring up the debris. In a drilling/milling operation, fluid is circulated from the surface through the bit or mill to flush the debris and cuttings from the well. The fluid carries the cuttings and debris to the surface where it is piped to a return tank. 
     At times it is necessary to work on these wells in an under-balanced condition where the pressures on the well must be controlled by using a choke or choke manifold. A choke is basically a restriction in the return line to hold pressure against the returning flow stream. With the pump rate being constant, the choke or choke manifold will control the downhole pressure. The larger the choke size/opening, the lower the back pressure and the lower the downhole pressure. Conversely, the smaller the choke size/opening, the higher the back pressure and the downhole pressure. 
     Chokes can be fixed or adjustable. Fixed chokes, also called positive chokes, are basically an orifice and come in a variety of sizes. An adjustable choke is variable and can be controlled electrically, hydraulically, pneumatically, or manually. 
     Because of their small openings, both fixed (positive) choke and variable chokes are susceptible to debris blocking. Inadvertent restrictions in the flow path can cause undesirable conditions in the well bore associated with drilling and/or milling operations. A restricted flow stream will reduce the ability of the circulated fluid to carry the debris and cuttings to the surface. This condition is serious as it may result in the pipe becoming stuck in the wellbore. 
     Plugs can be constructed of various materials, including composite materials and metals, such as brass, steel, aluminum, and cast iron. Depending on the material of the plug, the cuttings and debris may include small particulates and/or large rubber or fibrous shreds. Factors determining the size and composition of the debris and cuttings include the differential pressure across the plug when it is milled or drilled, the size of the mill or bit, and the techniques used to break up the plug. 
     The amount of debris and cuttings produced is dependent on the pipe diameter, pressure rating, plug style and plug manufacture. Common casing size can range from 2⅜ to 9⅝ inches. For example, a 4½ inch plug can produce 300 cubic inches of loose debris. The number of plugs used in a single well is dependent on the number of zones. It is not uncommon to have as many as 15 plugs in a single well. 
     When a choke or choke manifold is used during a milling or drilling operation, the debris can cause the choke to plug causing instability in the milling or drilling operation. There are two common practices for choke installations in a plug milling operation. One is a single fixed choke bean located in or at the return tank. The other is a choke manifold. 
     If a single choke bean method is used, when debris clogs the choke, the well has to be shut-in and milling operations stopped until the choke can be cleaned and put back into service. If a choke manifold is used and debris clogs one of the chokes, that choke can be bypassed to the other parallel choke. In this process, one person typically is cleaning the clogged primary choke while another person is trying to adjust the secondary choke back to the desirable backpressure. Not only does this process require extra manpower, but there is also the possibility that both chokes get clogged at the same time and the well has to be shut-in until a choke is cleaned. 
     As debris collects on a choke, holding a consistent backpressure can be difficult. The choke is opened farther to compensate for the debris restriction; but as the choke is opened, the debris can dislodge, reducing the backpressure, or the debris could clog further increasing backpressure. 
     In a drilling/milling operation, it is beneficial to remove the milling shavings before the flow stream reaches the choke. Filters or strainers can be placed upstream of the choke to prevent the debris getting to the choke. However, in such systems, parallel filtering systems with a bypass valving arrangement may be required. 
     The present invention provides the ability to drill continuously multi-plug zones under most common conditions without interrupting the drilling/milling operation to clear a clogged choke. In addition, the invention provides a compact, modular, single filtering system that is easily rigged and can be cleaned while in service. These and other advantages of the invention will be apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a modular filter system constructed in accordance with a preferred embodiment of the present invention. 
         FIG. 2  is a partially cut-away perspective view of the filter system shown in  FIG. 1 . 
         FIG. 3  is a perspective view of the filter screen preferably used in the system shown in  FIGS. 1 and 2 . 
         FIG. 4  is a table illustrating the process steps of the filter method of the present invention. 
         FIG. 5  is a flow chart illustrating the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the drawings in general and to  FIG. 1  in particular, there is shown therein a modular filtering system constructed in accordance with a preferred embodiment of the present invention and designated generally by the reference numeral  10 . The system  10  is adapted for filtering debris and other particulates out of a fluid stream received from a well, such as an oil or gas well (not shown) undergoing a drill out, flow back, well-test or other operation. While only one system  10  is shown in the drawings, multiple systems may be used in parallel. 
     The system  10  comprises a main filter line  12 , a flow back line  14 , and a bypass line  16 . The filter line  12  comprises a filter section  18 . The filter section  18  is adapted to allow the fluid stream from the well to pass through while separating solids from the fluid. A preferred filter section  18  comprises an outer tube or manifold spool  20  inside of which is mounted an inner filter tube  22  shown in  FIGS. 2 and 3 , which will be described in more detail below. 
     A pressure sensor or gauge  24  is provided on the manifold spool  20 . On the upstream end of the manifold spool  20  is an isolation valve  26  which connects to an inlet T  28 . Extending upstream from the inlet T  28  is a fitting, such as the wellhead connection  30 , which is adapted to connect to the wellhead (not shown). Thus, the valve  26 , the inlet T  28  and connector  30  form an inlet line  32 . A pressure sensor or gauge  34  is fixed to the inlet T  28  in the inlet line  32  to monitor the upstream pressure in the system  10 . 
     On the downstream end of the spool  20  is a debris transfer line  35  comprising a downstream isolation valve  36  that connects the filter  18  to the inlet end  37  of a debris tube, such as a 3-inch pup joint  38 . The outlet end  39  of the pup joint  38  is equipped with a T-joint  40  in a discharge line  41  to direct debris flow through a valved orifice, such as a choke valve, which may be an adjustable 2-inch orifice choke  42 . The open end  43  ( FIG. 2 ) of the pub joint  38  is provided with a removable cap  44 . A magnet (not shown) may be included in the cap  44  to attract and capture metal fragments in the debris flow. The outlet of the choke  42  is equipped with a connector  46  for connecting the system  10  to the debris pit (not shown). As used herein, “debris pit” denotes any excavation, vessel or collector for containing debris or other solids recovered from the return well fluids. 
     The filter tube  22  is shown best in  FIG. 3 , to which attention now is directed. The filter tube  22  comprises an elongate tubular body or member  50  with a plurality of slots, designated collectively at  52 , forming a perforated side wall. The perforations  52  allow fluid communication between the inside and outside of the tube  22 . The upstream or inlet end  50 A and the downstream or outlet end  50 B of the tubular member  50  are provided with collars  54  and  56  by which the tube  22  is mounted inside the spool  20 , as seen best in  FIG. 2 . 
     The outer diameter (O.D.) of the filter tube  22  is less than the inner diameter (I.D.) of the manifold spool  20  to provide an annulus  58  ( FIG. 2 ) to receive the filtrate, that is, the filtered fluid stream. In this way, during normal operation, the residue or debris in the fluid stream will be retained inside the filter tube  22  while the filtrate passes through the slots  52  in the annulus  58 . For example, in the embodiment shown, the O.D. of the filter tube  22  is 3½ inches while the I.D. of the spool  20  is 5½ inches, providing a 1-inch annulus  58 . 
     With continuing reference to  FIGS. 1 and 2 , the flow back line  14  preferably comprises a first outlet or flow back valve  60  connected to the downstream end of the manifold spool  20 . The flow back valve controls the fluid flow from the filter to the flow back line and. A second outlet or backflow valve  62  in a backflow line  64  may also be included for uses to be described and, when included, is connected to the upstream end of the spool  20 . A connecting pipe  66  makes a fluid connection between the first and valves  60  and  62 . That is, the connecting pipe  66  forms a part of both the backflow line  64  and the bypass line  16  and is a common fluid connection to the flow back line  14 . 
     An outlet T  70  in the flow back line  14  is connected to the outlet of the first outlet valve  60 . A fitting or connector  72  is provided on the outlet T  70  to connect the T to the flow back tank for directing the filtrate to the flow back tank (not shown). “Flow back tank” is used broadly and refers to any vessel or collector suitable for holding fluids processed by the filter system  10 . A purge valve  74  is connected to the outlet T  70 . A valved orifice, such as a choke valve  76 , is connected between the purge valve  74  and the main filter line  12  between the pup joint  38  and the downstream isolation valve  36  using a connecting joint  78  that forms a purge line. 
     Referring still to  FIGS. 1 and 2 , the bypass line  16  will be described. The bypass line  16  comprises a bypass valve  82  connected between the main filter line  12  and the second outlet valve  62  (or the first outlet valve  60 , if there is no second valve  62 ). The inlet of the bypass valve  82  is connected to the main filter line  12  between in the inlet T  28  and the upstream isolation valve  26 . The outlet of the bypass valve  82  is connected to the second outlet valve  62  (or first outlet valve  60 ) by a connecting joint  84  forming part of the bypass line  16 . 
     The use and operation of the inventive system is illustrated in the Process Logic Table shown in  FIG. 4  and flow chart shown in  FIG. 5 , to which attention now is directed. The fluid stream enters the system  10  at the wellhead connection  30 . With the upstream isolation valve  26  and the first outlet valve  60  open and the other valves closed, the fluid stream passes directly through the filter section  18 . The debris collects or stacks up inside in the filter tube  22  and the filtrate passes through the annulus  58 , out the outlet valve  60  in the flow back line  14 , and finally out the outlet T  70  to the flow back tank. 
     The operator monitors the system  10  to determine when the filter tube  22  is full or near full and needs cleaning. This determination may be made by monitoring the pressure differential between the upstream and downstream pressures as indicated by the gauges  24  and  34 . Alternately, cleaning intervals may be scheduled based on the filter capacity and the expected volume of debris generated by the milled plug. Still further, the cleaning mode may be scheduled at regular intervals to ensure that the filter never becomes overly clogged. The control of the system  10  as described herein is carried out manually by a human operator. However, it will be understood that the operation of the system  10  alternately be controlled by a computer-run control system (not shown). 
     The cleaning mode begins by equalizing the pressure across the downstream isolation valve  36  and then opening that valve. First, the purge valve  74  is opened and then the purge choke  76  is adjusted. Next, the purge valve  74  and choke  76  are both closed, and the isolation valve  36  is opened. Next, the debris choke  42  is adjusted to allow the debris to move into the pup joint  38 . The debris may then be isolated in the pup joint  38  by closing the isolation valve  36  and the debris choke  42 . It will be appreciated that this cleaning operation can be performed without disrupting the return flow from the well through the filter. 
     To remove the debris from the pup joint  38 , the purge valve  74  is opened, the choke  76  is adjusted, and the debris is purged from the system  10 . When the purge is completed, the purge choke  76  is closed, the debris choke  42  is closed, and the purge valve  74  is closed. The system  10  now is reset to the normal flow back mode. 
     In some instances, the filter may be cleared manually. To do so, the upstream isolation valve  26 , the purge valve  74 , and both the outlet valves  60  and  62  are closed, and the bypass valve  82  and the downstream isolation valve  36  are opened. This diverts the flow stream straight through the bypass line  16  and out the flow back line  14 , totally bypassing the filter line  12 . While the fluid stream is thus diverted, but not interrupted, the filter section  18  may be cleaned manually with a suitable tool. 
     The filter system  10  provides an important advantage during servicing of the system between uses, that is, when the system is disconnected from the well or other source. It will be seen from  FIGS. 1 and 2  that, in the preferred embodiment the filter section  18  and the pup joint  38  are both straight and aligned coaxially with each other and with the inlet  30  the capped end  43 . When the cap  44  is removed from the capped end  43 , a straight line of sight is formed from the end to the inlet  30 . This allows visual inspection of the inside of the inner tube  22  of the filter. 
     It will also now be apparent that during normal operation of the system, the flow stream flows first into the inside of the filter tube  22  and out through the slots  52  of the tube. In some situations, it is advantageous to reverse this flow, that is, to direct the fluid stream first into the annulus  58 , through the slots  52  to the inside of the filter tube  22 . This is accomplished by opening the bypass valve  82 , the downstream isolation valve  36 , and the second outlet valve  62 , and closing the upstream isolation valve  26 , the first outlet valve  60 , the purge valve  74 , and the purge choke  76 . This will direct the fluid first through the bypass line  16 , then through the second outlet valve  62  into the annulus  58  of the filter section  18 . The filtrate would flow through the slots  52 , then through the inside of the filter  22  and out through the open isolation valve  36 . The debris would remain trapped in the annulus  58  until removed. 
     As used herein, “valve” refers very broadly to any device capable of blocking or diverting fluid flow through a conduit. As used herein, a “choke” refers broadly to any device capable of modulating the flow rate of a fluid through a conduit. Thus, as used herein, a “valve” may or may not function as a “choke,” but a “choke” denotes a valve or other device with a fluid throttling capability and thus includes many types of valves. 
     The embodiments shown and described above are exemplary. Many details are often found in the art and, therefore, many such details are neither shown nor described. It is not claimed that all of the details, parts, elements, or steps described and shown were invented herein. Even though numerous characteristics and advantages of the present inventions have been described in the drawings and accompanying text, the description is illustrative only. Changes may be made in the details, especially in matters of shape, size, and arrangement of the parts within the principles of the inventions to the full extent indicated by the broad meaning of the terms of the claim(s).

Summary:
A system and method for separating solids from return fluids in well drill-out, flow back, well-test, and other production operations. Solids are collected in a filter comprising a perforate inner tube inside a solid outer tube with an annulus therebetween. The fluid stream from the well enters the filter through the inner tube so that the solids are captured inside and the filtrate flows out through the annulus. The filtrate is passed though a flow back line to a flow back tank. As needed, the solids are removed from the inner tube into a debris tube without interrupting the fluid flow through the filter. Chokes are included for equalizing the pressure along the flow path as the debris is moved from the filter to the debris tube and from the debris tube into to a debris pit so that dramatic changes in pressure are avoided.