Abstract:
A method and apparatus for retrieving magnetic casing fragments from a well using a shrouded magnet are disclosed. Retrieving casing fragments allows the determination of weight loss from the casing to assist in analyzing the integrity and the condition of the casing and to determine whether more expensive analysis is required. The shrouded magnet, that is formed by a non-magnetic container enclosing a source of magnetic field, recovers metal casing fragments by attracting them and other magnetic materials from oil and gas well fluids passing by the shrouded magnet device, which fragments are easily separated from the device by removing the source of magnetic field from the container.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus and method for removing magnetic materials from fluids discharged from a well, and more particularly to an apparatus and method for removing casing fragments from drilling and workover fluids which are circulated in oil and gas wells to ascertain the metal loss from the casing lining the bore of the well. In the drilling of oil and gas wells, drilling fluid, commonly referred to as “mud,” is used for a variety of purposes, including: (1) maintaining hydrostatic pressure on the zones being drilled to maintain control over high pressure zones; (2) removing drill cuttings from the well and the face of the bit; and (3) to assist in drilling by the jetting action of the drilling fluid through the nozzles of the bit. Drilling fluid is commonly circulated down the string of drill pipe, pumped through the nozzles of the bit, and circulated out of the well through the annulus between the drill pipe and the casing and/or open hole. Once the drilling fluid returns to the surface, the fluid is circulated through various pieces of equipment to remove cuttings and solids so that the drilling fluid may be recirculated back into the wellbore. 
     As a well is drilled, steel casing is commonly inserted and cemented in the well to line those portions of the well already drilled. The casing protects the well from collapse, cave-in, and provides control over pressurized zones. In the course of drilling a well, multiple strings of casing may be inserted into the well, each subsequently installed casing string a smaller diameter than the previously installed casing string. Once a casing string is cemented in place, drilling operations may continue by drilling out through the casing “shoe.” In some cases, such as when the lower portion of a well is lost, or if a well is being redrilled, the casing wall will be intentionally drilled through or milled in order to side-track the well, and drill in a different direction. However, at other times the casing wall is penetrated unintentionally. 
     It is known that when drilling, and when completion and workover tools are run through or operated inside of the casing, casing damage may occur. Often, the tolerances between the inside diameter of the casing and the outside diameter of the drill bit, drilling assembly, or other tools are tight, causing casing wear or puncture. Casing may also be damaged from continued rotation of the drilling assembly or drill pipe inside the casing, repeated trips of tools, the drilling assembly and drill pipe through the casing, or down hole conditions which result in the drill bit penetrating the wall of the casing rather than drilling through the casing shoe or formation. Because the casing protects the integrity of an oil and gas well, and protects the surrounding environment from releases of hydrocarbons from the well bore, it is important and useful to monitor the condition of the casing strings, particularly during drilling and workover operations. 
     Various means are known for monitoring casing integrity. Various downhole tools, such as mechanical calipers or electronic evaluation tools may be run through the casing to determine remaining wall thickness or to identify places where the casing wall has been damaged. However, running these tools is expensive, and generally requires removing the drill pipe, drilling assembly and bit from the well. Because of the expense, downhole evaluation tools are generally not run until there is reason to believe the casing may have been damaged, or where it is desirable to acquire a baseline analysis of the casing condition. 
     It is therefore desirable to have a cost-effective method of monitoring casing wear during drilling operations. One such method is to collect casing fragments contained within the drilling fluid, and weigh and record the weight of the fragments to estimate the total weight of casing loss and compare the amount of loss to the initial weight of the casing. Visual examination of the recovered fragments or more detailed analysis may also provide important information regarding the location or extent of the casing damage. If this method indicates an abnormal degree of metal loss from the casing, downhole tools may be run to determine the location and extent of damage. 
     Solids and cuttings are generally removed from drilling fluids at the surface by solids control equipment such as shale shakers and hydrocyclones, which dump solids into collection bins. In order to efficiently and accurately recover casing fragments, any device used to recover the fragments must be placed between the point of fluid discharge from the well and the solids control equipment. It is known to place a “ditch magnet” into the drilling fluid system to collect casing fragments from the drilling fluids. The typical ditch magnet is heavy, and requires at least two persons to lower it into the drilling fluid stream. As metal fragments adhere to the ditch magnet, the device becomes even heavier and difficult for personnel to remove. Removal of the metal particles from the ditch magnet is difficult because of the strong magnetic field. Drilling personnel usually run their hands over the surface of the ditch magnet in an effort to strip the magnetic materials from the magnet. This process is slow, laborious, and potentially dangerous to personnel because the metal fragments can be sharp enough to penetrate gloves and clothing. Removal and retrieval of all magnetic particles is therefore difficult, leading to injury and mistakes in determining the actual amount of metal loss from the casing. There is a need for an apparatus for inexpensive removal of casing fragments from drilling fluids without the disadvantages of the known devices. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for removing casing fragments from drilling and workover fluids circulated in oil and gas wells which meets the need identified above. 
     The disclosed apparatus is a shrouded magnet for retrieving metal fragments from oil and gas well fluids comprising three basic components, a container composed of non-magnetic material, a source of magnetic field disposed within the container, and a cap for sealing the container. The container has an opening, an outer surface, an inner peripheral surface and a base opposite the opening. The magnetic source is disposed within the container adjacent to the inner peripheral surface so that a magnetic field exists at the outer surface. Flow vanes may be attached to the outer surface; a handle may be attached to the cap; and an extension may be attached to the base to increase the stability of the device as it stands in the fluid stream. 
     An alternative embodiment of the device includes a plurality of magnets disposed within the container. The plurality of magnets may be assembled in three groups comprising a top group, a middle group and a bottom group in relative sequence from the opening of the container to the base, each group comprising a plurality of magnets in facing relation, the plurality of magnets in each group having the same magnetic pole orientation. Each group may be separated from the adjacent group with a non-magnetic spacer. The polarity of each group may be adjusted to increase the effectiveness of the device, such as orienting the north magnetic pole of the top group to face the cap and the south pole of the top group to face the base, orienting the south magnetic pole of the middle group to face the cap and the north pole to face the base, and orienting the north pole of the bottom group to face the cap and the south pole oriented face the base. 
     In another embodiment of the device, each magnet has a bore such that the bores of adjacent magnets are aligned along the vertical axis of the container and the magnets are disposed within the container adjacent to the inner peripheral surface so that a magnetic field exists at the outer surface. In this embodiment, a retention rod, having a top and a bottom, is inserted through the bore of each magnet and the bottom of the rod received within a receptacle on the inside surface of the base. The top of the retention rod may be attached to the inside surface of the cap, and a stop collar may be affixed to the retention rod between the bottom of the rod and the plurality of magnets, so that the magnets may be removed by removing the cap from the container. In this embodiment, as with other embodiments, the plurality of magnets may be assembled in groups, with the polarity of magnets in each group having the same magnetic pole orientation, and each group may be separated from the adjacent group with a non-magnetic spacer. The polarity of each group may be adjusted to increase the effectiveness of the device. 
     A method for recovering magnetic casing fragments from fluids discharged from a well is also disclosed. In this method the discharged fluids are passed through a magnetic field created by a shrouded magnet, where the shrouded magnet has magnets contained within a nonmagnetic container. The magnetic field separates the casing fragments and other magnetic materials from the fluids. When desired, the shrouded magnet is removed from the fluid stream, and the magnets are removed from the nonmagnetic container, so that the magnetic field attracting the casing fragments is removed and the casing fragments are released and collected. 
     A method for analyzing the condition of well casing by collecting magnetic casing fragments from fluids discharged from a well is also disclosed. In this method, the discharged fluids are passed through a magnetic field created by a shrouded magnet, where the shrouded magnet has magnets contained within a nonmagnetic container. The magnetic field separates the casing fragments and other magnetic materials from the fluids. When desired, the shrouded magnet is removed from the fluid stream, and the magnets are removed from the nonmagnetic container, so that the magnetic field attracting the casing fragments is removed and the casing fragments are released and collected. The casing fragments are weighed and the total weight of the recovered casing fragments are calculated. The total weight of the casing originally installed in the well is also calculated so that the percent of metal loss from the casing may be obtained by dividing the total weight of the casing fragments recovered from the well by the total weight of the casing originally installed in the well. The casing fragments may also be visually examined. 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of the exterior of the disclosed invention. 
     FIG. 2 is an exploded isometric view of the disclosed invention 
     FIG. 3 shows a side view of the exterior of the disclosed invention. 
     FIG. 4 shows a side view of the internal components of the disclosed invention. 
     FIG. 5 shows a top view of the exterior of the disclosed invention. 
     FIG. 6 shows a bottom view of the exterior of the disclosed invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     As shown in FIG. 1, the exterior components of the shrouded magnet  10  are a container  12  having an opening, and an outer surface  13 , a cap  14  for sealing the container, and a base  18 . While the shrouded magnet  10  may be placed in the fluid stream in any orientation, the operator may find that it is most convenient to place the device with the longitudinal axis of the container  12  perpendicular to the direction of fluid flow, so that the device is standing on the base  18 . When standing on the base  18 , the stability of the device may be increased by attaching a base extension  20  to the base  18 . It may also be desirable to attach flow vanes  22  to the container  12 , so that the long axes of the flow vanes are oriented generally parallel to the longitudinal axis of the container  12 . The flow vanes  22  may have fluid passages, such as holes, to direct fluid flow around the outer surface  13  of the container  12 . 
     As shown in FIG. 2, the internal components of the shrouded magnet  10  include a plurality of iron magnets  28  disposed within the container  12 . However, any means for creating a magnetic field extending through the container  12  to the outer surface  13  will accomplish the required purpose, including the iron magnets  28 , magnetic alloys or an electromagnet comprised of an iron core surrounded by a current-carrying coil. If an electromagnet is used, a means of producing electrical current is required. Such means may include either a direct current source such as a battery or an alternating current source such as a generator or utility power. A battery may be inserted inside the container  12 , thereby requiring no external leads to the coil. If an alternating current source is used, external leads to the coil will be required and the leads must be introduced into the container  12  so as to maintain a fluid-tight seal within the container, by methods well known in the industry. 
     The container  12  should be constructed of a non-magnetic material such as aluminum, fiberglass or plastic. The use of non-magnetic materials for the container  12  prevents the container from becoming magnetized, thereby allowing any magnetic materials attached to the outside surface  13  of the container to disengage when the magnets  28  are removed from within the container  12 . 
     In the embodiment shown in FIG. 2, a plurality of magnets  28  are disposed in facing relation. Each magnet  28  has a bore such that the bores of adjacent magnets  28  are aligned along the vertical axis of the container  12 , so that the outside edge of each magnet is adjacent to the inner peripheral surface  36  of the container  12 , resulting in the creation of a magnetic field extending to the outside surface  13  of the container  12 . As shown in FIG.  2  and FIG. 4, a retention rod  30 , having a bottom end  32  and a top end  34  may be inserted through the bores of the magnets  28 . A stop collar  38  or other type of retaining device may be affixed to the bottom end  32  of the retention rod  30 , which will allow the removal of all of the magnets  28  from the container  12  simply by removing the retention rod  30 . The top end  34  of the retention rod  30  may be attached to the inside surface of the cap  14 , so that removal of the cap  14  and pulling upwards will also remove the retention rod  30 , the magnets  28 , and the stop collar  38 . As shown on FIG. 3, a receptacle  33  may be fashioned on the inside surface of the base  18  for receiving and stabilizing the bottom end  32  of the retention rod  30 . 
     It has been found that if all of the magnets  28  within the container  12  are oriented so that the polarities (i.e., the north pole and south pole) of each magnet  28  are facing in the same direction, metal particles recovered from the drilling fluid tend to concentrate at that portion of the outside surface  13  of the container  12  where the magnetic field is the strongest. However, as shown in FIG. 4, the magnets  28  may be placed in groups, such that each magnet  28  in a group is oriented so that the polarities of each magnet in the group are facing the same direction, but the polarity of each group within the container  12  may be different from an adjacent group. For example, if three groups of magnets  28  are formed, the north magnetic pole of the top group  42  may be oriented facing the top end  34  of the retention rod  30  and the south pole oriented facing the bottom end  32 ; the south magnetic pole of the middle group  44  may be oriented facing the top end  34  and the north pole oriented facing the bottom end  32 ; and, the north pole of the bottom group  46  may be oriented facing the top end  34  and the south pole oriented facing the bottom end  32 . Alternating the magnetic polarity of each group of magnets  28  will result in distributing metal fragments recovered from the drilling fluid to be more evenly distributed on the outside surface  13  of the container  12 , allowing a larger accumulation of metal fragments before removal of the fragments is required. Each group of magnets  28  may be separated by a spacer  40 . 
     The cap  14  may be equipped with a handle  16  to assist the user in lifting or otherwise maneuvering the device. The container  12  should be equipped with sealing means  24 , such as threads and/or “O” rings and the cap  14  should have matching sealing means  26 , such as threads and/or “O” rings to prevent fluid flow into the interior of the container  12 . 
     Using the invention disclosed herein, a method has been developed for removing metallic casing fragments from fluids discharged from an oil or gas well. In this method, magnetic casing fragments are removed from fluids discharged from an oil or gas well by passing the fluids through a magnetic field created by a shrouded magnet  10 . The shrouded magnet  10  contains magnets  28  contained within a nonmagnetic container  12 , which act to separate the casing fragments and other magnetic materials from the well fluids. Upon accumulation of the metallic casing fragments upon the outside surface  13  of the container  12 , the shrouded magnet  10  is removed from the fluids and the magnets  28  are removed from the nonmagnetic container  12 , so that the magnetic field attracting the casing fragments is removed and the casing fragments may be released and collected. 
     Using the invention disclosed herein, a method has been developed for analyzing casing wear and making determinations of the casing integrity. In this method, as described above, magnetic casing fragments are removed from fluids discharged from an oil or gas well by passing the fluids through a magnetic field created by the shrouded magnet  10 , having magnets  28  contained within a nonmagnetic container  12 , which act to separate the casing fragments and other magnetic materials from the well fluids. Upon accumulation of the metallic casing fragments upon the outside surface  13  of the container  12 , the shrouded magnet  10  is removed from the fluids and the magnets  28  are removed from the nonmagnetic container  12 , so that the magnetic field attracting the casing fragments is removed and the casing fragments may be released and collected. The collected casing fragments are thereafter weighed and a total weight for all collected fragments is calculated. The total weight of the casing originally installed in the well is also calculated, based upon either recorded weights for each individual length of casing, or upon casing tables providing the weight per foot for the particular size and grade of casing. The percent of metal loss from the casing may then be calculated by dividing the total weight of the casing fragments recovered from the well by the total weight of the casing originally installed in the well. 
     The casing fragments are also visually examined to ascertain the nature of the casing wear. For example, large sections of casing wall with tool marks may call into question the integrity of the casing, but a small volume of small shavings may indicate uniform wear in the casing. Depending upon the material used for each length of casing, visual inspection may allow the determination of the particular casing string from where a particular casing fragment came. The location of a problem area may also be approximated by the depth of the drill pipe or tool string at the time the fragment is accumulated at the shrouded magnet, the volume of fluid within the well, and the displacement and speed of the pumps circulating the fluid. 
     While the above is a description of various embodiments of the present invention, further modifications may be employed without departing from the spirit and scope of the present invention. For example, the size, shape, and/or material of the various components may be changed as desired. Thus the scope of the invention should not be limited by the specific structures disclosed. Instead the true scope of the invention should be determined by the following claims.