This invention relates to a method of removing metal particles, in the form of swarf, from a liquid such as a drilling fluid. The invention also relates to apparatus for use in such a method.

When drilling for oil or gas, the drill is passed down a well, which is lined with a casing in the form of a steel tube. It is has now been found that, when a well is apparently exhausted, further oil or gas may be recovered by drilling off to the side of the well. In order to be able to do this, it is necessary to cut or mill a window in the side of the tube. This milling generates swarf, which is floated up to the well head using a drilling or cutting fluid such as a mud. The swarf then has to be removed from the upflow of drilling fluids, so that the fluids can be reused. Existing methods of removal of this swarf are either labour-intensive or require extremely large pieces of equipment. The swarf which is generated in this operation presents unique problems as regards its removal from fluids, because it is often generated in the form of large entanglements. These entanglements easily become jammed in any perforations if conventional classifiers such as shakers are used. Trommels in the form of rotating perforated drums are well known in the field of classifying solid particles, but they have the disadvantage that they are relatively inefficient in tenήs of the space which they occupy. The use of such equipment for removing swarf from drilling mud has not previously been considered, and produces surprisingly beneficial results.

The present invention therefore seeks to provide a method which alleviates the disadvantages associated with the prior art, and an apparatus which provides improved performance over the known arrangements. According to a first aspect of the present

-2- invention, there is provided a method of processing a drilling fluid having swarf particles entrained therein, the method comprising passing the fluid into an apparatus comprising a perforated rotating drum, such that the swarf particles are retained therein while the drilling fluids pass through the perforations.

According to a second aspect of the present invention, there is provided a method of processing a drilling fluid used in a process in which a window is milled in a side of a well, such that the drilling fluid has swarf particles entrained therein, the method comprising passing the fluid into an apparatus comprising a perforated rotating drum such that the swarf particles are retained therein while the drilling fluids pass through the perforations.

According to a third aspect of the present invention, there is provided a method of milling a window in a side of a well, the method comprising removing resulting swarf from the well using a drilling fluid, and removing the swarf from the drilling fluid by passing the drilling fluid into an apparatus comprising a perforated rotating drum such that the swarf particles are retained therein while the drilling fluid passes through the perforations.

The drum has an axis of rotation which is preferably inclined, so that the swarf falls out of the lower end of the drum.

The apparatus preferably comprises two coaxial perforated cylindrical drums, and means for rotating the drums.

The means for rotating the drums may comprise means for rotating the drums in the same direction or in opposite directions, and may allow either or both of the speed and direction of rotation of one or both drums to be altered.

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The apparatus preferably further comprises means, located between the two drums, for reducing the particle sizes of the solid particles. Preferably, this takes the form of a blade. This has the advantage that swarf particles do not cause the apparatus to become jammed.

Preferably, the outer drum has smaller perforations therein than the inner drum.

This has the advantage that the inner drum acts as a coarse classifier and the outer drum acts as a fine classifier for removing smaller solid particles.

In one embodiment of the invention, the outer drum is in the form of a screen of woven wire or similar material. According to a further preferred embodiment of the invention, the method comprises passing the fluid to apparatus which further comprises means for conveying a thin layer of liquid which has passed through the two drums, and further comprises means located adjacent to the thin layer of liquid for exerting a magnetic force on metallic particles entrained therein.

Preferably, the drum, or the inner drum, is made of anti-magnetic material. This is advantageous because the swarf which is generated by milling the steel tube tends to have some induced magnetism, and so making the drum out of anti-magnetic material reduces the possibility that swarf will be retained thereon.

According to a fourth aspect of the present invention, there is provided equipment for milling a window in a side of a well, comprising means for supplying a drilling fluid to the well to remove swarf particles therefrom, and apparatus for removing swarf particles from the drilling fluid, the apparatus comprising a perforated rotating drum, such that the swarf particles are retained therein while the drilling fluids pass through the perforations.

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For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Figure 1 is a schematic partial cross-section through apparatus in accordance with a first embodiment of the invention;

Figure 2 is an end view on arrow A of the apparatus shown in Figure 1; Figure 3 is a schematic partial cross-section through apparatus in accordance with a second embodiment of the invention;

Figure 4 is an end view of the apparatus shown in Figure 3; Figure 5 is a schematic illustration of a further processing stage in apparatus in accordance with the invention; and

Figure 6 is a schematic illustration of apparatus in accordance with the invention. Figure 1 shows a drum 2 having an inlet pipe 4 through which mud and swarf from a wellbore are supplied to the drum. The drum 2 is perforated with a large number of through-holes 6 over the majority of its length, but has a first non-perforated section 8 at the end nearest the inlet pipe 4 and a second non- perforated section 9 at the other end thereof. The sizes of the perforations may be chosen in dependence on the expected solid particle sizes.

The drum 2 is made of anti-magnetic material. As a result, the swarf, which often has some magnetism induced by the process of milling the steel tube, is less likely to be retained on the drum.

The apparatus further includes a drive motor 10. The drive motor preferably has a variable operating speed. The motor causes the drum 2 to rotate by means of a friction drive 12 which contacts the non-

-5- perforated section 8 of the drum. It will be understood that the drum may equally be driven by some other mechanical means, for example a gear mechanism.

The axis of rotation 14 of the drum is inclined to the horizontal at an angle θ, with the inlet end being uppermost. This angle θ is adjustable. At its lower end, the drum 2 is supported by rollers 16 and a thrust ring 18 which prevents it from sliding. The drum can thus be rotated as shown by arrow B, the speed and direction of rotation preferably being variable.

Liquid, together with particles which are small enough to pass through the perforations, fall through the perforations and then out of the drum into the collection area 20, and out through an outlet 22. The larger and most important swarf particles are collected in a container 24.

The apparatus is used in a process in which swarf is generated, and needs to be removed from a drilling fluid. When a window is milled in a side of a steel tube, for example in a deviation drilling process, some of the swarf is generated in the form of large balls or entanglements, which present great difficulties if one attempts to remove them using conventional classifiers, because they tend to become jammed in the perforations. However, using the present invention, this problem is alleviated, because, if a ball of swarf does become partly engaged in the perforations of the inner drum, the rotation of the drum will tend to allow the swarf to fall out. This is a major, otherwise unforeseen, advantage of using a rotating drum device of this type in this particular process.

Clearly, the amount of swarf collected will depend upon the size of the perforations 6 in the drum 2. However, it is also possible to vary the rate at which material is supplied via the inlet 4, the speed of rotation of the drum, or the angle θ of inclination of

-6- the drum. Variation of any or all of these parameters will affect the amount of swarf removed from the mud.

Figure 2 is a schematic end view of the drum 2, showing the rollers 16. Figure 2 also shows a cutter blade 26 (not shown in Figure 1). This blade is mounted fixedly along the length of the drum at a constant distance from the axis of rotation 14. Thus, it removes protruding strands of swarf and prevents the mechanism from becoming jammed. Figure 3 shows apparatus comprising a first inner perforated drum 52 and a second outer drum 54. The first drum 52 has comparatively coarse perforations 56, the sizes of which may be selected in accordance with the expected particle sizes of the solid materials to be removed from the liquid. The inner drum 52 is driven by a motor 58 and drive means 60. The motor may have a fixed or variable speed, and the drive means may be a friction drive or may be some other form of mechanical drive. The outer drum 54 may be in the form of a cylinder of metal with apertures in it, the apertures being of any convenient size and shape, and may be covered with screen panels made of woven wire or a similar material. The outer drum may be replaceable by a similar drum having different perforation sizes. Any such arrangement is intended to be considered as a drum herein. The outer drum 54 is driven by a motor 62 through a drive means 64. Again, the motor 62 may have a fixed or variable speed, while the drive means may be a friction drive or any other convenient form of drive. The drums 52, 54 may be drivable either in the same direction or in opposite directions, and the arrangement may be such that the direction of drive of one or both of the drums may be reversed, in order that contra-rotation or rotation in the same direction may be selected as required. The drums 52, 54 are preferably but not necessarily coaxial.

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Liquid with entrained solids, such as mud and swarf from a wellbore, enters the apparatus through an inlet 66, while mud leaves the apparatus through the outlet 68, and swarf is collected in the container 70. The inner drum 52 is supported on rollers 72, while the outer drum 54 is supported on rollers 74.

As described above in connection with Figures 1 and 2, the angle of inclination of the drums may be variable. Figure 4 is an end view on arrow C of the apparatus shown in Figure 3, and also shows a knife blade 76 mounted in the apparatus to remove strands of swarf which protrude from the inner drum 52.

In situations where it is desirable to remove from the drilling mud even very fine swarf particles, the apparatus as shown in Figures 1 and 2 or Figures 3 and 4 above may be supplemented by a second processing stage. Apparatus for such a stage is shown in Figure 5. A tank 100 collects mud and fine particles 102 from the outlet of the apparatus described above. The wall of the tank 100 has an overflow or weir 104, which may be at fixed height or may be at a variable height. Liquid thus exits the tank 100 and falls down a chute 106 in a thin layer. Located adjacent the chute 106 is a magnetic belt 108 driven by pulleys 110, 112. The belt 108 may be made of a permanently magnetic material, or may be provided with an electromagnetic generator. Any steel particles present in the liquid will be attracted to the belt 108, which rotates in a clockwise direction. These particles will then be removed from the belt 108 by a scraper 114, at which rejected metallic particles will be collected. The cleaned mud continues to fall down the chute 106. Figure 6 shows, schematically, a drilling system, in which a well 120 is formed in an underground

-8- formation 122. The well has a casing in the form of a steel tube 124. With the well exhausted, a further well can be drilled through a window 126 in the casing, formed by a milling tool 128. Drilling fluids are supplied to the milling tool from a supply 130 (again shown schematically), and the swarf from the milling operation is floated to the surface. To allow the drilling fluid to be reused, the swarf must be removed from the fluid, and so it is passed to apparatus 132 as described with reference to the earlier figures.

There is thus provided a method and apparatus which are extremely effective in removing metallic swarf particles from drilling muds, but which also have application in removing other solids (especially metallic) particles from other liquids.