Patent Publication Number: US-6901999-B2

Title: Swabbing tool for wells

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to equipment and methods used for the swabbing of wells, particularly oil and gas wells. 
   Swabbing is used to obtain production from an underground formation that has a pressure insufficient to overcome downhole hydrostatic pressure. A swabbing tool is lowered into a well on a wire line to near a producing formation. The swabbing tool typically has a mandrel and swab cups extending out from the mandrel. Passageways between the mandrel and swab cups allow fluid to bypass the swab cups when the swabbing tool is lowered in the well. As the swabbing tool is lowered into the well, fluid in the wellbore below the swabbing tool bypasses the swab cups into the wellbore above the swabbing tool. When a desired level of the swabbing tool is reached, the tool is pulled up and the swab cups slide down the mandrel onto a sealing plate that seals the passageways. Pulling up on the swabbing tool lifts the fluid above the swab cups and generates a strong suction force in the tubing below the swab cups. The suction tends to draw fluid from the producing formation. However, it often occurs that the wellbore fluid has entrained particulate matter introduced naturally or from production activities. This particulate matter, often including sand, can bypass the swab cups and settle onto the swab cups. With sufficient volume of sand, the swab cups and the swabbing tool can become stuck in the well. The present invention is intended to overcome this problem. 
   SUMMARY OF THE INVENTION 
   Therefore there is provided a swabbing tool for wells that filters particulates from fluids passing through the swabbing tool. According to an aspect of the invention, the swabbing tool comprises a mandrel, at least one swab cup on the mandrel, the mandrel and swab cup being arranged to allow fluids to pass from below to above the swab cup; and a sifter attached to the swabbing tool below the swab cup for filtering particulates from fluid passing from below to above the swab cup. The sifter is preferably barrel shaped and is provided with a sealing element attached to the sifter to force fluid into the sifter. According to a further aspect of the invention, the sifter has an interior and the sealing element is dimensioned to seal against a casing or tubing wall and force fluid into the sifter. The openings in the sand sifting element may be slots, with a size between 0.125 mm to 0.635 mm. 
   These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There will now be described preferred embodiments of the invention, with reference to the drawings, by way of illustration only and not with the intention of limiting the scope of the invention, in which like numerals denote like elements and in which: 
       FIG. 1  shows a swabbing tool according to the invention in place in tubing installed in a wellbore; 
       FIG. 2  is a section through a swab cup along the line  2 — 2  in  FIG. 1 ; and 
       FIG. 3  is a section through a seal plate along the line  3 — 3  in FIG.  1 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In this patent document, “comprising” means “including”. In addition, a reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present. The tool of the present invention is intended for use in a wellbore. The term axial refers to the direction along the wellbore, while the term lateral is indicative of a direction perpendicular to the axial direction and the term radial means outward from a central axis of the wellbore. 
   Referring to  FIG. 1 , a conventional swabbing tool  10  is suspended from a connector  13 , including a conventional socket, swivel, sinker bar and knuckle, terminating a wire line  12  within tubing  14  located in a well that penetrates a producing formation. The swabbing tool  10  includes a mandrel  16  and a number of swab cups  18  (only one is shown) attached to the lower end of the mandrel  16 . The swab cups  18  are sized for the tubing or casing for which the swabbing tool  10  is intended. Passageways  20  ( FIG. 2 ) between the swab cups  18  and the mandrel  16  allow fluid to bypass the swab cups  18  when the swabbing tool  10  is lowered into the well. The swab cups  18  are free to move up and down a limited amount along the mandrel  16  in conventional manner. Below the swab cups  18 , a sealing plate  22  terminates the mandrel  16 . The sealing plate  22  has several knobs  24  that create passageways  26  between the tubing  14  and sealing plate  22  for fluid to pass around the sealing plate  22 . When the swabbing tool  10  is raised in the well, the weight of fluid on the swab cups  18  forces them down onto the sealing plate  22 , which prevents fluid from entering the passageways  20 . The swabbing tool  10  may then be lifted, creating suction in the tubing  14 . The suction draws fluid from the underground producing formation penetrated by the well to enhance production. 
   In an embodiment of the present invention, a barrel adaptor  28  is installed on the lower end of the sealing plate  22  for example by threading or welding. The barrel adaptor  28  threads onto a hollow tubular sand sifter barrel  30  below the swab cups  18 . By this arrangement, the sifter barrel  30  extends in an axial direction downhole of the mandrel  16 . The sifter barrel  30  has an opening  32  at its lower end to allow fluid to enter the sifter barrel  30  from below as indicated by the arrow A. Openings such as slots  34  are machined, for example by laser cutting, into the wall of the sifter barrel  30  along the length of and around the sifter barrel  30 . Due to the axial extension of the barrel  30 , the slots  34  allow passage of fluid in a lateral direction, and due to the central location of the barrel  30 , the slots  34  permit radial fluid flow through the slots  34 . The openings typically are sized to filter sand from fluid passing through the slots  34 . To prevent wellbore fluid from bypassing the sifter  30 , an inverted swab cup  36  is attached to the lower end of the sifter barrel  30  and dimensioned to seal against the tubing  14 . The swab cup  36  has an interior bore, not shown but may be the same as the swab cup  18  illustrated in  FIG. 2 , to allow passage of fluid into the barrel  30  from below the swab cup  36 . 
   The barrel adaptor  28  may be a simple tubular connector that attaches to the lower end of the mandrel  16 . In the case of a casing swabbing tool, the barrel adaptor  28  may be replaced by a fish neck mandrel (not shown) that threads at its uphole end to the mandrel  16 , and on its downhole end to the sifter barrel  30  with conventional NPT threads. The swab cup  36  may be connected to the sifter barrel  24  using conventional fittings as shown such as an NPT coupler  38 , a male to male nipple  40  and an NPT coupler  42 , all of which are tubular with interior bores to allow passage of fluid through them. The swab cup  36  may be attached to the nipple  40 . The dimensions of the parts are chosen according to the intended application, with larger parts used for casing. The slots  34  in the barrel  30  may be 7.5 cm to 10 cm long, with a width in the order of 0.125 mm to 0.635 mm. If the slots  34  are laser cut, the width of the slots will vary due to expansion of the barrel  30  due to laser heat. Forty-eight slots have been found to be adequate in a barrel 96.5 cm long and 42.55 mm OD. The barrel may be made of light wall tubing. 
   In the operation of the swabbing tool  10 , as the swabbing tool  10  is lowered into a well, the inverted swab cup  36  pushes down on fluid  41  that contains sand and other particles. The pressure from the swab cup  36  forces fluid and suspended particles through the inside of the sifter barrel  30 . The slots  34  filter sand from the fluid and the remaining fluid passes into the annulus  44  above the inverted swab cup  36  and then through the passageways  20  between the mandrel  16  and swab cups  18  into the wellbore above the swabbing tool  10  as indicated by the arrows. Once enough fluid has enter the wellbore above the swabbing tool  10  and the swabbing tool  10  has reached the desired level in the wellbore, the swabbing tool  10  may be lifted in the wellbore in conventional manner. If the wellbore is completely filled with particles, the swab cup  36  will stop near the top of the fill, thus preventing the swabbing tool from becoming trapped in the particulates. 
   Using a sifter barrel  30  allows an arbitrarily large amount of open cross-sectional space in the sifter  30 . The barrel  30  may be lengthened as required to create more slots  34 . While it is possible to seal off the tubing with a filter that extends radially from the center of the wellbore into contact with the well tubing  14 , such a design is difficult to build with enough open cross-sectional area to match the cross-sectional area of the passageways  20 , particularly after the openings of the sifter have become partly clogged with sand or other debris or contaminants. Instead of an inverted swab cup  36 , the barrel  30  could have an expanded width downhole with a sealing element on its outer periphery at its downhole extremity, but such a design is more complicated than providing an inverted swab cup. Any of the parts making up the downhole end of the sifter may be perforated. 
   Immaterial modifications may be made to the invention described here without departing from the essence of the invention.