Down Hole Assembly

 
The picture at the right shows the "Mother Hubbard" mud anchor design that I have developed over the years (with some help).  It incorporates the design principles that were originally outlined by Gibson & Swaim. 
    This mud anchor is made by welding a 3 foot 2 7/8 stub into a 3 1/2" OD schedule 40 pipe.  Six 1/2" X 8" offset slots are cut into the top of the 3 1/2" pipe near the weld point.  The result is a distance of 4 feet between the seating nipple and the bottom of the bottom slot.  This makes proper gas anchor selection easy. 
    The six slots provide 24 sq. inches of inlet area.  Gibson & Swaim recommended an inlet area of at least 4 times the area between the outside of the dip tube and the inside of the mud anchor.  In  the case of the 3 1/2" OD mud anchor with an 1 1/4" dip tube the 6 slots offer 3 more sq. inches than the volume between the dip tube and the ID of the mud anchor. 
      I prefer the largest possible mud anchor because the speed of the fluid level draw from the mud anchor must be held to the lowest possible number.  This is a difficult to explain concept. 
      Assume that the stationary fluid level in the mud anchor is at the bottom of the bottom slot.  Then when the pump draws the fluid level drops in the mud anchor toward the inlet ports on the gas anchor.  It is the speed of that drop that must be controlled.  Gas bubbles rise at a speed of 0.5 fps.  Therefore, the rate of fluid level fall should be less than 0.5 fps or gas will be drawn into the pump.
    In my experience velocities in excess of 1 foot per second must be avoided.  Preferred velocities are near 0.5 fps. 
    To calculate the velocity in your mud anchor use the following formula:

                                 V= 0.00935 X BFPD / AMA

    WHERE:  BFPD is 100% pump efficiency, and AMA is the useable area of the Mud Anchor.  i.e. Area of the ID of the Mud Anchor Minus the area of OD of the gas anchor.
  Notice:  That I have included an HF Purge valve on the bottom of this mud anchor.  I was VERY skeptical at first but have come to the conclusion that these things actually reduce the accumulation of sand and trash in the mud anchor which could get into the pump.

One of the most often overlooked Gas-Lock causes is the length of the gas anchor or dip tube.  The chart below is for the 3 1/2" Mother Hubbard mud anchor that I prefer.  (Other charts for other Mud Anchor sizes are available on request)  It assumes a distance of 4 feet from the seating nipple to the bottom opening of the mud anchor.  To correct for a differing distance simply add or subtract the difference from the 4 foot base. 
    If the gas anchor is too short it will cause the pump to suck in "raw" fluid (which contains gas) and will lead to excessive fluid pound and/or gas lock.  If the gas anchor is too long the fluid friction in it will cause a pressure drop which can also lead to excessive fluid pound and/or gas lock.

 

 
"Muleshoes", "Porcupine Quills" and Snake Oil

 
 Muleshoe mud anchors are actually just a substitute for an open ended seating nipple.  If you have had success with the "Muleshoe" you will be equally impressed with an open ended seating nipple.  For either system to work you must be below the lowest set of perfs in the well.  In "gassy-foamy" wells I have found that neither works as well as the mother hubbard.  The reason I suspect, is that the path to the pump intake causes the fluid to "shear" somewhat in the mother hubbard and that leads to more gas breaking out.
  Porcupine Quills are a class of mud anchor that I have not had the courage to experiment with.  They use a series of external baffles which again "shear" the fluid for better gas break out.  Because of the external baffles they are very prone to getting stuck.  Hence the name.
  Open Ended mud anchors  are often run by inexperienced operators in an attempt to reduce sand failures after a frac job.  Since fluid can enter from the bottom of the mud anchor it completely defeats the gas separation capabilities of the mud anchor such that it is equal to the open ended seating nipple in gas separation.
  Perforated Subs serve no useful purpose due to their very small inlet area.  A typical perforated sub has about 6 square inches of inlet area.  You need far more.  Save some money, leave it out.
  Extra-Long Dip Tubes  too much fluid friction, you will constantly pound due to incomplete pump fillage.
  Cavins Tool most sand does not have enough mass to be spun out.  It does cause gas break-out though, and right at the pump intake.  Don't use them.
  Mud Anchor De-Centralizers  who's kidding who here.  Do you have a straight enough well that you actually BELIEVE your mud anchor is not already against the side of the casing?
  Filtering Systems Either the filter medium is too "tight" and plugs up or it is too "loose" and does no good.  Either way it's usually useless.
  Snake Oil widely sold in the oil patch, snake oil has two distinguishing characteristics, (1) It is very expensive.  (2)  It has no logical explanation for it's benefits.  Therefore, it usually has no benefit.

 
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