GB2119866A

GB2119866A - Oil wells

Info

Publication number: GB2119866A
Application number: GB08312108A
Authority: GB
Inventors: Geoffrey Ferguson Towner; John Edgar Thomas Towner
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-05-12
Filing date: 1983-05-04
Publication date: 1983-11-23
Also published as: DE3317239A1; GB8312108D0; AU1424383A; NL8301690A

Abstract

A reciprocable oil well pump comprises a plunger 11a and a hollow sucker rod string 12, 13, 14 and 15 for reciprocating the plunger, the interior of the sucker rod string forming a rechargeable pulsation dampener chamber open to the oil pressure immediately above the plunger and extending continuously to ground level, and a non-return valve 19 closing the upper end of the pulsation dampener chamber, through which valve the pulsation dampener chamber may be recharged with pressurised gas. The use of a hollow sucker rod string enables forces generated during pumping to be damped. <IMAGE>

Description

SPECIFICATION Oil wells The present invention relates to oil wells.

In the United States of America and elsewhere there are numerous oil wells which are operated by some form of artificial lift. A majority of wells operated by artificial lift use sucker rod pumps.

The sucker rod pump system is simple and is the most attractive method of artificial lift available in oil fields. Unfortunately, the pulsating nature of its operation introduces cyclic stresses in the sucker rods. The problems connected with cyclic stress increase with increasing depth. Despite the use of sucker rods manufactured to refined specifications and the use of high tensile steels the problems remain.

Failures occur in sucker rods and these are mostly due to fatigue as discussed in detail in an article in "Drilling" July 1981 page 170 entitled "Stress - The Bane of Sucker Rods" by Spencer E. Duke.

This reference offers a means of checking the fatigue failure potential of a sucker rod pump installation. It shows that for any minimum stress, the maximum stress must not exceed that given in a modified Goodman diagram for a particular grade of rod. The method requires measurement of the maximum rod load (P.P.R.L.) and the minimum rod load (M.P.R.L.). The measurement by dynamometer of the values for an existing installation is relatively simple. However, the calculation with accuracy of expected loads in a proposed installation is extremely difficult. A small under assessment of the maximum or minimum stress could lead to premature failure particularly in the case of deep wells.

The sucker rod pump is essentially a pipe line with a force pump at one end moving fluid through it. At the bottom of the structure of the pump where it is momentarily stationary, the sucker rod is carrying the weight of the vertical column of fluid in addition to the weight of the pump assembly. On the rising delivery stroke the load will be increased by the force necessary to accelerate the whole column of fluid and the sucker rods and to overcome frictional resistance which will develop as the fluid moves forward. Towards the top of the delivery stroke the column will decelerate and the force on the sucker rod will reduce correspondingly. During the downward stroke the fluid column is virtually stationary as oil passes through a valve in a plunger.

Further, any change in velocity of the fluid gives rise to a change in pressure. This change in pressure is transmitted through the fluid at the speed of sound. The pressure wave is reflected and rereflected until absorbed by friction or absorption.

As the plunger starts to move upwards it exerts a pressure on the adjacent liquid face in the production tube which liquid face begins to move with the plunger. However, the top of the oil column is still at rest and remains so until the pressure wave initiated by the plunger reaches the top of the column. Under the action of the pressure change the oil column has changed in volume. The column is encased in the steel of the production tube and this tubing is almost completely rigid. Thus, the change in volume is reflected completely as a change in length of oil column. Even a small change in velocity within the column can induce a large elastic pressure change.

The elastic transmissions of pressure are true waves and reflect negatively from open pipe ends or positively from a closed end. At any given point in the pipeline the pressure is the sum of the static pressure plus the algebraic sum of the pressure waves passing that point in either direction at that moment in time. Once generated the waves will move up and down a closed system with decreasing magnitude until the energy in the wave is completely dissipated.

It is extremely difficult to estimate the magnitude of the maximum and minimum pressures which will occur at any point in the column.

Even for a modest change in velocity of the pump plunger there is a substantial increase in cyclic stress.

The maximum load on the sucker rods is increased by fluid pressures opposing the motion of the plunger on the upstroke and decreased similarly on the downstroke. This increases the fatigue failure potential enormously. In extreme cases where the point of the elastic pressure wave is close to the period of the pump cycle the elastic pressure waves from successive strokes of the pump can add to another and build up quickly to completely destructive magnitude.

This type of problem is not unique to the oil industry. A similar problem was encountered long ago in the delivery lines of direct acting steam pumps forcing water into large cast iron mains to supply distant towns. It was found that the mains' burst under the combined load of the accelerative and elastic forces. The problem in this case was solved by placing a pressurized gas vessel in the pipe line adjacent the delivery valve from the pump.

This solution to the problem virtuaily eliminated all the cyclic stresses both accelerative and elastic.

Similar arrangements are used today in the oil industry as discussed in an article in "Drilling" April 1981 Page 90 entitled "How to take the Bumps out of Pumps" by Val Martin, Greer Hydraulics, Inc.

Further, in pipe lines above ground elastic forces have been reduced or eliminated in many ways. For example, pressure relief valves have been inserted at points of known high pressure and air valves have been inserted at points of known low pressure. As an alternative to the air valve there has been used a static water tank in which the level has been controlled by a float valve and filled through a small diameter pipe by passing a non-return valve. When the pressure drops the non-return valve opens and allows water to be drawn into the main pipe line.

Further, surge suppressors or dampeners are fitted to reciprocating pumps. Providing devices of this type are installed in the proper place and have an adequate response in that position they have been successful in removing elastic stresses.

In the oil well pump industry attempts have been made to limit stresses in sucker rods by a number of techniques. For example, the diameter of the pump plunger has been reduced in some instances so limiting the production for a particular velocity, reducing the static rod stress and allowing a larger cyclic stress to be accommodated.

An alternative approach has been to use large diameter highly polished sucker rods of very high tensile steel and sophisticated design and manufacturn to try to ensure no additional stress concentrations can occur such as at section changes or through flexing of bent rods.

Further, it has been proposed to limit acceleration and thus accelerative forces. To maintain the same rate of production this requires a much larger stroke and a complex mechanism to shorten the time of the down stroke. It must be emphasised that accelerative forces are only part of the problem and that elastic forces are dependent only on the amount of velocity change and not the rate of velocity change (the acceleration).

The present invention provides an improvement in an oil well comprising a sucker rod system whereby the elastic waves induced can be suppressed in a rapid manner which is fast enough to absorb at least partially the initial high pressure surge and to reduce the subsequent low pressure cell.

In accordance with one aspect of the present invention there is provided an oil well comprising a production tubular with a pump barrel attached to its lower end, a reciprocable oil pump having a plunger located within the pump barrel, a sucker rod string extending downwardly from ground level to the plunger for reciprocating the plunger, means for cyclically raising the sucker rod string, a rechargeable pulsation dampener chamber located and arranged so as to be open to oil pressure immediately above the plunger and means for charging pressurised gas into the pulsation dampener chamber.

Preferably, the gas chamber in the well is located within the sucker rod string which is hollow.

In accordance with another embodiment of the present invention there is provided a reciprocable oil pump comprising a plunger, a hollow sucker rod string for reciprocating the plunger, the interior of the sucker rod string forming a rechargeable pulsation dampener chamber open to the oil pressure immediately above the plunger and extending continuously to ground level, and a non-return valve closing the upper end of the pulsation dampener chamber, through which valve the pulsation dampener chamber may be recharged with pressurised gas.

The present invention also provides a sucker rod string comprising a plurality of hollow sections joined together so as to form a rechargeable pulsation dampener chamber located and arranged to be open to oil pressure immediately above a plunger, at least about a 50 foot length of said sections being of wide bore and joined together in a manner providing a substantially smooth internal shape, the endmost section at the end adjacent said wide bore section being arranged to be coupled to the plunger and the endmost section remote from the wide bore section being provided with a non-return valve to enable pressurised gas to be charged into the pulsation dampener chamber.

The present invention further provides a method of damping pressure surges when artificially lifting oil which comprises establishing a column of oil in an oil well, imparting lift to the column by means of a sucker rod string attached to a plunger at the bottom of the column, and damping increases in pressure in the column by exposing a repressurisable pulsation dampener chamber containing pressurised gas to oil pressure immediately above the plunger and recharging the pulsation dampener with pressurised gas as may be required.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a vertical section through the production tubular and pump barrel of an oil well in accordance with one embodiment of the present invention containing an oil pump; Figures 2a and 2e are schematic vertical sections through oil wells in accordance with the present invention showing an oil pump in various stages of operation.

Figure 3 is a vertical section through the production tubular and pump barrel of an oil well in accordance with a further embodiment of the present invention.

Figure 4 is a horizontal section along the line 4 - 4 of Figure 3.

In Figure 1, the well comprises a production tubular 10 with a pump barrel 11 attached to its lower end. A sucker rod string of jointed form reciprocates in the production tubular 10 to drive a pump 1 1a, located in the pump barrel 11, up and down to lift the oil on the up-stroke.

In this embodiment of the invention, the sucker rod string is formed of sectins 12, 13, 14 and 15 which sections 12,13, 14 and 15 are of special form to be described below, and are coupled between a reciprocating power source (not shown) and the plunger 11 a.

Each section 12,13, 14 and 15 is formed of a hollow tube of high-strength material such as hightensile steel. A threaded socket 17 is formed in the upper part of the section 13 and an externally threaded plug 16 is formed in the lower part of the section 12.

The plug 16 is connected to a central aperture 18 leading through the interior of the section 12.

The aperture 18 of the section is closed at its upper end buy a non-return gas valve 19 having means to attach it to a compressor or pump.

The section 13 is formed at its lower end with an externally threaded plug 20 which is arranged to be threadedly connected to a threaded portion 21 formed in the upper portion of the section 14.

Further, the section 14 has an externally threaded plug 22 at its lower end which is arranged to be threadedly connected to a threaded socket 23 in a section 15. The section 15 is tapered downwardly so as to be wider at its lower end than its upper end.

The lower end of the section 15 is closed off and formed with a depending externally threaded plug 24. Further, the sides of the section 15 are formed with a plurality of holes 25.

The plug 24 is arranged to threadedly engage with a threaded socket 26 in the upper end of the plunger section 1 la. Thus, the sucker rod string is formed of one or more of sections 12, one each of sections 13 and 15 and a plurality of the sections 14to achieve the required working depth. Each of the jointed sections is joined to its neighbour in a gas tight manner such as by the use of gaskets and the like.

The plunger section 11 a comprises an upper hollow portion 27 formed in its wall with a plurality of holes 28 and containing an upper portion 29 with the socket 26. The interior of the sucker rod string upwards from the aperture 25 defines a rechargeable pulsation dampener chamber.

Below the portion 27 the plunger section 11 a comprises a piston 30 having a central aperture 31 extending therethrough. The outer surface of the piston 30 is provided with a plurality of sealing rings 32 which provide a seal between the piston 30 and the pump barrel 11 in use.

A ball 33 is arranged to be seated in the upper end of the central aperture 31. Further, at the lower end of the pump barrel 11 there is provided a reduced diameter section 34 having a second ball 35 seated thereon.

The region within the sucker rod between the aperture 25 and the valve 19 defines a gas chamber which is open to the oil pressure immediately above the plunger 1 1a.

In use, the ball 33 will allow oil to pass through the piston 30 via the aperture 31 on a downward stroke of the sucker rod string. The second bail 35 remains seated during this movement. Thus, the oil will enter the space above the piston 30.

Similarly, on an upward stroke of the sucker rod string assembly the second ball 35 will rise from its seat and allow oil to enter the space below the piston 30. The first ball 33 remains seated during this movement.

As will be described in more detail in relation to Figure 2, the gas space within the sucker rod string acts as an absorber for changes of oil pressure.

In the arrangements shown in Figure 1,the sections 14 form the bulk of the length of the sucker rod string. The section 12 is required to hold the valve 19 at the upper end and the section 13 is required to couple to the section 12. Similarly at the lower end, the section 15 is required for entry of oil into the sucker rod string. As shown the sections 14 and the section 13 are of relatively large internal bore. In other words, the inner bores of these sections are defined by the outer walls. As far as possible, the bores of these sections are kept as uniform as possible so that a substantially smooth, wide passage extends upwardly from the section 15.

Of course, there are minor unavoidable constrictions in the bore caused by the threaded couplings between the sections of the sucker rod string.

However, these constrictions are kept as small as possible and no additional, avoidable constrictions are incorporated in the sucker rod string.

The relatively wide bore can extend the entire distance between the sections 15 and 12 if desired. In this case, the volume of the bore and the small size of the constrictions means that the entire volume is available to act to suppress elastic forces during pumping. As will be described, however, the oil may only enter the column for a small distance in a string which could easily be 10,000 feet long. Thus, it follows that, it is not essential for the whole of the string to be available for the suppression of elastic forces.

It is preferred that at least about the lower 50 feet be substantially smooth and of wide a bore as possible. More preferably, this applies to at least about the lower 1000 feet. Still more preferably to at least about the lower 200 feet even more preferably to at least about the lower 500 feet. Finally, in many instances it is envisaged that at least about the lower 1000 feet of the sucker rod string would be substantially smooth and of wide a bore as possible.

If the bore is too constricted then it may not be available to react to the changes in pressure rapidly enough to be of value. For example, if there was a sever constriction adjacent the lower end of the sucker rod string then this would prevent the response to change in pressure in the time available extending above the constriction. Thus, only the portion below the constriction would be available to damp the forces generated during pumping.

Turning now to Figure 2a there is shown an oil well in accordance with the present invention with a sucker rod string at its lowest position and showing lifting means 50 for the sucker rod string. The pulsation dampener chamber defined by the sucker rod string has been charged with pressurised gas through the valve 19 so that the oil-gas interface 52 is just above the apertures 25.

In Figure 2b, the lifting means 50 has commenced to lift the sucker rod string and the ball 35 has lifted from its seat to allow new oil into the pump barrel 11. Further, the plunger 11 a has commenced to lift oil up the production tubular 10. As described hereinabove, this has set up an elastic deformation in the fluid and as a result fluid has entered the sucker rod string under pressure so raising the gas-oil interface 52. This reduces the excess pressure build up in the production tubular 10 and reduces transmission of waves along the column of oil.

In Figure 2c, the lifting means 52 has reached the top of its stroke and the oil-gas interface is now stationary or beginning to fall due to the gas pressurised in Figure 2b now starting to expand.

In Figure 2d, the sucker rod string is now falling, the ball 33 has lifted off its seat and the oil is flowing out of the apertures as the gas expands to balance the gas-oil pressures. Thus, the interface 52 is falling back towards the apertures 25.

As shown in Figure 2e, the sucker rod string has returned to its iowest point when the gas-oil interface 52 will have returned to apertures 25.

After long use, the gas pressure in the sucker rod string may be found to be gradually reduced by solution in the oil. The gas-space may be recharged with pressurised gas by passing gas from a pump or compressor through the non-return valve 19 into the pulsation dampener chamber.

Similarly for deeper bores, it is essential to increase the pressure of gas in the pulsation dampener chamber, and this again may be achieved via valve 19.

It is comparatively simple to convert a standard well arrangement to incorporate this embodiment of the invention. All that is needed is to remove the usual sucker rod and substitute a hollow rod, apertured section 15 and a valve 19. The casing 10, production tubular 10, pump barrel and lifting means 50 for reciprocating the sucker rod string may be of prior known type.

The embodiment of the present invention shown in Figures 3 and 4 has some parts similar to the parts of the embodiment shown in Figures 1 and 2. Like reference numerals denote common parts.

The apparatus shown in Figure 4 comprises a sucker rod string 50 which is, as shown, solid. This sucker rod string 50 is of conventional known type and typically comprises a number of sections connected end to end in known manner.

The lower end of the string 50 is connected such as bythreaded engagement with plunger section 52 containing plunger parts 30,31,32,33,34 and 35 as previously described.

The plunger section 15 contains an upper hollow portion 53 with a plurality of apertures 54.

A jacket 56 is mounted aboutthe production tubular 10. The jacket 56 communicates with the interior of the production tubular 10 through a plurality of apertures 58 located just above the hollow portion 53 and defines a pulsation dampener chamber 57.

The jacket 56 has a non-return gas valve 60 mounted in its upper end, which gas valve 60 is connected via a conduit 62 to a source of pressurised gas (not shown) at ground level.

In use, the jacket 56 is pressurised with gas in similar manner to the interior of the sucker rod string in the embodiment of Figures 1 and 2. The plunger assembly of the present embodiment works in similar to that of the embodiment of Figures 1 and 2 and is reciprocated by the sucker rod string 50.

On the downward stroke, the hollow portion 53 filis with oil and this oil passes into the production tubular via the apertures 54. On the upward stroke the oil is lifted bythe plunger 11a. In order to absorb the elastic deformation set up in the column of oil, the chamber 57 defined by the jacket 56 acts as a pulsation dampener chamber and some oil tempor ariiy enters the chamber 57 through the apertures 58. The movement of the oil into the chamber 57 defined by the jacket 56 is dampened by the pressurised gas therein. A quantity of oil leaves the column by an upper outlet 64.

When the tendency towards pressure build up has eased the oil in the pulsation dampener chamber 57 defined by the jacket 56 will flow out through the aperture 58 back into the production tubular 10.

As with the embodiment of Figures 1 and 2, the pulsation dampener chamber can be recharged with gas and the pressure therein can be increased by the conduit 62 and valve 60.

Modifications and variations such as would be apparent to a skilled addressee are deemed within the scope of the present invention.

Claims

1. An oil well comprising a production tubular with a pump barrel attached to its lower end, a reciprocable oil pump having a plunger located with the pump barrel, a sucker rod string extending downwardly from ground level to the plungerfor reciprocating the plunger, and means for cyclically raising the sucker rod string, characterised in that it also comprises a rechargeable pulsation dampener chamber located and arranged so as to be open to oil pressure immediately above the plunger and means for charging pressurised gas into the pulsation dampener chamber.

2. An oil well according to claim 1, characterised in that the sucker rod string is hollow, the pulsation dampener chamber is located within the sucker rod string, and the means for charging pressurised gas into the pulsation dampener chamber is a non-return valve located adjacent the upper end of the sucker rod string.

3. An oil well according to claim 2, characterised in that the sucker rod string is, at least adjacent its lower end, of wide bore and substantially smooth internal shape.

4. An oil well according to claim 3, characterised in that at least about 50 feet of the sucker rod string extending upwardly from the plunger is of wide bore and substantially smooth internal shape.

5. An oil well according to claim 3, characterised in that at least about 100 feet of the sucker rod string extending upwardly from the plunger is of wide bore and substantially smooth internal shape.

6. An oil well according to claim 3, characterised in that at least about 200 feet of the sucker rod string extending upwardly from the plunger is of wide bore and substantially smooth internal shape.

7. An oil well according to claim 3, characterised in that at least about 500 feet of the sucker rod string extending upwardly from the plunger is of wide bore and substantially smooth internal shape.

8. An oil well according to claim 3, characterised in that at least about 1,000 feet of the sucker rod string extending upwardly from the plunger section is of wide bore and substantially smooth internal shape.

9. An oil well according to any one of claims 2 to 8 characterised in that the plunger comprises a piston sealingly engaged with the pump barrel and having an aperture extending through it, and an upper portion provided with an outlet aperture through which oil flows on the down stroke ofthe sucker rod string.

10. An oil well according to any one of claims 2 to 9 characterised in that the sucker rod string has, adjacent its lower end, an aperture through which oil is arranged to flow to enter the sucker rod string upon the plunger being raised by the sucker rod string and to flow to leave the sucker rod string upon the plunger being lowered.

11. An oil well according to claim 1, characterised in that the pulsation dampener chamber is located externally of the production tubular, the chamber is in communication with the interior of the production tubular and the means for charging pressurised gas in to the pulsation dampener chamber is a non-return valve arranged to be in communication with a source of pressurised gas at ground level.

12. A reciprocable oil pump comprising a plunger, characterised in that it also comprises a hollow sucker rod string for reciprocating the plunger, the interior of the sucker rod string forming a rechargeable pulsation dampener chamber open to the oil pressure immediately above the plunger and extending continuously to ground level, and a nonreturn valve closing the upper end of the pulsation dampener chamber, through which valve the pulsation dampener chamber may be recharged with pressurised gas.

13. A reciprocable oil pump according to claim 12, characterised in that the sucker rod string is, at least adjacent its lower end, of wide bore and substantially smooth internal shape.

14. A reciprocable oil pump according to claim 13, characterised in that at least about 50 feet of the sucker rod string extending upwardly from the plunger is of wide bore and has a substantially smooth internal shape.

15. A reciprocable oil pump according to claim 13, characterised in that at least about 100 feet of the sucker rod string extending upwardly from the plunger is of wide bore and has a substantially smooth internal shape.

16. A reciprocable oil pump according to claim 13 characterised in that at least about 200 feet of the sucker rod string extending upwardly from the plunger is of wide bore and has a substantially smooth internal shape.

17. A reciprocable oil pump according to claim 13, characterised in that at least about 500 feet of the sucker rod string extending upwardly from the plunger is of wide bore and has a substantially smooth internal shape.

18. A reciprocable oil pump according to claim 13, characterised in that at least about 1,000 feet of the sucker rod string extending upwardly from the plunger is of wide bore and substantially smooth internal shape.

19. A reciprocable oil pump according to any one of claims 12to 18, characterised in that the plunger comprises a piston sealingly engaged with the well casing and having an aperture extending through it, and an upper portion provided with an outlet aperture through which oil flows on the down stroke of the sucker rod string.

20. A reciprocable oil pump according to any one of claims 12 to 19 characterised in that the sucker rod string has, adjacent its lower end, an aperture through which oil is arranged to flow to enter the sucker rod string upon the plunger being raised by the sucker rod string and to leave the sucker rod string upon the plunger being lowered.

21. A sucker rod string characterised in that it comprises a plurality of hollow sections joined together so as to form a rechargeable pulsation dampener chamber located and arranged to be open to oil pressure immediately above a plunger, at least about 50 foot length of said sections being of wide bore and joined together in a manner providing a substantially smooth internal shape, the endmost section at the end adjacent said wide bore section being arranged to be coupled to the plunger and the endmost section remote from the wide bore section being provided with a non-return valve to enable pressurised gas to be charged into the pulsation dampener chamber.

22. A method of damping pressure surges when artificially lifting oil characterised in that is comprises establishing a column of oil in an oil well, imparting lift to the column by means of a sucker rod string attached to a plunger at the bottom of the column, and damping increases in pressure in the column by exposing a repressurisable pulsation dampener chamber containing pressurised gas to oil pressure immediately above the plunger and recharging the pulsation dampener with pressurised gas as may be required.

23. An oil well substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

24. A reciprocable oil pump substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

25. A sucker rod string substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

26. A method of damping pressure surges when artificially lifting oil substantially as hereinbefore described with reference to the accompanying drawings.