GB2055978A

GB2055978A - Oil well gas lift technique

Info

Publication number: GB2055978A
Application number: GB8004676A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-02-22
Filing date: 1980-02-12
Publication date: 1981-03-11
Also published as: EG14142A; GB2055978B; IN153262B; AU5549180A; DE3004656A1; AU535958B2; BR8000991A; NL8000945A

Abstract

An oil well gas lift system includes apparatus for compressing, cooling and liquifying production and other gases, apparatus to introduce a vapour-liquid mixture into the annulus of a well where the mixture is heated and passes through a throttle, valve to provide for gasification of at least a portion of the mixture at a location downhole, and another valve to introduce gasified material into the tubing string.

Description

SPECIFICATION Gas lift technique The present invention relates generally to a novel method and apparatus for gas lift. In particular, the present invention relates to a gas lift system which utilizes a phase conversion to provide pressurized gas at a desired downhole location.

The concept of utilizing gas and/or air as a means of artificial lift evolved in the late 1 700's.

The early methods and apparatus were designed primarily for continuous flow operations.

Continuous flow gas lift has been defined as a means of artificial lift where gas is continually injected from the surface down the tubing-casing annular space, through a gas lift valve, and up into the tubing string. The gas aerates the fluids in the tubing string, forcing them to the surface.

Historically, gas was injected either around the bottom or through a piece of equipment commonly called a "foot-piece." A technology developed which provided for the selective injection of gas into the tubing string principally through valves now well-known in the art. Intermittent gas lift is a means of artificial lift where a slug or column of liquid is allowed to accumulate in the tubing string, whereupon gases are injected through a gas lift valve underneath the liquid slug to propel it to the surface in the form of a piston. A wide variety of gas lift valves have been designed specifically for intermittent lift.

With both continuous and intermittent gas lift, it is required that substantial pressures be produced at the surface of the well to achieve desired results. In addition, numerous valves are required in known systems to provide suitable pressures at the point where gas is introduced into the tubing string. Relatively large pressures must be produced at the surface which in turn require that relatively heavy duty pumps or compressors, tanks, conduit, valves, and the like, be provided. It will, of course, be appreciated that an alternative to injecting gas into the tubing string to aerate the well fluids therein is to introduce pressurized fluid into the tubing string to aerate well fluids in the annulus and thus achieve production from the annulus.

The high pressures required with known systems and apparatus introduce additional problems of system reliability. High pressure components such as pumps, compressors and valves require careful maintenance to avoid expensive or dangerous failures. Large amounts of energy are consumed in operating heavy duty and high pressure pumps and compressors.

Whereas known arrangements have exhibited a degree of utility in artificial lift, room for significant improvement remains. The few problems enumerated in the foregoing are examples of many which tend to impair the effectiveness of previously known methods and apparatus for gas lift. Other noteworthy problems may also exist; however, those presented should be sufficient to demonstrate that known methods and apparatus in the art have not been all together satisfactory.

Recognizing the need for an improved gas lift system, it is a general object of the present invention to provide an improved method and apparatus for gas lift.

This and other objects of the invention are attained by a gas lift method characterized by the steps of liquifying a gas, introducing the liquified gas into an interior zone of a well, heating the liquified gas, and changing at least a portion of the liquified gas to gas at a subsurface location.

Another aspect of the invention is directed to a gas lift apparatus characterized by liquifying means for liquifying a gas, first conduit means for placing the liquified gas in fluid communication with an interior liquid zone in a well, pressure relief means for relieving pressure on the liquid in the interior liquid zone to facilitate a phase change to a gas, second conduit means for placing the gas in fluid communication with well fluids, and third conduit means for placing the gas and well fluids in fluid communication with the surface of the well.

A gas lift system according to a presently preferred embodiment of the invention intended to substantially accomplish the foregoing objects includes a compressor and a condenser to liquify a portion of the gas produced from a well. The liquid may be mixed with gases or vapors, and the resulting mixture is introduced under pressure into, say, the annulus of a well. A zone in the annulus is straddled by a pair of packers with the upper packer having a throttle valve which places the zone above the upper packer in fluid communication with the straddled zone. The liquid above the upper packer is maintained in compression both by surface pumps and by column weight.

Inasmuch as regions downhole are typically at slightly higher temperatures than the surface, at least a portion of the liquid above the upper packer may be vaporized by introducing it into the straddled zone which is at a relatively low pressure. The gas under pressure in the straddled zone may then be introduced by means of a conventional gas lift valve into the tubing string to aerate the well fluids contained therein.

The well liquids and gases which are artificially lifted to the surface may be separated into liquid and gaseous components. The liquid components may be carried off by conventional means and the gas components may be recompressed and condensed for further gas lift operations.

The invention will now be described, by way of example only, with reference to the accompanying drawing, which is a flow diagram schematically depicting a gas lift system in accordance with the present invention.

FIGURE 1 is a flow diagram which schematically depicts a system embodying the present invention.

With reference now to the flow diagram there is schematically depicted a well 10 defined by an outer casing 12 which extends below the surface 14 of the earth to a production zone 16. The casing may have perforations 1 8 made in a known manner to place the producing zone 1 6 in fluid communication with an interior of the casing 1 2.

A tubing string 20 extends coaxially down the well to a location near the producing zone 1 6. The outside of the tubing string 20 and the inside of the casing 12 define a zone 22 commonly referred to as the "annulus" of the well. Inasmuch as the gas lift system of the present invention may be characterized as a closed system, any point may be designated the beginning or end of the loop.

For convenience, a point designated "A" will be considered a starting point.

At the starting point "A" a vapor-liquid mixture is produced from the tubing string 20 of the well.

This mixture comprises crude oil, working media vapors, and produced and/or injected liquid petroleum gases. This mixture is at a pressure, say, P 1, and is fed into a first separator 24 of conventional design. In the separator 24, the mixture is separated into a liquid and gas fraction.

The gas fraction leaves the first separator at a first outlet 26, and the liquid fraction leaves the first separator at a second outlet 28.

The gas fraction which leaves the first separator via the first outlet 26 is directed by means of a conduit 30 to a first three-way valve 32. If the pressure of the gas at the three-way valve 32 drops below P 1, then this condition may be sensed by conventional apparatus at the threeway valve 32, and the three-way valve is automatically opened to direct the gas to a compressor 34 which increases the pressure of the gas to a new level P4. The gas is then routed to a mixer 36 via a conduit 38. If the pressure at the three-way valve 32 is equal to or greater than P1 ,then the three-way valve directs the gas directly to the mixer 36 via the conduit 38, and the compressor 34 is by-passed.

The liquid fraction leaves the first separator at the outlet 28 and is directed through a throttle valve 40 where the pressure is reduced to approximately atmospheric level. As a result of this pressure change, a new vapor-liquid mixture is formed and is transmitted via a conduit 42 at a pressure P2 into a second separator 44. In the second separator 44 the new mixture is separated into liquid and gas phases. The liquid phase may be removed through an outlet 46 and represents the oil output of the well 1 0 which is removed from the system.

The gas fraction in the second separator 44 comprises heavy hydrocarbons and is directed via a conduit 48 to a conventional compressor 50 where it is compressed to condensation pressure P3. The value of P3 depends upon the gas composition and upon cooling water temperature in a conventional condenser 52 downstream from the compressor 50. The cooling water is schematically shown cycling through the condenser 52.

If condensation is incomplete at the output of the condenser 52, the uncondensed portion of the output of the condenser may be removed through conventional means and may be fed back to the gas fraction output 26 of the first separator 24 as shown by a broken line 54 in the FIGURE. If condensation is complete, the output of the condenser 52 is directed via a conduit 56 to a second three-way valve 58.

The condensate output of the condenser is sensed by conventional means at the second three-way valve 58. If the sensing apparatus indicates that further compression is desirable, the second three-way valve 58 directs the condensate output to a pump 60 via a conduit 62. If the sensing apparatus indicates that no further compression is required, the output condensate by-passes the pump 60 via a conduit 64 and is routed directly to the mixer 36. The gas fraction at the output 26 of the first separator 24 and the liquid fraction output of the condenser 52 are mixed in the mixer 36. The output of the mixture is a vapor-liquid working mixture which is introduced into the annulus of the well via a conduit 66.

In the practice of the present invention, the apparatus should desirably be arranged by conventional means to provide a working mixture comprised of relatively light or volatile fractions of hydrocarbons and relatively heavy fractions of hydrocarbon. The mixture of the fractions may depend upon the particular composition of produced fluids, as well as upon the particular downhole temperature, surface temperature, and geometry of the well. In any event, it is believed that a mixture of heavy and light fractions will facilitate the liquification of the light fractions at a reduced pressure level thus facilitating the introduction of a liquid from conduit 66 into the well. It will be appreciated that on occasion it may be desirable to add light fractions from a source other than produced fluids in order to achieve a satisfactory composition of the working mixture.

In a preferred embodiment of the present invention the tubing string 20 is placed in fluid communication with the annulus by means of a conventional gas lift valve 68 installed near the bottom of the tubing string by conventional means. This is the valve through which gas may be.introduced beneath production fluids in order to lift those production fluids to the surface. Two packers 70 and 72 "straddle" a zone 76 in which the conventional gas lift valve is situated. Below the gas lift valve 68 there is positioned a first packer 70 to isolate the annulus above and below the packer. Above the gas lift valve 68, there is a second packer 72 which contains a throttle valve 74. The throttle valve 74 may be an aperture in the packer or may comprise another well-known arrangement.

The vapor-liquid mixture or composition from the mixer 36 is introduced into the annulus at a pressure at least slightly greater than atmospheric pressure and accumulates above the upper packer 72. Due to the weight of the column of fluid between the upper packer 72 and the surface 14, a significant pressure is imposed on the vaporliquid mixture in a region above the upper packer 72. The specific gravity of the mixture and the length of the column between the upper packer and the surface will determine to a significant extent the particular pressure immediately above the upper packer. It will be appreciated that the composition of the working mixture may be adjusted to provide more or less pressure downhole depending on the particular parameters of the well being treated.In addition, supplemental surface pumps may from time to time be added to the system to enhance the pressure in the annulus to maintain the appropriate pressure on the working mixture when temperature changes or changes in production gases cause variations in the rate of vaporization down hole.

As the vapor-liquid mixture moves down the annulus, a heat exchange occurs between the mixture and the casing and also between the mixture and the tubing 20. The tubing 20 is at a relatively high temperature as a result of the high temperature of the oil lifted out during production, and the casing 12 may likewise be at an elevated temperature. As a result of this heat exchange, the working mixture is heated.

The heated mixture passes through the throttle valve 74 into a zone of reduced pressure. As a result of the increase in temperature and a drop in pressure of the mixture, at least a portion of the mixture vaporizes. It should be noted that the extent of vaporization is controlled by the composition of the vapor-liquid mixture introduced into the annulus. In any event, the vapor is then directed through the gas lift valve 68 and artificially lifts the produced oil in the usual manner.

The vapor-liquid mixture of the working medium and the produced fluids are carried up the tubing 20 to the surface 14 to the arbitrary starting point "A".

It will be appreciated that in constructing a gas lift system according to the present invention, certain significant advantages are provided. In particular, the pressure increase in the vapor-liquid mixture introduced into the annulus is enhanced by gravity with the result that there will be provided a considerable savings in the energy consumed to achieve the pressurization of the vapor-liquid mixture in a zone immediately above the upper packer. Clearly, energy savings may be readily transferred into cost savings. It is estimated that the horsepower requirements in the present system will be reduced by a factor of fifteen over known artificial lift systems.

In addition, system reliability will be improved in that the number of gas lift valves downhole will be reduced. Typically, more than one gas lift valve is required in known systems. Although additional hardware, e.g., a mixer, separator, compressor, and condenser may be required in the practice of the present invention, these items are all on the surface where there more readily serviced and maintained. It is believed that the maintenance or replacement of downhole gas lift valves requires significantly greater down time and manpower than maintaining or replacing, say, a compressor.

The heat required to vaporize the working mixture may be obtained from the oil being produced and from the formation. It is believed this heat will be adequate to satisfactorily achieve the required phase change.

The foregoing description of the invention has been directed to a particular preferred embodiment in accordance with the requirement of the Patent Statute and for purposes of explanation and illustration. It will be apparent, however, to those skilled in this art that modifications and changes in both apparatus and method may be made without departing from the scope and spirit of the invention. For example, the working mixture may be introduced into the tubing string and production may be achieved through the annulus. In addition, artificial heating apparatus may be introduced into the well to enhance vaporization of the working mixture in the staddled zone. Moreover, it is within the scope of the present invention to achieve at the surface the increased vaporization of the vapor-liquid mixture which preferably takes place downhole. That is, the heating of the working mixture which is the output of the mixer 36 may in some instances be satisfactorily carried out at the surface and the pressurization achieved as a result of the column weight on the working mixture may be carried out through other means at the surface. Whereas such arrangements may not fully achieve the energy savings of the described preferred embodiment, counter-vailing circumstances could make such an arrangement desirable with at least some of the benefits of the invention still available. These and other modifications of the invention will be apparent to those skilled in this art. It is applicant's intention in the following claims to cover all such equivalent modifications and variations which fall within the true spirit and scope of the invention.

Claims

1. A gas lift method characterized by the steps of: liquifying a gas; introducing the liquified gas into an interior zone of a well; heating the liquified gas; and changing at least a portion of the liquified gas to gas at a subsurface location.

2. The method of claim 1 characterized in that the interior zone is an annulus of a well.

3. The method of claim 1 or 2 characterized in that the interior zone is in a tubing string of a well.

4. The method of claim 1,2 or 3 characterized in that said changing step is carried out by placing the liquid in fluid communication with an inlet of a valve, the outlet of said valve being in fluid communication with a zone of reduced pressure.

5. The method of claim 1 characterized by the step of introducing the subsurface gas into a region containing well fluid.

6. The method of claim 5 characterized by the step of controlling the rate at which the subsurface gas is introduced into the region containing well fluids.

7. The method of any one of the previous claims characterized in that the liquifying step comprises: liquifying relatively heavy fractions from gas produced from a well; supplying relatively light fractions from gas produced from a well; and mixing the relatively heavy fractions together with the relatively light fractions to provide a vapor-liquid mixture.

8. The method of claim 7 characterized in that the changing step comprises: heating the mixture to vaporize at least a portion of the liquid in the mixture.

9. A gas lift apparatus characterized by: liquifying means for liquifying a gas; first conduit means for placing the liquified gas in fluid communication with an interior liquid zone in a well; pressure relief means for relieving pressure on the liquid in the interior liquid zone to facilitate a phase change to a gas; second conduit means for placing the gas in fluid communication with well fluids; and third conduit means for placing the gas and well fluids in fluid communication with the surface of the well.

10. The gas lift apparatus of claim 9 characterized by separating means for separating the gas and well fluids into liquid and gaseous components.

11. The gas lift apparatus of claim 9 or 10 characterized by first valve means for controlling a rate at which pressure is relieved by said pressure relief means.

12. The gas lift apparatus of claim 9, 10 or 11 characterized by second valve means for controlling a rate at which the gas flows in said second conduit means.

13. The gas lift apparatus of any one of the previous claims characterized in that the gas liquified by said liquifying means is comprised of gases produced by the well.

14. The gas lift apparatus of any one of the previous claims characterized by heating means to heat the liquified gas at a location downhole.

1 5. A gas lift method substantially as herein described with reference to the accompanying drawing.

1 6. Gas lift apparatus substantially as herein described with reference to the accompanying drawing.