U.S. patent number 3,646,953 [Application Number 05/025,985] was granted by the patent office on 1972-03-07 for gas lift apparatus.
This patent grant is currently assigned to Macco Oil Tool Co., Inc.. Invention is credited to Douglas W. Crawford, James Kelly Elliott.
United States Patent |
3,646,953 |
Elliott , et al. |
March 7, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
GAS LIFT APPARATUS
Abstract
A casing pressured operated, variable orifice valve is employed
as the operating valve in a single point injection well assembly
with tubing pressure (fluid) operated unloading valves acting above
the operating valve to provide a continuous flow system. The
operating valve includes a throttling range which extends between
optimum injection gas pressure levels based on the amount of
injection gas pressure available at the well head to thereby
increase the overall efficiency of the system. The variable orifice
valve includes a pressure charged bellows and a coil spring in
series to produce a linear resultant load rate which in turn
cooperates with contoured closure surfaces in the valve to produce
a linear relationship between orifice size and injection gas
pressure. Gas is thus injected into the production column or tubing
at a rate which is linearly related primarily to the casing
pressure with the throttling range of the operating valve producing
a broad response in the gas injection rate. The injection rate is
surface regulated by controlling the injection gas pressure at the
wellhead.
Inventors: |
Elliott; James Kelly (Baytown,
TX), Crawford; Douglas W. (Houston, TX) |
Assignee: |
Macco Oil Tool Co., Inc.
(Houston, TX)
|
Family
ID: |
21829170 |
Appl.
No.: |
05/025,985 |
Filed: |
April 6, 1970 |
Current U.S.
Class: |
137/155; 417/110;
417/112; 417/118; 417/115 |
Current CPC
Class: |
E21B
43/123 (20130101); Y10T 137/2934 (20150401) |
Current International
Class: |
E21B
43/12 (20060101); F04f 001/20 () |
Field of
Search: |
;137/155 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cohan; Alan
Claims
What is claimed is:
1. A gas lift valve for conveying gas from a gas containing column
into a fluid containing column comprising:
a. a valve body having an inlet means for admitting gas into said
body from said gas containing column, an outlet means for emitting
gas from said body into said fluid containing column, an outlet
means for emitting gas from said body into said fluid containing
column and a communicating means for conveying gas between said
inlet and outlet means;
b. closure means disposed within said communicating means for
opening and closing said communicating means to permit or stop the
flow of gas between said inlet and outlet means and to regulate the
rate of gas flow through said communicating means when said
communicating means is open; and
c. control means connected with said closure means and responsive
primarily to the pressure of the gas at the inlet means for
controlling the opening or closing of said communicating means and
for controlling the rate of gas flow by varying the effective size
of said communicating means in a linear relationship with the
pressure value of the gas at said inlet means.
2. The valve as defined in claim 1 wherein said closure means
includes seat means and movable valve stem means whereby said
communicating means is closed by moving said valve stem means into
sealing engagement with said seat means, said valve stem means
having a contoured surface thereon cooperating with said seat means
whereby movement of said valve stem means with respect to said seat
means alters the effective size of said communicating means in said
linear relationship for controlling the rate of gas flow through
said communicating means.
3. The valve as defined in claim 2 wherein said control means
includes biasing means operable with said valve stem means for
biasing said valve stem means toward sealing engagement with said
seat means.
4. The valve as defined in claim 3 wherein said biasing means
includes a pressure charged, variable size chamber cooperating with
a spring means.
5. The valve as defined in claim 3 wherein said biasing means
includes a variable size, pressure charged chamber exposed to and
responsive to the pressure of the gas at said inlet means and
secured to said valve stem means whereby said contoured surface
cooperates with said seat means to alter the size of said
communicating means in said linear relationship with the pressure
of the gas at said inlet means.
6. The valve as defined in claim 5 wherein:
a. said charged chamber includes a bellows;
b. said biasing means includes spring means cooperating with said
bellows; and
c. said biasing means includes adjustment means for altering the
biasing force exerted by said spring means.
7. The valve as defined in claim 2 wherein said closure means
includes a check member urged toward sealing engagement with second
seat means by biasing means to prevent flow of fluid from said
fluid column through said communicating means when the pressure at
said outlet means is greater than the pressure at said inlet
means.
8. The valve as defined in claim 5 wherein said biasing means is
provided with stop means to limit the movement of said stem means
away from said seat means and to limit the size to which said
pressure charged chamber may be reduced.
9. A gas lift valve for conveying gas from a gas containing column
into a fluid containing column comprising:
a. a valve body having an inlet means for admitting gas into said
body from said gas containing column, an outlet means for emitting
gas from said body into said fluid containing column and a
communicating means for conveying gas between said inlet and outlet
means;
b. closure means disposed within said communicating means for
opening and closing said communicating means to permit or stop the
flow of gas between said inlet and outlet means and to regulate the
rate of gas flow through said communicating means when said
communicating means is open;
c. check means disposed within said communicating means, including
a closure member urged toward sealing engagement with seat means by
biasing means, for preventing flow of fluid from said fluid column
through said communicating means when the pressure at said outlet
means is greater than the pressure at said inlet means and for
isolating the pressure at said outlet means from the pressure
acting on said closure means when said closure means is in the
closed position; and
d. control means connected with said closure means and responsive
primarily to the pressure of the gas at said inlet means for
controlling the opening or closing of said communicating means and
for controlling the rate of gas flow through said communicating
means as a direct function of the gas at said inlet means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to apparatus for elevating fluids
through a flow path. More specifically, the present invention
relates to apparatus for surface controlled gas lifting of
petroleum fluids from a subterranean formation through a well
conduit to the earth's surface.
2. Brief Description of the Prior Art
In conventional gas lift systems, petroleum fluids are artificially
lifted to the surface of a well by injecting a gas under pressure
into the column containing the fluid to be elevated. By this means,
a desired flowing bottomhole pressure is created in the well which
permits elevation of the petroleum fluids to the wellhead at a
desired rate. In a typical continuous flow gas lift system, gas is
introduced into the well casing and injected into the petroleum
fluid column contained within the tubing string. The casing gas is
injected into the tubing through a single gas lift valve referred
to as the operating valve which is generally disposed in the tubing
string below a series of vertically spaced unloading valves.
When tubing pressure operated (fluid operated) gas lift valves are
employed to unload the well, each unloading valve closes
automatically when the tubing pressure opposite the valve decreases
below a predetermined minimum valve. By this means, each succeeding
lower valve is closed as the fluid level in the annulus is
successively lowered and the tubing fluid level is raised until the
operating or bottommost injection valve is exposed to the injection
gas in the casing. The latter valve, the operating valve, is
thereafter employed as the single point injection valve for a
continuous flow gas lift system. Where casing pressure operated
rather than fluid operated valves are employed to unload, closure
of each succeedingly lower casing pressure operated valve generally
necessitates a succeeding reduction in the casing pressure. For
this reason, a lower effective injection gas pressure is available
for injection into the tubing at the operating valve as compared
with that available with the use of fluid operated unloading
valves.
In conventional systems, the regulation range, i.e., the injection
gas pressure range over which the operating valve remains open,
also presents problems in that the regulation range of a casing
pressure operated valve is generally relatively limited resulting
in little or no control over the rate of gas injection into the
tubing. It is well known that as the well conditions change or as
production rates are altered to conform to restrictions imposed by
regulatory agencies, the most effective or efficient rate of gas
injection may be different from that existing in the well at the
beginning of the gas lift operation and it is often desirable to
alter the injection rate with such changing conditions or
restrictions. Where casing pressure operated unloading valves are
used, raising the gas pressure in an attempt to increase the
injection rate may only serve to move the point of gas injection up
the well to a higher gas lift valve. This is usually undesirable
since optimum producing conditions normally require injection at
the lowermost valve possible, thus making it clear that the lifting
efficiency is reduced whenever the point of injection moves above a
specified injection point.
While controlled variation in the rate of introducing gas into the
production fluid column is highly desirable, it will be understood
that it is impractical to attempt to control the injection rate by
replacing the operating valve each time a new injection rate is
desired. In short, the prior art apparatus and methods available
for gas lift operations have lacked suitable means for effecting
surface controlled regulation of the rate at which gas is injected
into the fluid column through the operating valve. Moreover, the
conditions normally imposed by conventionally employed casing
pressure operated unloading valves limit the operating range of
prior art systems to pressure levels significantly below the
injection gas pressure available at the wellhead.
SUMMARY OF THE INVENTION
In the apparatus of the present invention, a variable orifice,
casing pressure operated valve is employed as the operating valve
in a single point injection, continuous flow gas lift installation
with regulation of the gas injection rate being controlled by
varying the injection gas pressure at the wellhead. In the
preferred form of the invention, fluid operated valves are employed
above the operating valve to unload the well thereby permitting gas
injection through the operating valve at the highest possible
injection gas pressures based upon the gas pressure available at
the wellhead.
Thus, in a method designed for use with the apparatus of the
present invention, fluid operated unloading valves are employed
whereby opening and closing of the unloading valves is regulated by
the pressure of the production fluid in the tubing rather than the
pressure of the injection gas in the casing. For this reason, the
casing pressure at the wellhead may be maintained at the maximum
desirable level while maintaining injection of gas into the fluid
column through a single operating valve. The net result is that a
substantially higher gas pressure is available at the operating
valve for injection into the tubing which in turn increases the
overall efficiency of the system.
The operating valve of the present invention includes a spring and
a pressure charged bellows connected in series to form a resultant
load rate which produces a linear relationship between valve stem
travel and changes in casing pressure. The valve stem includes a
contoured closing surface which is adapted to move toward and away
from a valve seat under the influence of the injection gas pressure
with the contour and load rate producing a linear relationship
between the effective orifice size and the injection gas pressure.
Operation of the variable orifice valve is controlled by varying
the injection gas pressure at the well head which in turn alters
the orifice size of the operating valve to change the rate of gas
injection into the production fluid column. The design of the valve
is effective to produce a throttling range in which large changes
in the rate of gas injected into the production fluid column are
possible with relatively small changes in the pressure of the
injection gas. The linear relationship between orifice size and
injection gas pressure provides a linear relationship between
changes in the injection gas pressure effected at the surface and
changes in the rate of gas injection into the fluid column.
Where casing pressure operated valves are employed as unloading
valves, the highest maximum operating range for the variable
orifice operating valve of the present invention is decreased,
however, improved regulation and response is also possible when
compared with that possible with fixed orifice valves operating
over the same injection gas pressure range.
The foregoing and other features and advantages of the present
invention will become more apparent from the following detailed
description and claims when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevation, partially in section schematically
illustrating a gas lift installation;
FIG. 2 is a vertical section of the variable orifice gas lift valve
of the present invention;
FIG. 3 is a detailed view illustrating the contour of the valve
stem closure member in the valve of FIG. 2;
FIG. 4 is a graph illustrating the linear relationship between
injection gas pressure opposite the operating valve and equivalent
orifice diameter;
FIG. 5 is a gas passage chart for various orifice sizes
illustrating the improved regulation range of the variable orifice
gas lift valve of the present invention;
FIG. 6 is a graph of well depth versus injection gas pressure
illustrating features of the variable orifice valve of the present
invention with fluid operated valves as unloading valves; and
FIG. 7 is a graph of well depth versus injection gas pressure
illustrating characteristics of a system employing the variable
orifice gas lift valve of the present invention with casing
pressure sensitive valves employed as the unloading valves.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates a representative gas lift installation which
includes a casing C surrounding a production tubing string T. The
casing C and tubing T extend downwardly into a petroleum bearing
formation F. Perforations P formed in the casing C permit petroleum
fluids in the formation to flow into the casing and into the open
lower end of the production tubing string T. A suitable well packer
W is employed to form a leakproof seal between the tubing T and the
casing C to thereby isolate the annular area A from the
formation.
Under many conditions, the natural pressure of the formation F is
sufficient to produce flow of petroleum fluids through the
perforations P and into the production tubing T where the fluid is
forced up to the wellhead. In low pressure formations, however, the
natural formation pressure is insufficient to elevate the
production fluid through the tubing string T or, in some cases, the
formation pressure is adequate to maintain a continuous flow of
fluid through the tubing string, but is insufficient to initiate
flow with the well fluids in static, nonflowing condition. Under
any of the foregoing conditions, when the existing formation
pressure is insufficient to produce or sustain natural flow of the
production fluid at the wellhead, artificial means of lifting the
production fluid, such as gas lift are required.
In the system illustrated in FIG. 1, a suitable gas under pressure
is forced into the annular area A' (annulus) formed between the
tubing T and the casing C where it then enters gas lift valves V
(V.sub.i, V.sub.2, V.sub.3 -V.sub.10 ) which transmit the gas from
the casing C into the tubing string T to aerate or lighten the
fluid contained in tubing string T. As the column is aerated by gas
introduced through the valves V, the total weight of the column is
reduced to the point required to permit the existing formation
pressure to elevate the petroleum fluid to the surface.
Under certain conditions, maximum efficiency dictates that the gas
be injected into the tubing string through only a single valve
located as low as possible in the tubing string T. For this reason,
the upper valves located above the lowermost operating valve must
close after the well has been unloaded and remain closed while gas
is injected through the operating valve. It will be understood that
the term "unloading" as used herein refers to the initial operation
of the gas lift installation which requires removal of fluids from
the annular area A' to permit the pressurized gas to reach the
various gas lift valves in the system. During the unloading, fluid
in annulus A' above the packer W is forced into the tubing by the
gas injected into the annulus which U-tubes the fluid into the
unloading valves and into the tubing T.
In a preferred application of the gas lift apparatus of the present
invention, the operating valve indicated generally at V.sub.10 is
casing pressure operated and the unloading valves V.sub.1, V.sub.2,
and V.sub.3 are tubing pressure operated (fluid operated). The
operating valve V.sub.10 is a variable orifice, casing pressure
operated valve which includes a throttling range wherein the valve
remains open between two different injection gas pressure levels in
the casing C. By the described means of the present invention, the
pressure of the injection gas in the casing C may be varied to
regulate the rate of gas injection into the tubing string T through
the operating valve V.sub.10 . As will be seen, the specific design
of the valve V.sub.10 produces a linear relationship between
injection gas pressure in the casing and the rate of gas injection
into the tubing string T to produce a broad, linearly regulated
response range.
Referring to FIG. 2, the valve V.sub.10 of the present invention is
illustrated mounted in operative position in a mandrel side pocket
S partially drawn in by dotted lines. The valve V.sub.10 includes a
pressure charged bellows 11 secured to the upper portion of an
axially movable lower adapter 12 which in turn is secured at its
lower end to a valve stem 13. A helical coil spring 14 encircles
the lower adapter 12 and acts in series with the bellows 11 to
exert an axially directed force on the valve stem 13. The bellows
11 and spring 14 govern the "load rate" of the valve V.sub.10 which
is the relationship between injection gas pressure and stem
travel.
The bellows 11, lower adapter 12, spring 14 and valve stem 13 are
housed within a composite outer valve body which includes an upper
housing body 15 threadedly engaged at its lower end to a lower
housing body 16. A head adapter 17 secures the lower housing body
16 to a valve head 18 forming the lower part of the valve V.sub.10.
At the bottom of the valve, a tapered nose section 19 is threadedly
engaged to check body housing 20 which in turn is threadedly
engaged to the lower end of the valve head 18. The outer housing is
sealed by means of resilient O-rings 21 and 22 positioned
respectively between the upper and lower housing body sections 15
and 16 and the valve head 18 and check body 20.
The upper end of the gas lift valve V.sub.10 is sealed by a
retrievable bellows top 23 which is threadedly engaged into the top
of the upper housing body 15. The external surface of the bellows
top 23 is provided with threads 23a which are adapted to engage a
suitable positioning or fishing tool (not shown) employed to
position the valve V.sub.10 within or retrieve it from the pocket
S. A resilient O-ring 24 is disposed between the housing body 15
and the bellows top 23 to provide a leakproof seal between the two
mated components. A seal plug 25 is threadedly engaged in the
bellows top 23 and a seal plug gasket 26 and a resilient O-ring 27
are disposed between the seal plug and the bellows top 23 to seal
the enclosed bellows area A. A dill core valve 28 is positioned
within the central opening formed in the bellows top 23 to permit
pressurization of the bellows chamber with nitrogen or other
suitable gas from an external source.
A pair of drag rings 29 and 30 are positioned between the lower
adapter 12 and the upper housing body 15 to form a guide between
the adapter and the housing. As will be seen, gas pressure acts
against the bellows to move the adapter 12 axially in a direction
dependent upon the pressure differential existing between the
injection gas and the internal bellows area A. It may also be seen
that an axial force is imparted to the lower adapter 12 by the coil
spring 14 which is compressed between the lower end of the upper
housing member 15 and an axially movable adjustment nut 31 which
may be moved axially over external threads formed on the lower
adapter 12. A locknut 32 may be employed to fix the position of the
adjusting nut 31 in a conventional manner.
It will be understood that advancing the adjusting nut 31 axially
toward the threaded lower end of the body 15 increases the
compression of the spring 14 to thus increase the downwardly
directed bias on the lower adapter 12. The total axially directed
forces induced by the bellows 11 and spring 14 are also conveyed to
the valve stem 13 which is threadedly engaged to the lower end of
the lower adapter 12. A guide baffle 37 directs the axial movement
of the valve stem 13 to maintain the contoured closure surfaces of
the valve stem in a substantially concentric relationship with a
valve seat 18a and central bore 18b extending axially through the
valve head 18.
The check body 20 houses a check valve closure member 38 which is
biased axially upwardly under the influence of a coil spring 39
into sealing engagement with a check disc 40. Coaxial alignment
between the closure member 38 and the seat in the check disc 40 is
maintained by means of a check baffle 41. As will hereinafter be
more fully explained, the closure member 38 acts to prevent a
reverse flow of fluid from the tubing T into the casing C when the
tubing pressure exceeds the casing pressure.
With the valve V.sub.10 in its open position, it will be understood
that injection gas in the casing enters the valve through radial
ports Sa extending through the mandrel pocket S. The valve V.sub.10
is positioned in leakproof engagement within the mounting pocket S
by means of suitable packing 42 carried on the external valve body.
The gas entering the pocket S enters radial ports 18c formed in the
valve head 18 where it then flows between the contoured valve
closure surface formed at the lower end of the valve stem 13 and
the valve seat 18a formed on the valve head 18 through the central
head bore 18b past the check disc 40 and check baffle 41 in the
open check valve where it enters the check body 20 and flows from
the valve into the production tubing through bores 19a formed in
the nose 19. Gas leaving the bores 19a formed in the nose of the
valve flows into the tubing through a suitable bore Sb extending
from the side pocket mounting S.
OPERATION OF THE VARIABLE ORIFICE VALVE
The valve V.sub.10 of the present invention may be secured to an
external side mounting lug (not shown) in a nonretrievable gas lift
installation as the tubing string T is being lowered into the well,
or if desirable, may be retrievably positioned as illustrated in
FIG. 2 within the internal mandrel pocket S after the tubing T is
in place in accordance with standard wire line practice. The well
is then unloaded as previously described until the annulus A' is
free of fluids in the area above the valve V.sub.10. The pressure
of the injection gas which enters the radial ports 18c of the valve
body, acts against the bellows area less the port area to move the
lower adapter 12 axially upwardly through the surrounding upper
housing body 15. An additional upward force is imposed on the
adapter by the effect of the tubing pressure acting against the
valve stem 13. The upward axial movement of the valve stem 13 is
resisted by the force induced by the charge in the bellows area A
and by the axially directed force exerted by the compressed spring
14. The amount of casing pressure required to move the lower
adapter 12 axially upwardly is thus substantially determined by the
amount of charge in the bellows area A and the compressive force
exerted by the spring 14. Of course, the upward axial movement of
adapter 12, and consequently stem 13, is limited by the engagement
of adapter 12 with bellows top 23.
When the casing pressure is sufficiently high, the valve stem 13 is
forced away from the seat 18a to permit the gas to flow through the
head bore 18b . The pressure of the injection gas in the bore 18b
forces the check valve closure member 38 downwardly to overcome the
biasing force of the spring 39 which thereby frees the opening in
the check disc 40 to permit the injection gas to flow into the
tubing mandrel. Continued increase in the casing pressure moves the
valve stem 13 even further away from the valve seat 18a to increase
the effective orifice area through which the injection gas may
enter the tubing. As stated heretofore, the movement of valve stem
13 away from the valve seat 18a is limited by the engagement of the
upper end of adapter 12 with bellows top 23.
The valve design causes the valve to throttle, i.e., remain open
between different values of casing pressure with the valve stem
moving axially within the throttling range to different positions
producing variations in the effective orifice size. Thus, the valve
includes a throttling range over which the effective orifice area
of the gas injection passageway may be varied with corresponding
variations in the rate of gas injection through the valve.
As may best be seen by joint reference to FIGS. 3 and 4, the
closure surface formed at the lower end of the valve stem 13 is
contoured to produce a linear relationship between the injection
gas pressure and the effective orifice opening. Thus, as may be
shown by reference to FIG. 4, as the casing or injection gas
pressure increases, the effective orifice size of the valve
increases at a linear rate. It should be noted that the contour
illustrated in FIG. 3 of the drawings is a presently preferred form
which produces the desired linear relationship between injection
gas pressure and effective orifice size. Variation in the specific
contour employed in the closing surface will produce a
corresponding variation in the relationship between injection gas
pressure and effective orifice size. It will therefore be
understood that a given configuration may be employed to produce
any desired relationship, linear or nonlinear, between injection
gas pressure and the valve's effective orifice size.
One of the advantages in the ability of the valve of the present
invention to vary its orifice size over a throttling range may be
readily perceived by reference to FIG. 5. FIG. 5 is a composite
chart illustrating injection gas pressure (P.sub.c ) along the
lower axis (upstream pressure) and gas throughput along the upper
axis. The group of curves S.sub.1 formed on the chart below a
critical flow curve CC represent standard curves for the indicated
pressure values. The curves S.sub.2 above the critical flow curve
represent different valve orifice sizes. Lines X and Y on the graph
illustrate valve performance under the following given
conditions:
1. Valve orifice is 14/64ths inches.
2. Tubing pressure is 800 p.s.i.g.
3. The maximum casing gas pressure P.sub.c is 1,060 p.s.i.g.
4. The minimum casing pressure is 935 p.s.i.g.
Line X illustrates the gas throughput for a 14/64-inch fixed
orifice operating valve indicated by curve S.sub.2 ' wherein the
casing pressure is 935 p.s.i.g., and the tubing pressure is 800
p.s.i.g. The gas throughput which is read at the top of the chart
is seen to be approximately 735 million cubic feet per day (mcfd).
Using the same valve but operating at an increased setting of
casing pressure up to 1,060 p.s.i.g. and maintaining the tubing
pressure at 800 p.s.i.g., it is seen that the gas throughput
represented by the intersection with line Y is approximately 1,000
mcfd. The regulation range for the valve represented by the lines X
and Y is therefore 1,000 mcfd minus 735 mcfd which equals 265
mcfd.
By contrast, when using the variable orifice valve of the present
invention with a 5/16th-inch orifice (bore 18b ) and operating at
the same values of maximum and minimum injected gas pressure and
the same tubing pressure, the regulation range represented by the
spread between the lines T and Z (2,050 mcfd- mcfd) is 2,050 mcfd.
With both valves, the following values were employed:
Gas gravity equals 0.65 (air equals 1.0 );
Temperature equals 60.degree. F.;
Atmospheric pressure equals 14.65 p.s.i.;
Gravity minus temperature correction;
Correction factor equal 0.0544 x .sqroot.GT;
g equals gas gravity, T equals temperatures, .degree.R.
It will be readily appreciated from the results illustrated in FIG.
5 that the variable orifice gas lift valve of the present invention
significantly increases the regulation range of a specific gas lift
installation. Moreover, as will also be readily appreciated, the
linear relationship between injection gas pressure and effective
orifice size ensures that the response of the entire regulation
range is linearly related to variations in injection gas
pressure.
FIG. 6 of the drawings illustrates the relationship between
injection gas pressure and well depth in a system employing a
variable orifice valve of the present invention as the operating
valve at the bottom of a series of vertically spaced fluid operated
unloading valves. The graph of FIG. 6 represents a well having the
conditions specified therein employing the operating valve of the
present invention with a throttling range of 125 p.s.i.g. and the
load rate of the spring and bellows in series having a resultant
load rate equal to 125 p.s.i.g. per 0.25 inch of valve stem travel.
In the graph of FIG. 6, the line R represents the static gradient
of load fluid (G.sub.SL =0.465 p.s.i. per foot); the curves O.sub.1
and O.sub.2 represent the operating casing pressures, the curve W
represents the static gradient of well fluid (G.sub.SW =0.446
p.s.i. per foot) and extends to a point (not shown) representing
the static bottomhole pressure (SBHP =1,400 p.s.i.g. at 10,300').
The point W.sub.1 represents the static fluid level. The range
between the curves 0.sub.1 and 0.sub.2 corresponds to the
regulating range of the valve.
The graph illustrates the 125 p.s.i.g. regulation range between
O.sub.1 and O.sub.2 where casing pressure P.sub.c can be altered to
produce changes in O.sub.rate of gas O.sub.injected into the
tubing. The graph is based on conditions where the maximum
available gas pressure at the wellhead is approximately 800
p.s.i.g. The resultant injection gas pressure at the operating
valve, 10,000 feet below the wellhead, is 890 p.s.i.g.
With a valve V.sub.10 designed to have a 125 p.s.i.g. throttling
range, the injection gas pressure P.sub.1 required to move the
valve to full open condition is 125 p.s.i.g. greater than the
minimum pressure value P.sub.2 required to prevent the valve from
closing. The valve will, of course, remain open for any pressure
value greater than P.sub.1 and will close with any injection gas
pressure value below P.sub.2 . Over the range of pressure between
P.sub.1 and P.sub.2 , the orifice size of the valve is variable.
For pressures in excess of P.sub.2 the orifice size remains fixed
as with conventional, fixed orifice valves. The important control
area is therefore in the pressure range between P.sub.1 and P.sub.2
since orifice size varies with pressure to alter the rate of gas
injection into the tubing over this range.
FIG. 7 of the drawings illustrates a graph depicting the variable
orifice valve of the present invention in a system employing casing
pressure operated unloading valves. The curves OP.sub.0, OP.sub.1,
OP.sub.2, OP.sub.3 and OP.sub.4 represent the operating casing
pressure; the curve G represents the relationship between pressure
and depth for the fluid having a zero gas to liquid ratio (OGLR).
The following table represents the injection gas to liquid ratios
(IGLR) required to attain the minimum gradient:
---------------------------------------------------------------------------
Depth IGLR (ft.) (c.f.b.)
__________________________________________________________________________
2000 600 3000 800 4000 1000 5000 1400 6000 1400 6300 2000
__________________________________________________________________________
The curves P.sub.1 and P.sub.4 represent tubing pressure for
vertically spaced valves with the designations "min" and "max"
representing the minimum and maximum fluid pressure in the tubing
for the respective valves.
The 125 p.s.i.g. throttling range in the system depicted in FIG. 7
is from 800 p.s.i.g. to 925 p.s.i.g. As with the preferred system
employing fluid operated unloading valves, the system in FIG. 7 may
be regulated by varying casing pressure at the wellhead to effect a
remote control over the rate of gas injected into the tubing.
The foregoing disclosure and description of the invention is
illustrative and explanatory thereof, and various changes in the
size, shape and materials as well as in the details of the
illustrated construction may be made within the scope of the
appended claims without departing from the spirit of the
invention.
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