Abstract
A method for controlling the liquid load size of a plunger lift well during the shut in time of the well to facilitate a controlled plunger rise. Intra-cycle control allows dynamic adjustments within a cycle to keep the plunger running and not stalling out or rising too fast. The method includes the steps of shutting in the well to build up pressure within the well, adjusting a size of a liquid slug within the tubing while the well is shut in, opening a valve to relieve pressure within the well and raise the plunger within the tubing, pushing the liquid slug out of the well with the plunger, and closing the valve wherein the plunger falls within the tubing. The intra-cycle adjustments include reducing the size of the liquid slug for preventing fluid loading and increasing the size of the liquid slug for controlling a rise rate of the plunger.
Claims
1. A method of intra-cycle adjustment in a plunger well having a plunger within tubing comprising the steps of: shutting in the well to build up pressure within said well; determining a size of a slug of liquid in the tubing: comparing the size of the slug of said liquid with a large threshold value and a lower limit; controlling the size of said slug of said liquid within the tubing while said well is shut in to ensure that said size of said slug of said liquid is between said large threshold value and said lower limit; opening a valve to relieve pressure within said well and raise the plunger within said tubing; pushing said slug of said liquid out of said well with said plunger; closing said valve wherein said plunger falls within said tubing; reducing said size of said liquid slug for preventing fluid loading; and lowering pressure in an annulus defined by said tubing and casing for equalizing tubing pressure and casing pressure.
2. The method according to claim 1 wherein said step of controlling comprises: increasing said size of said liquid slug for controlling a rise rate of the plunger.
3. The method according to claim 2 wherein said step of increasing comprises: lowering pressure within said tubing for allowing more liquid to enter said tubing.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 shows a plunger lift well of the invention;
(2) FIG. 2 shows a spring loaded check valve for locating at the bottom of the well of FIG. 1;
(3) FIG. 3 lists the events of a plunger cycle of the plunger lift well of FIG. 1;
(4) FIG. 4 is a graphical representation of surface recorded casing and tubing pressures during the plunger cycle shown in FIG. 2;
(5) FIG. 5 is a pressure versus time plot showing the effects of controlling, i.e., reducing to a smaller size, a large liquid slug during the shut-in period of the plunger cycle;
(6) FIG. 6 is a pressure versus time plot showing the effects of controlling, i.e., increasing to a larger size, a liquid slug of small size during the shut-in period of the plunger cycle;
(7) FIG. 7 is a graphical representation of changing liquid load due to multiple plunger cycles.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(8) Referring to FIG. 1, shown is a plunger lift well 10 having casing 12 that extends below a ground surface 14. Tubing 16 extends into casing 12, defining annulus 17 therebetween. A tubing stop 18 is affixed at a lower end of tubing 16. A bumper spring 20 is supported by tubing stop 18 for engaging plunger 22 when plunger 22 falls during shut in of well 10.
(9) The bumper spring assembly 20 may include a spring loaded ball and seat assembly 24 (FIGS. 1, 2) made up of ball 26 received within seat 28. Relief spring 30 communicates with seat 28. Spring 30, having a correctly set spring compression, will prevent liquid in tubing 16 from falling out of tubing 16 during a shut in period of the plunger lift cycle. However, if the compression force of relief spring 30 is low enough, then equalizing pressure between casing 12 and tubing 16 for short intermittent times should still allow for compression of spring 30 and for some liquid to be pushed through seat 28. If the compression of spring 30 is set too high, then liquids will not be forced through seat 28 when tubing and casing are equalized. The spring loaded ball and seat assembly 24 should not substantially affect inflow of liquids if tubing valve 32 is opened as ball 26 can open over the seat 28 as with any standing valve. In a well where liquids are not been falling out of tubing 16 during well shut in, then a spring loaded ball and seat assembly 24 or other type of check valve is not required.
(10) An upper portion of tubing 16 may be closed off with tubing valve or master valve 32. A catcher 34 with arrival sensor is located above tubing valve 32 and a lubricator 36 is affixed to an upper end of tubing 16. Production line 38 communicates with lubricator 36 above tubing valve 32. Bypass line 40 communicates annulus 17 between casing 12 and tubing 16 with production line 38. Upper vent line 41 communicates production line 38 with lubricator 36. Lower vent line 45 also communicates production line 38 with lubricator 36. Upper vent line valve 43 is provided to adjust the pressure drop across plunger 22 when plunger 22 has risen to a location within lubricator 36 by controlling an amount of gas flowing through the upper and lower vent lines 41 and 45.
(11) Motor valve 42 is provided on production line 38. Motor valve 42 is preferably a diaphragm-operated device controlled by controller 48 to selectively open and close production line 38. Shutoff valve 44 is provided on production line 38 upstream of motor valve 42. Bypass line valve 46 is located on bypass line 40.
(12) The apparatus of well 10, described above, is used to control a size of liquid slug 23 at a bottom of tubing 16 during the shut in phase of a plunger cycle. The method for controlling the size of liquid slug 23 includes the steps of closing one or both of shutoff valve 44 and motor valve 42 to achieve shut in of well 10. Bypass line valve 46 on bypass line 40 is opened for a short period of time while shutoff valve 44 and motor valve 42 are closed. Opening bypass line valve 46 communicates annulus 17, which contains casing pressure, with tubing 16, which contains tubing pressure. This will begin to equalize pressure in the casing 12 and pressure in tubing 16 as measured at the surface. The pressure equalization will allow liquids in the bottom of tubing 16 to begin to flow back into the casing 12.
(13) Measurements are taken to determine whether a pressure differential between pressure in tubing 16 and pressure in annulus 17 of casing 12, measured at the surface, is below a predetermined threshold value. An example of a desirable pressure differential may be determined by the Foss and Gaul method, described in SPE 120636, Modified Foss and Gaul Model Accurately Predicts Plunger Rise Velocity by O. Lynn Rowlan, Echometer Company, SPE Member 0917344 and James F. Lea, PLTech LLC, SPE Member 009772-5 and J. N. McCoy, Echometer Company, SPE Member 0017843, said article incorporated herein by reference. Alternatively, the upper limit for the pressure differential could be determined from a previous plunger cycle wherein liquid slug 23 was found to be large enough to prevent cycling of plunger 22. A lower limit could be set to ensure that a specific quantity of liquid 23 remained in tubing 16, e.g., 10% of a barrel of liquid.
(14) The step of opening bypass line valve 46 for a short period of time is repeated if the pressure differential between pressure in casing annulus 17, i.e., the casing pressure, and pressure in tubing 16, i.e., the tubing pressure, is above the predetermined threshold value. Maintaining the pressure differential below the threshold value prevents an accumulation of a large slug of fluid 23 in tubing 16. Bypass line valve 46 may be opened repeatedly for brief periods to allow liquid to flow from tubing 16 to the casing 12. Bypass line valve 46 is then shut and measurements are taken to determine if the difference between the pressure in casing 12 and the pressure in tubing 16 has dropped below the threshold value.
(15) The phases of a plunger cycle are shown graphically in FIG. 3. As explained above, motor valve 42 is shut after a flow period and liquid 23 accumulates downhole, allowing plunger 22 to fall back downhole. FIG. 3(1) shows plunger 22 downhole. FIG. 3(1) shows well 10 closed, or shut-in, wherein pressure in casing 12 is building. Plunger 22 rests on bottom hole bumper 20 (not shown in FIG. 3(1)) at the base of well 10. FIG. 3(2) shows motor valve 42 in an open condition to allow gas to flow from tubing 16 into flow line 38. Plunger 22 and liquid 23 rise within tubing 16. FIG. 3(3) shows plunger 22 held at ground surface 14 as gas flows through lubricator 36 into production line 38 and through motor valve 42. FIG. 3(4) illustrates that most liquids 23 accumulate when gas velocity drops before motor valve 42 shut. FIG. 3(5) shows that when motor valve 42 shuts, plunger 22 falls toward liquid 23.
(16) During the time the motor valve 42 is shut, i.e., during the shut-in phase, as shown in FIGS. 3(5) and 3(1), plunger 22 falls through gas, then falls through liquid 23 and then rests on bottom hole bumper spring 20.
(17) FIG. 4 shows surface recorded pressures for casing 12 and for tubing 16 during a typical plunger cycle described above. Pressure in casing 12, i.e., the casing pressure (Csg P) is higher than the pressure in tubing 16, i.e., the tubing pressure (Tbg P), due to liquid load downhole. As shown in FIG. 4, casing pressure (Csg P) and tubing pressure (Tbg P) rise from event (A), when motor valve 42 (FIG. 1) shuts. From event (A) through event (1), plunger 22 falls through gas. From event (1) to event (2), plunger 22 falls through liquid 23. From event (2) to event (B), plunger 22 rests on bumper spring 20. At event (B), motor valve 42 is opened. At event (B), the pressure differential between the casing pressure (Csg P) and the tubing pressure (Tbg P) is indicated by the vertical arrow. From event (B) to event (3), plunger 22 rises within tubing 16. From event (3) to event (4), liquid slug 23 and plunger 22 arrive at lubricator 36. From event (4) to event (C) casing pressure (Csg P) and tubing pressure (Tbg P) continue to drop during an after flow period with plunger 22 in lubricator 36. At event (C), motor valve 42 closes again and the plunger cycle repeats.
(18) If, during the shut in portion of the plunger cycle, i.e, from event (A) to (B) in FIG. 4, liquid leaves the bottom of tubing 16 and flows back to casing 12, which it sometimes does, pressure in casing 12 and in tubing 16 will begin to equalize. During the shut in portion of the cycle, pressure in casing 12 (Csg P in FIG. 4) and pressure in tubing 16 (Tbg P in FIG. 4) rise as gas from well 10 pressurizes casing 12 and tubing 16. Liquid 23 may or may not exit from the bottom of the tubing 16 if no check valve, e.g., ball and seat assembly 24, is present. To control the conditions under which liquid 23 can escape from the bottom of tubing 16, a check valve may be added at the bottom of well 10 so that pressure exerted from the surface in tubing 16 will open the check valve, e.g., check valve assembly 24, and force out liquid 23 from tubing 16 only if pressure in tubing 16 is greater than a desired threshold. This may allow tubing 16 to be unloaded without swabbing or pulling tubing 16 if too much liquid is present in tubing 16. In the case of moderate liquid loading, liquid 23 may remain in tubing 16 for lifting by plunger 22 as described above.
(19) In summary, so long as liquid level is not too high, liquid 23 may be allowed to build up and be subsequently lifted by plunger 22. However, if the liquid level is too high, then the casing pressure and the tubing pressure may be equalized during the plunger cycle, e.g., from event (A) to event (B) in FIG. 4. Upon pressure equalization, which may be partial or full, liquid 23 flows out of tubing 16 either through a lightly compressed spring check valve, i.e., through check valve 24, or out of a bottom of tubing 16 having no check valve. Pressure is preferably partially equalized in short spurts during the shut in phase of the plunger cycle to control the amount of liquid 23 present in the well for avoiding a potential liquid loading of well 10.
(20) Referring now to FIG. 5, shown is a graphical representation of the steps for controlling a liquid slug 23 that is too large during the shut in period of a plunger cycle. Event (1) indicates well shut in. After event (1), casing pressure (CP) and tubing pressure (TP) begin to rise. Plunger 22 falls through gas, then through liquid 23. Plunger 22 will then remain for a short time on bottom of tubing 16, e.g., on bumper spring 20. At event (2), controller 48 equalizes casing pressure (CP) and tubing pressure (TP) for short time by opening bypass line valve 46. As shown in FIG. 5, a drop in casing pressure-tubing pressure differential occurs after event (2), which is indicative of a decrease in the size of liquid slug 23. Pressure equalization action is taken if a difference between casing pressure minus tubing pressure is larger than a predetermined threshold. A large pressure differential indicates a liquid slug 23 at the bottom of tubing 16 that is too large during the shut in period of the plunger cycle. If necessary, at event (3), controller 48 partially equalizes casing pressure and tubing pressure by briefly opening bypass line valve 46 during shut in. The size of liquid slug 23 then decreases, as is indicated by a drop in the casing pressure-tubing pressure differential. By repeatedly opening bypass line valve 46, the casing pressure-tubing pressure differential is reduced below an input acceptable value. At event (4), the size of liquid slug 23 is now below a maximum set point as determined by the set difference between casing pressure and tubing pressure. This keeps a large slug of liquid from stopping the plunger cycles. Plunger 22 is given time to fall through gas, liquid 23 and then arrive at the bottom of tubing 16. Motor valve 42 is then opened to communicate tubing 16 with production line 38 and plunger 22 rises. A height of liquid slug 23 in the bottom of tubing 16 may be determined from the following equation:
Height of liquid, ft=(CP?TP, psi)/(0.433 psi/ft?SpGr of liquid)
(21) Control of the size of liquid slug 23 can occur earlier in the plunger cycle and can occur more than the 2 times illustrated in FIG. 5.
(22) Referring now to FIG. 6, shown are the steps for controlling a liquid slug 23 that is too small during a shut-in period of a plunger cycle. Event (1) indicates well shut in, e.g., by closure of motor valve 42. After event (1), casing pressure (CP) and tubing pressure (TP) rise. Plunger 22 falls through gas, then through liquid 23 and then remains on the bottom of tubing 16 for a short period. Event (2) indicates that tubing pressure is briefly vented to production line 38 (FIG. 1), e.g., by opening tubing valve 32. This action may be taken when the difference between the casing pressure and the line pressure is determined to be too small. Venting tubing pressure to production line 38 ensures that tubing 16 is at lower pressure than the pressure in annulus 17 of casing 12, i.e., than the casing pressure. If liquids are proximate to the bottom of tubing 16 in casing 12, then the liquids will flow into the bottom of tubing 16. At event (3) of the shut in period, tubing pressure is briefly vented to production line 38 for a second time. Venting to production line 38 is undertaken when a difference between casing pressure and line pressure is determined to be too small. Venting tubing pressure to production line 38 allows more fluid in casing 12 to enter bottom of tubing 16. By venting tubing pressure to production line 38, a larger casing pressure-tubing pressure differential is achieved. Event (4) indicates that a size of liquid slug 23 is above a minimum set point as determined by a predetermined set difference between casing pressure and tubing pressure. Plunger 22 is then given time to fall through gas, liquid 23 and then locate on bottom of tubing 16. Well 10 is then opened, e.g., motor valve 42 is opened, to communicate tubing 16 to production line 38. Plunger 22 then rises. Control the size of liquid slug 23 can occur earlier in the plunger cycle and can occur more or less than the two times illustrated in FIG. 6.
(23) FIG. 7 is a graphical representation of changing liquid load and how the difference between the pressures in casing 12 and tubing 16 can change during controlled plunger cycles to avoid liquid slug 23 becoming too large, which could result in a stoppage of the plunger cycle and liquid loading of well 10. Lower vent line valve 46 is opened while motor valve 42 and shut off valve 44 are closed during the shut in portion of the plunger cycle. Casing pressure and tubing pressure rise, indicating shut in. Rising pressures allow higher pressure in casing 12 to act on the top of tubing 16 during short trial openings of bypass line valve 46, which equalizes the casing pressure and the tubing pressure, at least to some extent. If casing pressure and tubing pressure are allowed to completely equalize then liquid slug 23 in tubing 16 falls completely back into casing 12 or drops to a very low level in the bottom of tubing 16 as liquids flow from tubing 16 back into annulus 17 of casing 12, i.e., into casing 12, which is at a lower pressure.
(24) In one aspect of the invention, the pressure difference between the pressure in casing 12 and the pressure in tubing 16 is lowered during the shut in portion of the plunger cycle. Preferably, the two pressures are not equalized, but rather the differential between the pressures are lowered below a threshold input value. By avoiding a large pressure differential, plunger 22 does not have to lift a large slug of liquid 23 and possibly fail to arrive at the surface. Therefore, controller 48 should open bypass line valve 46 for a short time during the shut in period of the plunger cycle to reduce the difference in the tubing pressure above liquid 23 in tubing 16 and the casing pressure to below a threshold input value as measured at the surface. If the pressure differential is above the threshold input value, then the process is repeated. Even if the pressure difference is not reduced by repeating the procedure and checking the pressures, the size of liquid slug 23 may be reduced and the total plunger cycle will have a much better chance to continue to repeat the open and close portions of the normal plunger cycle. The method of the invention prevents plunger 22 from operating with a randomly sized, possibly larger than normal liquid slug 23 in tubing 16. A large liquid slug 23 is undesirable because it could stop operation of the plunger cycles and result in a need for a restarting procedure. A restarting procedure takes time, manpower, and may stop well production for a period of time.
(25) If the difference between the surface measured pressures in casing 12 and tubing 16 during the shut in period of the plunger cycle is too small, then this condition indicates that liquid slug 23 in tubing 16 is too small or may be non-existent. To increase the size of liquid slug 23, motor valve 42 is briefly opened while casing bypass valve 46 is closed and shut off valve 44 is open, to allow some gas to leave tubing 16 and allow more liquid to enter tubing 16. Controller 48 will repeat this process and measurements will be taken to determine if the tubing pressure and casing pressure differential has risen above the input minimum value. By ensuring that the pressure differential has risen above a minimum value, plunger 22 is prevented from rising with no liquid slug 23. The presence of only a small amount of liquid 23 or the absence of any liquid 23 can cause rapid arrivals at ground surface 14 of plunger 22, which can damage well equipment.
(26) In general, described above is a method to control the size of liquid slug 23 at the bottom of tubing 16 during the off portion, or shut in portion, of a plunger cycle. By controlling the size of liquid slug 23, controller 48 is allowed to continue cycling and not stop due to a large liquid slug 23. Additionally, damage to well equipment due to operating with too small of liquid slug 23 may be avoided. Various types of plumbing and valves might be present at the well head but would still allow operation of the invention as described herein.
(27) Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those of ordinary skill in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims.
