Gas Interference in Sucker Rod Pump Abdus Samad
Citation: AIP Citation: AIP Conference Proceedings Proceedings
1298,
274 (2010); doi: 10.1063/1.3516315
View online: http://dx.doi.org/10.1063/1.3516315 View Table of Contents: http://aip.scitation.org/toc/apc/1298/1 Published by the American the American Institute of Physics
Gas Interference in Sucker Rod Pump Abdus Samad Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai-600 036, India Abstract. Commonly used artificial lift or dewatering system is sucker rod pump and gas interference of the pump is the biggest issue in the oil and gas industry. Gas lock or fluid pound problems occur due to the gas interference when the pump has partially or completely unfilled plunger barrel. There are several techniques available in the form of patents to solve these problems but those techniques have positive as well as negative aspects. Some of the designs rely on the leakage and some of the designs rely on the mechanical arrangements etc to break the gas lock. The present article compares the existing gas interference handling techniques.
Keywords: Sucker rod pump, gas lock, fluid pound, gas interference PACS:00
INTRODUCTION If there is insufficient reservoir pressure in an oil well to overcome the hydrostatic head of the fluid in the well pipe, the oil and other fluids in the well can not flow unaided to the surface for collection. In such wells the fluid must be mechanically assisted or pumped to the surface. Available pumping methods are sucker rod pumping, gas lift, electric submersible pumping, hydraulic piston pumping, hydraulic jet pumping, plunger piston pumping, hydraulic jet pumping, plunger (free-piston) lift, and other methods [1]. Selection of mechanical assisted lift of fluid from well is called artificial lift and the lifts are selected based on the pressure/flow rate diagrams combining inflow performance relationships with tubing intake curve. The sucker rod pumps are commonly used pump in the world of artificial lifts and the causes of using this pump extensively because of simplicity, reliability, efficiency, flexibility, economy, relatively low maintenance, and high resale value of this pump [2]. The sucker rod pumping can work in very low intake pressure without damaging the pumping equipment [3]. The rod pump are called sucker rod pump, nodding donkey, beam pump, thirsty bird and horse head. Gas interference (GI) is a common problem in sucker rod pump (SRP) and several articles have been published by Society of Petroleum Engineers (SPE). Surface valve checks and dynamometer cards are the observing instruments of GI [3]. The GI in SRP results in gas lock and fluid pound and uncontrolled pump speed etc. [4]. Controlling the pump run time with a pump-off controller or a percentage timer can adjust the number of strokes so that the pump displacement will equal the volume of liquid that flows into well bore [5]. The incomplete pump fillage creates fluid pound and as a result a shock is produced. This shock produces buckling, pump wear, tubing wear, severe rod loading changes and pumping unit vibration. The gas lock [5-6] can be defined as the complete non delivery of liquid during pump reciprocating action. Cortines and hollabaugh [7] and Schmidt and Doty [8] reported that placing SRP at proper depth in well is important to avoid gas lock or fluid pound. If pump is placed below the perforation zone or pay zone, a natural gas anchor will be created and a reduction of gas interference can be observed. Experimental study [9] shows if vapour pressure of the oil noticeably exceeds the submerged pressure, efficiency declines noticeably. Slightly increase in oil vapour pressure or decrease in pump submerged pressure results a gas lock. There are several efforts been made to solve the GI problem modifying the plunger, valve etc and those are mainly reported in the patents [10-19]. Those designs tried to avoid GI by mechanical arrangements. If the cylinder between travelling valve (TV) and standing valve (SV) is filled with gas, the TV does not get opened during down stroke and SV does not gets opened during upstroke. A mechanical plunger has been designed to unseat the TV [16-19], a large clearance has been allowed CP1298, International Conference on Modeling, Optimization, and Computing, (ICMOC 2010) edited by S. Paruya, S. Kar, and S, Roy © 2010 American Institute of Physics 978-0-7354-0854-8/10/$30.00
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[15], gas vent hole in cylinder has been placed [13, 14] or top check valves design have been reported [10-12]. Present article compares the existing designs to solve the gas interference problem in SRPs. The inherent weaknesses and strengths of the designs have been presented.
PUMPING MECHANISM Figure 1 shows a simple reciprocating pump or SRP and this pump operates vertically. It has a plunger which travels inside a barrel. The plunger is connected to a valve rod and the rod supplies tensile or compressive force to the plunger giving a reciprocating motion. ‘Up’ and ‘down’ strokes are defined by the plunger movement opposite to or to the gravitational pull. The plunger has a TV which opens only during down stroke of the plunger. An SV which seats on a valve seat threaded to the plunger barrel opens only during the upstroke of the plunger. The valve seats and valves (TV and SV) are made up of hardened metal because during pumping operation the valves constantly will hits the seat. This creates high rate of wear. The valves are simply ball valves. Valve cages are fitted at the top of the balls so that valves can not move too long distance from valve seat during opening period. The valve should be seated as soon as the plunger stops movement. Cavity C 1 is the volumetric space between the TV and SV. Similarly, cavity C 2 is the volumetric space above the TV. The valves are gravity controlled; hence the pump does not work horizontally. The top and bottom most plunger of plunger positions are called top dead centre (TDC) and bottom dead centre (BDC) respectively. The following events occur during the strokes: Up stroke: Pcyl
Phead , SV closes, TV opens and fluid passes through the TV port. Here, Pwell, P cyl, and P head are the well pressure, cylinder pressure and pump delivery head (pressure), respectively. −
−
GAS INTERFERENCE The literatures explain the gas interference results in gas lock and fluid pound.
Gas Lock Gas (or steam) is solely responsible for gas lock and accumulation of gas in the cylinder does not allow the opening of the valves. Several efforts have been made to alleviate the gas lock problem [1019]. Gas lock can appear because of: Insufficient net positive suction head (NPSH): the well pressure ( Pwell) gets reduced ( Pwell
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(a) Down stroke
(b) Up stroke
FIGURE 1. Sucker rod pump −
−
. High gas to oil ratio (GOR): The gas enters with the liquid into the cylinder and expands if the GOR is high during upstroke. Gradually efficiency gets reduced and after a certain time the valves do not open the ports and gas lock problem appears. Soluble gas in the oil: The gasses in the liquid come out because of low pressure during upstroke. Gradually the gas gets accumulated i n the cylinder reducing volumetric efficiency and finally stopping liquid flow through the pump.
Fluid Pound If the gas fills half of the cylinder ( C 1 in Fig. 1), the high velocity plunger moves through the gas during down stroke and hits the fluid surface resulting in mechanical failure of the sucker rod. At the midway of down strokes, the plunger velocity is highest and if plunger travels through the gas and hits the liquid surface, a vibration stars propagating. The rod starts rubbing the tubing and tubing and rod failure probability is high.
AVOIDANCE OF GAS INTERFERENCE Using the pump off control the pump is shut down when the barrel becomes no less than 80 to 85 % full 4. Beam pump diagnostic tool is surface dynamometer card and this shows the nature of pump fillage. Load and position of polished rod is plotted on graph and the shape of the graph is dependent on the load on the rod. The card shows normal trend if the pump is ‘full’ 4. High GOR wells have high chance to gas lock. Easy way to avoid gas lock is to set the pump at a depth below the perforations [4] in gas wells. The liquid and gas will enter into the perforations of casing and the gas will move up because of lighter weight while the liquid will move downward to enter into the pump. Setting the pump below the perforations may have difficulty and gas separators are used [4].
Mechanical Arrangements to Handle the Gas Interference Gradual enlarging barrel pump (GEBP) [15] has been shown in Fig. 2. The cylinder gradually enlarges near the TDC. The leakage past plunger increases as the plunger enters into the enlarged section of barrel. The delivered liquid enters into the cavity C 1 and the differential pressure ( Ptop = Phead - Pcyl) across tubing (Phead ) and barrel (Pcyl) becomes zero. This adds volumetric loss. Gas vent pump [13-14] has a small hole on the cylinder near the TDC (Fig. 3). As the plunger crosses the vent the tubing fluid starts leaking into the cavity C 1. This has similar mechanism as the GEBP. 276
In Fig. 4, a mechanical plunger pin (MPP) [16-19] is used to open the TV when the plunger comes near the BDC. At this location if the cylinder pressure is not high enough to open the TV, the plunger forces the TV to open the port and gas passes through the port. Here, effective swept volume of plunger is lower because of pin and hence a volumetric loss is expected. A two valve plunger pin pump [19] is shown in the Fig. 5. The lower TV has higher projected area and during down stroke the valve faces higher opening force. The valve forces the upper TV through a plunger pin. Hence this system resembles the MPP pump in terms of leakage described above. Figure 6 shows a gas compression chamber pump [13] having a plunger with a gas compression chamber. During down stroke the lower TV opens the port and fluid enter into the cavity above the plunger. During up stroke, the fluid passes through the narrow path inside plunger. If there is any gas inside the cavity above lower TV, the gas gets compressed during down stroke and lower valve opens the port by differential pressure across this valve. The top valve pump (TVP) [11] has similar configuration as shown in Fig. 1 except that the TVP has a sliding valve on the top of barrel. The sliding valve or top valve opens during upstroke and closes during down stroke. During down stroke, the cavity C 3 (shown in Fig. 7(a)) pressure gets reduced gradually and becomes lower than Pcyl. The TV opens and fluid enters into C 3. The top valve holds the tubing fluid column load. As a result, TV opens at a low differential pressure (Pcyl- PC3). During upstroke, the top valve opens and fluid leaves the pump.
(a) Down Stroke
(b) Up Stroke
(c) Plunger at TDC
(a) Down Stroke
FIGURE 2. Gradual enlarging barrel pump
(b) Up Stroke
(c) Plunger at TDC
FIGURE 3. Gas vent pump
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(a) Down Stroke
(b) Up Stroke
(c) Plunger at BDC
(a) Down Stroke
FIGURE 4. Mechanical plunger pin
(b) Up Stroke
(c) Plunger at TDC
FIGURE 5. Two valve plunger pin
DISCUSSIONS The mechanical modifications of the SRPs have been compared in the table 1. The modified pumps have their positive as well as negative aspects and those are presented in the table.
(a) Down stroke (a) Down stroke
(b) Up stroke
(b) Up stroke
FIGURE 7. Top valve pump
FIGURE 6. Compression chamber
278
TABLE 1. Comparisons of existing gas lock breaker systems
Gas lock breaker system Two valve plunger pin pump Gradually enlarging barrel pump Gas vent pump
Positive side
Negative side
The lower valve has higher surface area and the higher force helps opening the lower valve. Gas can not be accumulated. Metal to metal sealing works. Gas can not be accumulated. Metal to metal sealing works.
Gas compression chamber pump
High compression ratio will help to get the valve opened during down stroke.
Mechanical plunger pin pump Top valve pump
TV opens during down stroke (neat BDC) of the plunger. Top valve remains closed during down stroke and the differential pressure in C 3 gets reduced. Hence differential pressure on the TV becomes lower which helps to open TV easily.
After opening the lower valve, there does not remain any existing force to open the upper valve. Large leakage, low volumetric efficiency [summary]. Seal can not be used. Large leakage, low volumetric efficiency. Seal can not be used. The top section of plunger requires a dynamic seal and the seal is exposed to sand and well fluid. The seal wear limits the pump life. Large leakage, low volumetric efficiency. The top valve requires one set of seal and the seal is exposed to sand and well fluid. The seal life becomes lower.
If valve cage height is 0.5in and pump has 100in stroke length, then unswept volume is 0.5%. From Fig. 8, i t can be seen t hat effective stroke l ength gets reduced by approximately 10% because of expansion of gasses if 0.5% of volume of the barrel is filled with gas at tubing pressure ( Phead ). Hence, if pump is gas locked and lowering the pump does not solve the problem completely because there is still existing gas in C 1. Figure 8 is drawn using equation 1. Gas equation: γ γ (1) P1V 1 = P1V 1 γ
T 2 T 1
P2 γ 1 P1 −
=
(2)
where, γ =1.25 for natural gas [25]. P, V and T are the pressure, volume and temperature, respectively. Form Fig. 8, if the cylinder is filled with more than 5% of the cylinder barrel with gas, the gas will expand during upstroke of the plunger and the expansion will continue till the plunger reaches TDC. The cylinder pressure will gradually reduce from 345bar to a minimum value when plunger moves from BDC to TDC. If cylinder pressure ( Pcyl) goes down to less than well pressure or net positive suction head available (NPSHa), the SV will open and fluid will enter into the barrel. If the Pcyl does not go below NPSHa, then the bottom valve will not open. If the cylinder is filled with 5% purely gas, then there will be no flow and gas lock will appear. Temperature (equation 2) has effect if the gas compression or decompression is too high; the temperature variation will be high which will create problem of galling or temperature stress.
279
350
Gas: 0.5% Gas: 2% Gas: 3% Gas: 5% Gas: 10% well pressure cylinder pressure
300
r a b , r 250 e d n i l 200 y c n i
150
e r u s 100 s e r P
50 0
0
20
40
60
80
100
Plunger movement during upstroke,%
FIGURE 8. Effective stroke when plunger moves from BDC ( γ = 1.25).
CONCLUSION This article presents the efforts made by the engineers to handle gas interference problem. The designs have their inherent weaknesses and strengths. No solutions are sufficient to solve the gas interference problem. Adding mechanical arrangement, increasing leakage etc have their positive sides to cope the problem somehow. Adding larger unswept volume will reduce production rate while reducing unswept volume. Reducing the unswept volume has problem of pump tag and temperature rise. Much more research and effort needed to enhance the production rate so that pump life as well as production rate can be enhance in an economic way.
REFERENCES 1.
K.E. Brown, “Overview of Artificial Lift Systems,” Journal of Petroleum Technology, Volume 34, Number 10, 1982, pp. 2384-2396. 2. J.P. Byrd, “Pumping Deep Wells With a Beam and Sucker Rod System,” SPE Deep Drilling and Production Symposium, April 1977, Amarillo, TX, USA. 3. M. W. Mahoney and R. D. Werff, “New Methods and Equipment for Dewatering Unconventional Gas Resources Using Sucker-Rod Pumping Equipment,” SPE Eastern Regional Meeting, October 2006, Ohio, USA. 4. J. F. Lea, H. V. Nickens and M. R. Wells, “Gas well Deliquification,” 2008, Gulf Professional Publishing, Oxford, UK. 5. J. N. McCoy, O.L. Rowlan, D.J. Becker and A.L. Podio, “How to Maintain High Producing Efficiency in Sucker Rod Lift Operations,” SPE Production and Operations Symposium, March 2003, Oklahoma, USA. 6. E.J. Dottore, “How to Prevent Gas Lock in Sucker Rod Pumps,” SPE Latin America/Caribbean Petroleum Engineering Conference, April 1994, Buenos Aires, Argentina. 7. J.M. Cortines and G.S. Hollabaugh, “Sucker-Rod Lift in Horizontal Wells in Pearsall Field, Texas,” SPE Annual Technical Conference and Exhibition, October 1992, Washington, D.C. USA. 8. Z. Schmidt and D.R. Doty, “System Analysis for Sucker-Rod Pumping,” SPE Production Engineering, Volume 4, Number 2, 1989, pp. 125-130. 9. G.M. Stearns, “An Experimental Investigation of the Volumetric Efficiency of Sucker-Rod Pumps,” Drilling and Production Practice, 1943, American Petroleum Institute. 10. C. L. Rupart, “Sliding valve pump,” 1980, US patent no. 4211551. 11. E. L. Crow, “Top loading fluid pump,” 1962, US Patent no. 3046904. 12. J. R. Brewer, “sucker rod pump,” 1992, US patent no. 5104301. 280
13. T.L. Brown, “Apparatus and Method for Reducing Gas Lock in Downhole Pumps,” 2005, US Patent. No. US 2005/0226752 A1. 14. B. L. Douglas, “Oil Well Pump Having Gas Lock Prevention Means and Method of Use Thereof,” 1975, US Patent no. 3861471. 15. C. K. Fischer Jr and B.J. Williams, “Downhole Pump With Bypass Around Plunger,” 2001, Patent no. US6273690B1. 16. Nelson, J. A., “Pump Barrel Seal Assembly Including Seal/ Actuator Element,” 1996, US Patent no. 5533876. 17. Short, C. G., “Travelling Valve Ball Displacing Tool,” 1997, UA Patent no. 5642990. 18. Hart, G. E. “Method and Apparatus for Breaking Gas Lock in Oil Well,” 1989, US Patent no. 4867242. 19. Ritchey, R. C., “Well Pumping,” 1954, US Patent no. 2690134.
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