Gas Interference Sucker Rod Pumps

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. −

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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