The Measured Impact Of Erosion On The Rotodynamic And Performance Characteristics Of A Mixed Flow ESP
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Three stages of a mixed flow electric submersible pump were subjected to 117 hours of erosion using a 2 gram/liter concentration ratio of 100 mesh hydraulic fracture sand to water. The sand passed through the pump one time and is discarded. The wear of flow paths and bearings is presented along with variation in pump performance and vibration as the pump wears. The pump was operated at its Best Efficiency Point (BEP) which is 1150 GPM (30100 BPD) at 3600 RPM with a specific speed of 1600. The hydraulic performance of the pump decreased 6.3% due to wear occurring in the secondary flow paths. Specifically, the bearings and seals were severely worn with bearing orbits up to 50 mils peak to peak. At the end, the bearings had worn more than the seals resulting in the seals carrying the rotordynamic loads of the rotor. Microscopic investigation of the tungsten carbide sleeve bearings showed what are believed to be thermal stress cracks which were revealed after the surface of the bearings were eroded. INTRODUCTION The natural flow of mature oil or gas fields drops below economic production rates after a period of production. When reservoir pressure drops, the need for artificial lifting systems become crucial to support the required operating flow rates. The Electrical Submersible Pump was introduced to the petroleum industry by the pump inventor Armais Artunoff in 1927. Since then, ESP systems continued to grow, especially and historically in high water cut oil wells. According to Schlumberger’s Oilfield review of summer 2004 [1], the Electrical Submersible Pump (ESP) system is the second most applied artificial lift system globally after the Sucker Rod. In 2004, there was an estimated number of 100,000 operating ESP systems in producing wells worldwide and this number continues to grow. ESPs are known for their high volume flow rates and have been widely used by the oil and gas industry for artificial lift. As on shore oil reservoir resources face depletion, oil and gas explorations turn to offshore, subsea and ultra-deep subsea reservoirs to continue extracting the precious minerals and naturally ESPs followed. Today’s challenges and requirements of ESP systems continue to push ESP research to have longer life spans, handle multiphase flows, resist abrasive erosion and maximize operating efficiency. There are two types of impellers used in ESPs: radial flow and mixed flow impellers. Radial flow impellers are typically used for lighter applications where the flow is less than 3,000 BPD and mixed flow impellers are used on larger capacity pumps (up to 40,000 BPD). The current pump under investigation has mixed flow impellers. The inherent problem created by the placement of ESPs in subsea wells, besides the impracticality and difficulty of periodic maintenance, is the high cost of installation and removal of the pump. The cost of removing and installing a faulty pump could be, depending on the depth, up to forty times the cost of a new pump. For this reason, more emphasis is now being placed on research to extend the useful life of the pump to at least five years, and to locate, predict and mitigate any and all causes of pump failure. One possible cause of submersible pump failure, depending on the nature of the oil bearing strata, is the damage incurred to the bearings, shaft and other components due to the presence of fine sand in the fluid. The sand present in the fluid originates either from the well itself or from the hydraulic fracturing of the well and thus its type, size and concentration will vary from well to well. Most wells have some type of sand control, usually gravel pack covered with a mesh. This will filter out the coarser gravel, but finer sands will seep through the mesh. The fine sand can pass through the productlubricated bearings and small clearances inside the pump, creating erosion problems, which in turn can lead to wear, vibration and possible pump failure. A custom-made for laboratory environment, fixed impeller mixed flow electrical submersible pump is used for this study (referred from now on as ESP). For space constraint issues and unavailability of a well in the laboratory, only three stages of the pump were tested and, since standard ESP motors