When operating the Electrical Submersible Pumps (ESP) in conditions of intense scaling deposition, a dispersed dense stone-like precipitate forms on their working parts and surfaces, which thickness reaches 0.6-1 mm, and disrupts heat transfer and leads to a "thermal shock" of the electric motor. At the moment the development of artificial lift methods of oil production the problem of ESP failures due to the «thermal shock» of the electric motor due to the formation of a mineral deposits layer on the motor housing is relevant. Generally time to failure of downhole pumps in the presence of scaling is reduced by 3 – 5 or more times.
A significant role in formation of the ESP motor temperature is played by such factors as the producing fluid temperature past it, the heat transfer coefficient of the gas-liquid mix, depending on the flow regime and water-cut, the scaling thickness on the motor housing, the motor shaft load, etc. Operational experience has shown that the most common motor failure root-cause is wear or damage of the stator winding insulation of the motor.
A mechanism of scales deposition on the motor surface is considered. The models of bulk scales crystallization from solutions and deposition on the surface are given. A procedure for temperature rise of the submersible asynchronous electric motor under different loading operational conditions and taking into account the total heat transfer coefficient change due to the depositing scale minerals on the ESP external surface is developed. Dependences of cooling fluid heat transfer coefficient on pump intake pressure for various flow rates, cooling fluid heating depending on the net motor power at different degrees of production water-cut, the motor temperature on net power, at different scale layer thicknesses on the motor housing are given.
A method for surface deposition of CaSO4 on the motor housing has been developed, which allows calculating the growth rate of the scale thickness, depending on the cooling liquid flow rate and the salts concentration in the solution.
The developed methods allow carrying out a model forecasting of risks associated with the motor operating conditions in wells with scaling problems.
References
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