Analysis of OAO "NNP" wells for 2004 showed that total number of premature failures made 618 and compised 167caused by cable faults, 371 caused by ESP faults, 43 through fault of downhole pump and hydraulic protection, and 37 due to tubing failures.

The main part of failures falls to ESP units. That is why quality of materials used in ESP operating parts should be of top priority. The main causes of ESP failures across OAO "NNP" are the following: aggressive environment (corrosion) - 13%, mechanical impurities - 43%, scales - 25%, proppant washout - 11%, tubing leaks - 11%, poor inflow - 1%.

The main factors aggravating the ESP work are salt deposits on the operating parts and their corrosion. Salt deposits and corrosion result in flow characteristics deterioration, vibration amplification, what consequentially shortens failure free operation time of ESP units. The average failure free operation time of these wells made 95 days due to scales and 103 days due to corrosion.

Salt deposit and corrosion causes

High watercut level (exceeding 80%) significantly intensifies the aggressive environment impact and corrosion process.

Having analyzed composition of salts and deposit formation process we can specify 3 root causes of salt deposits in ESP parts.

The first one is based on the definite reservoir fluid properties - watercut level exceeding 80%, presence of dissolved and undissolved minerals as wells as aggressive environment.

The second cause is of operational nature and may be entailed by:

  • change of thermobaric conditions in a well or in ESP, leading to precipitation of some compounds of the first group. So, shift of the work zone to the left side of the flow dynamic properties (i.e. when the pump is forced to decrease the flow rate) leads to rising of the flowing fluid temperature and enhancement of cavitation processes, what subsequently leads to precipitation of CaCO3 (calcium carbonate), in particular;

  • Mixing of formation fluid with injected water of different composition in the process of water injection for formation pressure maintenance may result in creation of salt-forming compounds and aggressive environment.

The third cause is design of ESP which may lead to:

  • formation of vortex and stall zones;

  • corrosion of working parts surfaces which leads to damage of smooth surfaces where salt deposits can easily form then;

  • Besides, salt deposits may be resulted from influence of electrostatic forces (initiation of galvanic couples).

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