Paper: USE ON NODAL TECHNIQUES TO IDENTIFY AND ELIMINATE THE HARMFUL EFFECTS OF PRODUCTION CHOKES ON ESP WELLS

Electrical submersible pumping is perhaps the most inflexible of any artificial lift method because any given ESP pump can only be used in a specific, quite restricted range of pumping rates. If used outside its recommended liquid rate range, the hydraulic efficiency of the pump rapidly deteriorates; efficiencies can go down to almost zero for pumping rates lying well outside of the lower or upper limits. In addition to the loss of energy and the consequent decrease in profitability the ESP system, when operated under such conditions, soon develops mechanical problems that can lead to a complete system failure. An improper installation design or inaccurate information on the well's inflow capability always results in a mismatch between the design rate and the actual pumping rate. The usual result of these troubles is a workover job and the running of a newly-designed ESP system of the proper lifting capacity. Since the capacity of the ESP system, without using an expensive VSD (variable speed drive) unit, cannot be easily changed, wellhead chokes are often used to restrict the pumping rate and to force the ESP pump to operate within the recommended liquid rate range. This solution, of course, is very detrimental to the economy of the production system because the pressure drop across the choke causes high hydraulic losses and a considerable waste of energy. The paper investigates the negative effects of surface production chokes on the energy efficiency of ESP systems using NODAL analysis tools. The proper way of conducting a NODAL analysis for this purpose is detailed along with the description of power flow in the ESP system. The calculation of energy losses in system components is detailed and the relative importance of the individual losses is shown. The elimination of the problems associated with the use of surface chokes is investigated and the proper parameters of the necessary VSD unit are found.