How to improve poor system efficiencies of ESP installations controlled by surface chokes

Abstract

Electrical submersible pumping is the most inflexible of any artificial lift system because a specific ESP pump can only be used in a definite, quite restricted range of pumping rates. If it is used outside the specified range, pump and system efficiencies rapidly deteriorate and eventually mechanical problems leading to a complete system failure develop. When serious deviation from the design production rate is experienced, the possible solutions are (a) running a different pump with the proper recommended operating range, or (b) using a variable speed drive (VSD) unit. However, in case the ESP system produces a higher than desired liquid rate, a simple and frequently used solution is the installation of a wellhead choke. The wellhead choke restricts the pumping rate and forces the ESP pump to operate within its recommended liquid rate range. This solution, of course, is very detrimental to the economy of the production system because of the high hydraulic losses across the choke that cause a considerable waste of energy. The paper utilizes NODAL analysis to investigate the negative effects of surface production chokes on the energy efficiency of ESP systems as compared to the application of VSD drives. The power flow in the ESP system is described and the calculation of energy losses in system components is detailed. Based on these, a calculation model is proposed to evaluate the harmful effects of wellhead choking and to find the proper parameters of the necessary VSD unit. By presenting a detailed calculation on an example well using the proposed model the detrimental effects of wellhead choking are illustrated and the beneficial effects of using a VSD drive are presented. Using data of a group of wells placed on ESP production a detailed investigation is presented on the field-wide effects of choking. The energy flows and the total energy requirements are calculated for current and optimized cases where VSD units providing the required electrical frequencies are used. Final results clearly indicate that substantial electric power savings are possible if production control is executed by VSDs instead of the present practice of using surface chokes.