Abstract

                Sucker Rod Pumping (SRP) systems have used various control methods to optimize production, improve energy efficiency, and reduce maintenance. These systems typically use Rod Pump Controllers (RPCs) or timers to cycle the rod pumping unit on and off. Variable Speed Drives (VSDs) are occasionally used in conjunction with RPCs to improve operation by modulating pump speed. An improved SRP control that combines the RPC and VSD functions in an integrated system was installed on approximately 100 rod pumping systems at PDVSA starting in January 2004. The improved control resulted in a median production increase of 21%.Embedded optimization controls in the VSD allow PDVSA engineers to preset a desired target pump fill based upon an Optimizer ID routine that samples production rates at various operating speeds and pump fill values over several days.  The control subsequently adaptively modulates the pumping speed profile accordingly, thereby maintaining continuous operation at the identified optimal target pump fill. By maintaining a constant fluid level rather than the traditional cycling of fluid level between two limits, production losses associated with cycling above the target IPR well flowing pressure are eliminated. Furthermore, continuous and adaptive downhole pump speed control within each stroke allows the target IPR well flowing pressure to be reduced while improving equipment protection.In addition, the VSD exercises the pump through controlled sequences that automatically identify other valuable system characteristics such as pump leakage, traveling/standing valve problems, pumping unit characteristics, counterbalance characteristics, and motor electrical parameters.
            

Introduction

VSD well controllers were installed on approximately 100 SRP wells at San Tome Field Orinoco Belt PDVSA in an effort to improve production and control. The results from each well proved to be unique, but in general production increases ranged from 10% to 160% with a median increase of 21%. These production differences can likely be attributed to several factors, but in general the increases are associated with a sustained reduction in fluid level made possible by better pump speed management.

Sucker rod pumped wells had traditionally been controlled with fixed timers or "pump off" controllers (POCs) that cycle the pumping unit on and off either in response to a changing pump fill or arbitrarily based upon time. In the case of a fixed timer control, fluid level is not closely managed. In the case of the POC, a common control technique involves shutting the pumping unit off for a time delay upon detecting a "pump off," or more specifically, an incomplete pump fill. When incomplete pump fill is the result of gas breakout in the pump barrel rather than low fluid level, it is possible that such a control technique will result in limited production due to excessive fluid over the pump (excessive well flowing pressure) and reduced well inflow. As such, production increases can be achieved by lowering the fluid level with higher operating speeds at reduced pump fills.

Traditional fixed speed pumping systems do not easily lend themselves to the experimentation necessary to determine this "optimal" pump speed and fill. Furthermore, for a given pumping speed the rod and pump speed during plunger impact are solely dependant upon pumping unit geometry and rod string makeup, and so the fluid/gas "pound" and rod compression associated with higher operating speeds can be unmanageable.