It is generally known, that oil production of residual oil reserves is connected with higher technical costs. The present article deals with optimisation processes of oil production in the tail-end phase fields. The selection of an artificial lift system for residual oil resources is often defined by economical implications and profitability of the tail-end phase fields.
A high water cut, often low to middle production rates, between 5–100m3/day, and difficult production conditions as high sand content and high oil viscosity are characteristic for these wells.
Over the past decade, due to the essential technological innovations PCP (Progressing Cavity Pump) has developed from a production equipment for only special cases to a field proven production technology, which meets different applications in oil production.
Progressing Cavity Pumps being driven from the surface drive head by sucker rods are generally used for oil production. A complete PC Pump Unit configuration involves: Downhole PC Pump Equipment, Rod String Equipment, Well Head Equipment and Drive Head Equipment. The downhole rotor and stator make up the essential assembly of this system. To ensure an average production rate between 1m3/day and 300m3/day, it was necessary to design a rotor-stator assemblies appropriate for various diameters, eccentricities and pitches.
Besides, a long experience in operating pumps has proven, that the optimal pump speed range is between 100 and 350rpm. The necessary pressure capacity between 60 and 300bar could be only executed at a very high number of stages. The production rates in dependence of pressure and speed are given In Fig.2.
Introduction of the extremely new production technology was caused also by optimisation of stator’s elastomer and a bond material between elastomer and steel pipe.
At an early stage it was already recognised, that any universal solution in material selection is not available to handle the wide range of complex requirements. For this reason various oil resistant elastomer types have been developed; the selection of an optimal material is often possible only based on material test (swelling tests).
Step by step not only a pump assembly but the whole pump system had to be optimised. Particularly using PCP Systems in marginal oil fields proved evidently technical and commercial advantages of this production system, such as high efficiency, low capital cost, economical in operation, long service life also under difficult conditions.
In Turkmenistan, the state oil company was looking for an alternative technology to replace Sucker Rod Pumps used before. The reason for this change was both a very short run-life of the downhole plunger pump due to the high content of sand and very frequent repairs of the surface equipment due to the environmental site conditions with sandstorms (Fig. 3). Using Progressing Cavity Pumps has doubled a run-life of the downhole pumps. The failure rate of the surface equipment has decreased, since considering the environmental conditions it was more constructive, to seal up a rotated shaft than to seal up a polished rod moving vertically.
The sealing and wearing of the polished rod in Sucker Rod Pumps applied before with a stroke of approximately 3–4m were often the failure causes. Besides the important advantages of PCP are obvious: power consumed by the pump system was reduced to 50percent, because the weight of the moving parts is considerably lower (Fig. 4).
The positive results were achieved by changing to the new PCP production system. Reducing sand content to 20percent contained in the production fluid was possible due to the fact, that replacing a piston pump by a rotating progressing cavity pump without valves enabled to almost eliminate the pulsation while oil producing, which consequently caused a considerably constant production flow rate
In the past the production of high viscous crude oil containing a very high asphalt content was featured by low efficiency and a considerable environmental impact.
Sucker Rod Pump with its significant leakage at the sealing used before and replaced by PCP system have step by step stimulated the environmental awareness in oil production. Using Progressing Cavity Pumps featuring a pressure capacity of 300bar enables to triple the efficiency of the production system. Thus, yet today approximately 30percent of total Albanian oil is produced using Progressive Cavity Pumps.
The main problem the operator encountering today in the oilfields of Germany, Austria and Romania is the oil production at water cuts of 98percent, which means the threshold of profitability. Using Progressing Cavity Pumps featuring the highest efficiency between 60 and 80percent it was possible, to restart and to profitably take in operation the closed wells. As well a much more cost-intensive gas lift system were successfully replaced by special PC Pumps equipped with Gas Separator.
Based on the constant research and optimisation referring both materials and design, and production data analysing of pumps being already in operation, it is possible to optimise oil production in the marginal field using Progressing Cavity Pumps.

Joerg Eitler is vice-president and sales director with Netzsch Oilfield Products GmbH, Selb, Germany. www.nop.netzsch.com