Comparison of ESP technologies for operation at high gas content in pump based on NK Rosneft field tests

Abstract

Abstract One of limitations of achieving full potential in pumped wells is due to the presence of excessive free gas at pump intake. The presence of free gas at pump intake has several effects on the pump performance. A reduction in the pressure drop developed by the pump is usually observed when pumping multiphase flow mixtures. This performance reduction sometimes may be severe, resulting in unstable pump operation, production losses and may lead to premature equipment failure. Electric submersible centrifugal pumps (ESP) are commonly used in Russia and other parts of the world for oil production. Currently operators are being required to produce wells under non conventional and challenging operational conditions. One of such conditions is the use of ESP in wells with a high free gas liquid ratio that requires the use of special technology. Under those circumstances it is very important to understand more profoundly the processes of ESP operation as any mistake in the equipment design may have severe economical consequences. One of the crucial information needed is the technical limiting operational conditions for different ESP technologies when handling high gas liquid ratio mixtures. Determination of the technical operational envelope is itself a challenge. Experimental work conducted in Universities and industry research centers are very important for an understanding of the problem and advance of the technologies. But it is difficult to recreate in the lab the same conditions that exist in a well bore. Working with actual fluids at high temperatures and pressures are still challenging conditions to be reproduced in the lab. Therefore theoretical models, experimental correlations manufactures technical guidelines need to be verified against the conditions existing in a real oil well. This work presents the conclusions of the use of state of the art engineering methods describing the reservoir-pump production system to analyze the performance of gas handling technologies based on real well field test results. The field tests were conducted under the New Technologies System project of Rosneft Oil Company. Field tests estimated technical limits for ESP gas handling technologies for up to 75% of volumetric gas fraction at pump intake conditions, as well as confirmed the possibility of oil production enhancement with the tests (the NPV for 11 ESP tested was more than 40 mln. rubles in 2007). It is estimated that more than a 100 wells would benefit from ESP gas handling technologies in Purneftegas alone, yielding significant economic impact for the company, increasing oil production by more than 700 tons/day. Review of gas handling approaches Typically, wells with high GLR are produced by natural flow until conditions require an artificial lift system. At this point, gas lift is usually chosen as an artificial lift method unless some of the free gas can be separated and vented allowing the use of a conventional pump. The performance of ESPs under two phase flow conditions depends on factors such as liquid flow rate, amount of free gas, fluid properties (surface tension, densities and viscosities), rotational speed and of course pump geometry. The industry has innumerous simple "rules of thumb" (usually only based on the amount of free gas and pump intake pressure) that try to capture this complex behavior. As a consequence, when those "rules of thumb" fail to reproduce the phenomena they either hinder the applicability of ESP in gassy wells or in the worse case permit the installation of equipment under non acceptable pumping conditions. Both cases bring undesirable economic consequences. Therefore application of ESP in high GLR wells has been limited. Recent advances in ESP gas handling technology indicate the possibility of extending ESPs to some of the more gassy applications in the industry.