Pump-Scaling Issues in ASP Flooding in Daqing Oil Field
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 109165, "Technical Breakthrough in PCPs' Scaling Issue of ASP Flooding in Daqing Oil Field," by Cao Gang, SPE, and Liu He, SPE, University of Science and Technology of China, and Shi Guochen, SPE, Wang Guoqing, Xiu Zhongwen, Ren Huaifeng, SPE, and Zhang Ying, Daqing Oilfield Company, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11–14 November.
The alkaline/surfactant/polymer (ASP) –flooding pilot tests in Daqing oil field commenced in 1982. Although the recovery rate can be improved by 20%over that with waterflooding, severe scaling formed in downhole equipment and in artificial-lift systems of ASP producers. Sucker-rod failure was extremely high for beam-pumping wells. The running life for progressing-cavity-pump (PCP) systems was longer but still not economical. Severe scaling on the surface of the pump and string was the major factor causing drastic oscillation of operating load. Stator-surface modification and fluid treatments increased the average running life from several months to longer than 1 year.

Introduction
ASP-flooding pilot tests commenced after waterflooding and indicated that the recovery rate was improved by 20% on average. However, a negative issue emerged: Severe scaling formed on the surface of production systems and resulted in abnormal operation.
In beam-pumping systems, when scaling particles dropped out and accumulated in the space between pump barrel and piston, the pump would be blocked in a short period. The minimum running life was less than 1 month. In electrical-submersible-pump systems, the main problem was motor failures resulting from separators being blocked by scaling. The shortest running life was 1/2 month. The structure of PCPs resulted in a failure rate much lower than that of the other lifting methods. The average running life was approximately 3 months. Fig. 1 shows some of the pump and rod abrasion problems.

PCP-Scaling Mechanism
In the process of ASP flooding, alkaline lye reacts with rock minerals and formation water, increasing scaling-ions concentration (e.g., Ca2+, CO32−, SiO23−) in the formation. As production fluid moved close to or into the wells, the change of temperature and pressure breaks the balance of scaling ions and scaling forms near the wellbore, on the surface of downhole equipment, and on the inner surface of tubulars.
The study indicated that ASP scaling is a mixture of carbonate and silicate. The main mineralogical compositions are amorphous-state silicon dioxide, hexagonal-spherical calcite, and conventional calcite. Minor mineralogical compositions are clastic quartz, clastic feldspar, clastic clay particles, and pyrite. The hardness of silicate is much higher than that of chromium coating, causing rotor-coating wear after a short period.

Rod Fatigue.
PCP-well operating loads in the ASP-flooding area were less stable than in other areas. Severe oscillation of the operating load resulted in a high rod-fatigue-failure rate in PCP wells in the ASP-flooding area. A study indicated that rod-fatigue failure resulted from the comprehensive influence of several factors.