Key Technologies of Polymer Flooding in Offshore Oilfield of Bohai Bay

Abstract

Abstract Bohai Bay is one of the most important offshore oil production areas in China, and 74% of oil is heavy oil. For conventional heavy oil reservoirs, water flooding is applied normally with an inverted 9-spot flood pattern. The recovery is about 18~20% because of the heterogeneous and unconsolidated reservoirs. The current strategy for application of polymer flooding to enhance heavy oil recovery is based on the fully understanding of mechanisms and core technologies of polymer flooding in the laboratory research and field practice. Single well pilot test and 5-spot flood pattern pilot test have been conducted in Bohai oilfield. The significant increase of oil production and reduction of water cut were observed. The limited life span of offshore platforms challenges operators to more effectively develop oil, thus it is necessary to identify the appropriate enhanced oil recovery (EOR) technology for use in a reservoir. Therefore, polymer flooding in early stage of oilfield development has been designed and conducted in Bohai oilfield. Five key technologies of polymer flooding concluded from the field practices were critical to the success of an EOR project and discussed in this paper. (1) Characteristics of polymer: polymer should tolerate high salinity and hardness of make-up brine. (2) Polymer injection technology: the technology should satisfy the requirement of offshore platform. (3) Modification of Polymer displacement under the big well spacing and multi-layer reservoir conditions. (4) Evaluation technology of polymer flooding in the early stage of oilfield development. (5) Production treatment technology of polymer flooding. This paper will focus on the impact of the five key technologies on development of offshore oilfields through introducing polymer flooding in Bohai Bay. Introduction It is estimated that about 74% of oil reserve in Bohai Bay is heavy oil, with the viscosities ranged from 50mPa·s to 10,000mPa·s. Currently the oil recovery by water flooding is around 20% OOIP, and most of oil is still remained in the reservoir. As a promising EOR technology, polymer flooding for offshore oilfield was studied for more than 10 years. Though some unsolved problems challenged the application of polymer flooding in offshore oilfield 1,2, Polymer flooding was adopted for improving heavy oil recovery in 2003. As a driving agent, polymer reduces the mobility of aquifer phase to force the remaining heavy oil. And as a mature EOR technology, polymer flooding is suitable for operation in offshore platform because it is not require complex and additional surface facilities in the relatively limited platform spacing. Based on the encouraging result of incremental oil in the single well pilot test, the polymer flooding was expanded to a larger pattern in October 2005. The limited life span of offshore platforms challenges operators to more effectively develop oil, thus polymer flooding in early stage of oilfield development has been designed and conducted in Bohai oilfield. The success of the single well pilot test provided not only the amount of extra oil, but also some unexpected emerged situations with the polymer flooding scaled-up during field application. When it comes to the expansion of application of polymer flooding, more work has to be done in advance to avoid unexpected risks. Based on the understanding and experience from the previous work, five key technologies of polymer flooding were considered to show impact to encourage the widespread use of polymer flooding in offshore oilfield.