Application of EOR Technology by Means of Polymer Flooding in Bohai Oilfields

Abstract

Abstract Enhanced oil recovery by means of polymer flooding is considered as an important technology for the strategic development of offshore oilfields in China. The implementation of polymer flooding in the offshore oilfields requires overcoming several major technical problems caused largely by the absence of a fresh water source and limited space on the platform. A pilot test of polymer flooding was conducted in SZ36–1 Oilfield located in Bohai Bay, China. The problems mentioned above were solved through the development of (1) a hydrophobically associating polymer tolerant to salts in the brine and produced water and (2) a portable automatic injection unit. The significant incremental oil and decrement of water cut were observed after 8 months of polymer injection. The successful implementation of polymer flooding makes three breakthroughs:the first polymer flooding pilot test in offshore oilfields in China;initial application of a hydrophobically associating polymer as the driving agent in the field;significant increment of oil in the trail of a single well injection of polymer. The practice illustrates the feasibility of polymer flooding under current production conditions of offshore oilfields that lead to applications in a larger scale. Introduction There are about 20 offshore oilfields in Bohai Bay, which are referred to Bohai Oilfields[1], in which 70% crude oil with viscosity ranged from 50 mPa·s to 150 mPa·s. Water flooding is the dominant technique for the development of viscous oil in Bohai Bay. The problem is that the oil recovery by water flooding is quite low 18∼20% compared to the average oil recovery of 32% in onshore oilfields. The reasons lie in the high viscosity of the oil, thick pay zones with high heterogeneity, as well as the present offshore oil development mode: the oil is produced normally in an inverted 9-spot flood pattern with 350m well spacing that is much larger than that of onshore oilfields. The operation of the oilfields follows the policy of " less wells with high production rate", which is realized in the limited offshore platforms with a great concern on the investment and balance between production rate and final oil recovery. SZ36–1 Oilfield, located near the coast of Suizhong County, Liaoning Province, China is a typical offshore oilfield of Bohai Oilfields in reservoir characteristics, fluid properties and development mode. This oilfield was put into operation in 1993. From the very beginning, the technologies on enhanced oil recovery were investigated in order to improve water flooding efficiency. Among the technologies, polymer flooding showed great potential for the oilfield from a study of numerical simulation[2]. It is know that polymer flooding has been successfully used in onshore oilfields like Daqing Oilfield in an industrial scale with an proved incremental oil recovery over waterflooding of 12% OOIP[3]. However, the implementation of polymer flooding on the platform met great challenges on the lackness of fresh water supply and the limited operation space on the platform. Other challenges involved cost, safety, environmental concerns. The problems mentioned above were solved through the development of (1) a hydrophobically associating polymer tolerant to salts in the brine and produced water and (2) a portable automatic skid injection unit. Based on a series of work on laboratory evaluation and numerical simulation, the first pilot test of polymer flooding was conducted in SZ36–1 Oilfield in September 2003. The purposes of the pilot test were (1) to examine the solubility and injectivity of the polymer under platform conditions; (2) to examine whether the injection facilities meet the requirements of polymer flooding process; (3) to collect produced fluids containing polymers for further study. The pilot test has been conducted successfully with a significant increment of oil. It is followed by another pilot test in SZ36–1 Oilfield. Enhanced oil recovery by means of polymer flooding is considered as an important technology for the strategic development of offshore oilfields in China. This paper focuses on presenting the challenges for the application of polymer flooding in offshore fields, design of the pilot test including polymer and facilities, and field results of the pilot test. Field History The candidate areas for the pilot and further applications of polymer flooding are in SZ36–1 Oilfield. The reservoir was sedimented in a fluvial environment, founded in 1987 and has put into production since 1993 by two phases: Phase I and Phase II. It is a mono-anticline extending in NE direction. The reservoir is structurally controlled and lithologically influenced[5]. Reservoir Characteristics With stable distribution and good connectivity, the reservoir is distributed in lower Dongying Group with a reversed sedimentary rhythmic feature. The buried depth is in the range of 1300∼1600m and average thickness of pay zone is 61.5m. The whole oil-bearing formation is divided into 4 groups, which are subdivided into 14 layers. Lithologically, reservoir rocks are feldspathic quartz sand composed mainly of fine sands. The sand is unconsolidated and poorly cemented with porosity of 28∼35% and average permeability of 2,600×10–3µm2. The original reservoir pressure is 14.28MPa and the reservior temperature is about 65°C.