How to Effectively Develop a Marginal Off-Shore Heavy Oil Field - A Case Study
Authors: Yang QingHong , Shu William , Liu Ming , Wang Lirong , Wang Yunxian , Feng Xin , Hou Dongmei , Zhao Xiujuan , Zhang Fengli 
Published: July 2010

Abstract
BZ25-1S field located at Bohai Bay of China is characterized as a complex fluvial reservoir with small (100-300m wide) and thin (2-8m thick) sand channels. Many channels are isolated or poorly connected with overall reservoir connectivity of only about 67%. It is classified as a marginal field because of the thin sand channels, low reserves abundance, weak reservoir energy, and high oil viscosity (30-260mPa.s).
Information obtained from early development wells revealed a different, yet much more complicated geological system than the geological model built from appraisal wells and used in the Overall Development Plan (ODP). By integrating information of different sources obtained during drilling, significant modifications have been made to the original ODP in order to reduce the drilling risk and improve development efficiency. These included:Changed the regular well pattern to irregular pattern;Drilled horizontal/multilateral wells;Applied new cost-effective completion method that combines open hole and premium screen with external casing packer.
A large number of potential dry holes or low efficiency wells were moved to better locations through such adjustment. This resulted in the increase of average pay thickness encountered per well from 10.7m to 20.8m.
However, the early production still showed steep declines of pressure (0.6MPa/month) and production rate (4.4%/month). Hence, an integrated approach that incorporates geological, well log, seismic and production data was developed to optimize the field production. Following this approach, (1) an optimal water injection plan was designed and implemented to maximize the water flood efficiency; (2)new geological information revealed from dynamic analysis was timely used to update geological model; and (3) production/injection schedule and profile were dynamically adjusted to control the rapid water breakthrough in single layer or at single well.
Field production data showed that 96% of the wells displayed positive responses from the water injection resulting in stable reservoir pressure, slower production decline, and slightly water cut increase. The overall recovery factor is expected to increase by more than 5%, which significantly improved the development efficiency of this marginal oil field.

Introduction
BZ25-1 is a first developed fluvial-dominated sandstone heavy oil field deposited in shallow water deltaic environment in Bohai Bay, China, and one of the most complex tertiary fluvial fields in the area with poorly-connected narrow channels strip-shaped (less than 300m wide) and large vertical and lateral variations in reservoir properties (Fig. 1). Water injection efficiency is expected to be low for heavy oil fields but still widely used because of its economy1-5. Meanwhile, lots of small faults have been developed in some areas with complicated oil-water relationships. The distributions of oil, gas, and water are controlled by both faults and lithology and oil properties vary greatly6. It was a very challenging task to drill development wells in such low reserve abundance reservoir.