BZ25-1S oilfield is a large heavy oil (30-260mPa.s) reservoir with narrow (100-300m wide) and thin (2-8m thick) fluvial channels first developed in Bohai bay, China, by CNOOC and Chevron. Due to relative large well spacing (350-450m), it is difficult to characterize the connectivity between sandbodies and determine the response of oil production on water injection. The early field oil production and pressure declined at the rate of 4.4%/month and 0.6 MPa/month respectively because of lacking natural energy. The results of water-oil displacement in cores explicit that there is no non-water production period and the recovery factor would not surpass 5% when field water cut reaches 70%.
Targeting these prominent problems in the reservoir, we have applied comprehensive water injection techniques based on study of detailed reservoir characteristics. We mainly: 1) Combining research on geology and field outcrops and analysis of reservoir dynamic performance, we carried out detailed reservoir description of sandbodies and systematically analyzed the response of oil production on water injection for this type of reservoirs; 2) An innovative technology of water injection optimization has been developed. It takes the nature of offshore production technology into account, constantly adjusts water injection rate so that water displacement efficiency is improved; 3) An advanced surface water injection and production technology such as selective water injection supporting the implementation of the water injection optimization techniques has been also developed.
Field production data showed that almost all producers within injection-production well patterns displayed positive responses from water injection resulting in stable reservoir pressure, oil production, and slightly water cut increase. The filed oil rate has been stabilized for about four years without water breakthrough in large scale and the overall recovery factor is expected to improve more than 6%.
Introduction BZ25-1S field located in Bohai Bay, eastern of China, was first discovered in 1998 and the development plan was approved in 2003. The first phase (three platforms: B, D, and E) was put on production in August of 2004 and the second phase (two platforms: C and F) in August 2005. The reservoirs are small and complex fluvial channels with complicated geometry. Those channels were formed at different geological times and result in poor reservoir connectivity. Early production showed steep pressure and production decline. Quick implementation of water injection is needed to arrest the fast production decline and to stabilize reservoir pressure. Water injection efficiency is expected to be low for heavy oil fields but still widely used because of its economy1-5. Feng et al6 published a paper in 2007 on integrating all data from geology, seismic, and production to optimize the water injection in BZ25-1S and Yang et al7 presented an article in May 2010 summarizing the best practices and lessons learned during the implementation of ODP. This paper is an extension of those two publications concentrating on reservoir management to optimize production during the rising period of water cut.