BZ25-1S oilfield is a large heavy oil reservoir with narrow and thin fluvial channels in Bohai bay developed by CNOOC and Chevron. Due to large well spacing (350-450m), it is difficult to characterize the connectivity between sand bodies and determine the response of oil production on water injection. The early field oil production and pressure declined at the rate of 4.4% per month and 0.6 MPa per month respectively because of insufficient natural energy. The results of water-oil displacement in cores indicate that there was no non-water production period and the recovery factor would not more than 5% when field water cut reaches 70%.

Focusing on these main problems in the reservoir, comprehensive water injection and production optimization technology was applied based on study of detailed reservoir characteristics: 1) Combining research on geology and field outcrops and analysis of reservoir dynamic performance, we carried out detailed reservoir description of sand bodies and systematically analyzed the response of oil production on water injection for this type of reservoirs; 2) An innovative technology of production optimization has been developed. It takes the nature of offshore production technology into account, constantly adjusts water injection rate and controls water production amount so that water displacement efficiency improved; 3) An advanced surface water injection and production technology such as selective injection supporting the implementation of the production optimization techniques has been also developed.

Field production data showed that 95.6% of the wells displayed positive responses to water injection in which led to stable reservoir pressure, slower production decline, 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 5%.

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