Innovation and Practice of Reservoir Architecture Detailed Characterization and Remaining Oil Potential Tapping Technology in High Water-cut Stage of Large Offshore Fluvial Heavy Oilfield
Authors: Su Yanchun , Li Tingli , Huo Chunliang 
Published: September 2015

Qin Huang Dao 32-6 (QHD32-6) oil field located at Bohai Bay of China is a large fluvial heavy oil field discovered by CNOOC. This oilfield has come into high water-cut (88.6%) &low annual recovery rate (0.88%) stage after 14 years of exploitation, so a secondary infilling plan is needed to improve the development efficiency. However, the ultimate challenge is how to improve the forecast accuracy and tapping efficiency of remaining oil at high water-cut stage.

A new technology of accurate anatomizing of fluvial reservoir architecture has been proposed for the first time for the offshore fluvial oil field aiming at improving geological reservoir model. Using high resolution 3D seismic data and guided by the high resolution sequence stratigraphy, we established a fine isochronous stratigraphic framework considering multiple factors including constant-time slice, river entrenchment, sand body superimposition, and lateral facies change. Then the stratigraphic correlations were established for the subzones and single layers of the investigated area in this framework. Based on the seismic response characteristics and fine stratigraphic correlation and guided by the modern sedimentology and outcrop prototype model, we accurately anatomized the reservoir architecture in four levels, including compound channel, single channel, point bar, and lateral accretion body. After these work, the technology of accurate anatomizing of reservoir architecture and its quantitative characterization was carried forward for help us establish 3D visualization of geological reservoir model with millions of nodes and improve the accuracy of production history matching and remaining oil distribution forecast, thus enabled us to better tap the potential of remaining oil in high water-cut stage.

Further, a new strategy of separate-zone production by horizontal wells has been proposed for the first time in the offshore oilfield. We separated the field from the original one-set development to three-set development. The potential of remaining oil is tapped by horizontal wells replacing the original directional wells, and totally 127 new horizontal wells were proposed. The infill redevelopment project has been successfully executed with 127 horizontal wells drilled and put into production with excellent results of high single-well average productivity (64m3/d) and low water-cut (15%). In contrast, the single-well productivity of old deviated commingled wells is only 20m3/d with water-cut up to 92%. After the implementation of horizontal well infill redevelopment, field water-cut has reduced from 88% to 79%, annual oil recovery rate has increased from 0.8% to 2.1%, and the incremental recovery factor is 12.8%.

This technology has been widely applied in Bohai Bay offshore fluvial oilfields, including BZ25-1S, BZ28-2, BZ19-4 and CFD11-1. Good development results are shown in these oilfields at medium to high water-cut stage and the field development economic efficiency has been improved significantly.