With the development of waterflooding in an oil field, water breakthroughsare a common problem. Polymer solutions are widely applied to resolve thewater-channeling problem in a deep formation. In practice, polymers are used asagents in deep formation treatment either to block in-depth high permeabilityzones or to lower water mobility. However, for in-depth profile modification, apolymer solution has to be injected at a relatively high concentration so thatsuch treatment sometimes becomes uneconomical. On the other hand, polymerflooding is ineffective primarily due to the viscosity loss caused bymechanical and chemical degradations. This paper presents a weak gel system, which can be used as an in-depth profile modification agent and as an oildisplacement agent in a heterogeneous reservoir. This gel has relatively lowstrength but can still be cross-linked in the reservoir formation. Themechanisms of weak gel system are described in detail. In addition to studyingthe effects of polymer concentration, cross-linker concentration, temperatureand pH value on gelation time, rheological behavior, gelation ranges and gelstrength are examined. The coreflood results not only provide evidence that theweak gel acts both as an in-depth profile modification agent and as an oildisplacement agent but show an enhanced oil recovery as well. Furthermore, thepilot tests have been performed in four areas of Gudong oil field and Gudao oilfield in China since 1992. As a result, the injection pressure has beenincreased for all testing wells and the field water-cut has been reduced. Bythe end of 2001, the accumulative incremental oil production has been over58,000 tons.

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