Application of a novel polymer system in chemical enhanced oil recovery (EOR)

Water breakthrough is a common technical problem during the course of water flooding in an oil field. Polymer solution is widely applied to resolve the water-channeling problem in a deep formation. In practice, polymer is used in deep formation treatment either to form a gel blockage and plug high permeability zones in situ, or to act as a mobility control agent and increase the viscosity of injected fluids. However, for an in-depth profile modification operation, a polymer solution has to be injected at a relatively high concentration so as to maximize its penetration. Thus such a treatment may be uneconomical. On the other hand, a traditional polymer flooding may become ineffective mainly due to the viscosity loss caused by mechanical and chemical degradations. This paper presents a novel polymer solution, which is classified as a weak gel system and can be used In chemical enhanced oil recovery (EOR) as an in-depth profile modification agent, and as an oil displacement agent simultaneously in a heterogeneous reservoir. The weak gel system has relatively low strength but can still be cross-linked in the reservoir formation. In comparison with a conventional conformance control agent, this weak gel can form a blockage in a deeper formation due to its mobile feature. The EOR mechanisms of the weak gel system are described in detail. In addition to the effects of temperature, concentration, and pH value on gelation time, the rheological property, gelation range, and gel strength are examined. The core-flood test results not only provide experimental evidence that the weak gel improves injection profile and increases sweep efficiency but also show enhanced oil recovery. Furthermore, the pilot tests have been performed in Gudong and Gudao oil fields in China since 1992. It has been found that this weak gel system can significantly enhance oil recovery and effectively reduce water-cut. By the end of 2001, the cumulative incremental oil production had been over 58000 tons.