Discontinuous Chemical Flooding: An Effective Approach to Achieve Sustainable Oil Production for Mature Reservoir

Abstract

The offshore oilfields in China have entered the mature stage with high water cut and recovery rate. Oilfield Q has an oil viscosity of only 5 cp, with a water cut of 95% and a recovery rate of more than 40%. It is challenging to maintain sustainable oil production with only waterflooding. The field production is close to the economic limit. Therefore, finding an effective and economical EOR technique is crucial to mature oilfield development. In this paper, we proposed a new EOR method, called discontinuous chemical flooding (DCF) for mature oilfield development with a case study of oilfield Q. Four types of chemical slug, which are primary slug, main slug, secondary slug, and supplementary slug are designed. Primary slug injection well is determined based on the interwell connectivity analysis model and tracer test. The CMG CMOST module is applied to determine the optimal slug size and concentration of primary and main slug. According to the predicted produced agent concentration and water cut curve trend from numerical simulation, the implement criteria of secondary slug, and supplementary slug are provided. The interwell connectivity analysis indicates that the permeability between injector I3 and producers A1, I1 as well as A5 are relatively high, which is an indication of a potential thief zone. Primary slugs of gel injection are designed for all injectors with channeling effects. The CMOST evaluation results show that the optimal formula of strong primary slug is polymer with a concentration of 5000 mg/L plus cross-linker with a concentration of 4000 mg/L. As for weak primary slug, the optimal formula is polymer with a concentration of 3000 mg/L plus cross-linker with a concentration of 2000 mg/L. The optimal main slug concentration for injectors, I1, I6, and I9 is 1400mg/L. The optimal main slug concentration for injectors, I5, I7, I8, and I11 is 1000mg/L. The optimal main slug concentration for the rest of injectors is 1200mg/L. The optimal main slug size is 5 years. The final reservoir simulation results yield a maximum water cut reduction of 6.7%, and an incremental oil production of 53.2×104 m3 compared with the optimal waterflooding scheme. The scheme of oilfield Q is the first DCF scheme for mature offshore thin oil reservoir in China. It gives the approach of further enhance oil recovery for mature oilfields with ultra-high water cut and recovery rate. It also promotes the application of chemical flooding in offshore reservoir.