The development program for processes to reduce watercut during production ofthe lower Brent reservoir unit ofthe Gullfaks field culminated by a large scale injection of alkaline Na-silicate gel in a lower Brent production well durmg summer 1993. The operation was prepared and designed through an extensive laboratoiy program as well as small scale onshore field tests.The injection sequence consisted of a preflush of fresh water added low èoncentratkns of KC1 or Na-silicate. The purpose ofthe preflush was to condition reservoir parameters to improve injectivity ofthe Na-silicate gel. A volume ofapproximately 5500 m3 4-6% Na-silicate gel was mjected and displaced into the reservoir with one tubing volume fresh water. In total 9700 m 3 fluid was injected. An extensive sampling and anaJysis program of production fluids was worked out and results will be presented.Analysis of ion composition of the produced water allowed to determine the fractions of sea water, formation water and fresh water used as solvent for the injected fluids. Aftertreatment, the fraction of fresh water was initially about 50%, rapidly raising to 90% and higher. The injected volume was backproduced during a period of about five months. The reduction of permeability of the treated volume was also observed asan increased draw-down m the well. The resuits show that permeability is reduced significanlJy in the treated volume, and that an efficient water diversion has been achieved. The water-cut was initially at the same level as before treatment, but decreased within three months to 72-74%. The water-cut stayed at this level for another three months before rising to 77%. The treatment gave initially an increase in sand production and the well had temporarily to be produced at reduced rates. After this initial period, the total fluid rate could be increased to about 2000 Sm3 /d, the same level as before treatment, implying that the oil rate has been significantly higher most of the period after treatment than before.Based on history matching ofthe reservoir section with the Eclipse black oil simulator, predictions of well performance after treatment were made. The bistory matching included an active use of well influx data, including results from four production logs. The result indicated that the oil saturation in the drainage voluine ofthe well was rather low at the time oftreatment. Predictions with the history matched model reconstructed the water-cut development, and an estimate of incremental oil production can be done by comparing with predictions ofwell productivity without gel treatment. Using the fresh water as tracer, the perineability reduction factor in the treated volume is calculated to be approximately hundred.The conclusion is that a further testing and application of the technology iri North Sea reservoirs seems to be very promising for the pupose of improving oil recovery and well productivity.
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