ABSTRACT
The Forties Field sea water injection system was commissioned in 1976 and rapidly achieved design injection capacity of 450,000 bwpd at 80 bar WHIP [71,500 m3/d @ 8 MPa]. The system originally incorporated a cartridge type fine filtration system. In recent years our understanding of both water injection and water quality effects has advanced such that the fine filters were removed during 1989 and 1990. No adverse effects have been experienced or are anticipated. It was recognised that well performance when injecting fine solids or emulsions would depend on the state of the formation around the wellbore. Well-pulsing techniques were used to demonstrate the existence of fractures or cavities in the poorly-consolidated sandstone of the formation. Results suggested that changes in injected water quality did not lead to fracture growth. Some additional information indicating the degree of formation disturbance around the wellbore was provided by analysis of step rate and pressure fall-off tests.
We also report on the results of sustained produced water re-injection into the Forties reservoir. A pilot reinjection trial was conducted in 1985. Produced water was injected into a Forties well at 12,000 bwpd, 89 bar WHIP [1,900 m3/d @ 8.9 MPa] for 6 weeks. The oil in water content varied from 501200 ppm [50-1200 cm3/m3] but there was no evidence of injectivity impairment throughout the trial.
Based on the success of the 1985 trial, a long term reinjection test was initiated on the same well in March 1989. Injection performance was initially difficult to analyse due to persistent pump seal failures, control system problems and, at times, a lack of sufficient produced water. However an injectivity decline of 20-30% was established and various damage and stimulation effects have been investigated.
