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Waterflooding at Prudhoe Bay in Alaska was initiated in 1983. By the 1990s a significant number of producers had watercuts of 90% or higher due to waterflood maturity. Wellbore treatments with cement had been successfully used to shut off unwanted water influx from cement channels or from hydraulically isolated zones in unfractured production wells, however, wellbore shutoff had been only partially successful in controlling water influx from offending zones that were not hydraulically isolated from desired producing intervals.
In the early 1990s, the decision was made to apply crosslinked polymer gel technology to address the problem of high perm water influx. Candidates for the program included wells:
- Suffering water influx by a known mechanism, i.e. high permeability with respect to the major oil producing intervals.
- Sufficient hydraulic limitation due to water influx that successful shutoff could allow a decrease in bottomhole flowing pressure (increasing drawdown)
- Sufficient reserves to warrant spending money for their recovery
- Good mechanical condition.
Virtually all of the wells treated were “throw-away” wells, slated for abandonment or sidetracking. Each treatment has at least a measure of technical success in terms of water cut decreasing significantly. A total of 19,000 bwpd decrease was attained and in two cases the decrease maintained for over a year. Oil rates immediately after treatment were generally equal to or lower than pre-job rates, but in virtually all cases increased over time.
The project showed that:
- Selective water shutoff with high molecular weight chromium (III)-carboxylate crosslinked PHPA polymer gel can be accomplished without selective placement of treatment chemicals when the water influx mechanism involves a high permeability path.
- Improvement in wellbore hydraulics is a valid and attractive candidate selection criterion for polymer gel water shutoff treatment; oil rate may be enhanced in such cases.
- Although the oil productivity index may be damaged to some degree by the gel, improved hydraulics can overcome this.
- Damage to oil PI appears to be temporary in some cases, and high drawdown may enhance oil rate cleanup
- Penetration of high molecular weight gelant into rock matrix can occur in the Prudhoe Bay reservoir, but resistance to flow is greater than into a natural fault or created fracture.
- The shear-thinning nature of high molecular weight gelant leads to reversible thickening if pumping is stopped for a short time during a job; downtime should be minimized.
- A Hall plot is a useful indicator of formation behavior during pumping and should be constructed during the job to provide real-time performance analysis.
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