UNCONVENTIONAL EOR PROGRAM INCREASES RECOVERY IN MATURE WAG PATTERNS AT PRUDHOE BAY

A large scale conventional WAG process was implemented in the Prudhoe Bay field in 1987. Many of the original patterns are no longer receiving MI due to pattern maturity and diminishing response to the WAG process. The EOR process is being revived in the gravity dominated eastern part of the Prudhoe Bay Field waterflood area by using horizontal sidetracks to place MI at various locations within existing patterns which have a significant remaining EOR target.

The sidetracks are initially completed at the toe of the well using a limited perforation interval, followed by injecting a large slug of MI, and a water chase. The injector is then re completed sequentially along the length of the horizontal wellbore, and the process is repeated to allow up to 7 independent events of miscible gas injection. Independent distribution of injection points enables the MI to contact additional pore volume not swept by the original WAG injector due to the large gravity impact which enables the MI to segregate, and override oil away from the injector.

The MIST process (MI Sidetracks) increases EOR recovery up to 400% over a conventional WAG pattern. Pattern oil rate responses begin within two months of injection, and peak within 7 months in proximal offset wells. Oil rate increase within the MIST patterns has been as large as 200% during each individual MIST event.

The following paper will explain the reservoir characteristics that contribute to such large incremental recoveries, well design, operational programs, plans to further expand the program, along with options to technically review for further upside recovery. A number of additional MIST patterns are currently planned over the next several years.

This technology could be applied to many gravity dominated reservoirs with miscible gas injection.