A detailed geological and compositional simulation study of the 4A Dakota Sand in the Henry Field, Uinta County, Wyoming was authorized by the Henry Field Technical Committee in April, 1991. The 4A Sand is a retrograde gas condensate reservoir. In April 1991, ownership of the reservoir involved three separate sets of working interests; the Henry Unit Participating Area (PA), South Henry Unit PA, and the Marathon Federal #16-1 well. The Henry Unit PA initiated an ongoing pilot injection program in December, 1985. The study was undertaken to determine the economic feasibility of further developing the 4A Sand in the Field by expanding the pilot injection to increase recovery of liquids in the 4A Sand.

Following a geological study of the structure, thickness and continuity of the 4A Dakota Sand in the Field, a compositional simulation model was used to obtain a match to the Field's 3 years of primary and 6 years of cycling history. This model was then used to study alternate producing schemes, ranging from continuation of historical practices to expansion of the dry gas cycling project by the addition of three injectors, to accelerated expansion of the project by adding injectors and producers and hydraulically fracturing all new wells and the lowest productivity existing wells.

The compositional model predicts that 16% of the original 13,880,000 barrels of condensate in place in the 4A Dakota Sand would have been recovered if gas injection had never been initiated. The FULL EXPANSION CASE predicts recoveries as high as 64.6% of the maximum possible oil recovery.

Prior to initiation of the accelerated expansion project, a Henry Field Enhanced Recovery Unit (HERU) was formed from existing working interest tracts based on participation parameters approved by the Technical Committee and approved by the Working Interest Owners. At the end of 1992, injection well drilling, compression installation, and gas plant construction were in progress at the HERU with an expanded project start up date of early 1993.

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