Summary This paper presents the results of coupling a reservoir simulator with an acid fracture design program. This model is used to identify program. This model is used to identify refracture candidates and to optimize acid refracturing treatments. Methods used to study treatment efficiency for specific reservoir properties are given for various acid systems. Introduction The Cottonwood Creek field in Washakie County, WY, was discovered in 1953 ad produces primarily from the Phosphoria produces primarily from the Phosphoria formation, a low-permeability, dolomite oil reservoir. This reservoir, covering about 60 sq miles, is one of the largest stratigraphic oil columns in Wyoming. Early development of the Cottonwood Creek Unit was concentrated in the updip portion of the field because of good production response obtained from high-permeability streaks. These streaks were originally interpreted as natural fractures. Reservoir modeling ad core data indicate that the reservoir is not naturally fractured. A waterflood was initiated in 1958 in the updip portion of the field, which maintained pressure support to the high permeability streaks. Productivity of these permeability streaks. Productivity of these wells was high ad stimulation was not believed to be necessary. The low-permeability portions of the reservoir, vertically adjacent portions of the reservoir, vertically adjacent to the high-permeability streaks were virtually unswept by the waterflood and contain a large amount of recoverable oil. As development continued in the Cottonwood Creek Unit, less prolific producers were drilled. Because of prolific producers were drilled. Because of the low permeability encountered, most of the wells required fracture stimulation to be commercial. During 1987, a detailed reservoir and logical engineering study of the Phosphoria formation in the Cottonwood Creek Unit area was initiated to determine whether the producing rate ad ultimate oil recovery producing rate ad ultimate oil recovery could be improved. The study indicated that may wells in the area had short fracture half-lengths. Extensive reservoir simulation indicated that increasing the effective fracture half-length would increase the producing rate ad ultimate recovery. Because of producing rate ad ultimate recovery. Because of these results, a acquisition program was pursued in the area. The most significant pursued in the area. The most significant acquisition was the Cottonwood Creek Unit, acquired Jan. 1, 1989. Geology The Cottonwood Creek Unit is located in the Big Horn basin about 8 miles east of Worland. The field produces primarily from the Ervay member of the Permia, Park City formation (Phosphoria), covers about 33 sq miles, and includes 150 oil wells. The Phosphoria reservoir is a stratigraphic trap Phosphoria reservoir is a stratigraphic trap covering roughly 60 sq miles at depths varying between 4,500 and 11,000 ft. The downdip limit is controlled by a oil/water contact.

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