Performance and Operation of a Crosslinked Polymer Flood at Sage Spring Creek Unit A, Natrona County, Wyoming
Authors: Mack James C. , Warren John 
Published: December 2007

Summary
This paper reviews field geology and development, characterizes the reservoir, evaluates secondary performance, and describes the design and benefits of a performance, and describes the design and benefits of a polymer program. Performance of the Sage Spring Creek polymer program. Performance of the Sage Spring Creek Unit A confirms a high flood efficiency and superior oil recovery. The sweep improvement program is a technical and economic success.

Introduction
The Sage Spring Creek Unit A was discovered in 1949 and produces from a fractured Dakota sandstone. The field was under fluid-expansion/solution-gas-drive primary recovery until a five-injection-well peripheral primary recovery until a five-injection-well peripheral waterflood began in 1972. Injection rates were low to establish a flood front and to prevent channeling. In 1978, a volumetric sweep program was initiated in three injection wells to divert injection into the lower-permeability rock. Cationic and anionic polymers and aluminum citrate were injected to provide in-depth diversion of injection water.

Field Discovery
The Sage Spring Creek Unit is located in Natrona County (WY) approximately 25 miles [40 km] northeast of Casper. The field makes up part of the first line of folding along the southwestern margin of the Powder River basin. It lies 10 to 15 miles [16 to 24 km] south of the prolific Salt Creek and Teapot (U.S. Naval Reserve) fields. The discovery well, Amerada 1, was drilled by Amerada Hess Corp. to a total depth of 9,539 ft [2908 m] in Aug. 1949, plugged back, and completed in the Dakota sandstone at 7,200 ft [2195 m], and pumped 68 B/D [10.8 m3/d] oil. Ten producing wells were drilled between discovery and May 1954. No further development took place until 1971, when Farmers Union Exchange (Cenex) purchased the property.

Geological and Reservoir Characteristics
From Ref. 1: "The Sage Spring Creek Unit lies on the Sage Spring Creek anticline, a surface structure, which is a south-plunging nose with no north closure. The anticline plunges southward two degrees and dips on the east and west. Flanks of the structure average about seven degrees. Structural relief from the south end of the Unit to the north end amounts to about 2,800 feet (853.4 m). No faulting has been noted on this structure."The only producing horizon within the Unit is the Dakota sandstone. Oil is trapped as a result of changes in porosity and permeability of the Dakota sandstone in an porosity and permeability of the Dakota sandstone in an updip direction. The Dakota sandstone is Lower Cretaceous in age and present throughout the southwestern Powder River basin. The sandstone varies in amounts of siltstone and shale and is related to shoreline environment. The clean, well-developed sands are probably channel or offshore bar-type deposits, while the tight sand/siltstone/shale facies are deeper-water accumulations. The areal extent and well location of the Sage Spring Creek Unit A is shown in Fig. 1. The reservoir limits are defined on the southwest edge by an oil/water contact and to the north and east by a pinchout of permeability and porosity. The reservoir covers a total surface area of 1,456 acres [5892 × 103 m2] with an average net pay thickness of 21.6 ft [6.6 m]. The total volume of the oil zone is 31,487 acre-ft [38 839 × 103 m3]. The reservoir is split into two areas: a high-capacity channel and a low-capacity area. The high-capacity channel lies along the southwestern edge of the reservoir and trends from the southeast to the northwest. Air permeabilities range from 1 to 281 md, giving a permeabilities range from 1 to 281 md, giving a Dykstra-Parsons permeability variation of 0.82. Some fracturing may exist in the high-capacity channel with directional permeability and fracture orientation parallel to the permeability and fracture orientation parallel to the channel. The low-capacity area is located in the northeastern portion of the reservoir and exhibits much lower portion of the reservoir and exhibits much lower permeability. Production capacity is limited in this area permeability. Production capacity is limited in this area of the field. The combination of permeability variation and fracturing makes the reservoir very heterogeneous. Produced oil from the Dakota sandstone is 36 deg. API [0.84-g/cm3] sweet crude. The viscosity of the oil at reservoir temperature (160 deg. F [71 deg. C]) is near 1.0 cp [0.001 Pas], giving a favorable water/oil mobility ratio. Average reservoir porosity is 13% and connate water saturation is 25%. Original oil in place (OOIP) is 630 STB/acre-ft or 19,800,000 STB [3148 × 103 stock-tank m3]. Reservoir and fluid properties are presented in Table 1.
Primary Performance Primary Performance The primary producing mechanism was solution gas drive with a partial edgewater drive.
JPT
P. 1145