Strategies for Reserve Growth in Tirrawarra Field

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The Tirrawarra Sandstone, a fluvial-lacustrine sandstone reservoir of earliest Permian age, lies at a depth of approximately 10,000 feet, and current volumetric mapping shows an original oil in place (OOIP) of 127 million barrels. The cumulative production to December 1992 is 17.2 million barrels. The average net pay thickness is 60 feet over an area of 12,000 acres, with an average porosity of approximately 10 percent and an initial oil saturation of roughly 70 percent. The reservoir contains a volatile oil with a solution gas-oil ratio (GOR) of 2714 standard cubic feet per reservoir cubic foot (scf/rcf) (C5+/C4-). At the initial pressure of 4,280 pounds per square inch absolute (psia) and temperature of 285°F, the oil formation volume factor was 2.9 reservoir barrels per stock tank barrel (rb/stb) (C5+), and the oil viscosity was 0.09 centipoise.

Early reviews and production trends indicated that recovery would be low without some method of improved or enhanced recovery. Laboratory and screening studies using reservoir oil and various injection fluids resulted in the conclusion that satellite separator gas from several local Patchawarra Formation gas (PCSG) reservoirs, gas from the de-ethanizer tower at the nearby Moomba gas plant, and CO2 were all first contact miscible at or above 3,200 psia. A pilot seven-spot pattern in the area of best reservoir productivity was started up in 1984, using PCSG as an injection fluid. After the production decline in this area was largely arrested, four more patterns (approximately seven spots) were added over the next couple of years. Three more have been added during 1992-93. Injection of the Moomba de-ethanizer gas was phased in during 1987.

Due to the low permeability range in the Tirrawarra Sandstone (0.1-10 millidarcies), all but three production and injection wells have undergone hydraulic fracture stimulation to ensure injection and offtake is on a commercial scale. The induced fracture orientation is unknown. Fracture half-lengths are between 200 and 500 feet with well spacing of 1,800 to 2,400 feet. Radioactive tracers were placed in the first five injectors in 1985 and 1986, and active sampling occurred until mid-1991. Results were disappointing, most likely due to injection of insufficient amounts of two of the tracers and questionable analytical practices by the laboratory; however, there was a clear indication of uniform sweep.


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