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The AAPG/Datapages Combined Publications Database

CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 39 (1991), No. 2. (June), Pages 226-226

"Water and Miscible Flood Optimization in a Layered Reservoir [Abstract]"

Styan, W.1, Mullane, T.1

ABSTRACT

Approximately 33 E6m3 (200 m bbls) of 832 kg/m3 (38 API) crude and over 36.2 E9m3 (1.3 TCF) of condensate rich natural gas was originally trapped in karsted Turner Valley carbonates along the Mississippian subcrop at Harmattan. Discovered one year after the gas cap, in 1957, the oil column was produced by solution gas and gas cap expansion until 1974 when a field wide waterflood was implemented. A limited CO2 miscible flood was initiated in 1988.

Predominantly vuggy, leached fossil moldic porosity is developed in dolomitized crinoidal mud supported fabrics near the top of storm generated fining upward cycles. Syntaxially cemented crinoidal grainstones at the base of each cycle are impermeable and effectively segregate the reservoir into layers. In intershoal areas, where grain supported fabrics are absent, horizontal stratification with limited kv replaces distinct permeability barriers. Near the top of overall restricting upward mega-cycles, dolomitization occurs prior to calcite stabilization. In this environment, thin but areally extensive, high permeability layers are developed in grainstone fabrics. Low permeability wackestones and mudstones intercalated with green shales cap the sequence to form a laterally continuous horizontal flow barrier.

Both inverted 9 spot and line drive patterns were configured on 80 acre spacing during initiation of the waterflood. Completion of all injector well pay occurred during this phase of development with offset producers perforated only in lowermost poorer reservoir layers. Following breakthrough, wells were completed uphole with limited success. High water volumes with limited incremental oil suggested large scale migration of oil had occurred in more permeable layers. Bubble maps, production and perforation history plots integrated with both pressure surveys and reservoir models were used to follow fluid migration. Periodic injection and production logging in gas lift wells confirmed that the waterflood was effectively sweeping the more permeable uppermost layers, and that several poorer reservoir units near the base contributed little and may contain bypassed pay. Updip migration of oil is being captured by large pumpjacks and ESP's.

Early breakthrough by CO2 in the miscible flood mimicked fluid migration behavior observed during waterflood confirmed the geological/reservoir model, but contradicted early reservoir simulations. Experience has shown that a detailed cross sectional model must be employed to properly model early breakthrough caused by override and vertical permeability variations. Profile logging has shown that gravity override plays a significant role in this reservoir. A field test will be conducted to examine performance at higher injection rates by injecting the fixed volume of CO2 to fewer injectors at a time (3 to 1).

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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES

1 Shell Canada Limited, Calgary, Alberta T2P 2H5

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