One of Mobil's principal heavy-oil reservoirs, the Miocene Monarch, in the South Midway-Sunset Field, Kern County, California, is a thick sequence (400 ft.) of unconsolidated deep-sea turbidite gravels, sands, silts, and muds. Disappointing steam injection rates and poor production responses over the anticline prompted a laboratory and simulation investigation of the causes of the shortfalls. High-temperature coreflood experiments were conducted on selected facies, backed by liquid and solids analyses. The experiments showed that fines migration and smectite clay swelling are the primary causes of the restricted injectivity and quantified the severe damage.

Numerical thermal simulators lack the ability to model dynamic formation damage, but it was necessary to capture this progressive loss of permeability to match the field performance. Using a commercial simulator, we represented our laboratory data through the artificial regulation of the temperature dependence of the relative permeabilities. With this, and the recognition of some secondary effects, a very good history match of wellhead rate and pressure responses was achieved. The work shows that the use of standard nitrogen permeabilities gravely over-estimates field performance in this type of reservoir.

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