The Midway-Sunset Field

Located in the southern San Joaquin Basin of California and discovered in 1894, the Midway-Sunset Field is one of the largest in the USA with a STOIIP of 4400 MMBO.

The Midway-Sunset Field is highly complex (Fig. 1), with hundreds of different pools with different trapping mechanisms, and contains compartments within the Miocene Monterey Formation turbidite reservoirs. Furthermore, the crude oil is heavy (average API gravity of 12°) and viscous (ranging from 11 cP at 120 °F to 7000 cP at 100 °F), adding to the complexity of this field. Initial oil production peaked in 1914 at 92,772 BOPD under primary recovery with EUR at the time forecast to be 1000 MMBO.

Initially in the 1940s and ‘50s, gas and water injection was attempted but due to the viscous nature of the oil these projects had limited impact. In the 1960s, thermal recovery techniques were implemented resulting in a four-fold increase in production that peaked in 1991 at 167,619 BOPD (Fig. 2) with EUR now thought to be 3478 MMBO, yielding an ultimate recovery factor of 79%.

Tertiary recovery started in 1960 with an in situ combustion project that ran until 1992 where it was estimated to have an incremental recovery factor of 23%. This was followed by the implementation of cyclic steam stimulation in 1963 (Fig. 3) as well as continuous steam injection. Within five years of its commencement, the steamflood had already achieved incremental recovery of 9.8%. Most thermal recovery projects have since been converted to steamflood which proved more economical and efficient than the in situ combustion projects.

Although the field was unitized with thermal recovery projects being small and fragmented, tertiary recovery was highly successful. The steeply dipping beds in some areas (up to 50-70° on the flanks of tight folds) helped increase the effectiveness of thermal recovery due to gravity drainage, and combined with very close well spacing (average of 2 ac), it yielded an extremely high ultimate recovery factor of 79%.

As of 2021 cumulative production was 3236 MMBO, equating to 73.5% of the STOIIP.

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