This paper discusses the successful application of reverse osmosis membranes at the San Ardo Oil Field in California to desalinate produced water for beneficial re-use through surface discharge to the local groundwater.

The field produces heavy oil via a recovery process called steam flooding, in which steam is injected into the formation to heat up the crude and decrease its viscosity, thereby allowing the normally tar-like substance to flow by gravity to producing wells. This process typically results in the production of ten or more barrels of water for every barrel of oil recovered. Historically, a portion of this water has been recycled and softened to provide water for steam generation, with the remainder going to local EPA class II injection wells for disposal. However, the injection zone capacity is limited, which has constrained full field development. In October 2007, a desalination facility was commissioned to allow a portion of the produced water to be treated and discharged to the shallow fresh water aquifer, thereby providing an alternate outlet for produced water, and allowing field development to progress.

At the San Ardo site, the process is sized to treat 50,000 barrels of water per day, and consists of de-oiling followed by the OPUS™ technology, which consists of multiple treatment processes, including degasification, chemical and ion exchange softening, multi-media filtration, cartridge filtration, double-pass reverse osmosis, pH neutralization, and partial remineralization. The treated water is discharged to post-treatment constructed wetlands and aquifer recharge basins. This technology, portions of which were developed jointly and patented separately by Chevron U.S.A., Inc. and Veolia N.A. Water Systems, has proven to be a reliable and robust process for successfully treating produced water for surface discharge.

This paper details the treatment process, water quality specifications, and challenges faced during the design and operation of the facility.


The San Ardo Oil Field comprises ~2,500 acres of land adjacent to the Salinas River in Monterey County, California, approximately fifty miles north of San Luis Obispo (Figure 1). With an estimated ultimate oil recovery of 530 million barrels, it is the 13th-largest oil field in California (California Department of Conservation Annual Report, 2006). It was discovered in 1947 and is operated in two separate units by Chevron North American Exploration and Production (Chevron) and Aera Energy LLC, respectively.

The field is an anticlinal structure with two productive zones: the Aurignac and Lombardi Sands, respectively. These zones are part of the Monterey Formation, a sedimentary rock unit which underlies much of coastal California. In places, the sands are several hundred feet thick and contain abundant quantities of 11–13 degree API gravity crude oil (Wikipedia, 2009). The average formation depth ranges from 1,800 to 2,200 feet below ground level.

Steam flooding is used to recover oil from the field, a process in which injection wells force steam into the formation to heat up the crude and decrease its viscosity. Steam flooding has been used continuously in the Aurignac and Lombardi sands since the 1960s; however, oil production has been declining in recent years due to limited capacity for disposing of the produced water, which can range from 10 to 20 times the oil production rate. Produced water comes to the surface as part of the oil and gas extraction process. The challenge for producing the remaining heavy oil, particularly in the Lombardi formation, is to remove the excess water from the reservoir. Dewatering reduces the formation pressure, thereby allowing the injected steam to contact the remaining heavy oil for production. Dewatering, in essence, would allow the area of steam-enhanced production to be expanded.

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