The Midway Sunset oil field in the San Joaquin Valley of California is one of the United State's largest fields, having produced approximately three billion barrels of oil out of various reservoirs. Many of these reservoirs contain heavy oil, requiring recovery by steam to reduce the viscosity of the oil and allow it to flow. In steamflood operations, monitoring the steam's distribution and behavior is necessary for effective steam management and identification of bypassed oil. Cased-hole neutron logs are critical to this surveillance and are acquired throughout the life of the steamflood operation using cased observation wells. Each subsequent cased-hole neutron porosity is compared to the original open-hole neutron porosity to determine the increase in steam saturation and corresponding decrease in oil saturation that has taken place since the well was drilled. These logs are used to define current steam-oil contacts throughout the field, allowing reservoir teams to modify steam injection accordingly. There are many cased-hole neutron tools available, but they vary in tool characteristics, benefits and costs. Some tools can only be run in temperatures lower than 300 degrees Fahrenheit (degF), while steam chest temperatures can reach 350 degF or more. This constraint leaves fewer tool options for some wells, and the data from these tools has not always been reliable. To better define the quality, limits and value of this data, we test multiple hightemperature neutron tools, including both chemically sourced and electrically-sourced tools, by running them in a new well immediately after completion to compare them directly to the open-hole neutron log and to each other. We analyze the data quality, benefits and costs of the different tools and design a plan for our future surveillance.


The Midway Sunset oil field is located in Kern County at the southern end of the San Joaquin Valley in central California (Figure 1). It is one of the United State's largest oil fields by total production, having yielded approximately three billion barrels of oil since the beginning of its development in the late 1890s (Miller, 2008). It has produced out of many different reservoirs that represent various depositional environments, from alluvial fans to deep-water turbidites. These reservoirs contain oil from less than 10 to greater than 30 API gravity, with the majority being heavy-oil reservoirs that currently require secondary recovery methods to access remaining reserves. The Potter and Webster are two such reservoirs.

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