California Oil, Gas, and Groundwater Program
Publication: Conference and Public Meeting Abstracts or Presentations
Chemical and isotopic data for water and gas from hydrocarbon reservoirs are commonly used to determine whether these fluids are mixing with groundwater. Understanding the geochemical variability of oil-field water could improve identification of those mixing processes across a wide range of settings such as the diverse site-specific conditions in which California’s onshore oil fields exist. We define oil-field water as water associated with hydrocarbon production, such as native formation water, fluids used for enhanced recovery and well stimulation, and oil-field wastewater-injection fluids. As part of the California State Water Resources Control Board’s Regional Monitoring Program (RMP) of Water Quality in Areas of Oil and Gas Production, geochemical data were collected from 22 oil wells, 3 water sources intended for injection, and 2 disposal ponds in four oil fields – Fruitvale, Lost Hills, North Belridge, and South Belridge – for the dual purpose of building a library of oil-field fluid chemistry in the southern San Joaquin Valley, and to support field-specific investigations of fluid transport between oil and gas operations and protected groundwater.
Water isotopes indicate that Sierran recharge naturally flushed oil-bearing zones in the Fruitvale oil field on the east side of the valley. On the west side of the San Joaquin Valley (Lost Hills, North Belridge, and South Belridge fields), meteoric-water patterns were found in places where groundwater is used in enhanced oil recovery (EOR) injection and where surface water is used in hydraulic fracturing (HF). Recycled produced water from oil wells is also a dominant source of fluid for EOR, but its presence in oil-field water can be difficult to discern because of its chemical similarity to formation waters. The use of noble gases together with water isotopes and other chemical tracers is a promising approach for detecting injected water of various origins in oil-field water. Meteoric-water flushing, particularly on the east side, lowered concentrations of total dissolved solids, trace elements, and radium in oil-field water, and promoted biodegradation of dissolved organic carbon and hydrocarbon gases like propane. Data for the low molecular weight organic acid acetate and carbon isotopic values of dissolved inorganic carbon indicated biogenic methane production also occurred in some shallow oil zones, although methane signatures were still largely consistent with thermogenic (petroleum) sources. Natural diagenetic processes and water injection for EOR and HF produced diagnostic chemical and isotopic signals in some samples. Natural and human processes produced substantial variability in the geochemistry of oil-field water. This variability needs to be considered in evaluating potential mixing between oil-field waters and groundwater as part of the RMP.