Multiple Controls on Petroleum Biodegradation and Impact on Oil Quality

Abstract

Abstract Biodegradation of oils in nature is important in reservoirs cooler than approximately 80° C. Oils from shallower, cooler reservoirs tend to be progressively more biodegraded than those in deeper, hotter reservoirs. Increasing levels of biodegradation generally cause a decline in oil quality, diminishing the producibility and value of the oil as API gravity and distillate yields decrease; and viscosity, sulfur, asphaltene, metals, vacuum residua, and total acid number increase. For a specific hydrocarbon system (similar source type and level of maturity), general trends exist for oil quality parameters versus present-day reservoir temperature < 80° C. However, additional controls on biodegradation may have significant effects, making pre-drill prediction of oil quality difficult. It has long been observed that fresh, oxygenated waters in contact with reservoir oil can cause extensive aerobic biodegradation. More recently it has been recognized that anaerobic sulfate-reducing and fermenting bacteria can also degrade petroleum. Highly saline formation waters may inhibit bacterial degradation and effectively shield oils from oil-quality deterioration. The timing of hydrocarbon charge(s) and the post-charge temperature history of the reservoir can have major effects on oil quality. Currently charging reservoirs may overwhelm the ability of bacteria to degrade the oil, resulting in better-than-anticipated oil quality. Fresh charge to reservoirs containing previously degraded oil will upgrade oil quality. Calibrated methods of oil quality risking, based on a detailed evaluation of reservoir charge and temperature history and local controls on biodegradation, need to be developed on a play and prospect basis. Introduction Biodegradation of hydrocarbons, and the resulting decline in oil quality, is common in reservoirs cooler than approximately 80° C. Petroleum biodegrading organisms have a specific order of preference for compounds that they remove from oils and gases. Progressive degradation of crude oil tends to remove saturated hydrocarbons first, concentrating heavy polar and asphaltene components in the residual oil. This leads to decreasing oil quality by lowering API gravity while increasing viscosity, sulfur, and metals contents. In addition to lowering reservoir recovery efficiencies, the economic value of the oil generally decreases with biodegradation due to a decrease in refinery distillate yields and increase in vacuum residua yields. Furthermore, biodegradation leads to the formation of naphthenic acid compounds, which increase the acidity of the oil (typically measured as Total Acid Number or TAN). Increased TAN may further reduce the value of the oil and may contribute to production and downstream handling problems such as corrosion and the formation of emulsions. Reservoir gas caps and solution gases also undergo biodegradation in cool reservoirs. C2+ gas components, particularly propane (C3) and n-butane (n-C4), are preferentially removed from natural gas, making biodegraded gases drier through enrichment of methane (C1). Most biodegrading organisms also generate carbon dioxide (CO2) as a by-product when they degrade hydrocarbons, increasing the CO2 content of solution gas or gas caps. Elevated CO2 contents can negatively impact development economics by necessitating the use of special steels to resist corrosion. Evaluating the decline in hydrocarbon quality associated with biodegradation has become critical in recent years as offshore drilling has progressed into deeper water depths. In many areas (e.g., offshore West Africa, Brazil, Mid-Norway, South Caspian, eastern Canada), reservoir targets in deep-to-ultradeep water are shallow and geothermal gradients low. These factors make oil quality a major risk as decreased recovery efficiency and oil value compound with higher deepwater operating costs to significantly impact economics, even on major discoveries.