Summary

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; in addition, viscosity, sulfur, asphaltene, metals, vacuum residua, and total acid numbers increase. For a specific hydrocarbon system (similar source type and level of maturity), general trends exist for oil-quality parameters vs. present-day reservoir temperatures of <80°C. However, other controls on biodegradation may also have significant effects, making predrill prediction of oil quality difficult in some areas.

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 also can 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. Reservoirs undergoing current charging with hydrocarbons 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 owing to a decrease in refinery distillate yields and an 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 the enrichment of methane (C1). Most biodegrading organisms also generate carbon dioxide (CO2) as a byproduct when they degrade hydrocarbons, increasing the CO2 content of solution gas or gas caps. Elevated CO2 contents can impact development economics negatively 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 are low. These factors make oil quality a major risk because decreased recovery efficiency and oil value compound with higher deepwater operating costs to significantly impact economics, even on major discoveries.

Risking oil quality predrill in shallow compartments is a major challenge. Reservoir temperature and the consequent level of biodegradation must be estimated. To do this, knowledge of the primary (generative), undegraded oil composition is essential. The major controls on primary oil composition are characteristics of the source rock:

  1. organic-matter type,

  2. depositional environment, and

  3. level of maturity.

These controls exert the dominant influence on as-generated gravity, viscosity, gas/oil ratio (GOR), sulfur, and residua contents. Geochemical analyses of natural oil seepage on the sea bottom often provide direct evidence for evaluation of source-rock characteristics (sedimentary facies) and maturity. Thermal modeling of hydrocarbon generation timing and mapping of source rocks from seismic also contribute to the understanding of the hydrocarbon system.

After emplacement in cool reservoirs, hydrocarbons are subject to biodegradation, in addition to a number of other potential alteration processes, including water washing, phase separation, gravity segregation, and de-asphalting. Some reservoirs have a complex history with multiple episodes of charge and degradation. Fresh charge to a reservoir may upgrade quality, while earlier episodes of severe degradation (possibly when the reservoir was shallower and cooler than at present) may downgrade quality. Thus, knowledge of generation and charge timing and reservoir temperature history can help improve predrill predictions of oil quality.

Oil and Gas Quality

Oil and gas quality reflects the compositional characteristics of hydrocarbons that impact the economic viability of an exploration, development, or production opportunity. Compositions may affect the direct value of the product (e.g., crude valuation relative to a benchmark oil) or the development or facility costs (e.g., additional wells required, emulsion processing, use of special steels), or they may even cause the oil to be unrecoverable. Typical oil-quality properties include API gravity, viscosity, sulfur, asphaltene, and metals (e.g., vanadium, nickel, and iron content), residua (e.g., vacuum residua or Conradson carbon content), acidity (TAN), wax content or pour point, and sensitivity to emulsion formation upon production. Biodegradation impacts essentially all oil-quality properties.

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