Olefinic Hydrocarbon

Key Takeaways

• Olefin formation mechanisms in the subsurface and in drilling operations distinguish naturally occurring trace olefins from those generated by drilling-related contamination: naturally occurring olefins in crude oil are extremely rare and generally below detection limits (sub-ppm concentrations) because the reducing conditions, catalytic activity of clay minerals, and long geological timescales of petroleum maturation essentially convert all thermally generated olefinic intermediates to stable paraffinic or naphthenic molecules; when elevated olefin concentrations are detected in formation gas samples, mud gas logs, or produced fluid analyses, they are almost invariably artifacts of the drilling process, specifically from the thermal cracking of the lubricating components of the drilling fluid (synthetic base oils or olefin-based fluid loss additives in oil-based muds that decompose at bottomhole temperatures above approximately 200 degrees Celsius), from the volatilization of olefin-based synthetic lubricants used in the drill bit bearings, or from the thermal degradation of polymer-based additives (polyacrylates, polyacrylamides) in water-based muds that generate low-molecular-weight olefinic fragments at high temperatures; the discrimination between formation-sourced gas and contamination-sourced olefins in mud gas analysis is important because the presence of olefinic compounds in the gas chromatograph output can interfere with the interpretation of the hydrocarbon gas ratios (the Pixler plot and Haworth plot ratios used to classify gas shows as wet or dry) by adding artificial olefinic signal to what should be an exclusively paraffinic hydrocarbon gas mixture.
• Petrochemical significance of olefins as primary building-block molecules for the chemical industry makes olefin production one of the most important downstream processing functions of the petroleum refining complex, with ethylene, propylene, butenes, and butadiene being the highest-volume olefins produced globally: ethylene (annual global production approximately 200 million metric tons) is the feedstock for polyethylene (PE, the world's most produced plastic, used in packaging film, containers, pipes, and coatings), ethylene oxide (for ethylene glycol antifreeze and polyester fibers), vinyl chloride monomer (for PVC), and ethylbenzene (for polystyrene); propylene (annual global production approximately 130 million metric tons) is the feedstock for polypropylene (PP, the second most produced plastic, used in automotive parts, packaging, and textiles), acrylonitrile (for acrylic fibers and ABS plastic), cumene (for phenol and acetone), and propylene oxide (for polyurethanes); butenes are feedstocks for polyisobutylene, butyl rubber, and methyl tert-butyl ether (MTBE, a gasoline oxygenate); the steam cracker, which cracks ethane, propane, naphtha, or gas oil at 750-900 degrees Celsius in the presence of steam, is the primary production route for olefins in the petrochemical industry, and the growth in steam cracker capacity globally tracks directly with the growth in plastics and synthetic fiber demand.
• Olefin content as a drilling fluid quality indicator in oil-based mud (OBM) systems provides a measure of the thermal stability and degree of degradation of the synthetic base oil used in the mud formulation: high-quality synthetic base oils for OBM applications (isomerized and dewaxed paraffinic hydrocarbons, linear alpha olefins (LAOs), or internal olefin sulfonates) are selected for their low olefin content in the fresh fluid and their thermal stability at bottomhole temperatures up to 200 degrees Celsius; linear alpha olefins (LAOs) are a specific class of terminal olefins (with the double bond at the end of the carbon chain, alpha position) used directly as the base fluid in some OBM formulations because their low pour point, low viscosity, and good lubricity make them suitable base oils, and their environmental profile (more readily biodegradable than mineral oil) makes them preferable for offshore applications under strict discharge regulations; when an LAO-based OBM is exposed to high bottomhole temperatures over extended periods, isomerization (migration of the double bond from the terminal to internal positions) and other degradation reactions alter the olefin profile, and monitoring the olefin content and double bond position distribution in the mud by gas chromatography provides a thermal stability assessment of the fluid that predicts the remaining useful life of the OBM before performance degradation requires reformulation or replacement.
• Environmental regulations for olefin content in offshore drilling discharge govern which OBM base oils can be discharged with drill cuttings into the marine environment and which must be recovered and transported to shore for disposal: the North Sea countries (UK, Norway, Netherlands) and Gulf of Mexico regulate the discharge of OBM-contaminated cuttings based on the biodegradability and aquatic toxicity of the base oil adhering to the cuttings; linear alpha olefins (LAOs) and synthetic paraffins derived by Fischer-Tropsch or hydrocracking have favorable biodegradability profiles (>80% biodegradation in 28 days in OECD 306 seawater biodegradability tests) and low toxicity (LC50 above 1,000 ppm in marine organism tests), qualifying them for environmental discharge under the North Sea CHARM (Chemical Hazard and Risk Management) scheme; highly aromatic mineral oils and heavy mineral oils have poor biodegradability and higher aquatic toxicity, and their cuttings must be collected and transported to shore for treatment or incineration; the shift from mineral oil-based OBMs to synthetic olefin-based OBMs in North Sea deepwater drilling during the 1990s and 2000s was driven substantially by these discharge regulations, which made the environmental cost of mineral oil cuttings disposal a significant factor in the total well cost comparison.
• Gas chromatography analysis of olefins in hydrocarbon gas samples uses the specific elution order of olefin isomers relative to the corresponding paraffin isomers to identify the olefinic peaks in the chromatogram and quantify their concentration: ethylene (C2=) elutes just before ethane (C2) in a standard gas chromatography separation, propylene (C3=) elutes before propane (C3), and the butene isomers (1-butene, 2-butene, isobutylene) elute at various positions among the butane isomers (n-butane, isobutane) depending on the column temperature program; the presence of olefinic peaks at the expected elution positions in a mud gas chromatogram, when the olefin-to-paraffin ratios are higher than typical background contamination levels, indicates either severe thermal degradation of OBM components, thermal cracking of retained oil from a previous formation, or inadvertent olefin contamination of the mud from lubricant additions; the interpretation of these olefin peaks in the context of formation evaluation must account for the drilling contamination source before attributing the olefinic signal to geological hydrocarbon shows, which requires knowledge of the OBM type in use, the bottomhole temperature history, and the background olefin levels in the uncontaminated mud baseline sample taken at the start of the section.