2013: Advent Oil & Gas: Components

by on December 7, 2013

Having spent millions and millions of dollars surveying, exploring, appraising, designing, and developing a site, what do we have to show for it?  This depends very much on the type of reservoir.  ‘Oil and gas’ isn’t just one category (or even two), but covers a very broad range of mixtures defined by their chemical components.

(Before we start – I’m an engineer, not a chemist.  This article contains simplifications and generalisations that would probably make a real chemist sad.  Ask Bunny if you want to read this stuff from someone who really knows what they’re talking about!)

From a commercial point of view, everything in oil or gas can be considered a hydrocarbon, an inert substance, or a contaminant.  In the vast majority of reserves, hydrocarbons make up the vast majority of the mass and volume.  This is just as well since they are the part we are most interested in.  Hydrocarbons are, as the name implies, compounds made entirely of carbon and hydrogen, which come in an astonishing variety of shapes and sizes because of carbon atoms’ tendency to bond together in chains and rings.

The more common type of hydrocarbons is the aliphatic group, consisting of ‘chains’ and most types of ‘rings’.  The simplest is methane, which is a gas has just one carbon atom and four carbon atoms.  As we add carbon atoms in a line we get ethane, butane, propane, and pentane, all of which are gasses at room temperature; six carbon atoms make up the backbone of hexane, which is a liquid.  These form a class of compounds called alkanes, which continue as you add more carbon atoms, forming increasingly viscous liquids and eventually solids (polythene is technically a very large alkane).  Variants on the alkanes are the cycloalkanes, which are simple loops; alkenes and alkynes, in which there are double and triple bonds respectively between carbon atoms; and branched chains.  Natural gas is primarily methane with some other light alkanes, while crude oil contains a wide mix of mostly heavier alkanes.

The less common type is the aromatic group.  These are defined by the presence of ‘rings’ in which each carbon atom shares one and a half bonds with each of the two adjacent to it.  Almost always, the rings are six-membered benzene rings, and these are often fused together to form polycyclic aromatic hydrocarbons or PAHs.  Aromatics, particularly PAHs, are often very toxic and constitute a serious threat to the environment – they are most common in coal tar and heavy oil but occur other fossil fuels, and can also be produced by partially burning almost anything organic (including wood, fat, and cigarettes).

Inert substances are generally less interesting since (by definition) they don’t do much chemically.  These are most important in natural gas, where the presence of large amounts of nitrogen often brings down the calorific value.  However, they can be useful: unreactive but valuable helium is extracted from natural gas.  Normally the proportion of inerts is kept within a specific range to control the energy content of the fuel.

Contaminants are any of a variety of nasty chemicals that cause problems in combustion.  The most important type in mineral oil and gas is sulphur compounds – especially hydrogen sulphide, which not only smells of rotten eggs but is extremely toxic and corrosive.  Oil and gas with a high concentration of sulphur is called ‘sour’.  Some biofuels, which I’ll discuss properly in another article, also suffer from contamination by siloxanes and their derivatives the silicones.  These silicon-based compounds are mostly an issue where gas is produced from waste, since siloxanes are used as additives in conditioner, cosmetics, and food (McDonalds uses a silicone oil called PDMS in some meals – no, really); when they are burned they form glassy silicate deposits that tend to wreck turbines, cookers, and other equipment.  All types of contaminant need to be carefully controlled in fuels.

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