A mixture of hydrocarbons and varying quantities of non-hydrocarbons that exist in the gaseous phase or in solution with crude oil in underground reservoirs.
This is a subcategory of petroleum that is naturally occurring complex mixture of hydrocarbons. It may contain a minor amount of inorganic compounds not all of which may be combustible and able to fetch a price on the market.
Gas in tight sands and shales, coal bed methane, gas in geo-pressured reservoirs and gas from gas hydrates.
It can be either associated or non-associated. Associated gas and dissolved gases are both found with crude oil. Dissolved gas is that which is dissolved in the crude. Associated is free gas in contact with crude oil. All crude oil reservoirs contain dissolved gas and may or may not contain associated gas. Non-associated gas is found in a reservoir that contains a minimal quantity of crude oil.
Gas in tight sands occur in formations of permeabilities 0.001 to 1.0md. Gas hydrates are snow like solids in which each water molecule forms hydrogen bonds with 4 of the nearest water molecules to build a crystalline lattice structure that traps gas molecules in its cavities.
It has a phase diagram that is smaller than that of oils with a critical point further down the left side of the envelope. They are the result of the gases containing fewer heavy hydrocarbons than the oils. The critical temperature is less than the reservoir temperature and a cricondentherm greater than reservoir temperature. Initially they are totally gas in the reservoir but as reservoir pressure decreases the gas exhibits a dew point. As the pressure is reduced liquid condenses to form free liquid in the reservoir. This will normally NOT flow and can’t be produced.
Predominantly smaller molecules will lie below reservoir temperature. They exist solely as a gas in the reservoir, throughout the reduction of reservoir pressure. No liquid is formed in the reservoir. The separator conditions lie within the phase envelope as such liquid is formed on the surface.
Primarily methane with some intermediates. No liquid is formed in the reservoir and at the surface. They are the easiest to deal with because no liquid condenses as the gas comes to the surface. Surface separation and densities equal that of the reservoir.