 | This article possibly contains original research. (September 2017) |
Heavy oil production is a developing technology for extracting heavy oil in industrial quantities. Estimated reserves of heavy oil are over 6 trillion barrels, three times that of conventional oil and gas.
Factors that affect the difficulty of putting reserves into production include permeability, porosity, depth and pressure. The density and viscosity of the oil is the determining factor.[1] Density and viscosity determine the method of extraction.[2]
Oil viscosity varies with temperature and determines the ease of extraction; temperature can be controlled so that oil can be moved without employing additional techniques.[3] Density is more important for refiners since it represents the yield after distillation. However, no relationship links the two.[2]
Oil reservoirs exist at varying depths and temperatures. Although viscosity varies significantly with temperature, density is the standard in oilfield classification. Crude oil density is commonly expressed in degrees of American Petroleum Institute (API) gravity which are associated with specific gravity. The lower the API gravity, the denser the oil. The API gravity of liquid crude oil ranges from 4º for tar rich in bitumen to condensates that have an API gravity of 70º. Heavy oils are classified between ultra-heavy oils and light oils. They have API gravities ranging between 10º and 20º.[4]
Crude oil generated by petroleum source rocks has an API gravity of between 30º and 40º. Crude oil becomes heavy after considerable degradation, after entrapment and during devolatilization. Degradation occurs through chemical and biological processes when oil reservoirs become contaminated by bacteria through subsurface water.[5] The bacteria then break down some crude oil components into heavy components, making it more viscous. Water carries away low molecular weight hydrocarbons in solution form since they are more soluble. When crude oil is enclosed by a poor quality seal, lighter molecules separate and escape, leaving behind the heavier components through devolatilization.[6]
Heavy oils are commonly found in geologically young formations since they are shallow and have less efficient seals, providing the conditions for heavy oil formation.