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. Density and viscosity determine the method of extraction.
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. Density is more important for refiners since it represents the yield after distillation. However, no relationship links the two.
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.
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. 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.
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.
Terminology
Injection pattern
The injection pattern refers to the arrangement of the production and injector wells to the position, size, and orientation of flow of a reservoir. Injection patterns can vary over the well lifetime by moving the injection well to areas where maximum volume of contact can be achieved.
Geographical heterogeneity
Geological heterogeneity is the spatial distribution of porosity and permeability in a reservoir rock.
Permeability
Permeability depends on the size of the sediment grains that formed the rock and the manner in which they were packed. Permeability is the number of pores, and their interconnectedness in a rock and the existence of different layers in a rock with different permeability is a manifestation of geological heterogeneity. When steam injection takes place, water flows through the more permeable layers, bypassing the oil-rich less permeable layers. This causes low sweep efficiency and early water production with the volume of oil in contact with the water.
Sweep efficiency
Sweep efficiency is the measure of the effectiveness of an EOR method that depends on the total volume of the reservoir that the injected fluid contacts. Sweep efficiency is affected by multiple factors: mobility ratio, directional permeability, cumulative water injected, flood pattern, geological heterogeneity and distribution of pressure between injectors and producers.
Displacement efficiency
Displacement efficiency is the fraction of oil that is recovered from a zone that has been swept by a steam injection or any other displacement method. It is the percentage volume of oil that has been recovered through displacement by an injected fluid or displacing element injected into the reservoir. It is the difference between the volume of the reservoir before the displacement begins and the volume after the displacement has ended.
Amplitude versus offset
Amplitude Versus Offset (AVO) is a technique used in seismic inversion to forecast the existence of reservoirs and the rock types surrounding it. Literature reviews and studies incorporate the analysis of AVO and seismic inversion in oil exploration and rock physics studies.
