Definitions
Chapter 3: Technical potential
Permeability
The ease with which a liquid or a gas flows through porous media. Measured in Darcys.
1 Darcy = 1 000 milliDarcys (mD).
Tight reservoir
A reservoir with low permeability, often defined by the following values:
- lower than 0.1 mD for a gas-filled reservoir
- lower than one mD for an oil-filled reservoir
By comparison, unconsolidated sand has a permeability of more than 5 000 mD. Concrete can have a permeability between 0.1 and one mD. Unconventional tight shale-gas reservoirs may have a permeability below one mD.
Conventional and unconventional petroleum resources
Unconventional petroleum resources is a collective term for oil and gas deposits which cannot be recovered commercially with conventional production wells and technology. This is usually because flow to the wells would be very low.
Chlorite
Chlorite is converted from iron minerals washed out in brackish water and then buried, and grows on quartz grains. It can hinder quartz cementation and thereby preserve permeability.
Quartz cementation
This can arise at depths below 4 000 metres, and reduces reservoir properties by filling pore spaces with quartz. That in turn means poorer flow properties for oil and gas.
Condensate blocking
When reservoir pressure falls below the dew point, condensate precipitates out and liquid may accumulate around the well. This in turn reduces the relative permeability of the gas, worsens flow properties in the reservoir, and may result in poorer well productivity.
HPHT
A high pressure, high temperature reservoir, often defined as a pore pressure gradient of 0.8 psi/ft and a downhole temperature greater than 149°C.
Porosity
Pore spaces as a percentage of the total volume of a rock.
Recovery factor
The relationship between the quantity of petroleum which can be recovered from a deposit and the amount originally in Place.