2 Migration & trapping
Are there lakes of crude oil and natural gas underground? Where is it stored?
Porosity and permeability are two of the primary factors that control the movement and storage of fluids in rocks and sediments. Porosity is the ratio of the volume of openings (voids) to the total volume of material. Porosity represents the storage capacity of the geologic material. Porous rocks, such as sandstone and limestone, contain tiny spaces, called pores, that can hold natural gas, water and crude oil, like a sponge. Non-porous rocks, such as granite, do not have pore spaces. After crude oil and natural gas are formed, they tend to migrate upward through the rock layers due to pressure within the earth. Both liquids and gases move based on their properties and the natural flow of fluids from areas of higher to lower pressure.
The ability of liquids and gases to move through these spaces is called permeability. The permeability of a rock is a measure of the resistance to the flow of a fluid or gases through a rock. If it takes a lot of pressure to squeeze fluid through a rock, that rock has low permeability. If fluid passes through the rock easily, it has high permeability. Recently, an increasing amount of U.S. gas production is coming from shale. Shale, a source rock, is generally more porous than limestone, but has extremely low permeability and traditionally it has been a poor producer of hydrocarbons. Evolving well stimulation techniques have rendered low-permeability shale reservoirs more permeable.
Permeability is measured in millidarcies. The permeabilities normally encountered in reservoir rocks range from less than ten millidarcies in low porosity sandstones, to about ten thousand millidarcies in fractured rock. There are some general trends of increasing permeability with increasing porosity. If the sand grains are large, then the pore diameters are large and the permeability is high. Smaller pores mean larger surface areas around them, and therefore more resistance to flow (lower permeability). Different fluids have different viscosities. For example, if gas and oil have equal relative permeabilities, more gas than oil will flow within the rock because of the dramatic difference in viscosity.