The oilfield unit is the millidarcy. A darcy is a large value. Conventional reservoir rock is commonly tens or hundreds of millidarcies; tight rock is a small fraction of a millidarcy; an unconsolidated sand can be several darcies.
Absolute permeability is the single-phase value, measured with one fluid in the pores. Effective permeability is what one phase gets when other phases occupy part of the pore space. Relative permeability is that effective value divided by a reference permeability. A high porosity does not guarantee a high permeability.