Borehole: The hole as drilled by the drill bit.
Casing: Pipe cemented in the well to seal off formation fluids or keep the hole from caving in. Casing remains in the well as a permanent reinforcement after the drilling is complete.
Dry hole: A well that contains no oil or gas, or too little of either to make production economically viable.
Annulus: The space between (1) the casing and the wall of the borehole; (2) two strings of casing, and (3) tubing and casing.
Core: A cylindrical sample of a formation penetrated in a rotary drilling operation. Samples are examined to obtain geological information.
Permeability: The property of a rock formation that quantifies the flow of a fluid through the pore spaces and into the wellbore. A tight rock formation has low permeability and lower capacity to flow O&G. Wells in such formations typically require additional stimulation via fracking or other techniques.
Porosity: Refers to the pore space present in the underground formation that enables the rocks composing the formation to hold fluids.
Reservoir: The underground formation where O&G has accumulated. It consists of a porous rock to hold the oil or gas, and a cap rock that prevents its escape.