Underground energy and waste storage require repurposing of existing oil and gas wells for gas storage, compressed air, hydrogen, methane, and CO₂ disposal, requiring an impermeable cap rock (e.g. shales) over the porous reservoir. We measured shale permeability over a range of burial pressures and gas pore pressures. Permeability decreases markedly as effective pressure on the rocks is increased. Knowing these relationships is essential to the safe design of engineered gas reservoirs.