PTA is a Reservoir Engineering tool when we infer drive mechanics. Gas caps reveal themselves in the pressure buildup because of a dramatic change in reservoir fluid viscosity (µ) — the gas/water interface manifests as a deviation in derivative trend. This can be mistaken as increasing transmissivity (kh/µ). Gas caps are important to identify because it may be better to defer depletion in favour of retaining down-dip hydrocarbon drive pressure. Down-dip water can be a coning issue in high permeability gas systems. Aquifer drive appears in the pressure derivative pattern as a dramatic upward spike, followed by pseudo steady-state, which can be confused with enclosing no-flow boundaries. Water drive is important to understand because of the real possibility of water coning.