PTA is a Reservoir Engineering tool when we detect fluid volatility. Free gas caps, solution gas, volatile oil, and retrograde condensate gas all leave signature pressure buildup response patterns. Shut-in pressure humps may indicate phase redistribution, gas going back into solution, or a gas cap; while water hammer and adiabatic heating cause sharp spikes. Pressure dips immediately after shut-in may be indicative of volatile oil; but towards termination of a test may be indicative of a gas cap. Retrograde condensate and volatile oil pools display a perpetually dynamic, widening, near wellbore pressure drop due to increasing ‘skin’ (actually it’s decreasing relative permeability in the near wellbore region). Optimal depletion strategies and production operations, for both volatile oil and retrograde gas pools, depend on insitu bottom hole fluid samples and PVT analyses (Pressure-Volume-Temperature properties). So, having a welltest infer multi-phase issues on discovery is certainly a bonus!