PTA is a Reservoir Engineering tool when we identify interference or communication or isobaric pressure boundaries. Interference can occur while frac’ing up-hole zones or nearby wells in dense drill spacing units (DSU). Communication due to production of offset wells can happen in high permeability zones. Drawdown for production causes an isobaric pressure gradient extending out to all external boundaries such that new wells drilled into existing pools may encounter less-than-virgin pressure. Pressure testing may reveal a radius-of-investigation (rinv) to the isobaric pressure wave. Interference within a wellbore during operations is not good: failed packer seat, bad casing or cement job, frac’d out-of-zone, or unconsolidated rock are all potential problems to identify.