PTA is a Reserves tool when we model reservoir boundaries. Analysts often assume a single drilling spacing unit (DSU) for external boundaries and, thus, volumetric reserves. This is an acceptable standard for initial tests, which are usually (hopefully) infinite acting. Modelling to estimate reserves can be done two ways. First one must be able to history match welltest data. Pressure derivative pattern changes include several reserve defining characteristics which can be synthetically matched: no-flow boundaries like faults and channel edges, underlying water legs, active aquifers, gas caps, and channel sands all allude to volumetrics via the radius-of-investigation (rinv). A second method is to purposefully deviate the synthetic history match away from the data, forcing a pseudo steady-state curve, thus estimating a minimum volumetric reserve, again effectively based on the radius-of-investigation (rinv).