- A mathematical model has been developed on the principle of mass balance of the gas component for an oilwell (pumping or flowing) which enables one to calculate the annular gas volume, including gas bubbles in the oil column. The knowledge of annular gas volume at a given instant of time can then be used to find the pseudo length of both oil and gas columns and hence the bottomhole pressures. This technique can be used to obtain pressure buildup data. In addition the flowing bottomhole pressure prior to the shut-in can be determined in the same manner to allow calculation of productivity index (PI) and to evaluate pumping well performance. The method has distinct advantage over the conventional acoustic well sounder (AWS) technique which measures the actual gas-liquid interface in the annulus.