During the flow period permeability is inferred from the rate of influx of fluid (oil, water, mud, or gas) into the drill pipe. This is reported as the length of the recovered fluid in the pipe or the measured gas rate. The larger the length of the recovery the higher the inferred permeability. Care should be taken to note the total duration of the flow periods, since by implication we are attempting to derive a rate from an estimation of length. (i.e., 100m of oil recovered in 30 minutes implies a higher rate than 200 m of oil recovered in 90 minutes.)
The build-up data, collected while the well is shut-in, reflect the rate at which the pressure transients induced by the flow of fluid from the reservoir return to the original state. As fluid is removed from the formation, an incremental drop in pressure (DP) is induced. This induced pressure drop travels outward from the well decreasing in amplitude until DP is zero. The rate at which the pressure transients return to the well is reflected in the shape of the build-up curve and is proportional to the inferred permeability. i.e., the faster the build-up curve returns to reservoir pressure the higher is the inferred permeability.