It is common practice in pumping oil wells to use acoustic well sounder instruments (#AWS) to obtain reservoir pressure buildup data without pulling rods and pump. With this method each pressure measurement is tied to a liquid level shot, which becomes the primary input signal. Sparse sample rates and noise (due to inaccuracy of the acoustic shot) are inherent problems. Both deficiencies can severely affect pressure derivative analysis techniques.
A number of historical AWS #welltests were reviewed, evaluating only surface casing pressures. Analytically, it was presumed that the period of active liquid movement, after shut-in, was a function of wellbore storage (afterflow). The analyzable portion of the buildup was, thus, after the liquid movement stabilized, in which case one final static liquid level measurement (AWS shot) should be sufficient.
The first plot shows conventional acoustic data while the second plot shows casing pressures only, adjusted to the sandface by the final AWS shot only. The semi-log straight-line slopes, thus permeability and p*, are virtually identical. This is because the liquid level is constant throughout the valid radial flow regime. Skin damage, however, will not be resolved correctly.