Differentials should be created for both #welltest pressure and temperature traces, for quality control and data validation. Liquid shifts are most clearly exhibited by differential diagnostics. Drift is also evident from the differential. Every effort should be undertaken to identify an erroneous or malfunctioning pressure gauge.
The final test pressure should be taken immediately prior to purging the lubricator, since this operation causes a slight flow, effectively ending the buildup. Purging the lubricator at the end of the test is very much like purging production facilities: in a gas filled wellbore (#LNG) a downward spike occurs, but if liquids are present an upward spike may occur.
Sometimes a static gradient survey is conducted while running gauges into the well, before they are left on bottom. Pulling a static gradient survey, at the end of the buildup, while recovering the recorders, is known as a reverse gradient. While these practices save a bit of money and time, they often miss critical information on wellbore fluid composition. The fluid gradient between recorder run depth (RRD) and mid-point of perforations (MPP) remains unknown. This can result in only a partial liquid gradient being measured.
Conventional static gradient surveys are run to MPP, often to PBTD, and definitively measure wellbore fluid interfaces and gradients. Conventional gradients are the preferred method.
Initial and final tubing and casing pressures are important for welltest engineering. These measurements help with quality control and can suggest annular liquid levels or possible gas composition differences between tubing and annulus (i.e. air in the casing¾ a volatile situation which must be rectified before tie-in).