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Chapte
r 3 – P ressure Transient Analysis (PTA) -p114/743
3.I
More on QA/QC
3.I.1
Diagnosing phase segregation from differential pressure
Differential Pressure Analysis is an intelligent evaluation observed anomalies, and an attempt
to identify the fluid phases in the wellbore near the gauge sensing points. It gives a reasonable
idea of wellbore occurrences and the fluid distribution.
This only gives approximate results and it cannot be used for all types of wellbore phenomena.
Nevertheless it enhances the operational understanding and allows for an intelligent correction
to reservoir datum with more confidence. Such analysis will also enhance the confidence in the
material balance calculation and the identification of drainage patterns.
Practically it involves a choice of several recognizable events on the difference channel (blue
circles in the figure below) and transfers the different delta pressure values to a simple
spreadsheet. Estimation of fluid gradients is based on the known fluid densities and resulting
changes in the pressure gradient, that makes the total picture logical by applying a global
gauge offset.
Fig. 3.I.1 –
QAQC
plot with events marked
on difference channel
This method does not only reveal wellbore anomalies. It also determines the pressure gauge
offset, which will have to be within the range of the gauge manufacturer’s claimed accuracy
and resolution.