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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapte
r 3 – P ressure Transient Analysis (PTA) -p116/743
3.I.2
Impact on the diagnostic
The impact on the Pressure Transient Analysis can be substantial, as illustrated below. The
upper gauge is affected by phase segregation and the shape of the derivative is clearly
distorted, resulting in the wrong choice of interpretation model.
Fig. 3.I.2 – Lower gauge; homogeneous
Fig. 3.I.3 – Upper gauge; double porosity
3.I.3
Gauge drift
Gauge drift is caused by unstable electronic components and fatigue of the sensing material
used in the instruments. Strain gauges are particular susceptible to this problem.
Drift during a relatively short well test is uncommon, especially today as the quality of
electronic gauges has increased immensely. However, it still does happen, and a severe drift
will lead to a wrong PTA diagnostic.
The drift problem is more common during long term measurements and can therefore be a
real problem in permanent downhole gauges (PDG).
In any case it is important to check the data for validity through the QA/QC procedures
described in this document before attempting any serious analysis. To identify a drifting gauge
it is necessary to have two or more measurements and study the difference channel
constructed between a reference gauge and all the other gauges.
The following figures illustrate the difference channel and the impact on the analysis of the
drifting gauge id we did not know that such drift exists.
0.01
0.1
1
10
100
1000
0.01
0.1
1
10
100
1000