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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapte
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5 –
Wellbore models
OH – OSF – DV
5.A
Introduction
Until we are able to beam the fluid directly from the pore space into the ship cargo bay we will
need to use this route called the wellbore. Wellbore effects are seen very differently,
depending where you stand:
For the Pressure Transient Analysts anything related to the wellbore is a nuisance. Wellbore
effects will spoil the early part of the pressure response, and may even persist throughout
the whole test or shut-in survey.
So to the PT-Analyst Wellbore Effects = BAD.
Production Analysts are a little luckier, because they work on a time scale where transient
effects are not that important, and addressing wellbore effects amounts to connecting a lift
curve. In fact, playing with the lift curves and implementing ‘what if’ scenarios is part of
their jobs.
So to the Production Analyst Wellbore Effects = OK.
This Manichean split can be presented another way:
The steady-state component of wellbore effects is a key element of the well productivity. It
may be modeled using lift curves, or VLP curves, and this in turn requires flow correlations
that are present in both Production Logging and Well Performance Analysis, a.k.a. Nodal
Analysis™ (Trademark of Schlumberger).
Correction to datum may be either applied to the data in order to correct the real pressure
to sandface, or integrated in the model in order to simulate the pressure at gauge level.
Correction to datum and integration of VLP curves are detailed in the PTA (QA/QC) and the
Well Performance Analysis chapters of this book.
The transient component of wellbore effects often ruins the life of the PT-Analyst. The
action at the origin of a sequence of flow (opening and shut-in of a valve, change of a
choke) is occurring at a certain distance from the sandface, and any wellbore volume
between the operating point and the sandface acts as a cushion. This induces a delay
between what we want to see and what effectively occurs at the sandface.
In welltest operations, it is highly recommended to reduce this nuisance as much as
possible by means of downhole shut-in tools.
In Rate Transient Analysis it is not much of an issue, as transient wellbore effects occur at
a time scale of little interest for rate decline.
This chapter deals with the modeling of some of the simplest transient wellbore models, and is
mainly applicable to Pressure Transient Analysis only.