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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapte
r 3 – P ressure Transient Analysis (PTA) -p119/743
3.J
The case of dry gas
3.J.1
Diffusion of real dry gas
As mentioned in the ‘Theory’ chapter, in order to extend the methodology of Dynamic Data
Analysis to gas cases, we introduce a function of the pressure called the pseudopressure. It is
given by:
Gas pseudopressure:
dp
Z
p
pm
p
0
2
Using the gas pseudo pressure instead of the pressure,
the diffusion equation remains valid
the same methodology presented above can apply.
In addition to these methods, the gas case presents few particularities presented here.
3.J.2
Correcting the pressure to sandface
Usually the pressure gauge is not set at the sandface due to mechanical constraints in the
completion. Because of this, the results obtained from the pressure transient analysis are at
the level of the gauge and not the sandface which, in terms of pressure (Pi) and skin (S), will
not reflect the true sandface conditions.
It is necessary first to define the vertical pressure profile in the well. The Saphir/Topaze
internal flow correlations or an actual lift curve generated by some specialized program (i.e.
Amethyste) can be used for this.
The available correlation for gas is an external lift curve or the internal Cullender & Smith
method, but with two modifications for handling water and condensate.
The correlation is based on the gas properties as defined in the PVT setup, and a general
friction factor is calculated using the Colebrook and White equation. Note that when you deal
with a condensate case with equivalent gas gravity and total rates, the proper gradient and
rates are used in the correlation to account for the condensate presence. The presence of
water can be accounted for, based on a constant water to gas production ratio.
The solution selected in Saphir is to include both the hydrostatic and friction pressure loss in
the model and correct the generated model response to the actual gauge depth, and then to
return all the results at the sandface.
In fact, the gauge pressure is not transformed, nor corrected hence it is the model that is
brought to the level of the gauge.