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r 4 – R ate Transient Analysis (RTA)- p147/743
4.D.3.d
Flowing gas material balance plot and
P-Q diagnostic plot
Like the P/Z vs q plot, these methods, specific to gas, and not ‘adapted’ oil analyses methods,
are presented in the chapter ‘The right stuff’.
4.D.4
General major gas issues
4.D.4.a
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 a specialized program
(Amethyste) can be used for this.
The available correlation for gas, in Topaze, 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 Topaze is to include both the hydrostatic and friction pressure loss in
the model and correct the measured pressure to the sandface depth.
An important consequence is that the dependent skin attributed to the formation can be a lot
smaller because a large part is now attributed to the pressure loss through.
4.D.4.b
Gas material balance correction
The Rate Transient Analysis is performed on large duration data set and the material balance
in gas diffusion is a critical issue.
The diffusion equation can be considered linear for as long as the diffusion terms left outside
the time and pressure variables remain constant. Diffusion equation:
) (
0002637
.0
) (
2
pm
c
k
t
pm
t
As soon as we have a pressure gradient in the reservoir, the diffusion term, and especially the
product
c
t
, will become different from one reservoir block to the next.
If we look at a real gas simulation for a long term production survey and use it to match with
an analytical model where the diffusion was taken at initial pressure, we can see a divergence
between the simulated pressure and the measured data, even though the reservoir geometries
and the PVT used are strictly the same.
There are two ways to handle this problem in Rate Transient Analysis.