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Chapte
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Fig. 3.J.1 – Analysis at gauge level
Fig. 3.J.2 – Analysis after model correction to gauge level
You can observe that results (Pi and skin) are now returned at the sandface and that the rate
dependent skin attributed to the formation is now a lot smaller and should be attributed to the
pressure loss through, in this example, the 1000 ft of 1.5”ID tubing.
Once the sandface parameters have been returned by the model match, the sandface pressure
can be properly corrected to a common reservoir datum though the, hopefully appropriate,
knowledge of the fluid gradients in the reservoir.
3.J.3
Gas material balance
The issue of material balance in gas diffusion has become increasingly critical in recent years
with the spread of permanent gauges. The origin is the same as for changing wellbore storage.
Even with the use of pseudopressures, the diffusion equation can be considered linear for as
long as the diffusion terms left outside the time and pressure variables remain constant. This
time, we are not facing problems of changing wellbore storage, a process only linked to the
pressure in the wellbore, but to the whole diffusion process throughout the reservoir.
Diffusion equation:
) (
0002637
.0
) (
2
pm
c
k
t
pm
t