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VA – GP - OA: Numerical Multiphase PTA
p 23/29
4. Condensate gas production
We now analyze multiphase effects in the case of condensate gas production. In this situation,
mass transfer exists between the two phases, since the heavy component initially present in
the rich gas can condensate during depletion, when the pressure drops below the dew point.
4.1. Test Case 4
Test description
The reservoir is circular, with a central well. No water phase is considered. The initial pressure
is Pi=5,000 psi. The porosity is
Φ
= 0.20 and the rock compressibility is cr=3e-6 psi
-1
.
The gas fluid is defined using the “Condensate (Dew point fluid)” option, using T=300 °F and
GOR = 3500 scf/stb at first stage (500 psi, 90 °F), with Pd=4500 psi. Gas gravity is 0.65. All
other data are kept at their default values. The resulting maximum liquid deposit is 7.9% at
3140 psi. The Bg curve is fitted with Bg=0.00451 cf/Scf at Pi, while the viscosity curve is fitted
at
g=0.053 cp at Pi. The production history is:
Production for 10,000 hr: Qg=10,000 Mscf/D.
Build-up for 10,000 hr.
The relative permeability curves are of power-law type with exponent 2, using Sorg =0.25,
Sgr=0.05, Krogmax=0.8 and Krgomax=0.5.
In order to investigate different levels of depletion, we ran about 20 simulations on this case,
varying both the reservoir radius R (from 5,000 ft to 50,000 ft) and the reservoir permeability
k (from 8 mD to 50 mD).
Results
Figure 33 presents the loglog obtained for mild depletion (R=50,000 ft, k=20 mD). The dew
point is reached around the well after 16hr of production. Large oscillations are visible as soon
as oil appears in the system, not only on the derivative, but also on the pressure curve. As the
depletion is limited, the oil deposit extends only in a few cell rings around the well.
Because the process leading to oscillations is the same as for black-oil, previous observations
made for black-oil remain true (figures 33 and 34):
reducing the gridding progression ratio reduces the oscillations level, while increasing their
frequency.
the Sor value also has a great influence on oscillations
Figure 33: Influence of grid refinement and Sor on oscillations (R=50,000 ft, k=20 mD)
0.01
0.1
1
10
100
1000
10000
Time [hr]
100
1000
K=20mD
K=20mD refined
K=20mD - Sor=0.