This section will mainly focus on the investigation of low rate effects on both models.
Since the injected gas amount is the fraction of the produced gas, injection rate will go down with a reduction in production rate. Production and injection rates of 1,000 and 750 Mscf/day were used. With the low rates, compositional effects will become more important since more condensate drops out before the gas-gas miscible displacement takes place. Consequently revaporization of retrograde condensate will also come into play as a recovery process with lower production as injection rates.
The difference between MBO and compositional models vanish as the reservoir is depleted with higher gas rates. However it is observed that there exists a maximum rate above which the performance of models does not get any closer.
For a gas condensate reservoir, with high production and injection rates, rich reservoir gas can be produced by a gas-gas miscible displacement before a significant liquid drop-out takes place and even if these rates are increased, the gas-gas miscible displacement will not improve any further and the amount of condensate in the reservoir will not change. So the match between the models will remain the same after that limit production and injection rates are reached.
MBO model gives a better representation of oil production rate and gas-oil ratio when it is initialized with Rv versus depth table. Oil production rate plot exhibits different performances for both models especially between 1,800 and 3,000 days.
This difference has also been reflected in gas-oil ratio values. Around 1800 days average condensate saturation in MBO model is about to reach its maximum value. This makes the oil production rates go down.
At 2400 days in MBO model, condensate saturation around the wellbore exceeds critical saturation, becomes mobile and flows. This changes the trend in decline for oil production rate. The first liquid drop-out in compositional model is observed around 1,900 days in the bottom layer. For the compositional model the condensate around the wellbore begins to flow around 3,000 days. After 3,000 days the models begin to produce the same amount of oil. The mobility of the condensate around the wellbore makes the differences get smaller. At 4,000 days condensate saturation in compositional model reaches its maximum value and the performance of the models depart slightly again.
Fig. 4.69 Average reservoir pressure for low production and injection rates 3000
3500 4000 4500 5000 5500 6000
0 1000 2000 3000 4000 5000 6000
Time, days
Pressure, psia
MBO w ith Rv vs Depth Compositional MBO Pd vs Depth
Fig. 4.70 Oil production rate for low production and injection rates
Fig. 4.71 Average oil saturation for low production and injection rates 0
20 40 60 80 100 120 140 160
0 1000 2000 3000 4000 5000 6000
Time, days
Production Rate, stb/day
MBO w ith Rv vs Depth Compositional MBO Pd vs Depth
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
0 1000 2000 3000 4000 5000 6000
Time, days
Saturation
MBO w ith Rv vs Depth Compositional MBO Pd vs Depth
Fig. 4.72 Gas-oil ratio for low production and injection rates
Fig. 4.73 Recovery factor for low production and injection rates 0
10 20 30 40 50
0 1000 2000 3000 4000 5000 6000
Time, days
GOR, Mscf/stb
MBO w ith Rv vs Depth Compositional MBO Pd vs Depth
0 0.1 0.2 0.3 0.4 0.5
0 1000 2000 3000 4000 5000 6000
Time, days
Recovery Factor
MBO w ith Rv vs Depth Compositional MBO Pd vs Depth
4.2.4 Effect of Completion
To investigate the effects of completion interval on the performance of models, initially the producer and injector were completed at the bottom part of the reservoir, which is layers 10 to 18, and later on the wells were completed at the top of the reservoir from layer 1 to 9.
The pressure distribution for compositional and black-oil models for the two cases mentioned aboveis very similaras can be seen in Figs. 4.74 and 4.75.
The mole fraction of the lighter components decreases with depth, while the mole fraction of the heavy components increases from the top to the bottom of the reservoir.
If the wells are completed at the bottom part of the reservoir, the production performance of the models differs from each other (Fig. 4.76, Fig. 4.80, Fig. 4.82) and if the completion location is in the region of lighter components the models exhibit similar production performances (Fig. 4.77, Fig. 4.81, Fig. 4.83).
In other words richest part of the gas is located at the bottom of the reservoir due to gravitational forces. When considering gas injection, one must be aware that the compositional effects, such as development of miscibility changes with depth. Also miscibility cannot be represented with black-oil models. More discrepancies are expected in the regions where highly miscible processes take place i.e. around the bottom completions.
In MBO model liquid content of the gas at the bottom of the reservoir is higher and this liquid holding capacity is a function of pressure.
Figs. 4.78 and 4.79 give field wide oil saturation distribution for two different completion strategies. When the bottom layers are completed initialization method can
make a big difference in results. For the particular fluid used in this study bottom layer completion resulted in high amounts of condensate accumulation initially in MBO and an unrealistic vaporization of all the condensed oil until the saturation becomes to the level given by compositional model.
The larger vaporization forms a richer gas phase flowing towards wellbore in black-oil model compared to the compositional model. This process is represented by higher gas-oil ratios in compositional model and higher gas-oil production rates in modified black-gas-oil model.
Higher recovery factors are obtained with the bottom completions. The deviation in the recovery of models starting from 1000 days with the bottom completions disappear as the excess condensate drop-out is vaporized.
Only 2 % deviation between compositional and MBO models is observed in recovery factors with the model completed in the upper layers. No difference observed for the bottom completion since less condensate is left in the bottom layers compared to the upper completion case in both models.
Fig. 4.74 Average field pressure with wells completed at the bottom of the model
Fig. 4.75 Average field pressure with wells completed at the top of the model 0
1000 2000 3000 4000 5000 6000 7000
0 1000 2000 3000 4000 5000 6000
Time, days
Pressure, psia
MBO w ith Rv vs Depth Compositional MBO w ith Pd vs Depth
0 1000 2000 3000 4000 5000 6000 7000
0 1000 2000 3000 4000 5000 6000
Time, days
Pressure, psia
MBO w ith Pd vs Depth MBO w ith Rv vs Depth Compositional
Fig. 4.76 Oil production rate with wells completed at the bottom of the model
Fig. 4.77 Oil production rate with wells completed at the top of the 0
100 200 300 400 500 600
0 1000 2000 3000 4000 5000 6000
Time, days
Production Rate, stb/day
MBO w ith Rv vs Depth Compositional MBO w ith Pd vs Depth
0 100 200 300 400
0 1000 2000 3000 4000 5000 6000
Time, days
Production Rate, stb/day
MBO w ith Pd vs Depth MBO w ith Rv vs Depth Compositional
Fig. 4.78 Average oil saturation with wells completed at the bottom of the model
Fig. 4.79 Average oil saturation with wells completed at the top of the model 0
0.02 0.04 0.06 0.08 0.1
0 1000 2000 3000 4000 5000 6000
Time, days
Saturation
MBO w ith Rv vs Depth Compositional MBO w ith Pd vs Depth
0 0.05 0.1 0.15 0.2
0 1000 2000 3000 4000 5000 6000
Time, days
Saturation
MBO w ith Pd vs Depth MBO w ith Rv vs Depth Compositional
Fig. 4.80 Gas-oil ratio with wells completed at the bottom of the model
Fig. 4.81 Gas-oil ratio with wells completed at the top of the model 0
50 100 150 200
0 1000 2000 3000 4000 5000 6000
Time, days
GOR, Mscf/stb
MBO w ith Rv vs Depth Compositional MBO w ith Pd vs Depth
0 20 40 60 80 100 120
0 1000 2000 3000 4000 5000 6000
Time, days
GOR, Mscf/stb
MBO w ith Pd vs Depth MBO w ith Rv vs Depth Compositional
Fig. 4.82 Recovery factor with wells completed at the bottom of the model
Fig. 4.83 Recovery factor with wells completed at the top of the model 0
0.2 0.4 0.6 0.8 1
0 1000 2000 3000 4000 5000 6000
Time, days
Recovery Factor
MBO w ith Rv vs Depth Compositional MBO w ith Pd vs Depth
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0 1000 2000 3000 4000 5000 6000
Time, days
Recovery Factor
MBO w ith Pd vs Depth MBO w ith Rv vs Depth Compositional
4.3 Horizontal Wells