**4.3.2 Experimental procedures**

Before testing, firstly, oil and gas sample under normal temperature are transferred into the intermediate container and put the middle container in a thermostatic oven. Then the oven is being heated up to 60Ԩ for 24 hours in general. The pressure of oil and gas sample under high-temperature is increased to the testing pressure—20*MPa*. Meanwhile, the temperature and pressure of PVT cell is increased to the experimental temperature and pressure, and then, the height of plunger is recorded. Secondly, transfer the oil sample into PVT cell and record the height of plunger again when the oil sample becomes steady. The difference of the two recorded heights is the oil volume. Thirdly, transfer the gas sample into PVT cell from the top of PVT cell. During the transferring process, it is necessary to keep a low sample transfer rate so that it would not lead to convection. Record the height of plunger and liquid level once completing sample transfer. Fourthly, start the diffusion test and make a record of time, pressure and liquid level. If variation of pressure is less than 1 psi during an interval of 30 minutes, it means gas-oil have reached the diffusive equilibrium and the

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14

and testing method are both reasonable.

**4.4.2 Experimental analysis 4.4.2.1 Equilibrium time** 

16

18

Pressure(MPa)

20

22

of Gas-Oil System Under High Temperature and High Pressure 9

the testing P of CH4-oil experiment the calculating P of CH4-oil experiment the testing P of N2-oil experiment the calculating P of N2-oil experiment the testing P of CO2-oil experiment the calculating P of CO2-oil experiment

0 20 40 60 80 100 time(hour)

The diffusion coefficient is obtained by using established model to match the variation in pressure. Pressure matching is shown in fig.4. The matching result is fairly good. Normally, diffusion coefficient of gas in oil phase is most practical problem in engineering project; the diffusion coefficients of gas in oil phase of the three diffusion tests are shown in fig.5. Fig. 5 indicates that the diffusion coefficient, which increases with the decrease of pressure till the system reaches balance, is variable. The final calculated mole fraction of N2 in oil phase when in balance is 12.86%, testing value varies from 16.7464%—10.8767% in the different positions at the end of the experiment; For CH4-oil, the calculated result of CH4 is 35.34%, the testing value ranges from 34.3391% to 37.6201%; and for CO2-oil, the calculated result of CO2 is 67.262% and the testing value ranges from 66.6284% to 66.3558%. The calculated value of component is close to the actual tested ones, which shows the established model

The comparison of the equilibrium time of N2-oil, CO2-oil and CH4-oil system under the condition of 20MPa, 60Ԩ is displayed in Tab 4 which shows that the equilibrium time of CO2-oil system is obviously less than that of N2-oil and CH4-oil system, because the diffusion velocity of CO2-oil is higher than that of the other two gases. The equilibrium time of N2-oil is less than that of CH4-oil; however, it doesn't mean that the diffusion velocity of N2-oil is higher than CH4-oil. In fact the main reason is that the solubility of N2 in the oil is lower, and after a certain time, N2-oil has reached saturated at the testing temperature and pressure so it appears that the equilibrium time of N2 is less than that of CH4. Another reason is that dry gas is used in the experiment instead of CH4 and there are some heavy components, such as N2 and C3H8 in the dry gas, so the diffusion equilibrium time increases.

Fig. 4. Contrast of pressure variation of three groups of experiments

diffusion test is finished. And then, test the composition and density of oil phase and the composition of gas phase at different positions. Finally, wash the equipments with petroleum ether and nitrogen gas to prepare for the next experiment.

### **4.4 Experimental results and analysis 4.4.1 Experimental results**

The test results are shown in Tab 3 and Fig 4.

Tab 3 has shown that the property of upper oil is different from that of lower oil in a certain extent. The component concentration of C11+ and flash density of the oil at upper position (upper oil) are lower than those at lower position (lower oil), but GOR of upper oil is obviously higher than that of the lower oil. Comparing the oil property of the three groups of experiment, it is found that the CO2 concentration in oil phase and GOR in CO2–oil diffusion experiment is higher than those of the other two gases diffusion experiments when the gas-oil system reaches balance. It shows that the high diffusion velocity, strong dissolving power and extraction to heavy components of CO2 are the theory to explain the above phenomena.


Table 3. Comparision of oil component and composition at different position at the end of test

Fig4 has shown that system pressure drawdown curve due to diffusion displays that pressure is declining gradually with time. The pressure history curve of CO2-oil diffusion test lies below, CH4-oil lies middle, N2-oil lies above. Hence, we can see that different diffusion tests have different rates of pressure drawdown. It shows that the diffusion velocity of CO2 is the fastest, CH4 is slower and N2 is the slowest. For each group of diffusion experiment, the pressure drawdown is also different. The pressure drop of N2-oil is 1.14MPa, CH4-oil is 4.55*MPa* and CO2-oil is 3.9*MPa*.

diffusion test is finished. And then, test the composition and density of oil phase and the composition of gas phase at different positions. Finally, wash the equipments with

Tab 3 has shown that the property of upper oil is different from that of lower oil in a certain extent. The component concentration of C11+ and flash density of the oil at upper position (upper oil) are lower than those at lower position (lower oil), but GOR of upper oil is obviously higher than that of the lower oil. Comparing the oil property of the three groups of experiment, it is found that the CO2 concentration in oil phase and GOR in CO2–oil diffusion experiment is higher than those of the other two gases diffusion experiments when the gas-oil system reaches balance. It shows that the high diffusion velocity, strong dissolving power and extraction to heavy components of CO2 are the theory to explain the

component upper oil phase lower oil phase

CO2 —— 1.1115 66.6284 —— 0.7231 66.3558 N2 16.7464 0.8037 0.1354 10.8768 1.9091 0.0549 C1 0.0256 34.3391 2.8402 0.0711 30.6201 1.9226 C2 0.0052 0.7732 0.0231 0.0045 0.3081 0.0000 C3 0.0394 0.1065 0.0397 0.0279 0.0240 0.0245 iC4 0.1532 0.2481 0.1208 0.1084 0.1225 0.1035 nC4 0.1981 0.3724 0.1715 0.1594 0.2431 0.1499 iC5 0.4111 0.9540 0.4520 0.4545 0.4554 0.2850 nC5 0.3091 0.7560 0.3594 0.3594 0.5611 0.2056 C6 1.2669 5.6477 2.6848 1.6267 2.4097 0.8201 C7 1.9029 5.6401 2.2140 2.9228 3.3796 1.0394 C8 4.3693 7.1465 3.5759 5.7419 3.8080 2.1943 C9 3.4355 5.2515 2.1883 4.9054 2.7312 1.6908 C10 3.9898 4.6165 1.5017 4.5018 2.6389 1.9596 C11+ 67.1475 32.2331 17.0647 68.2393 50.0661 23.1940 GOR(m3/m3) 13.62 71.78 255 11.53 61 232.8

*<sup>o</sup>* (kg/m3) 822.6 821.9 825 823.8 822.9 830.2 Table 3. Comparision of oil component and composition at different position at the end of test Fig4 has shown that system pressure drawdown curve due to diffusion displays that pressure is declining gradually with time. The pressure history curve of CO2-oil diffusion test lies below, CH4-oil lies middle, N2-oil lies above. Hence, we can see that different diffusion tests have different rates of pressure drawdown. It shows that the diffusion velocity of CO2 is the fastest, CH4 is slower and N2 is the slowest. For each group of diffusion experiment, the pressure drawdown is also different. The pressure drop of N2-oil

is 1.14MPa, CH4-oil is 4.55*MPa* and CO2-oil is 3.9*MPa*.

N2 CH4 CO2 N2 CH4 CO2

petroleum ether and nitrogen gas to prepare for the next experiment.

**4.4 Experimental results and analysis** 

The test results are shown in Tab 3 and Fig 4.

**4.4.1 Experimental results** 

above phenomena.

Fig. 4. Contrast of pressure variation of three groups of experiments

The diffusion coefficient is obtained by using established model to match the variation in pressure. Pressure matching is shown in fig.4. The matching result is fairly good. Normally, diffusion coefficient of gas in oil phase is most practical problem in engineering project; the diffusion coefficients of gas in oil phase of the three diffusion tests are shown in fig.5. Fig. 5 indicates that the diffusion coefficient, which increases with the decrease of pressure till the system reaches balance, is variable. The final calculated mole fraction of N2 in oil phase when in balance is 12.86%, testing value varies from 16.7464%—10.8767% in the different positions at the end of the experiment; For CH4-oil, the calculated result of CH4 is 35.34%, the testing value ranges from 34.3391% to 37.6201%; and for CO2-oil, the calculated result of CO2 is 67.262% and the testing value ranges from 66.6284% to 66.3558%. The calculated value of component is close to the actual tested ones, which shows the established model and testing method are both reasonable.

### **4.4.2 Experimental analysis**

### **4.4.2.1 Equilibrium time**

The comparison of the equilibrium time of N2-oil, CO2-oil and CH4-oil system under the condition of 20MPa, 60Ԩ is displayed in Tab 4 which shows that the equilibrium time of CO2-oil system is obviously less than that of N2-oil and CH4-oil system, because the diffusion velocity of CO2-oil is higher than that of the other two gases. The equilibrium time of N2-oil is less than that of CH4-oil; however, it doesn't mean that the diffusion velocity of N2-oil is higher than CH4-oil. In fact the main reason is that the solubility of N2 in the oil is lower, and after a certain time, N2-oil has reached saturated at the testing temperature and pressure so it appears that the equilibrium time of N2 is less than that of CH4. Another reason is that dry gas is used in the experiment instead of CH4 and there are some heavy components, such as N2 and C3H8 in the dry gas, so the diffusion equilibrium time increases.

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Table 4. Balance time for different gas-oil systems

**4.4.2.2 Pressure comparison** 

of Gas-Oil System Under High Temperature and High Pressure 11

The diffusion experiments of CO2-dead oil have been conducted under the pressure of 1.36MPa, 0.8*MPa* and temperature of 20Ԩ. abroad and the final equilibrium time was 35 and 27 minutes respectively. Compared with our test at high temperature and pressure, there is a great difference. It shows that pressure, temperature and oil composition have a dramatic influence on diffusion velocity. For the actual case of reservoir gas injection, the accurate shutin time for the maximum oil recovery can be determined according to the testing results.

dissuasive gas experimental condition balance time, hour N2-oil 20*MPa*,60℃ 42 CH4-oil 20*MPa*,60℃ 91.5 CO2-oil 20*MPa,*60℃ 27.33

The comparison of pressure variation of the four diffusion experiments is shown in fig.6. It can be seen from fig.6, the pressure drop curve caused by the diffusion shows the pressure curve for CO2 lies in the bottom, CH4 lies in the middle, N2 lies at the top. From the first phase of each pressure history curve, we can see, speed differences of different gases' pressure drop are significant. Therefore, the CO2 diffusion rate is the fastest, CH4 is second and N2 is the slowest. Each diffusion experiment didn't have the same degree of pressure drop. The diffusion pressure drop of N2-oil diffusion was 1.14MPa, diffusion pressure drop of CH4-oil was 4.55MPa. CO2-crude oil reduced to 3.7MPa; CO2-crude oil diffusion pressure under the condition of 20MPa 80 Ԩ reduced to 3.9MPa. The equilibrium pressure of four experiments was 18.68MPa, 15.57MPa, 16.4MPa and 16.3MPa respectively. CO2-crude oil under the condition of 20MPa, 60 Ԩ, had a tendency of a period of diffusion pressure upward phase. From the two pressure curves of CO2-crude oil, we can see that temperature on the early diffusion of CO2 has some influence, the higher the temperature, the higher the rate of diffusion, but the final balance pressure has almost no difference. The shape of the pressure curves, except that of the pressure curve of CO2-crude oil under the condition of

20MPa, 60Ԩ has abnormal pressure trend, the other three are essentially the same.

Fig. 6. The comparison of pressure variation of four diffusion experiments

Fig. 5. Diffusion coefficient in liquid phase

The diffusion experiments of CO2-dead oil have been conducted under the pressure of 1.36MPa, 0.8*MPa* and temperature of 20Ԩ. abroad and the final equilibrium time was 35 and 27 minutes respectively. Compared with our test at high temperature and pressure, there is a great difference. It shows that pressure, temperature and oil composition have a dramatic influence on diffusion velocity. For the actual case of reservoir gas injection, the accurate shutin time for the maximum oil recovery can be determined according to the testing results.


Table 4. Balance time for different gas-oil systems

### **4.4.2.2 Pressure comparison**

10 Mass Transfer in Chemical Engineering Processes

0 10 20 30 40 50 time(hour)

0 20 40 60 80 100 time(hour)

0 5 10 15 20 25 30 time(hour)

(c)

the D of CO2 in oil phase

(b)

the D of CH4 in oil phase

(a)

the D of N2 in oil phase

5.540E-12

2.24E-12

1.850E-11

1.855E-11

1.860E-11

D(m2/s)

Fig. 5. Diffusion coefficient in liquid phase

1.865E-11

1.870E-11

2.26E-12

2.28E-12

D(m2/s)

2.30E-12

5.544E-12

5.548E-12

D(m2/s)

5.552E-12

5.556E-12

The comparison of pressure variation of the four diffusion experiments is shown in fig.6. It can be seen from fig.6, the pressure drop curve caused by the diffusion shows the pressure curve for CO2 lies in the bottom, CH4 lies in the middle, N2 lies at the top. From the first phase of each pressure history curve, we can see, speed differences of different gases' pressure drop are significant. Therefore, the CO2 diffusion rate is the fastest, CH4 is second and N2 is the slowest. Each diffusion experiment didn't have the same degree of pressure drop. The diffusion pressure drop of N2-oil diffusion was 1.14MPa, diffusion pressure drop of CH4-oil was 4.55MPa. CO2-crude oil reduced to 3.7MPa; CO2-crude oil diffusion pressure under the condition of 20MPa 80 Ԩ reduced to 3.9MPa. The equilibrium pressure of four experiments was 18.68MPa, 15.57MPa, 16.4MPa and 16.3MPa respectively. CO2-crude oil under the condition of 20MPa, 60 Ԩ, had a tendency of a period of diffusion pressure upward phase. From the two pressure curves of CO2-crude oil, we can see that temperature on the early diffusion of CO2 has some influence, the higher the temperature, the higher the rate of diffusion, but the final balance pressure has almost no difference. The shape of the pressure curves, except that of the pressure curve of CO2-crude oil under the condition of 20MPa, 60Ԩ has abnormal pressure trend, the other three are essentially the same.

Fig. 6. The comparison of pressure variation of four diffusion experiments

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diffusion coefficient in gas phase (final value)

Table 7. Diffusion coefficient of identical component in different systems

capacity is, the higher the molar concentration would be, whereas the lower.

ignored due to the small rate of change (<2%) under this experimental condition.

The gas injection is applied widely not only in oil-field, but also in condensate gas-field. Hence, further researches need to be done to make sure whether the diffusion phenomena of gas-gas and gas-volatile oil agree with the research result in this paper. The porous media has impact on the phase state of oil and gas, the diffusion in porous media should be the

According to literature review, there are two different opinions about the problem whether component concentration has an influence on diffusion coefficient or not at present. Some scholars think that there is an influence of component concentration on diffusion coefficient while others think that there is no influence. Taking the component of injected gas diffusing into liquid phase at 60Ԩ as an example, the relationship of content and diffusion coefficient is shown in fig7, 8 and 9. These figures show that diffusion coefficient of gas changes with the concentration variation of gas diffusing in the liquid phase. Compared with the initial values, the molar concentration changing level of N2,CH4,CO2 are 12.86%,34.087% and 67.262% respectively and the changing level of the diffusion coefficient of the three gases is 0.211%,1.88% and 0.934% respectively at the end of tests. The data above show that the rate of change of concentration differs from that of diffusion coefficient in different systems. N2 has the smallest rate of change while the rate of change of CH4 diffusion coefficient is the largest. Theoretically, the component concentration does have a certain impact on diffusion coefficient. But in engineering application, the impact on the diffusion coefficient can be

**4.4.2.5 Influence of molar concentration on diffusion coefficient** 

component

of Gas-Oil System Under High Temperature and High Pressure 13

N2 1.932E-11 8.281E-11 2.403E-10 5.555E-12 3.978E-12 1.082E-11 C1 1.944E-11 6.081E-11 2.690E-10 3.559E-12 2.287E-12 1.263E-11 CO2 —— 6.743E-11 2.723E-10 —— 3.985E-12 1.869E-11

Table 5 and Table 6 shows that the contents of intermediate hydrocarbon components in lower gas is higher than those in upper gas. The content of C11+ components in upper oil, density of single-off oil is lower than the latter, but the upper part of the oil phase gas-oil ratio was significantly higher than the lower oil phase. From the component data of different locations, we can see that the oil and gas properties are not the same, the concentration difference of C11+ components of N2, CH4, CO2 and CO2 (80Ԩ) between the upper and lower oil is respectively 10.8330%, 7.0842 % and 6.5924%, so during the phase calculation, we must consider physical heterogeneity which is caused by molecular diffusion and others of the oil and gas. From the content of the pseudo-component, we can also see that solubility in oil and extraction capacity of N2 are very low. Since the cause, the property of N2-oil experiment between upper and lower oil have little difference. Because of CH4 and CO2 have the higher solubility in the oil and powerful extraction capacity, the property between the upper and lower oil has great difference. In addition, the content of the diffusion gas are not the same, and their content of the same diffusion experiment in upper oil is higher than that in lower oil. For different experiments,CO2 gas diffusion experiments is the highest content of gas diffusion(66% -74%), which is followed by CH4 (34%-37%) and a minimum of N2 (10%-16%), the final molar concentration differences of the gas diffusion reflect the size of the gas diffusion capacity, the stronger the diffusion

N2-oil CH4-oil CO2-oil N2-oil CH4-oil CO2-oil

diffusion coefficient in oil phase (final value)
