**4.4.2.3 Composition changes**

The C2-C6 hydrocarbon compositions of four group of experiments are shown in Table 5, the comparison of oil phase composition is shown in Table 6.



Table 5. C2—C6 content contrast of gas phase

Table 6. Oil content contrast of oil phase

### **4.4.2.4 Influence of system on diffusion coefficient**

The calculated results of diffusion coefficient show that the diffusion coefficients of a certain component in different systems are not the same under the same temperature and pressure. Taking the injected gas for an example, as shown in Tab7, diffusion coefficient of each component of gas and liquid phase in the CO2-oil system is higher than that of N2-oil and CH4 oil system, which is consistent with the diffusion phenomenon observed within the experiment. In the same system, diffusion coefficients of the identical component in different phases are not the same. The diffusion coefficient of gas phase is higher than that of liquid phase. For the phenomena above, there are two reasons, one is interaction between components; the other is the influence caused by the system's state. Molecular motion in gas phase is quicker than that in liquid phase, so diffusive velocity in gas phase is faster.

The C2-C6 hydrocarbon compositions of four group of experiments are shown in Table 5, the

experiment upper gas,% lower gas,% remark

CH4—oil 1.4974 5.5255 20MPa,60Ԩ

CO2—oil 0.9445 1.7420 20MPa,80Ԩ

upper oil lower oil

(80Ԩ) N2 CH4 CO2

CO2 (80 Ԩ)

CO2

CO2 1.1115 74.6707 66.6284 0.7231 66.3558 66.5355 N2 16.7464 0.8037 0.0606 0.1354 10.8768 1.9091 0.0549 0.0564 C1 0.0256 34.3391 2.8120 2.8402 0.0711 37.6201 1.9226 1.8397 C2 0.0052 0.7732 0.0000 0.0231 0.0045 0.3081 0.0000 0.0000 C3 0.0394 0.1065 0.0252 0.0397 0.0279 0.0240 0.0245 0.0229 iC4 0.1532 0.2481 0.1155 0.1208 0.1084 0.1225 0.1035 0.1274 nC4 0.1981 0.3724 0.1666 0.1715 0.1594 0.2431 0.1499 0.1856 iC5 0.4111 0.9540 0.3145 0.4520 0.4545 0.4554 0.2850 0.3851 nC5 0.3091 0.7560 0.2260 0.3594 0.3594 0.5611 0.2056 0.2813 C6 1.2669 5.6477 0.7177 2.6848 1.6267 2.4097 0.8201 0.7089 C7 1.9029 5.6401 0.7219 2.2140 2.9228 3.3796 1.0394 0.8206 C8 4.3693 7.1465 1.5241 3.5759 5.7419 3.8080 2.1943 1.9411 C9 3.4355 5.2515 1.1743 2.1883 4.9054 2.7312 1.6908 1.5711 C10 3.9898 4.6165 1.3611 1.5017 4.5018 2.6389 1.9596 1.8674 C11+ 67.1475 32.2331 16.1098 17.0647 68.2393 43.0661 23.1940 23.6572 GOR(m3/m3) 13.62 71.78 363.2 255 11.53 61 232.8 208.2

*<sup>o</sup>* (kg/m3) 822.6 821.9 827.7 825 823.8 822.9 830.2 831.4

The calculated results of diffusion coefficient show that the diffusion coefficients of a certain component in different systems are not the same under the same temperature and pressure. Taking the injected gas for an example, as shown in Tab7, diffusion coefficient of each component of gas and liquid phase in the CO2-oil system is higher than that of N2-oil and CH4 oil system, which is consistent with the diffusion phenomenon observed within the experiment. In the same system, diffusion coefficients of the identical component in different phases are not the same. The diffusion coefficient of gas phase is higher than that of liquid phase. For the phenomena above, there are two reasons, one is interaction between components; the other is the influence caused by the system's state. Molecular motion in gas

phase is quicker than that in liquid phase, so diffusive velocity in gas phase is faster.

**4.4.2.3 Composition changes** 

composition

comparison of oil phase composition is shown in Table 6.

Table 5. C2—C6 content contrast of gas phase

Table 6. Oil content contrast of oil phase

**4.4.2.4 Influence of system on diffusion coefficient** 

N2—oil 0.3142 0.4740

CO2—oil 1.1392 1.1524

N2 CH4 CO2


Table 7. Diffusion coefficient of identical component in different systems

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