New natural gas composition

The aim of this section is to study the impact of the change of the raw natural gas composition. The motivation of this new study is based on the fact that for the previous natural gas (NG1) the rich gas cricondenbar was never a problem. Hence it was not used for the different optimisations.

1. Comparison with the previous natural gas composition

The offshore plant is the same as for the other natural gas (Figure 11) and the process parameters are set as in the base case model. It means that for the condensate stabilization unit the second level of pressure (P2) is equal to 9.50 bar, the third one (P3) to 2.56 bar and the heater temperature Th to 82°C.

The temperature after the coolers are equal to 30°C (T1, T2, Tf and Td). The flow rate of raw natural gas is still equal to 1 000 kmol/h.

The oil TVP specification is the same as before, 0.965 bar. However, the rich gas cricondenbar specification is reduced to 105 bar for this new natural gas.

The new composition of natural gas (NG2) is presented in appendix (Appendix 11: NG2 composition).

The following figure shows the comparison between NG1 and NG2 composition.

Figure 27: reservoir composition comparison 0

73 Chapter 8 – Further studies

The new natural gas is richer in methane and ethane. Since they are light hydrocarbons, the increase of their content in the reservoir increases the production of rich gas (by 497 kmol/h). Whereas for NG1, the production of gas and oil were approximately the same, rich gas production is predominant for NG2 (more than 95 % of the raw natural gas is converted into rich gas).

The following table introduces the main results of the simulation. It shows that by changing the composition of the gas keeping the same process parameters ¹⁸, TVP specification of crude oil is not met.

Table 27: results comparison between NG1 and NG2

2. Parametric studies

In this section, the influence of process parameters on the specifications are studied.

Note that for each parameter study, the other parameters are the ones of the base case model ¹⁸. Figures representing the following points can be found in appendix (Appendix 12: parametric studies with NG2 composition).

2.1 Condensate stabilization unit

The minimum heater temperature (Th) to meet the good TVP is 100.3°C (Figure 72).

It is not possible to find a value of P2 that meets the TVP (Figure 73). However, it is different for P3. Indeed, it must be selected below 2 bar to have an oil TVP in the good range of values (Figure 74).

In any case, changing a parameter of the condensate stabilization unit influences the cricondenbar. It remains around 102.5 bar.

2.2 Gas recompression train

TVP is not affected by the values of the temperature after the coolers in the gas recompression stages (T1 and T2). It is never below 0.965 bar (Figure 75 and Figure 76).

As for the condensate stabilization unit, rich gas cricondenbar is always around 102.5 bar.

18 With process parameters from the base case model: P2 = 9.5 bar, P3 = 2.56 bar, Th = 82°C, T1 = T2 = Tf = Td = 30°C

NG1 NG2

Gas production [kmol/h] 470 967

Oil production [kmol/h] 498 33

Rich gas cricondenbar [bar] 98.85 102.5

Oil TVP @30°C [bar] 0.9604 1.288

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2.3 Gas processing

In the gas processing part, Tf and Td cannot be selected to obtain a TVP within the good range of value if the other parameters are the same as in the base case model (Figure 77 and Figure 78).

However, these two parameters influence the cricondenbar. It is reduced when the temperatures are reduced. A too high Td leads to an off-spec rich gas.

2.4 Conclusion

On one hand, these studies show that the oil TVP is significantly influenced by Th, P3 and less significantly by Tf. On the other hand, rich gas cricondenbar is dictated by Tf and Td.

These studies enable to conclude that even with a natural gas with a larger proportion of light hydrocarbons, cricondenbar is never a problem. Indeed, in all the simulations which were carried out, when the oil TVP is met, cricondenbar is automatically met. As a result, cricondenbar specification cannot be used for optimisation.

3. More restrictive cricondenbar specification

In order to identify if the cricondenbar can be used for the optimisation of the plant, the specification is reduced to 100 bar. It should be noted that it can also be the result of a margin taken when sizing the plant.

Previous parametric studies indicate that the process parameters which dictates the value of the cricondenbar are Tf and Td. These two need to be reduced to reduce the cricondenbar (see appendix A12.3 Gas processing). The feed gas cooler temperature must be below 12.5°C ¹⁹ (Figure 77) or the temperature before dehydration below 25°C ²⁰ (Figure 78).

Process parameters must also be changed to meet the correct oil TVP. Graphical results can be found in appendix (Appendix 13: cricondenbar specification of 100 bar). The following sub-sections present these results.

3.1 Cricondenbar controlled by Tf

If Tf is equal to 12.5°C, cricondenbar is met but not the TVP. As introduced before, Th, P3 influence the value of TVP. Hence, parametric studies on these two parameters are carried out with the new value of Tf. In this case the minimum heater temperature is 94°C to meet the TVP ²¹ (Figure 79) and the maximum pressure P3 is 2.17 bar ²² (Figure 80).

19 P2 = 9.5 bar, P3 = 2.56 bar, Th = 82°C, T1 = T2 = Td = 30°C

20 P2 = 9.5 bar, P3 = 2.56 bar, Th = 82°C, T1 = T2 = Tf = 30°C

21 P2 = 9.5 bar, P3 = 2.56 bar, T1 = T2 = Td = 30°C, Tf = 12.5°C

22 P2 = 9.5 bar, Th = 82°C, T1 = T2 = Td = 30°C, Tf = 12.5°C

75 Chapter 8 – Further studies

3.2 Cricondenbar controlled by Td

Another possibility to control the rich gas cricondenbar is to change the value of the temperature before the dehydration unit. If Td is set equal to 25°C, TVP of oil is off-spec. In this case, Th must be above 99.1°C ²³ (Figure 81) or P3 below 2.04 bar ²⁴ (Figure 82). Changing Tf does not bring the oil within the good range of TVP values.

3.3 Cricondenbar controlled by Tf and Td

The last possibility to control the cricondenbar is to reduce both Tf and Td. Different couples of temperatures can be obtained.

3.4 Conclusion

Optimisation with a natural gas richer in light hydrocarbons and with a low value of cricondenbar specification can be done in two distinct steps. The first one is to obtain a correct cricondenbar. If it is too high, Tf and/or Td can be reduced. Then, if the oil TVP is not correct, adjustments on the relevant parameters can be achieved: increase Th, reduce P3 and/or reduce Tf (Figure 77).

23 P2 = 9.5 bar, P3 = 2.56 bar, T1 = T2 = Tf = 30°C, Td = 25°C

24 P2 = 9.5 bar, Th = 82°C, T1 = T2 = Tf = 30°C, Td = 25°C

76 Chapter 8 – Further studies