Injection profiles

Another method, cross-correlation chromatography, was also reported for detection sensitivity enhancement (see Figure 7.12). Multiple injections were performed in a continuous but random sequence. A single point detection output was recorded. This output was correlated with the injection profile. This correlation enhanced the detection sensitivity due to the multiplex advantage [705]. 

Figure 7.3 Illustration of possible injection profiles, (a) ideal profile (theoretical) (b) ideal situation (practical) (c) practical profile (d) practical profile with tail cut off .

Another important aspect of acid gas injection is the calculation of the injection profile and ultimately the injection pressure. Regardless of other criteria in the design of the compressor, the ultimate factor is that the acid gas must arrive at the wellhead with sufficient pressure that it can be injected into the formation for disposal. 

Gases are compressible, that is, their density is a function of the temperature and the pressure. Thus when calculating the injection profile for a gas this fact must be accounted for. 

One of the major drawbacks to the current version of AGIProfile is that it does not independently determine the nature (i.e., the phase) of the acid gas. The user must tell the program what phase the user expects and then it is up to the user to verify this assumption. This is done by plotting the calculated profile on the phase envelope. This is also done to determine whether or not the injection profile intersects the phase envelope. 

There is a second effect of the presence of methane in the injection gas. Methane, being more volatile than acid gas, tends to broaden the phase envelope. That is, the dew point pressures are significantly increased. The consequence of this is that it is now easier for the injection profile to intercept the phase envelope. In practical terms this means that presence of methane in the gas will tend to increase the likelihood that the acid gas will vaporize in the injection well. 

In this section, a few existing injection wells will be examined. The calculated injection profiles are from AGIProfile. 

Figure 9.1 shows the injection profile and the phase envelope for this case. This is a relatively simple case since the fluid does not change phase in the wellbore. The calculated injection pressure is 8770 kPa (1272 psia), which is about 17% larger than the value of 7446 kPa (1080 psia) given by Lock (1997). 

Figure 9.1 The calculated injection profile for the Chevron West Pembina Well.

Using AGIProfile, the injection profile is calculated. The integration scheme has problems with this well indicating a potential problem - probably a phase change. 

At a temperature of about 11°C (52°F) the injection profile intersects the phase envelope. From this point to the surface, the fluid remains in two phases. In this self-regulating system, any heat transfer from the surroundings to the fluid will condense or evaporate the liquid and thus the pressure will return to the equilibrium value. 

The injection profile for this well is shown in figure 9.2. Notice how the injection profile intersects the phase envelope. 

Wang. S. and J.J. Carroll. 2006. Model calculates acid gas injection profiles. Oil Gas J. Sept. 4. 

Measure the effects of surfactant use on the water injection profile. 

At a concentration as low as 0.1% the surfactant substantially improved the injection profile, increasing the vertical uniformity of the distribution of water into the pay zone. This uniformity improvement continued during all subsequent tests (described below). 

Instead, the test well was returned to water injection in its rotation as part of the regular WAG production cycle. After about three times as much water had been injected as surfactant solution had been used, the injectivity and injection profile returned to their pretest behavior and the benefit of the surfactant was lost. 

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