Phase Behavior of Highly Waxy Gas Condensate Systems

As in other presentations23,24, the JEFRI PVT

system was used in this work. The apparatus is a mercury-free,

high-pressure windowed PVT system equipped with video

camera-monitor-recorder facilities and computer-controlled

pumps. The maximum working pressure of PVT cell is

103.41MPa, the maximum operating temperature 180.0C and

the maximum volume 100cm3. The windows on the front and

back of PVT system are allowed to observe visually the phase

conditions in the cell. The special design of piston in the cell

aims at measuring accurately the small volume of liquid phase.

The system is calibrated and inspected routinely by the

Authoritative Management.

The laboratory fluids are all recombined with the same

separator gas/liquid according to the producing GOR

encountered in one gas-condensate field in China.

Table 1 shows the physical properties of the separator

condensate. The condensate does not contain resins and

asphaltene, but the content of wax is very high, even up to

14%. The freezing point of the condensate may be high up to

14C which is always higher than the separator temperature,

therefore, to keep the temperature of separator above the

freezing point is very important for sampling the condensate.

The original reservoir pressure and temperature are

56.0MPa and 138C, respectively. According to the GOR

range of the producing wells, four gas condensate fluids are

recombined with the same separator gas and liquid. Table 2

shows the GORs and compositions of the recombined fluids.

The GORs are 1367.4, 900, 800, 750m3/m3, respectively, the

corresponding C7+ content are 6.44, 9.22, 10.18, 10.73

mol.%. These values mean that these fluids are rich and nearcritical

gas condensate systems.

Each fluid sample is first transferred into the

PVT cell and pressurized up to the original reservoir pressure.

A series of isothermal constant composition expansion (CCE)

processes are conducted. At a given temperature, the onset of

dew- or bubble- point was judged visually through the window

of the PVT cell, and determined by the first appearance of a

liquid drop at the bottom / gas bubble at the top of the cell.

The equilibria in the CCE processes in this work are

achieved through gradual and progressive depressurization at

very small interval instead of the usual agitation of rocking the

cell. The good reproducible results are obtained in the

laboratory to check the reliability of this method.

In following sections, we will give the experimental results

of four fluids and discuss the characteristics of the phase

behavior with the changes of GOR and heavy hydrocarbon

contents.

. This fluid is subjected to eight

different isothermal CCE processes at temperature ranging

from 25.0 to 138C. The fluid exhibit retrograde dew-point

behavior at all test temperatures. The measured percentage

volume of liquid dropout is presented in figure 1. The results

shows that the dewpoint pressure decreases with increasing

temperature and the retrograde curves almost seems to be

symmetrical with respect to the pressure at which maximum

liquid condensed out of the vapor phase.

Figure 2 demonstrates the phase diagram of this fluid by

cross-plotting the curves shown in figure 1. It is seen that the

quality lines are in parallel to the dewpoint locus and does not

exhibit a converging behavior as reported normally in the open

literature. The dewpoint curve and the quality lines are

terminated due to the appearance of wax. The complicated

phase behavior in the course of isothermal pressure decline at

22.0C is colorfully shown in figure 3. Even the pressure

increased to 70MPa, a large amount of wax deposited in PVT

the cell, that is, a whole system exhibited gas-solid two-phase

equilibria condition. The upper gas shows bright yellow color,

the lower solid shows black-gray color. Surprisingly, when a

litter more pressure declined, the upper gas exhibited the

normal retrograde condensation performance, implying more

liquid accumulation. At the same time, the solid gradually

disappeared when more liquid condensed. At lower pressure,

the system contained in the PVT cell shows a layering

behavior. The observed behavior of wax may be consistent

with the retrograde condensation of wax as reported by

Nichita, Goual and Firoozabadi, however, the safety

consideration due to extremely high pressure did not allow us

to observe the increasing stage of wax deposition during first

depressurization.

. CCE measurements are

conducted at five different temperatures. During the five CCE

processes, the fluid exhibits retrograde dewpoint behavior.

The liquid dropout curves are shown in figure 4. It is seen

from figure 4 that the dewpoint pressure increases with

decreasing temperature. It should be noted that solid phase

was observed at temperatures of 30.0C and 18.2C and at

high pressure. However, retrograde dewpoint phase change

occurred in the presence of solid phase, thus the retrograde

curve at 30.0C seems to cross the curve at 61.0C.

Comparison with the results of fluid 1 shows that wax

deposition temperature is higher for the lower GOR fluid.

Figure 5 presents the phase diagram of fluid 2. The phase

diagram clearly demonstrates five regions where single phase

or different multi-phases occurred.

. Three isothermal CCE processes

are performed for this fluid. At one temperature the fluid

exhibits bubble-point behavior and at other two temperature

the fluid exhibits dew-point behavior. Thus the critical point

must lie in the range between 50.0C and 62.0C. The

percentage liquid volumes are plotted in figure 6 and the

subsequent phase diagram is shown in figure 7.

. This fluid is subjected to eleven

different isothermal CCE processes at temperature ranging

from 55.0 to 138C. Among them, the fluid exhibits bubblepoint

behavior at five temperatures and exhibits dew-point

behavior at other six temperatures. The liquid volume percent

is shown in figure 8, for clarity it is also enlarged in figure 9. It

can be determined from figures 8 and 9 that the critical point

of this fluid must locate in the narrow range of 68.5C to

70.0C.

Figure 11 shows

the phase envelope locus of the four fluids. It is found out that

the biphase regions expand with the increase in GOR. Herzog25

demonstrated that the two-phase region inside the phase

envelop of the produced gas will become smaller with

reservoir depletion in his figure 4. Thomas et al.26 suggested

that lean gas injection into the reservoir will results in a

suppression in the two-phase region as elucidated in their

figure 4. However, it can be concluded from this study that the

two-phase region may first expand in the process of gas

injection.

Also, the changes of bubble-point and dewpoint pressures

with increasing GOR are plotted in figure 12 which shows that

the saturation pressure increases dramatically with the increase

in GOR when GOR is less that about 900m3/m3, however,

increase gradually when GOR is above about 900m3/m3.

As

the same separator gas and liquid samples are used to

recombine the above four fluids, we can further investigate

effect of heavy hydrocarbons on the dewpoint and bubblepoint

pressure. The lower GOR fluid contains higher content of

heavy hydrocarbons. Figure 13 clearly shows the influence of

the amounts of C7+ fractions on the saturation pressure, that

is, the increase in C7+ content lead to decrease in saturation

pressure, which is in contrast with the literature reports9, 28,29.

The liquid condensed in the surface separators is usually

light-colored, brown, orange, greenish, water-white or

colorless. This field experience can help the engineers

conveniently classify the reservoir fluid as retrograde gas,

however, this empirical method sometimes lead to erroneous

judgments. In fact, black tank oil may occurred as gaseous

phase in the reservoir condition30-31, which means that the

heavy hydrocarbon fractions may have a more complicated

effect on the phase behavior of the subsurface fluids than the

expected.

Although the

dewpoint is usually judged from the emergence of the first

liquid droplet, we observed in our tests that the phase change

first occurred at the top of the PVT cell in the form of fluid

color turning into black as shown in Fig.14. This phenomenon

may be explained as follows.

Heavy hydrocarbons tends to locate at the lower part of

PVT cell due to gravity segregation effect, which leads to the

decrease in the dewpoint of fluid in the lower cell part.

However, the hydrodynamic effect makes the fluid pressure in

the top part lower than that in the lower part. Although the

difference may be very small, from the standpoint of phase

stability the small difference will make the upper fluid beyond

the stability limits. Therefore, it is necessary that the phase

transition first occur at the top part in the PVT cell due to

higher dewpoint pressure and lower fluid pressure in the top of

cell.

In order to check the capability of the

cubic EOS prediction, the measurements of fluid 1 is selected

as an example. A popular phase behavior package is first

tuned with the results of the standard phase behavior studies,

the predicted phase diagram is shown in figure 15.

Comparison of the calculated diagram with the experimental

data, in figure 2, shows poor agreement is obtained, this may

be due to wax deposition effect that is not considered in the

package. On the other hand, this implies that a more rigorous

thermodynamic model is needed to reliably predict the overall

thermodynamic states that are very important to the design of

optimum development planning.