3-Ikoku - Natural Gas Production Engineering

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Properb es

of slorsl ases

and Condensate

Syslems

Oil rerervoirr andenrate

Gas

rerervoin

i

Fig 2 2 Pressure-temperature phase diagram of a reservoir fluid

Fig 2 3 Phase daagrarns of a cap gas

and

oil lone fluid showing a) retrograde capgas and

(b) nanretrograde capgas

After

Craft and Hawkins.

begins from the reservoir and pressure declines, no change in the state of the

reservoir fluids occurs until the dew point pressure is reached at 2700 psia, point

r .

.

Below this pressure a liquid condenses out of the reservoir fluid as a fog or

dew. This is not considered t o be a normal situation since, for most hydrocarbon

fluids, a pressure reduction tends to increase the amount of gas. Therefore , this

behavior is usually referred t o as retrograde condensation, signifying that vapor-

ization generally occurs during isothermal expansion rather than condensation.

The condensation leaves the-gas phase with a lower liquid content. As the

condensed liauidadheres to thewallsofthe wr esoac esof the rock. it isimmobile.

.

Thus, the gas produced at the surface will have a lower liquid content, and the

producing GOR will rise. As the liquefiable portions of the reservoir fluids are

usually the most valuable components, the loss of part of these fluids could

substantially reduce the ultimate income from the property, which must be

considered in an economic evaluation.

Exmination of Fig. 2.2 will show that for a reservoir fluid to exhibit the

phenomenon of retrograde condensation, the initial conditions of pressure and

iemperature must exist outside the phase envelope to the right of themitical point

C and to the left of ooint T or within the ohase env el o~en the reeion marked X

The point T is cal id the cricondentherm and is the'maximum temperature at

which two phases can exist n equilibrium (300°F for the example). The process of

retrograde condensa tion continues until a point of maximum liquid volume is

reached, 10 at 2250 psia (point E .

In some cases, a sufficient volume of liqu~d ill be condensed in the reservoir

to ~rovi de obilitv of the liauid ~ha se .n such

cases

the surface fluid comoosition

depends on the relative mobilities of the vapor and liquid in the reservoir. As

production continues from point

E

to the abahdonment pressure 3 vaporization

of the ret roarade liauid occurs. This revaoorization aids liauid recoverv and mav

be evidenceh by decreasing GOR on th; surface.

This example assumesiha t the reservoir fluid composition remains constant.

Unfortunatelv. as retr oaa de condensation occurs. the reservoir fluid comwsi-

tion changes and the P-?envelope shifts, increasing retrograde liquid condinsa-

tion. Generally, for a particular initial hydrocarbon fluid, retrograde loss in-

creases at lower reservoir temperature, higher abandonment pressure, and for

greater shifting of the phase envelope to the right.

As a finalillustrat ion, conside ra reservoir;nitiall) at 350°F and 3600 psia,

revresented

bv

voint 4, in Fia. 2.2 . Since

theinttialreservoirconditionsexist

tothe

-

right of the criti'calpoint Cand outs ide the phase envelope, the reservoir fluid will

be 100 gas. Furthermore, since the reservoir temperature exceeds the cricon-

dentherm T,at no point in the isothermal depletioncycle (alongpath4,-4.) is the

phase envelope crossed. Therefore, the fluid

in

the reservoir never changes

composition; it is always in the gaseous state.

However, after the reservoir fluid leaves the reservoir and enters the well-

bore, the temperature, as well as the pressure, will decline until surface tempera-

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