Retrograde Condensation Curve

PVT Analysis of Reservoir Fluids › 10. Retrograde Condensation Curve

Reviewed by Coretest reservoir-fluids engineers · Updated 21 June 2026

The retrograde condensation (liquid dropout) test maps how much liquid phase forms inside a gas condensate as reservoir pressure declines below the dew point. It is one of the defining experiments for gas condensate reservoirs because, unlike black oils, these fluids exhibit the counter intuitive behavior in which lowering pressure first creates liquid, drives it to a maximum, and then partially re vaporizes it.

Purpose↑ Contents

The test answers the central engineering questions for a gas condensate reservoir:

• At what pressure does liquid first appear in the reservoir (the dew point), and therefore at what point does valuable condensate begin to drop out and become potentially unrecoverable?
• How much liquid accumulates as a function of pressure, expressed as a percentage of cell (or pore) volume, and where is the maximum?
• At what pressure does maximum liquid dropout occur (PMCD, Pressure of Maximum Condensate Dropout)? This pressure is critical because the dropped out condensate accumulates near the wellbore, building up a condensate bank that reduces gas relative permeability and well deliverability.
• How severe is the condensate blockage risk, and how should the reservoir be produced (pressure maintenance by gas cycling or lean gas injection, or blow down)?

In short, it quantifies the recoverable versus retained liquid hydrocarbons during depletion and underpins both reserves estimation and well productivity forecasting.

Physical principle↑ Contents

A gas condensate exists at reservoir temperature \(T\) that lies between the fluid critical temperature \(T_c\) and the cricondentherm (the maximum temperature on the two phase envelope). Within this band the isothermal pressure path crosses the dew point line twice. Starting above the upper (retrograde) dew point the fluid is a single dense gas. As pressure falls to the dew point pressure \(p_d\) the first droplet of liquid condenses. Continued depletion increases the liquid volume to a maximum at PMCD, after which further pressure reduction causes the liquid to re vaporize, so the dropout curve turns back down toward the lower dew point.

This non monotonic behavior is the thermodynamic signature of retrograde condensation. It arises because the composition of the equilibrium phases shifts as pressure changes. The vapor liquid equilibrium for each component \(i\) is governed by the equilibrium ratio (\(K\) value):

$$K_i = \frac{y_i}{x_i}$$

where \(y_i\) is the mole fraction of component \(i\) in the gas (vapor) phase and \(x_i\) is the mole fraction of component \(i\) in the liquid phase. At the dew point the system is at incipient liquid formation, so the equilibrium condition is:

$$\sum_i \frac{z_i}{K_i} = 1$$

with \(z_i\) the overall (feed) mole fraction of component \(i\). As pressure drops well below the dew point, the \(K\) values of the intermediate and heavy components increase enough that the dense components, which had condensed, preferentially partition back into the now leaner, larger gas phase, producing re vaporization.

The gas phase non ideality is captured by the real gas law:

$$Z = \frac{pV}{nRT}$$

where \(Z\) is the gas compressibility (deviation) factor (dimensionless), \(p\) is pressure, \(V\) is the gas volume, \(n\) is the number of moles of gas, \(R\) is the universal gas constant (\(8.314\ \text{J/(mol K)}\), or \(0.0831447\ \text{L bar/(mol K)}\)), and \(T\) is absolute temperature in kelvin.

The gas formation volume factor follows directly:

$$B_g = \frac{ZRT}{p}$$

In field practice \(B_g\) is often written \(B_g = \frac{p_{sc} Z T}{T_{sc}\, p}\) when referenced to standard conditions (subscript sc), giving reservoir volume per standard volume of gas (for example \(\text{rm}^3/\text{sm}^3\) or rb/Mscf). It expresses how much the gas shrinks when brought from reservoir to surface conditions.

The primary measured quantity is the liquid (condensate) dropout, the fraction of the total hydrocarbon volume present as liquid at a given pressure. By mass it is:

$$\text{Dropout}(\%) = \frac{m_{liquid}(p)}{m_{initial}} \times 100$$

where \(m_{liquid}(p)\) is the mass of condensed liquid at pressure \(p\) and \(m_{initial}\) is the total mass of the original single phase fluid charged to the cell. More commonly the dropout is reported volumetrically as the liquid volume \(V_L\) at pressure \(p\) divided by a reference volume \(V_{ref}\) (either the dew point volume or the cell volume), so that:

$$S_L(\%) = \frac{V_L(p)}{V_{ref}} \times 100$$

where \(S_L\) is the retrograde liquid saturation. The choice of \(V_{ref}\) (saturation volume at \(p_d\) versus total cell volume) must be stated explicitly because it changes the numerical value.

Apparatus and conditions↑ Contents

The experiment is run in a high pressure, high temperature, visual (windowed) or non visual PVT cell, typically a mercury free, variable volume cell with a floating piston. A visual cell or a calibrated cathetometer/capillary sight glass is strongly preferred because the dropped out liquid volume must be read directly or measured against a calibration. The cell is held at constant reservoir temperature in a thermostatic air bath or jacketed oven, and pressure is set by the piston driving fluid (hydraulic oil in modern mercury free systems, historically mercury). A magnetic or rocking stirrer ensures rapid phase equilibration.

• Temperature: held constant at reservoir temperature throughout (this is an isothermal test).
• Pressure: the controlled, varied parameter, stepped downward from above the dew point to deep depletion.
• The experiment is run either as a constant composition expansion (CCE), in which no fluid is removed and total composition is fixed, or, more representatively for depletion, as a constant volume depletion (CVD), in which gas is withdrawn at each step to return the cell to the saturation (dew point) volume so the test mimics a volumetric reservoir. The liquid dropout curve can be reported from either; the CVD version is the reservoir realistic one because it accounts for the changing remaining composition.

Procedure↑ Contents

1. Charge a representative recombined gas condensate sample (surface recombined or bottomhole) into the cleaned, evacuated PVT cell at a pressure above the expected dew point, recording the charged mass or moles.
2. Bring the cell to reservoir temperature and allow full thermal equilibration; verify single phase by confirming no liquid is visible and the pressure volume relation is that of a single phase gas.
3. Raise pressure above the dew point and record the single phase isothermal compressibility behavior to establish the starting volume \(V_{ref}\).
4. Reduce pressure in small, controlled increments. For accurate dew point and dropout definition use fine steps near the dew point, on the order of 5 to 20 bar per step (coarser steps, for example 30 to 50 bar, are acceptable deep in the two phase region).
5. After each pressure decrement, stir or rock the cell and allow the system to reach equilibrium (constant pressure at fixed volume, and a stable, settled liquid interface). Equilibration can take from minutes to hours depending on fluid richness and interfacial behavior near the critical region.
6. Detect and record the dew point as the pressure at which the first persistent liquid appears (visual cell) or at which the \(p\)–\(V\) trend shows a discernible break.
7. At each step, measure and record the equilibrium pressure, the total cell volume, and the volume of the settled retrograde liquid (read directly in a visual cell or computed from the piston position and calibration).
8. If running CVD, withdraw gas at constant pressure until the cell returns to the saturation volume \(V_{ref}\), measure the volume and composition of the withdrawn gas, and record the produced gas and remaining cell contents before moving to the next step.
9. Continue the pressure decline well past PMCD, into the re vaporization branch, to at least the abandonment pressure or to where the liquid volume clearly stops increasing and begins to decline.
10. At the end, optionally depressurize fully and perform a material balance check on total recovered hydrocarbons against the charged mass.
11. Convert all readings to the chosen reference basis and compute the derived parameters below.
12. Construct the liquid dropout curve and identify the dew point, PMCD, and maximum liquid fraction.

Measured parameters↑ Contents

At each pressure stage the following are recorded directly:

• Equilibrium pressure \(p\) (bar or psia).
• Cell temperature \(T\) (held constant, verified each step).
• Total cell volume \(V_{total}\) and the settled retrograde liquid volume \(V_L\).
• The dew point pressure \(p_d\) (first appearance of liquid).
• In CVD mode: the volume of gas removed at each stage and its composition (by chromatography), plus the standard conditions volume of that produced gas.

Calculated and derived parameters↑ Contents

• Retrograde liquid dropout (volumetric): \(S_L(\%) = \frac{V_L(p)}{V_{ref}} \times 100\), with \(V_{ref}\) defined as either the saturation volume at \(p_d\) or the total cell volume; state which.
• Liquid dropout by mass (when phase densities/compositions are known): \(\text{Dropout}(\%) = \frac{m_{liquid}(p)}{m_{initial}} \times 100\).
• Pressure of maximum condensate dropout PMCD: the pressure at which \(\frac{dS_L}{dp} = 0\) (the peak of the dropout curve).
• Maximum liquid fraction \(S_{L,max}\): the value of \(S_L\) at PMCD.
• Gas deviation factor at each stage: \(Z = \frac{pV}{nRT}\), where \(n\) is the moles of gas in the equilibrium vapor.
• Gas formation volume factor: \(B_g = \frac{ZRT}{p}\) (or \(B_g = \frac{p_{sc} Z T}{T_{sc}\, p}\) referenced to standard conditions).
• Equilibrium ratios \(K_i = \frac{y_i}{x_i}\) when phase compositions are measured, useful for tuning an equation of state model.
• In CVD mode: cumulative gas recovery and the produced wellstream composition versus pressure, from which surface condensate yield (for example STB/MMscf) and ultimate liquid recovery can be derived.

Reporting and typical curves↑ Contents

The principal deliverable is the retrograde liquid dropout curve, a plot of liquid volume percent (\(S_L\)) on the y axis versus pressure on the x axis, read from high pressure on the right to low pressure on the left as the reservoir depletes. The curve rises from zero at the dew point \(p_d\), climbs to a peak (\(S_{L,max}\)) at PMCD, then declines along the re vaporization branch. Supporting outputs include:

• A table of pressure, \(S_L\), \(Z\), and \(B_g\) at every stage.
• The reported dew point pressure \(p_d\) and its determination method.
• PMCD and the maximum liquid fraction.
• For CVD runs: tables of produced gas composition, cumulative recovery, \(Z\) and \(B_g\) versus pressure, and two phase \(Z\) factor.

The curve is read to classify the fluid: a high peak (rich condensate) signals a large condensate bank and severe near wellbore blockage, motivating pressure maintenance or gas cycling; a low, shallow peak (lean condensate) suggests blow down may be acceptable. The position of PMCD relative to the planned abandonment pressure tells the engineer how much of the depletion path lies on the damaging accumulation branch versus the favorable re vaporization branch.

Practical notes and pitfalls↑ Contents

• Sample representativeness is paramount. A non representative recombination ratio or any loss of heavy ends during sampling shifts the dew point and the entire dropout curve. Validate the recombined sample against separator GOR and compositions before testing.
• Use a mercury free, visual variable volume cell where possible. Mercury contamination is a safety and disposal hazard, and direct visual reading of the thin liquid film near the dew point greatly improves accuracy. The first traces of retrograde liquid form a faint mist or film that is easy to miss, leading to an underestimated dew point.
• Allow genuine equilibrium at each step. Near the critical region and near the dew point, interfacial tension is very low and the liquid settles slowly; reading volumes before equilibrium gives a smeared, inaccurate curve. Insufficient equilibration time is the most common error in this test.
• Use fine pressure steps near the dew point (5 to 20 bar) to resolve \(p_d\) and the early dropout, where small errors propagate strongly into reserves and deliverability estimates.
• Account for dead volume and small liquid holdup. Condensate can wet cell walls, ports, and the stirrer and not report in the readable interface, biasing \(S_L\) low. Calibrate cell dead volume and account for wetting films, especially for small absolute liquid volumes.
• Always state the reference volume basis for \(S_L\) (dew point saturation volume versus total cell volume), since the two conventions differ numerically and are routinely confused between labs.
• Prefer CVD for depletion forecasting. CCE keeps composition fixed and therefore overstates retained liquid late in depletion; CVD removes equilibrium gas and reproduces the changing remaining fluid, giving a reservoir realistic dropout and recovery profile. Choose the method to match the engineering question.
• Use the measured curve to tune an equation of state model rather than relying on raw lab points alone; a tuned EOS lets the dropout behavior be extrapolated to compositions, temperatures, and injection scenarios (for example lean gas cycling) not directly tested.